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IEEE Standard Requirements For Secondary Network Protectors

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IEEE Std C57.12.

44-2000
(Revision of
IEEE Std C57.12.44-1994)

IEEE Standard Requirements for

Secondary Network Protectors

Sponsor
Transformers Committee
of the
IEEE Power Engineering Society

Approved 10 August 2000

IEEE-SA Standards Board

Abstract: The performance, electrical and mechanical interchangeability, and the safety of the
equipment are covered. The proper selection of such equipment is established as a basis for use
in this standard. Certain electrical, dimensional, and mechanical characteristics are described; and
certain safety features of three-phase, 60 Hz, low-voltage (600 V and below) network protectors are
taken into consideration. This equipment is used for automatically connecting and disconnecting a
network transformer from a secondary spot or grid network.
Keywords: grid network, network, protector, spot network

The Institute of Electrical and Electronics Engineers, Inc.

3 Park Avenue, New York, NY 10016-5997, USA

Copyright © 2000 by the Institute of Electrical and Electronics Engineers, Inc.

Print: ISBN 0-7381-2520-2 SH94870

PDF: ISBN 0-7381-2521-0 SS94870

No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior
written permission of the publisher.

--``,-`-`,,`,,`,`,,`---

Copyright The Institute of Electrical and Electronics Engineers, Inc.

Provided by IHS under license with IEEE
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IEEE Standards documents are developed within the IEEE Societies and the Standards Coordinating Com-
mittees of the IEEE Standards Association (IEEE-SA) Standards Board. Members of the committees serve
voluntarily and without compensation. They are not necessarily members of the Institute. The standards
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well as those activities outside of IEEE that have expressed an interest in participating in the development of
the standard.

Use of an IEEE Standard is wholly voluntary. The existence of an IEEE Standard does not imply that there
are no other ways to produce, test, measure, purchase, market, or provide other goods and services related to
the scope of the IEEE Standard. Furthermore, the viewpoint expressed at the time a standard is approved and
issued is subject to change brought about through developments in the state of the art and comments
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sonable to conclude that its contents, although still of some value, do not wholly reflect the present state of
the art. Users are cautioned to check to determine that they have the latest edition of any IEEE Standard.

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Comments for revision of IEEE Standards are welcome from any interested party, regardless of membership
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Note: Attention is called to the possibility that implementation of this standard may
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Introduction
(This introduction is not a part of IEEE Std C57.12.44-2000, IEEE Standard for Secondary Network
Protectors.)

This is the ﬁrst revision of IEEE Std C57.12.44, which was originally published in 1994. The main purpose
of this revision is to incorporate the many improvement ideas received during the balloting for the original
standard and to expand fuse information. The revision ofﬁcially started with an IEEE Project Authorization
Request approval on 29 September 1995. The revision was accomplished by the Working Group on
Requirements for Secondary Network Protectors under the Underground Transformers and Network
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Protectors Subcommittee of the IEEE PES Transformers Committee. The major changes are the addition of
ﬁgures depicting the terminals in Clause 11, the addition of fuse pictures and fuse curves in Annex B, and
the conversion to metric as the primary dimensional system in accordance with IEEE policy. Clauses 4, 5, 6,
8, 10, and 11 of the standard were signiﬁcantly revised, as were Annexes B and C.

A task force working group consisting of EEI, NEMA, and IEEE PES delegates developed the original doc-
ument. The ﬁrst working group meeting was held 24 February 1988, in Washington, D.C. The document was
completed in late 1994 by the Working Group on Requirements for Secondary Network Protectors under the
Underground Transformers and Network Protectors Subcommittee of the of the IEEE PES Transformers
Committee. Mr. R. B. Robertson chaired the working group during the preparation and balloting of the orig-
inal standard.

This IEEE standard is a voluntary consensus standard. Its use becomes mandatory only when required by a
duly constituted legal authority or when speciﬁed in a contractual relationship. To meet specialized needs
and to allow innovation, speciﬁc changes are permissible when mutually determined by the user and the pro-
ducer, provided such changes do not violate existing laws and are considered technically adequate for the
function intended.

The Working Group on Requirements for Secondary Network Protectors had the following membership dur-
ing the revision process:
D. H. Mulkey, Chair
A. J. Alcantara, Jr. G. Miller E. Owen
E. A. Bertolini M. C. Mingoia B. Nutt
R. R. Butani J. R. Moffat P. G. Risse
R. W. Fisher C. G. Niemann A. L. Robinson, Jr.
R. D. Graham P. E. Orehek K. Romano
J. L. Harper W. G. Wimmer

Other individuals who have participated in the revision are:

A. Bartek N. P. McQuin J. Somma
R. B. Robertson

Copyright © 2000 IEEE. All rights reserved. iii

Copyright The Institute of Electrical and Electronics Engineers, Inc.
Provided by IHS under license with IEEE
No reproduction or networking permitted without license from IHS Not for Resale
The following persons were on the balloting committee:
S. H. Aguirre J. D. Huddleston, III C. G. Niemann
G. Andersen J. S. Hurst P. E. Orehek
R. L. Barker R. W. Ingham D. S. Patel
A. Bartek R. I. James W. F. Patterson
E. A. Bertolini B. Klaponski P. A. Payne
A. Bolliger B. Kumar L. W. Pierce
R. R. Butani S. R. Lambert P. G. Risse
D. Chu J. P. Lazar A. L. Robinson, Jr.
P. W. Clarke M. Loveless J. R. Rossetti
J. L. Corkran D. L. Lowe H. Jin Sim
J. N. Davis W. A. Maguire T. Singh
R. H. Fausch K. T. Massouda J. E. Smith
R. D. Graham J. W. Matthews R. J. Stahara
R. L. Grunert N. P. McQuin R. E. Sullivan
N. Wayne Hansen J. Melanson B. Scott Wilson
P. J. Hopkinson J. R. Moffat W. G. Wimmer
D. H. Mulkey

The ﬁnal conditions for approval of this standard were met on 10 August 2000. This standard was condition-
ally approved by the IEEE-SA Standards Board on 21 June 2000, with the following membership:

Donald N. Heirman, Chair

James T. Carlo, Vice Chair
Judith Gorman, Secretary

Satish K. Aggarwal James H. Gurney James W. Moore

Mark D. Bowman Richard J. Holleman Robert F. Munzner
Gary R. Engmann Lowell G. Johnson Ronald C. Petersen
Harold E. Epstein Robert J. Kennelly Gerald H. Peterson
H. Landis Floyd Joseph L. Koepﬁnger* John B. Posey
Jay Forster* Peter H. Lips Gary S. Robinson
Howard M. Frazier L. Bruce McClung Akio Tojo
Ruben D. Garzon Daleep C. Mohla Donald W. Zipse

*Member Emeritus

Also included is the following nonvoting IEEE-SA Standards Board liaison:

Alan Cookson, NIST Representative

Donald R. Volzka, TAB Representative

Noelle D. Humenick
IEEE Standards Project Editor

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iv Copyright © 2000 IEEE. All rights reserved.

Copyright The Institute of Electrical and Electronics Engineers, Inc.
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No reproduction or networking permitted without license from IHS Not for Resale
Contents
1. Overview.............................................................................................................................................. 1

1.1 Scope............................................................................................................................................ 1
1.2 Purpose......................................................................................................................................... 1
1.3 Word usage .................................................................................................................................. 1

2. References............................................................................................................................................ 1

3. Definitions............................................................................................................................................ 2

4. Service conditions................................................................................................................................ 4

4.1 Usual service conditions .............................................................................................................. 4

4.2 Unusual service conditions .......................................................................................................... 4

5. Design tests .......................................................................................................................................... 5

5.1 Continuous current thermal test ................................................................................................... 5

5.2 Electrical tests .............................................................................................................................. 7
5.3 Mechanical endurance test......................................................................................................... 10

6. Production tests.................................................................................................................................. 11

6.1 Operational tests......................................................................................................................... 11

6.2 Dielectric tests............................................................................................................................ 13
6.3 Insulation resistance tests........................................................................................................... 13
6.4 Current path resistance tests....................................................................................................... 14
6.5 Mechanical tests......................................................................................................................... 14

7. Relay characteristics .......................................................................................................................... 15

7.1 General....................................................................................................................................... 15
7.2 Closing characteristics ............................................................................................................... 15
7.3 Tripping characteristics.............................................................................................................. 15
7.4 Closing adjustment..................................................................................................................... 16
7.5 Tripping adjustment ................................................................................................................... 16

8. Fuses and fuse mountings .................................................................................................................. 18

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8.1 Fuse provisions .......................................................................................................................... 18

8.2 Functional performance specifications ...................................................................................... 19
8.3 Fuse types................................................................................................................................... 19

9. Rating requirements ........................................................................................................................... 20

9.1 Nameplates................................................................................................................................. 20
9.2 Interrupting rating ...................................................................................................................... 20
9.3 AC voltage ratings ..................................................................................................................... 20
9.4 Dielectric test voltage ................................................................................................................ 20

Copyright © 2000 IEEE. All rights reserved. v

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9.5 Control voltage........................................................................................................................... 20
9.6 Closing voltage .......................................................................................................................... 22
9.7 Tripping currents........................................................................................................................ 22

10. Mechanical performance specifications............................................................................................. 23

10.1 Submersible type........................................................................................................................ 23

10.2 Nonsubmersible type ................................................................................................................. 23
10.3 Open frame type......................................................................................................................... 23
10.4 Mounting application ................................................................................................................. 24
10.5 General requirements ................................................................................................................. 24
10.6 Finish ......................................................................................................................................... 27

11. Other requirements............................................................................................................................. 27

11.1 Submersible enclosure ............................................................................................................... 27

11.2 Nonsubmersible enclosure ......................................................................................................... 27
11.3 Open frame enclosure ................................................................................................................ 28
11.4 Mounting configurations............................................................................................................ 28
11.5 Network-side terminations......................................................................................................... 28

Annex A (normative) Classification of insulating materials ......................................................................... 30

Annex B (informative) Network fuse application ......................................................................................... 34

Annex C (informative) Pumping ................................................................................................................... 44

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Annex D (informative) The unprotected zone in network protectors............................................................ 45

Annex E (informative) Contact surface oxidation ......................................................................................... 46

Annex F (informative) Reverse current sensitivity........................................................................................ 47

Annex G (informative) Network protector enclosures .................................................................................. 50

vi Copyright © 2000 IEEE. All rights reserved.

1. Overview

1.1 Scope

This standard describes certain electrical, dimensional, and mechanical characteristics and takes into consid-
eration certain safety features of three-phase, 60 Hz, low-voltage (600 V and below) network protectors.
They are used for automatically connecting and disconnecting a network transformer from a secondary spot
or grid network.

1.2 Purpose

This standard is intended for use as a basis for establishing the performance, electrical and mechanical inter-
changeability, and safety of the equipment covered and to assist in the proper selection of such equipment.

1.3 Word usage

As used in this standard, the word “shall” indicates mandatory requirements. The words “should” and “may”
refer to matters that are recommended and permissive, respectively, but not mandatory.

NOTE—The introduction of this standard describes the circumstances under which the document may be used on a
mandatory basis.

2. References

All characteristics, deﬁnitions, tests, and voltage designations, except as speciﬁcally covered in this stan-
dard, shall be in accordance with the standards listed below.

When a standard referred to in this document is superseded by a revision, the revision shall not apply. The
referenced standard and the speciﬁc referenced edition shall be the applicable referenced standard until the
new version of the referenced document is incorporated by formal action or appropriate revision of the citing
standard.

Copyright © 2000 IEEE. All rights reserved.

--``,-`-`,,`,,`,`,,`--- 1
Copyright The Institute of Electrical and Electronics Engineers, Inc.
Provided by IHS under license with IEEE
No reproduction or networking permitted without license from IHS Not for Resale
IEEE
Std C57.12.44-2000 IEEE STANDARD REQUIREMENTS

ANSI B1.1-1989, American National Standard for Uniﬁed Inch Screw Threads (UN and UNR Thread
Forms).1

ANSI B1.20.1-1983, American National Standard for Pipe Threads, General Purpose (Inch).

ANSI C57.12.28-1996, American National Standard for Switchgear and Transformers—Pad-Mounted

Equipment-Enclosure Integrity.

ANSI C57.12.32-1994, American National Standard for Submersible Equipment—Enclosure Integrity.

ANSI C57.12.40-1994, American National Standard for Secondary Network Transformers—Subway and
Vault Types (Liquid Immersed)—Requirements.

ANSI C57.12.57-1987, American National Standard for Transformers—Ventilated Dry-Type Network

Transformers 2500 kVA and Below, Three-Phase with High-Voltage 34 500 Volts and Below, Low-Voltage
216Y/125 and 480Y/277 Volts—Requirements.

ANSI C57.12.70-1978 (Reaff 1992), American National Standard Terminal Markings and Connections for
Distribution and Power Transformers.

ANSI C84.1-1995, American National Standard Voltage Ratings (60 Hz) for Electric Power Systems and
Equipment.

IEEE Std 1-1986 (Reaff 1992), IEEE Standard General Principles for Temperature Limits in the Rating of
Electric Equipment and for the Evaluation of Electrical Insulation.2

IEEE Std 100, IEEE Standard Dictionary of Electrical and Electronics Terms, Sixth Edition.
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ANSI C37.09-1979 (Reaff 1988), IEEE Standard Test Procedure for AC High-Voltage Circuit Breakers
Rated on a Symmetrical Current Basis.

IEEE Std C37.108-1989 (Reaff 1994), IEEE Guide for the Protection of Network Transformers.

IEEE Std C57.12.00-1993, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power,
and Regulating Transformers.

IEEE Std C57.12.80-1978 (Reaff 1992), IEEE Standard Terminology for Power and Distribution
Transformers.

IEEE Std C57.91-1995, IEEE Guide for Loading Mineral-Oil-Immersed Overhead and Pad-Mounted Distri-
bution Transformers Rated 500 kVA and Less with 65 °C or 55 °C Average Winding Rise.

3. Deﬁnitions

An asterisk (*) following a deﬁnition indicates that the deﬁnition is identical to that which appears in IEEE
Std 100, IEEE Standard Dictionary of Electrical and Electronics Terms.

1ANSI publications are available from the Sales Department, American National Standards Institute, 11 West 42nd Street, 13th Floor,
New York, NY 10036, USA (http://www.ansi.org/).
2IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, P.O. Box 1331, Piscataway,
NJ 08855-1331, USA (http://standards.ieee.org/).

2 Copyright © 2000 IEEE. All rights reserved.

3.1 arcing contacts: The contacts of a switching device on which the arc is drawn after the main (and inter-
mediate, where used) contacts have parted.*

3.2 desensitizing relay: A relay that prevents tripping of a network protector on transient power reversals,
which neither exceed a predetermined value nor persist for a predetermined time.

3.3 intermediate contacts: Contacts in the main circuit that part after the main contacts and before the arcing
contacts have parted.*

3.4 main contacts: Contacts that carry all or most of the main current.*

3.5 network limiter: An enclosed fuse for disconnecting a faulted cable from a low-voltage network distri-
bution system and for protecting the unfaulted portions of that cable against serious thermal damage.*

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3.6 network master relay: A relay that functions as a protective relay by opening a network protector when
power is back-fed into the supply system and as a programming relay by closing the protector in conjunction
with the network phasing relay when polyphase voltage phasors are within prescribed limits.*

3.7 network phasing relay: A monitoring relay that has as its function to limit the operation of a network
master relay so that the network protector may close only when the voltages on the two sides of the protector
are in a predetermined phasor relationship.*

3.8 network protector: An assembly comprising a circuit breaker and its complete control equipment for
automatically disconnecting a transformer from a secondary network in response to predetermined electrical
conditions on the primary feeder or transformer, and for connecting a transformer to a secondary network
either through manual control or automatic control responsive to predetermined electrical conditions on the
feeder and the secondary network.
NOTE—The network protector is usually arranged to connect automatically its associated transformer to the network
when conditions are such that the transformer, when connected, will supply power to the network and to automatically
disconnect the transformer from the network when power ﬂows from the network to the transformer.*

3.9 network protector fuse: A back-up device for the network protector.

3.10 network secondary distribution system: A system of alternating-current distribution in which the sec-
ondaries of the distribution transformers are connected to a common network for supplying light and power
directly to consumers’ services.

3.11 phasing voltage: The voltage across the open contacts of a selected phase.
NOTE—This voltage is equal to the phasor difference between the transformer voltage and the corresponding network
voltage.*

3.12 pumping: The unintentional cyclical tripping and closing of a network protector.

3.13 removable breaker: The removable breaker consists of the circuit breaker, disconnecting provisions,
network relays, auxiliary panels, current transformers, control devices, other attachments, and all intercon-
necting wiring, which can be rolled out of the network protector enclosure on rails for maintenance or
removal.

3.14 short-time current: The current carried by a device, an assembly, or a bus for a speciﬁed short time
interval.*

3.15 shunt release (shunt trip): A release energized by a source of voltage.

NOTE—The voltage may be derived either from the main circuit or from an independent source.*

Copyright © 2000 IEEE. All rights reserved. 3

3.16 solid-state or microprocessor network relay: A relay with no mechanical parts, using solid-state com-
ponents, that performs the combined functions of the master and phasing relays, and that may include a time
delay function.

3.17 spot network: A small network, usually at one location, consisting of two or more primary feeders,
with network units and one or more load service connections.

3.18 trip-free: The capability of a switching device to have the moving contacts return to and remain in the
opening position when the opening operation is initiated after the initiation of the closing operation, even if
the closing force and command are maintained.*

4. Service conditions

4.1 Usual service conditions

4.1.1 General

Network protectors conforming to this standard shall be suitable for operation at rated current under the fol-
lowing conditions.

4.1.2 Temperature

The ambient temperature shall be between –20 °C and 40 °C.

4.1.3 Altitude

The altitude shall not exceed 1000 m.

4.1.4 Generation

The network protector is designed to be applied to a system with power generation only on the high voltage
side of the transformer that supplies networked secondary without any secondary power generation.
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4.2 Unusual service conditions

4.2.1 General

Conditions other than those described in 4.1.1 are considered unusual service, and when prevalent, shall be
brought to the attention of those responsible for the design and application of the apparatus. Examples of
some of these conditions are listed in 4.2.2 and 4.2.3.

4.2.2 Unusual temperature and altitude conditions

Network protectors may be used at higher or lower ambient temperatures or at higher altitudes than those
speciﬁed in 4.1, but special consideration must be given to these applications. IEEE Std C57.91-1995 pro-
vides information on recommended practices for the accompanying transformer.

4 Copyright © 2000 IEEE. All rights reserved.

4.2.3 Insulation at high altitude

The dielectric strength of network protectors, which depend primarily on air for insulation, decreases as the
altitude increases due to the effect of decreased air density. Network protectors design voltage shall be de-
rated using the correction factors of Table 1 in IEEE Std C57.12.00-1993.

The insulation level at 1000 m multiplied by the correction factor must not be less than the required insula-
tion level at the required altitude.

5. Design tests

The design tests shall be divided into three separate categories designated as continuous current thermal,
electrical, and mechanical endurance.

5.1 Continuous current thermal test

The continuous current test shall be performed to demonstrate that the network protector can carry its rated
continuous current at its rated frequency in an ambient temperature range between 10 °C and 40 °C without
attaining a temperature rise in excess of those listed in Table 1 (see Annex A for more information.) The net-
work protector shall be tested with the highest loss fuse that would normally be applied with that network
protector rating.

Table 1—Temperature rise

Material temperature class Temperature rise °C

90 50

105 65

130 90

155 115

180 140

200 160

220 180

5.1.1 Test conditions

The network protector in its enclosure, with fuses, shall be connected to a balanced three-phase current source
at the transformer side of the network protector with copper bus bar whose ampacity is rated to carry the full
rated current. This bus bar shall extend a minimum of 1.2 m from the network protector. This bus bar shall have
a current density range of 120 A/cm2 to 230 A/cm2.3 The transformer throat openings shall be suitably
enclosed with an insulating panel permitting the transformer extension bus to exit, but to seal all openings at
the throat area. The network side terminals shall be short circuited with the same size bus bar as used on the
transformer side connection and shall extend a minimum of 1.2 m from the network side terminals.

3800 A/in2 to 1500 A/in2.

Copyright © 2000 IEEE. All rights reserved. --``,-`-`,,`,,`,`,,`---

5
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IEEE
Std C57.12.44-2000 IEEE STANDARD REQUIREMENTS

Thermocouples shall be used to measure the temperature. Thermocouples used for measuring the tempera-
tures of insulation shall be located on the current carrying member or other metal part at a point as close as
practical to the accessible junction of the insulation and the current carrying member or other metal part.
Thermocouples used for measuring the temperature of the transformer or network side terminal connections
and other conducting joints shall be located approximately 13 mm from the terminal or other conducting
joints on the current carrying member. Thermocouples shall be held in intimate contact with the conductor
surface by such methods as welding, drilling and peening, or cementing. Thermocouples may be used to
determine the internal air temperature within the network protector enclosure by suspending the unmounted
thermocouple bulbs in air.

The bus entering or leaving the network protector shall not be used to add or remove heat during the contin-
uous current thermal test. To determine this, three thermocouples shall be placed 30 cm apart along the bus
attached to the network protector. The bus is then shielded from air convection ﬂow such that the three ther-
mocouples along the bus bar are within 5 °C of the adjacent thermocouple on the bus bar. These readings are
only applicable to the ﬁnal three readings taken at 30 min intervals and are not subject to the temperature
variation criteria of the internal parts of the network protector. Refer to Figure 1 for the minimum number of
thermocouples required.

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NOTE—Thermocouples 1–16 shall be duplicated on all three phases.

Figure 1—Thermocouple locations

5.1.2 Test duration

The rated continuous current of the network protector at rated frequency shall be applied continuously until
the temperature rise stabilizes. The temperature rise shall be considered stable when three successive
readings taken at 30 min intervals vary no more than ±1.0 °C. If the temperature rise at the end of the third
interval is equal to the established limit (see Table 1) and if the temperature rise has increased since the
previous reading, the test shall be continued one more interval.

5.1.3 Ambient temperature

The network protector shall be tested at an ambient temperature between 10 °C and 40 °C. No correction
factors need be applied. The temperature rise shall, in no case, exceed that speciﬁed in Table 1. The ambient

6 Copyright © 2000 IEEE. All rights reserved.

temperature shall be determined by taking the average of the readings of the three thermocouples placed
horizontally 30 cm from the projected periphery of the network protector enclosure and approximately on a
vertical line as follows:

a) One approximately 30 cm above the network protector or enclosure (including bushings).

b) One approximately 30 cm below the network protector enclosure. In the case of a ﬂoor mounted net-
work protector enclosure, it shall be 30 cm above the ﬂoor or mounting base.
c) One approximately midway between the above two positions.

All reasonable precautions shall be taken to reduce errors caused by the time lag between the temperature
rise of large apparatus and variations in the ambient temperature. Therefore, the ambient sensing thermocou-
ples shall be immersed in a suitable liquid, such as oil, in a suitable metal cup. The size of the metal cup
should be determined by the size of the apparatus being tested, but in no case shall the cup be any smaller
than 25 mm in diameter and 51 mm high.

5.1.4 Performance

Network protectors shall be considered to have passed this test if the limits of observable temperature rise as
listed in Table 1, for every point measured per Figure 1, are not exceeded. The resistance of the conductors
shall be measured on the network protector and its enclosure, either as one unit or separately. The resistance
values recorded should be representative of the values recorded after the continuous current thermal test and
serve as a basis for quality control monitoring of production of the individual designs and establishing the
maximum acceptable limits of the individual designs and conﬁgurations.

5.2 Electrical tests

The test circuit conditions shall be verified by measuring the current in the test circuit by short circuiting the
supply side of the test circuit at the network protector. For the purpose of determining the X/R ratio, the ac
current shall be measured in the first half cycle after the short circuit is initiated, this current being calculated
in accordance with IEEE Std C37.09-1979. The test circuit shall have an X/R ratio between 6.6 and 8 with X
and R in series. Any reactor used in the test circuit shall be an air-cored reactor. The test circuit impedances
shall be such that the three-phase symmetrical currents are essentially equal. The time at which the magni-
tude of the symmetrical current is required to meet a rating level of the network protector is specified for
each test separately.

In all cases the network protector enclosure shall be insulated from ground and shall be connected to ground
through a 30 A current limiting fuse, having an interrupting rating capacity not less than the interrupting rat-
ing of the network protector.

5.2.1 Interrupting rating test

The interrupting rating of the network protector, without fuses, shall be expressed in rms symmetrical amperes
at the maximum design voltage of the network protector.

5.2.1.1 Test conditions

All interrupting testing must be three phase with the removable breaker installed in its enclosure. The test
circuit impedance being such that the average symmetrical phase current at the end of 1 s is not less than the
interrupting rating of the network protector.

Two separate tests shall be performed. One test shall energize from the transformer side with the network
side shorted; the other test shall energize from the network side with the transformer side shorted.

Copyright © 2000 IEEE. All rights reserved.

--``,-`-`,,`,,`,`,,`---
7
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No reproduction or networking permitted without license from IHS Not for Resale
IEEE
Std C57.12.44-2000 IEEE STANDARD REQUIREMENTS

5.2.1.2 Operating duty

The operating duty of the network protector shall consist of an opening operation, followed by a 2 min inter-
val, and another opening operation (O-2 min-O). For each opening operation, the network protector shall
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carry its rated interrupting current for 1 s before opening.

5.2.1.3 Performance

At the end of the test the network protector shall be in substantially the same mechanical condition as before
the test. It shall be capable of withstanding rated voltage in the OPEN position, and it shall be capable of car-
rying rated current without exceeding the temperature rise limit

After the test, the network protector must be inspected, and may be maintained. The ground fuse must be
veriﬁed to be intact.

5.2.2 Short-time current test

A network protector without fuses shall be given a short-time current rating, expressed in rms symmetrical
amperes at the maximum design voltage of the network protector.

5.2.2.1 Test conditions

All short-time current testing must be three phase, with the test circuit impedance being such that the aver-
age symmetrical phase current at the end of 1 s is not less than the short-time rating of the network protector.
The maximum peak current at the initiation of current shall not be less than 2.3 times the short-time symmet-
rical current rating.

5.2.2.2 Operating duty

The rated short-time current shall be maintained for a period of 1 s without the network protector breaker’s
contacts parting. The test circuit is to be energized and de-energized only by the action of the test station
breakers.

5.2.2.3 Performance

At the end of the test, the network protector shall be capable of meeting its interrupting rating and capable of
carrying rated continuous current without exceeding the temperature rise limit.

After the test, the network protector must be inspected to verify that the main contacts are free of excessive
burning or pitting. The ground fuse must be veriﬁed to be intact.

5.2.3 Fault close test

For network protectors having either spring close mechanisms or stored-energy mechanisms, fault close lev-
els shall be tested to verify the network protector’s ability, less the fuses, to close and latch its contacts.

5.2.3.1 Test conditions

All fault close current testing must be three phase, with the test circuit impedance such that the rms total cur-
rent of the maximum phase during the ﬁrst half cycle is not less than 1.35 times the fault close and latch
symmetrical current rating of the network protector. The maximum peak current at the initiation of current
shall not be less than 2.3 times the fault close and latch symmetrical current rating.

8 Copyright © 2000 IEEE. All rights reserved.

5.2.3.2 Operating duty

The fault closing shall consist of electrically closing the network protector breaker into the test circuit. The
test circuit is then de-energized by the action of the test station breaker after 10 cycles.

5.2.3.3 Performance

Directly after this test is completed, the network protector breaker shall be checked to ensure the following:

a) The spring closing mechanism has completed its closing cycle successfully.
b) The main contacts are free from excessive burning or pitting.
c) The network protector breaker can be tripped manually.

At the end of this test, the network protector shall be capable of carrying rated continuous current without
exceeding its rated temperature rise and shall be capable of meeting its interrupting rating. The ground fuse
must be veriﬁed to be intact.

5.2.4 Fuse interruption test

The network protector in its enclosure with fuses shall be subjected to fault currents that are a minimum of
the interrupting rating to a maximum of 110% of the interrupting rating of the network protector. The trip
circuit of the network protector shall be made inoperable for this test. This test is intended to show that the
fuses are capable of clearing a phase to phase fault without arcing to ground.

5.2.4.1 Test conditions

All fuse interrupting testing must be three phase with the removable breaker installed in its enclosure. The
test circuit impedance shall be such that the average symmetrical phase current at the end of 1 s is not less
that the interrupting rating of the network protector.

Two separate tests shall be performed. One test shall energize from the transformer side with the network
side shorted. The other test shall energize from the network side with the transformer side shorted.

5.2.4.2 Operating duty

The operating duty cycle of the network protector fuse test shall consist of three separate three-phase fuse
interruptions at the maximum interrupting rating and maximum design voltage of the network protector.
Each test shall be conducted with new fuses. The network protector shall be inspected and may be main-
tained as required. The test circuit shall remain energized for 1 s after interruption.

5.2.4.3 Performance

Directly after each three-phase fuse interruption, the network protector shall be checked to ensure the
following:

a) At least two fuses cleared successfully

b) The 30 A ground fuse is intact

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Copyright © 2000 IEEE. All rights reserved. 9
Copyright The Institute of Electrical and Electronics Engineers, Inc.
Provided by IHS under license with IEEE
No reproduction or networking permitted without license from IHS Not for Resale
IEEE
Std C57.12.44-2000 IEEE STANDARD REQUIREMENTS

5.3 Mechanical endurance test

5.3.1 Test conditions

The removable breaker is to be mounted in its enclosure with the enclosure door in the closed position.
Rated voltage of the network protector shall be supplied directly into the control circuitry in such a manner
as to utilize as many control components as possible, such as auxiliary switch contacts, latch check contacts,
auxiliary relays, closing motor, tripping device, motor seal-in contacts, resistors, rectiﬁers, and control
transformers.

Electrical heaters shall be strategically placed throughout the network protector enclosure. The heaters shall
increase the internal enclosure air temperature to 95 °C ± 5 °C. On ventilated or open types of enclosures the

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entire network protector shall be placed into an environment whose ambient temperature is 95 °C ± 5 °C.
Three thermocouples shall be mounted at convenient locations within the network protector enclosure. One
thermocouple shall be located within 150 mm from the bottom of the enclosure. The second thermocouple
shall be located at half the total height of the enclosure, while the third thermocouple shall be located within
150 mm from the top of the enclosure. The average of the three temperature readings shall be used as the ele-
vated temperature reading. Temperature stability must be maintained for 1.5 hours prior to proceeding with
the mechanical test.

5.3.2 Operating duty

The mechanism shall be operated 10 000 consecutive times without malfunction or interim adjustment. One
operation is deﬁned as a CLOSING and OPENING of the breaker under no electrical load.

5.3.3 Rate of operation

The rate of operation shall be at least one operation every 2 min. During each operation, the network protec-
tor circuit breaker shall remain closed for no less than 1 s. The device used to control the input voltage shall
have a system that controls the opening and the closing of the network protector circuit breaker and stops the
test if the circuit breaker fails to OPEN or CLOSE. The control device should indicate the mode of failure.

5.3.4 Performance

At the end of this test, the network protector breaker shall be thoroughly inspected to verify and record that
the critical adjustments are in tolerance. Any change or modiﬁcation must be reviewed as to its impact on its
thermal and/or electrical test, if previously conducted.

Any failure of the external control circuitry does not require the test counter to be reset to zero. The external
control device can be replaced or repaired and the mechanical test may recommence.

Any component that has been scorched, discolored, or burned by the internal heaters shall not be considered
a failure.

Any and all changes and/or modiﬁcations must be fully documented during this phase of the testing along
with the starting/stopping counter readings.

6. Production tests

The production tests shall be performed on every network protector. They are divided into the following
categories:

10 Copyright © 2000 IEEE. All rights reserved.

a) Operational
b) Dielectric
c) Insulation resistance
d) Current path resistance
e) Mechanical

6.1 Operational tests

The purpose of the operational test is to prove the ability of the network protector to function both in closing
and opening the network protector breaker with and without relays under both nominal rated voltage and
minimum voltage conditions.

6.1.1 Electrical operations

6.1.1.1 Normal voltage test

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With the relays removed, the test control leads should be connected to that portion of the control circuitry
that will permit a remote CLOSE and OPEN operation of the removable breaker. The test source should be
connected to the rated voltage of the network protector and the removable breaker should be tested 50 oper-
ations per Table 2. The removable breaker contacts shall remain closed for a minimum of 1 s before the
opening sequence is initiated.

Table 2—Operational test summary

Test Removable Relays Voltage Number of

subclause breaker utilized level operationsa

6.1.1.1 Out of enclosure No Rated 50

6.1.1.2 Out of enclosure No Minimum 5

6.1.1.3 Out of enclosure Yes Rated 25

6.1.1.4 Out of enclosure No Rated 5

6.1.1.5 In enclosure No None 5

6.1.1.6 In enclosure No Rated 5

aOperation is defined as one opening and one closing.

6.1.1.2 Minimum voltage tests

The voltage level should be reduced to 73% of rated voltage and initiate a CLOSE sequence. At this voltage
level, the removable breaker must not close. Without changing the voltage level, the auxiliary motor relay
contact should be mechanically closed. The motor must have sufﬁcient torque at this voltage level to com-
pletely CLOSE the removable breaker. The removable breaker should be manually tripped and the input
voltage increased from 73–80% of rated voltage. Initiate a close sequence. The removable breaker shall
operate through its auxiliary motor relay supplying power to the motor. The voltage should be reduced to
7.5% of rated voltage. At this reduced level, an opening sequence should be initiated. The removable breaker
must completely OPEN. Perform this sequence ﬁve times per Table 2.

Copyright © 2000 IEEE. All rights reserved. 11

6.1.1.3 Relay tests

Three-phase test leads should be connected to the transformer side bus and the network side bus connec-
tions. The ground lead should be connected to the removable breaker. The calibrated network relay(s) should
be installed, and the input voltage adjusted to the rated voltage. Electrically close the removable breaker by
raising the phasing voltage on the test circuit. The removable breaker shall CLOSE its contacts at a predeter-
mined value. The removable breaker should be electrically tripped by applying reverse current on the test
circuit. The removable breaker shall OPEN its contacts completely at a predetermined trip level. Perform
this sequence 25 times per Table 2.

6.1.1.4 Trip free test

The trip free operation of the mechanism shall be checked by attempting to electrically CLOSE the remov-
able breaker contacts while mechanically holding the tripping component of the removable breaker in the
trip position. The removable breaker contacts shall not CLOSE under these conditions. Perform this
sequence ﬁve times per Table 2.

6.1.1.5 Manual operation

On removable breakers without a stored-energy or spring close mechanism, with the removable breaker
mounted in its enclosure, manually CLOSE the network protector by the use of the outside operating handle.
Then, OPEN the network protector by use of the outside operating handle. The network protector shall
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CLOSE and OPEN its contacts through the outside operating handle. Perform this sequence ﬁve times per
Table 2.

6.1.1.6 Operating handle test

On removable breakers having either stored-energy or spring closed mechanisms, the outside operating han-
dle shall electrically complete the motor close circuit when the handle is placed in the CLOSE position. This
test is performed with the removable breaker mounted in its enclosure and three-phase test leads connected
to the transformer side of the network protector. Moving the outside operating handle to the CLOSE position
shall complete the electrical circuit of the closing motor and the network protector shall close. Then, OPEN
the network protector contacts by use of the outside operating handle. Perform this sequence ﬁve times per
Table 2.

6.1.2 Performance

The operational test as deﬁned in Table 2 shall be performed without any malfunction of the network
protector.

6.2 Dielectric tests

Dielectric withstand tests shall be conducted on removable breakers and enclosures, either as one unit or
separately. All barriers and insulation shall be in place. These tests prove the condition of all control wiring
and bus insulation both phase-to-phase and phase-to-ground.

6.2.1 Tests conditions

The network relays are removed. The leads to the motor shall be disconnected from the circuit. Additional
line to ground circuits shall be disconnected from ground. All open points on the harness, such as sockets,
plugs, terminal boards, and relay connectors shall be electrically connected together and isolated from
ground. Solid-state devices, such as diodes and rectiﬁers, shall be shorted or removed from the circuit.

12 Copyright © 2000 IEEE. All rights reserved.

The dielectric test shall be conducted at 2200 V at 60 Hz for 1 min for each of the following points of appli-
cation discussed in 6.2.1.1 through 6.2.1.3.

6.2.1.1 Removable breaker

a) Between top phases and bottom phases, with the top phases grounded and the circuit breaker OPEN
b) Between all phases and ground with the circuit breaker closed
c) Between the outside phases and ground with the center phase grounded and the circuit breaker
OPEN

6.2.1.2 Enclosure

a) Between all phases on the network side (including secondary disconnects) and ground
b) Between the outside phases on the network side and ground, with the center phase grounded
c) Between all phases on the transformer side and ground
d) Between the outside phases on the transformer side and ground, with the center phase grounded

6.2.1.3 Component tests

a) The motor shall be tested at 900 V at 60 Hz for 1 min.

b) The network relay(s) shall be tested at 1500 V at 60 Hz for 1 min.

6.2.2 Performance

A dielectric breakdown shall constitute a failure.

6.3 Insulation resistance tests

Insulation resistance tests shall be conducted on the removable breaker and its enclosure, with all barriers
and insulation in place, either as one unit, or separately.

6.3.1 Test conditions

The network relays shall be removed. The leads to the motor shall be disconnected from the circuit. Addi-
tional line-to-ground circuits shall be disconnected from ground. All open points on the harness, such as
sockets, plugs, terminal boards, and relay connectors shall be electrically connected together and isolated
from ground. Solid-state devices, such as diodes and rectiﬁers, shall be shorted or removed from the circuit.

With the removable breaker CLOSED, the resistance should be measured at the following locations using
direct current from an instrument set at 2500 V:

a) Between the main bus bars; left to center, center to right, and left to right
b) Between the main bus bars and ground; left, center, and right

6.3.2 Performance

The direct current resistance values shall be equal to or greater than 25 MΩ measured at 2500 V.

Copyright © 2000 IEEE. All rights reserved.

--``,-`-`,,`,,`,`,,`---
13
Copyright The Institute of Electrical and Electronics Engineers, Inc.
Provided by IHS under license with IEEE
No reproduction or networking permitted without license from IHS Not for Resale
IEEE
Std C57.12.44-2000 IEEE STANDARD REQUIREMENTS

6.4 Current path resistance tests

The current path shall encompass all main current carrying conductors of both the enclosure and the
removable breaker. The acceptable limits shall be established by each manufacturer for each amperage class
of network protector and removable breaker, with and without fuses. These values shall be established from
the continuous current thermal test (see 5.1.4). The limits shall be tabulated and subdivided into speciﬁc
sections of the enclosure and the removable breaker.

6.4.1 Test conditions

With the removable breaker placed in its enclosure and all fuses and disconnect links mounted, measure the
resistance on each phase of the network protector using a 100 A minimum current test set.

6.4.2 Performance

All readings shall be within the manufacturer’s acceptable limits.

6.5 Mechanical tests

Mechanical tests shall be performed on network protectors to prove the mechanical ﬁt between the remov-
able breaker and its associated enclosure. It shall also prove the mechanical seals required on submersible
types of enclosures.

6.5.1 Mechanical ﬁt test conditions and performance

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With the removable breaker out on its enclosure extension rails, roll the breaker into the enclosure. It shall

a) Roll smoothly and with no tendency for the breaker rollers to ride off the extension rails.
b) Align properly on the rails or racking device.
c) Ensure that the securing hardware aligns properly and is complete.
d) Ensure that the fuses ﬁt properly in their designated location, and the alignment of the mating
surfaces provide solid contact with the fuse to insure proper current and heat transfer to the main
conductor.

6.5.2 Submersible seal integrity test conditions and performance

On submersible enclosures, the transformer throat area shall be temporarily sealed. The leak test shall be
performed by pressurizing the completed network protector to a gauge pressure of 50 kPa4 and submerging
it in a tank of water. Any sign of escaping air in the form of bubbles constitutes a failure. Minor leaks from
the removable throat plate bolts, which do not interfere with detection of other leaks, shall be disregarded.

Alternative methods of leak detection of equivalent sensitivity may be utilized.

47.0 1bf/in2

14 Copyright © 2000 IEEE. All rights reserved.

7. Relay characteristics

7.1 General

The relay characteristics shall be composed of the combination of the closing and tripping curves as well as
the adjustment of the relay.

Network protectors shall utilize 216 Y/125 V relays regardless of the three-phase, four-wire system voltage
for which they are to be applied.

7.2 Closing characteristics

The network protector shall close automatically if the net three-phase watt ﬂow is into the network from the
transformer and remains as a net three-phase watt ﬂow into the network following the closure of the network
protector.

To ensure that both the watt and var ﬂow in the network protector are into the network from the transformer,
following closure of the network protector, the phasing voltage phasor shall lie between 85° and –15° from

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the in-phase or 0° line.

7.3 Tripping characteristics

The network protector shall trip automatically if the net three-phase power ﬂow is into the transformer from
the network.

The network protector shall trip automatically for faults on the primary or for faults in the network trans-
former. It shall also trip automatically upon the reverse magnetizing current of its associated transformer in
the absence of a fault. This type of relay is described as having a watt characteristic, with a maximum
response angle of 180°. The network relay trip characteristic can be plotted as a function of current magni-
tude and the angle between the phase-to-ground voltage and phase current.

As an option, the network protector shall trip automatically for faults on the primary feeder where the pri-
mary feeder utilizes single-phase protective devices. It may also be applied where single phase to ground
primary fault on ∆-Y transformers may exist and the station breaker relaying may be unable to detect this
level of ground current. This type of single phase-to-ground fault relay is described as having a watt-var
characteristic, with a maximum response angle of 120°.

An additional option is the time delay relay or time delay function in which the normal tripping signal is
delayed for an adjustable period of time. As backup protection during the time delay period, instantaneous
overcurrent elements are engaged for primary feeder faults. The instantaneous overcurrent elements are
adjustable from 50–200% of the network protector current transformer rating.

7.4 Closing adjustment

The magnitude of the phasing voltage phasor shall be greater than a predetermined minimum threshold level
from the network voltage, which shall serve as the reference phasor determining the network relay charac-
teristic. This predetermined minimum level is chosen to assure that the network protector will not close when
the magnitude of the phasing voltage is approaching zero. The closing adjustment threshold value shall be
between 0.6 V and 2.0 V on a 216 V system. Refer to 9.6 for the closing voltage settings. The angle of the phas-
ing voltage phasor in relation to the network voltage reference phasor shall be between 90° and –25° from the
in-phase or 0-degree line. Any combination whereby the phasing voltage phasor exceeds the predetermined

Copyright © 2000 IEEE. All rights reserved. 15

minimum value above the reference network phasor and falls within the predetermined allowable angular dif-
ference between the two phasors shall call for the relay(s) contact(s) to close (see Figure 2).

Figure 2—Close characteristics

7.5 Tripping adjustment

The network relay shall make its trip contact whenever the balanced net three-phase power (watt) ﬂow is in
the reverse direction (180° from a constant network side voltage) and exceeds a predetermined threshold
value. The trip adjustment threshold value shall be 0.05–5% of the network protector current transformer
rating. Refer to 9.7 for trip current settings. For watt characteristic relays, the trip setting is the current in
amperes at 180° with respect to the phase-to-ground voltage to make the relay trip contact (see Figure 3).

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16 Copyright © 2000 IEEE. All rights reserved.

Figure 3—Watt characteristic trip

Copyright © 2000 IEEE. All rights reserved. 17

8. Fuses and fuse mountings

8.1 Fuse provisions

The network protector shall be equipped with provisions for installing three fuses mounted either internally
or externally. Internal fuse mounting provisions are shown in Figure 4 and Figure 5, and are determined by
the fuse selected. External fuse mountings may be in housings suitable for submersible operations, which
shall be capable of being pressure tested along with the network protector enclosure per 6.5.2

(a) (b)
NOTE—Stud diameter is 1/2-13 UNC. Stud length provided shall be of sufﬁcient length to permit removal of
fuse mounting assembly with a standard depth insulated socket. This length is determined by the fuse
selected.
Figure 4—One and two hole internal fuse mounting patterns:
(a) 800–3500 A; (b) 2250–3500 A

18 Copyright © 2000 IEEE. All rights reserved.

--``,-`-`,,`,,`,`,,`---

Copyright The Institute of Electrical and Electronics Engineers, Inc.

Provided by IHS under license with IEEE
No reproduction or networking permitted without license from IHS Not for Resale
IEEE
FOR SECONDARY NETWORK PROTECTORS Std C57.12.44-2000

(a) (b)
NOTE—Stud diameter is 1/2-13 UNC. Stud length provided shall be of sufﬁcient length to permit removal of
fuse mounting assembly with a standard depth insulated socket. This length is determined by the fuse selected.
Figure 5—Four hole internal fuse mounting patterns:
(a) 2500–3500 A; (b) 4500–5000 A

8.2 Functional performance speciﬁcations

Fuses used in network protectors shall be speciﬁed by the user.

Fuses shall be selected with time-current characteristics (corrected to fuse’s operating ambient temperature)
to provide maximum protection when compared to the thermal damage curve of its associated transformer.
The minimum acceptable crossover point between the two curves should be no less than three times rated
current. The maximum acceptable crossover point between the two curves should be no greater than seven
times rated current. Under no circumstances are fuses to be applied whose average melting curve is faster
than the network protector relay tripping response.

8.3 Fuse types

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Fuses for network protectors are not normal fuses and are specially designed for use in network protectors.
They fall into the following four general categories:

a) Copper
b) Low loss
c) Alloy
d) Silver-sand current limiting

Copyright © 2000 IEEE. All rights reserved. 19

9. Rating requirements

9.1 Nameplates

Nameplates shall contain the following minimum information for the removable breaker and the enclosure:

a) Manufacturer
b) Place of manufacture
c) Name of device (automatic network protector)
d) Model of device (example: MG-8; CM-22)
e) Identiﬁcation serial number
f) Rated system voltage
g) Rated continuous current
h) Rated symmetrical interrupting current
i) Frequency
j) Current transformer ratio
k) Wiring diagram
l) Weight (total and removable breaker)
m) Submersible or nonsubmersible
n) Installation and operating instructions reference
o) Month and year of manufacture (uncoded)

9.2 Interrupting rating

Network protectors shall have continuous current, interrupting, and close and latch ratings as indicated in
Table 3. As an aid in selection, the table shows recommended transformer sizes as well as the protector con-
tinuous current rating expressed as a percentage of the transformer nameplate current.

9.3 AC voltage ratings

The ac ratings shall be as indicated in Table 4.

9.4 Dielectric test voltage

The 60 Hz dielectric test voltage shall be 2200 V except for the motor, relays, and solid-state devices. The
motor and solid-state devices shall be tested at 900 V. The network relays shall be tested at 1500 V. For
equipment that has been in service, test voltages shall be not greater than 75% of the foregoing values.

The test voltage shall be applied continuously for 1 min.

9.5 Control voltage

When measured line-to-line (L-L) or line-to-ground (L-G), at the protector terminals, the range of voltage
for operation of the control mechanisms shall be as indicated in Table 5.

20 Copyright © 2000 IEEE. All rights reserved.

Table 3—Ratings for network protectors

Network protector Network transformer

Interrupting Close and latch Protector rating

Continuous rms Nameplate Nameplate
rating, rms rating, rms as % of
current rating rating rms current
symmetrical symmetrical transformer
(A) (kVA) (A)
(A)a,b (A)c nameplate current

System voltage 216 Y/125

800 30 000 25 000 225 600 133

1200 30 000 25 000 300 800 150

1600 30 000 25 000 500 1333 120

1875 30 000 25 000 500 1333 141

2000 35 000 35 000 500 1333 150

2250 35 000 35 000 500 1333 169

2500 60 000 40 000 750 2000 125

2825 60 000 40 000 750 2000 141

3000 60 000 40 000 1000 2667 112

3500 60 000 40 000 1000 2667 131

4500 60 000 40 000 1000 2667 169

System voltage 480 Y/277

800 30 000 25 000 500 600 133

1200 30 000 25 000 750 900 133

1600 30 000 25 000 1000 1200 133

1875 30 000 25 000 1000 1200 156

2000 35 000 35 000 1000 1200 167

2250 35 000 35 000 1000 1200 188

2500 45 000 40 000 1500 1800 139

2825 45 000 40 000 1500 1800 157

3000 45 000 40 000 2000 2400 125

3500 45 000 40 000 2000 2400 146

4500 60 000 40 000 2500 3000 150

5000 60 000 40 000 2500 3000 167

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aThe interrupting rating shall exceed the through fault rating of its associated transformer.
bShort-time rating of network protectors without fuses shall be equal to the interrupting rating.
cApplies only to network protectors having spring close or stored energy mechanisms.

Copyright © 2000 IEEE. All rights reserved. 21

Table 4—Alternating current voltage ratings

Rated voltage
Maximum design voltage (V)
(V)

216 Y125 250

480 Y/277 or 480 ∆ 500

575 Y/332 or 575 ∆ 600

Table 5—Control voltage

Closing relay Trip range

Rated voltage Closing motor range
Connect between range 80–106% 7.5–106%
(V) 73–106% (V)
(V) (V)

216 Y/125 L–G 100–135 90–135 10–135

216 Y/125 L–L 170–230 157–230 16–230

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480 Y/277 L–G 220–295 200–295 20–295

480 Y/277 L–L 385–510 350–510 36–510

480 ∆ L–L 385–510 350–510 36–510

575 Y/332 L–G 265–351 242–351 25–351

575 Y/332 L–L 460–610 420–610 43–610

575 ∆ L–L 460–610 420–610 43–610

9.6 Closing voltage

Network protectors shall have provisions for adjusting the closing voltage as indicated in Table 6. The clos-
ing voltage shall be the median value.

Table 6—Closing voltages

Available closing voltages (volts in-phase with network voltage)

Rated voltage
(V)
Low Median High

216 Y/125 1.0 1.5 2.0

480 Y/277 2.2 3.3 4.4

480 ∆ 2.2 3.3 4.4

575 Y/332 2.7 4.0 5.4

575 ∆ 2.7 4.0 5.4

9.7 Tripping currents

Network protectors shall have provisions for adjusting the tripping current as indicated in Table 7. The trip-
ping current shall be the nominal value.

22 Copyright © 2000 IEEE. All rights reserved.

Table 7—Tripping currents

Tripping currents (primary A)

Current
Rated current transformer
(A) rating Low 0.05% Nominal 0.2% High 5%
(A) (CT secondary (CT secondary (CT secondary
= 2.5 ma) = 10 ma) = 250 ma)

800 800 0.4 1.6 40

1200 1200 0.6 2.4 60

1600 1600 0.8 3.2 80

1875 1600 0.8 3.2 80

2000 1600 0.8 3.2 80

2000 2000 1.0 4.0 100

2250 1600 0.8 3.2 80

2250 2000 1.0 4.0 100

2500 2500 1.25 5.0 125

2875 2500 1.25 5.0 125

3000 3000 1.5 6.0 150

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3500 3000 1.5 6.0 150

3500 3500 1.75 7.0 175

4500 3000 1.5 6.0 150

4500 3500 1.75 7.0 175

10. Mechanical performance speciﬁcations

Network protectors shall be either submersible, nonsubmersible, or open-frame type.

10.1 Submersible type

A network protector removable breaker shall be mounted in a sealed enclosure.

10.2 Nonsubmersible type

A network protector removable breaker shall be mounted in an enclosure to prevent contact with internal live
parts.

10.3 Open frame type

A network protector removable breaker shall be mounted on a freestanding open frame.

Copyright © 2000 IEEE. All rights reserved. 23

10.4 Mounting application

Submersible and nonsubmersible network protectors may be either separately mounted or transformer
mounted. Open frame network protectors are mounted separately from transformers.

10.5 General requirements

The following shall apply to the removable breaker, submersible, and nonsubmersible network protector
enclosures unless noted.

10.5.1 Network protector barriers

The network protector shall include barriers, of insulating material, between phases and between phases and
ground. After the removable breaker has been rolled out of its enclosure, its interphase barriers shall be
readily removable.

10.5.2 Studs, bolts, and screws

All studs, bolts, and screws shall be secured to prevent loosening under normal service conditions and
transportation.

10.5.3 Inspection windows

Inspection windows shall be provided in the door of the enclosure so that the operation counter and position
indicator can be seen without opening the door.

10.5.4 Operating handle

An external operating handle shall be provided. Provisions shall be made with a latch to prevent accidental
movement and means to padlock the handle with an 11 mm shackle padlock in each position: OPEN, AUTO,
and CLOSE for manually operated protector; OPEN and AUTO for electrically operated protector. An elec-
trically operated protector may have a NULL position for padlocking. This is the spring return position from
the CLOSE position of the switch. The position of the operating handle shall be clearly indicated by name-
plates visible from the front of the enclosure. An interlock shall be provided so that the removable breaker
cannot be rolled into or out of the enclosure unless the removable breaker is open.

10.5.4.1 OPEN position

Placing the handle in the OPEN position shall cause the network protector to open and remain open.

10.5.4.2 AUTO position

Placing the handle in the AUTO position shall cause the network protector to be controlled by the relay(s).

10.5.4.3 CLOSE position

Moving the handle to the CLOSE position shall close the protector and shall permit the relay(s) to open the
network protector.

10.5.4.4 NULL position

This is the spring return from the CLOSE position; all contacts are open.

--``,-`-`,,`,,`,`,,`---

24 Copyright © 2000 IEEE. All rights reserved.

10.5.4.5 Submersible network protector operating handle

If the external handle is mounted on the side, mountings shall be provided on both the right-hand and left-
hand sides of the network protector enclosure so that the handle can be transferred from one side of the
enclosure to the opposite side whenever the need arises. The handle shall be connected to the network pro-
tector through the submersible enclosure by means of a watertight assembly. The seals used in this assembly
shall be replaceable from outside the enclosure.

10.5.4.6 Submersible network protector hinged door

The enclosure door shall be provided with hinges on the left-hand side that can be easily changed for hinging
on the right-hand side without modiﬁcation to the enclosure.

10.5.4.7 Submersible enclosure air test ﬁtting

Air test provisions shall be provided. This consists of a 1/2 inch NPT female ﬁtting on the enclosure with a
sampling device, made of corrosion resistant material, installed into the ﬁtting using a suitable thread sealer.

10.5.4.8 Nonsubmersible network protector operating handle

If the external handle is mounted on the side, it shall be provided on the left-hand side of the network protec-
tor enclosure.

10.5.4.9 Nonsubmersible network protector hinged door

The enclosure door shall be provided with hinges on the right-hand side.

10.5.5 Lifting facilities

The network protector enclosure and the removable breaker shall each include two lifting eyes with a
minimum inside diameter of 25 mm and shall be located on opposite sides of the unit, in a vertical plane
approximately through the center of gravity.
--``,-`-`,,`,,`,`,,`---

10.5.6 Rollout rails

Means shall be provided for rolling the removable breaker from its enclosure after it has been disconnected.
This shall be accomplished by means of self contained, or detachable rails, or the equivalent, provided in the
enclosure.

10.5.6.1 Rollout rail stops

Stops shall be provided to prevent the removable breaker from rolling off the rails. When extended, the rails
shall not extend beyond the open enclosure door.

10.5.6.2 Rollout rail storage

Provision shall be made for safely storing the rollout rails within the enclosure.

10.5.7 Special tools

If the removable breaker requires the removal and replacement of hardware, for its removal from the enclo-
sure, the hardware shall be a 1/2 inch and/or 3/4 inch hexagon head. An insulated straight “T” socket wrench
for the appropriate 1/2 inch and/or 3/4 inch hexagon hardware shall be available.

Copyright © 2000 IEEE. All rights reserved. 25

10.5.8 Ground pad

Enclosure-grounding provisions shall consist of a copper-faced-steel or stainless-steel pad with two holes
horizontally spaced on 44.5 mm centers and drilled and tapped for 1/2-13 UNC thread (refer to ANSI B1.1-
1989). The ground pad shall be welded to the enclosure. The minimum thickness of the copper facing shall
be 0.4 mm. The minimum threaded depth of holes shall be 13 mm. Thread protection for the ground pad
shall be provided.

10.5.9 Materials

Submersible enclosures shall be constructed of corrosion resistant 5 mm minimum stainless or 6 mm copper

bearing steel. When required to avoid excessive temperature, nonmagnetic stainless of equal thickness shall
be used adjacent to and surrounding the terminals and bus structure. All hardware shall be silicone-bronze or
300 series stainless steel or functional equivalent.

Nonsubmersible enclosures shall be constructed of 3 mm5 thick minimum steel.

10.5.10 Nameplates

A metal corrosion-resistant nameplate shall be afﬁxed by corrosion resistant screws to the network protector
enclosure and the removable breaker. It shall bear the rating and other essential operating data as speciﬁed in
9.1.

10.5.11 Cover plate

Submersible network protectors shall be provided with a gasketed cover plate to seal off and protect the net-
work protector throat area, suitable for outdoor storage.

10.5.12 Internal main bus

The network protector main bus structure shall consist of a copper bus bar of adequate size to provide the
speciﬁed ampere rating.

10.5.13 Secondary disconnect

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A secondary disconnect with a minimum of four points shall be provided to connect and disconnect the aux-
iliary and control circuits between the removable breaker and its enclosure

10.5.14 Spare auxiliary contacts

Spare contacts rated at 20 A and 600 V and mechanically driven shall be provided as follows:

a) One contact to be closed when the network protector is closed. Commonly known as an “a” contact.
b) One contact to be closed when the network protector is open. Commonly known as a “b” contact.

10.5.15 Operation counter

A nonresettable operation counter shall be provided that shall be easily read through the inspection window.

511 gauge or 0.1196 inch

26 Copyright © 2000 IEEE. All rights reserved.

10.5.16 Position indicator

A mechanical indicator shall be provided, which will indicate the OPEN and CLOSE positions of the
network protector. The mechanical indicator shall be easily read through the inspection window. When the
network protector is fully open, only the word OPEN shall be visible. When the network protector is fully
closed, only the word CLOSE shall be visible.

NOTE—An intermediate breaker position shall be indicated by partial visibility of the word OPEN or CLOSE.

10.5.17 Insulating material

All insulating materials in the network protector shall be track resistant per ASTM 495, ASTM D2303, and
UL746A; ﬂame resistant per ASTM D229 and UL94; and shall be asbestos-free.

10.5.18 Grounding

The operating mechanism and relay cases shall be grounded to the enclosure through the removable breaker.

10.6 Finish

The ﬁnish shall conform to ANSI C57.12.28-1996 or ANSI C57.12.32-1994, as appropriate.

--``,-`-`,,`,,`,`,,`---
11. Other requirements

11.1 Submersible enclosure

This is an enclosure that is designed for outdoor or vault application subject to submersion or high humidity.
It shall provide a degree of protection from unintentional contact with energized internal components when
the door is closed. The enclosure shall have sealed interfaces at every entrance or exit point as well as the
door having a gasketed surface. It shall be capable of withstanding an internal or external static gauge pres-
sure of 105 kPa6 without rupture and shall remain watertight.

11.2 Nonsubmersible enclosure

This is an enclosure that provides a degree of protection from unintentional contact with energized internal
components when the door is closed.

11.2.1 Ventilated enclosure

This is an enclosure that provides a degree of protection against limited amounts of falling dirt, but is not
required to prevent the entry of dust or liquids.

11.2.2 Semi dust-tight enclosure

This is an enclosure that provides a degree of protection against falling dirt or airborn dust, but is not
required to prevent the entry of liquids. This enclosure shall have no intentional openings in its design and
shall have a gasketed interface between the enclosure and the door.

615 1bf/in2

Copyright © 2000 IEEE. All rights reserved. 27

--``,-`-`,,`,,`,`,,`---

11.2.3 Drip-tight enclosure

This is an enclosure that provides a degree of protection against falling dirt and water, but is not required to
prevent the entry of dust. It shall have provisions for drainage.

11.2.4 Drip-tight/semi dust-tight enclosure

This is an enclosure that provides a degree of protection against falling dirt or water, and airborne dust. It
shall have no intentional openings in its design and shall provide a gasketed interface between the enclosure
and the door.

11.3 Open frame enclosure

This is an open frame in which the removable breaker is clearly visible and offers no protection from unin-
tentional contact with energized components. It offers minimal protection from falling debris, dirt, dust, or
liquids.

11.4 Mounting conﬁgurations

Network protector enclosure mounting conﬁgurations are divided into the following two main categories:

a) Transformer mounted
b) Separately mounted

11.4.1 Transformer mounted

This is an enclosure that can be close coupled to its associated transformer through throat mounting
provisions. The throat opening, bolting pattern, and lower housing support shall be standardized as to the
following amperage of the network protector:

— 800 A through 2250 A shall conform to Figure 3 of IEEE Std C57.12.40-1990

— 2500 A through 4500 A shall conform to Figure 4 of IEEE Std C57.12.40-1990

11.4.2 Separately mounted

This is an enclosure having the provisions of being supported without the aid of its associated transformer.

Separately mounted enclosures shall be divided into two categories: wall mounted or free standing.

11.4.2.1 Wall mounted

This is an enclosure that is designed to be attached to a vertical surface.

11.4.2.2 Free standing

This is an enclosure that is designed to be self supported with provisions for attachment to the ﬂoor.

11.5 Network-side terminations

Network protector enclosures shall be supplied with terminations for connection to a secondary spot or grid
network. These terminations shall be of either the stud or spade type.

11.5.1 Stud-type terminals

Stud-type terminals shall be threaded. Stud diameters and threading shall be as follows (see Figure 6):
— 1 1/2 inch diameter 12 threads/inch for network protectors in the 800–1875 A range
— 3 inch diameter 12 threads/inch for network protectors in the 2000–4500 A range

(a) (b)

Figure 6—Typical stud type terminals:

(a) 800–1875 A; (b) 2000–4500 A

11.5.2 Spade-type terminals

Spade-type terminals shall have 9/16 inch diameter holes as follows (see Figure 7):
— Network protectors in the 800–1875 A range shall have spades with 13 mm thick material with two
sets of two holes 44.5 mm apart and each set separated by 44.5 mm minimum.
--``,-`-`,,`,,`,`,,`---

— Network protectors in the 2000–3000 A range shall have spades with 19 mm thick material with four
sets of two holes 44.5 mm apart and each set separated by 44.5 mm minimum.
— Network protectors in the 3500–4500 A range shall have spades with 25 mm thick material with four
sets of two holes 44.5 mm apart and each set separated by 44.5 mm minimum.

(a) (b)

Figure 7—Typical spade type terminals:

(a) 800–1875 A (b) 2000–4500 A

Annex A
(normative)

Classiﬁcation of insulating materials

A.1 General

The temperature limits and ratings of electrical equipment are inﬂuenced signiﬁcantly by the characteristics
of the insulation systems used. It is important to recognize the distinction between insulating materials and
insulation systems.

Insulating materials include processed compositions of nonconductive materials and combinations thereof,
before they are fabricated into shapes and structures for use in speciﬁc electrical equipment. Electric conduc-
tivity is very small, approaching zero, and electric isolation is provided.

Insulation systems include fabricated and processed combinations of insulating materials speciﬁcally
designed to perform the insulation functions needed in the associated electrical equipment. Such equipment
may contain more than one insulation system.

A.2 Insulating materials

There are many different kinds of materials used for insulation purposes. Rapid advances in polymer chem-
istry have produced insulating materials, which are so numerous and complex that simple chemical descrip-
tion has become virtually impossible. Consequently, the traditional procedure of dividing insulating
materials into several thermal classes, based upon broad descriptive statements of chemical composition, is
no longer meaningful, adequate, or appropriate.

By means of thermal aging tests and evaluation of service experience, an insulating material may be
assigned a temperature index (TI). The TI provides a technical basis for comparing the thermal capability of
insulating materials, but should not be related directly to the appropriate operating or service temperature for
the equipment in which they are used. The latter depends on many factors including environment, service
severity, mechanical stresses, geometry of materials (e.g., thickness), and the design of the insulation system
in which the material is used.

To promote standardization and to provide continuity with past procedures, it is desirable that the TI for
insulating materials be grouped in material temperature classes as given in Table A.1.

As an example, if a particular material is given a TI of 140 °C by thermal aging test or service experience, it
is in the TI range of 130–154 and, therefore, is a Class 130 material.

The TI of an insulating material is a value obtained by test or from service experience, and may be used as a
guide, but does not imply an equipment thermal classification or a limitation on use in equipment. Tempera-
ture classification for the purpose of rating electrical equipment should be defined in terms of the thermal
endurance of the total insulation system.

--``,-`-`,,`,,`,`,,`---

Table A.1—Material temperature classes

Range of the
Material Former class Temperature rise
temperature index
temperature class designation °C
°C

90 O 90–104 50

105 A 105–129 65

130 B 130–154 90

155 F 155–179 115

180 H 180–199 140

200 — 200–219 160

220 — 220–249 180

An insulating material may be assigned more than one TI based upon different properties, environmental
conditions, or material geometry. For example, a material can be assigned a TI based upon retention of
mechanical properties and a different TI for retention of electrical properties. Thus, the TI describes perfor-
mance characteristics that provide the designer with information for the selection of materials based upon
engineering data rather than arbitrary classiﬁcation.

A.3 Insulation systems

The temperature limits for electric equipment should be selected so that the equipment will provide a satis-
factory service life under normal operating conditions. The temperature limit for an insulation system may
not be directly related to the TI of the individual material included in it. In an insulation system, the thermal
performance of insulating materials may be improved by the protective character of other materials used
with them. A speciﬁc material as part of a system may be satisfactory for use at different limiting tempera-
tures depending upon the type and design of equipment in which it is used and the kind of service to which
the equipment is subjected.

The ability of an insulation system to perform its function is also affected by the presence of other factors in
addition to thermal stresses. These include electrical stresses, mechanical stresses, and environmental
stresses, e.g., moisture, dirt, chemicals, or other contaminants.

The limiting insulation temperature consists of the sum of limiting ambient temperature plus the limiting
temperature rise. For most purposes, the temperature of the outdoor air is taken as the ambient, and 40 °C is
normally chosen as the limiting ambient temperature. Other values for ambient temperature may be chosen
in special circumstances.

The temperature rise values generally used for electrical equipment are the results of long experience and
have been proven reasonably satisfactory. This suggests that any changes in existing standards should be
made only if they are clearly indicated in light of new test data, new or improved materials, additional
operating experience, new measurement techniques, or changes in service requirements. Only carefully
evaluated service experience and/or adequately accepted tests provide the basis for rational thermal
classiﬁcation of electrical equipment and the temperature limit of insulation systems.

Based upon such an evaluation of the insulation system in a particular type of electrical equipment, the
temperature rise value may be selected from Table A.1. These suggested temperature rise values have a
numerical relationship to the material temperature classes but are not necessarily directly related to the TI of
the materials used.

31
Copyright The Institute of Electrical and Electronics Engineers, Inc.
Provided by IHS under license with IEEE
No reproduction or networking permitted without license from IHS Not for Resale
IEEE
Std C57.12.44-2000 IEEE STANDARD REQUIREMENTS

As indicated previously, a material with a given TI may be acceptable for a different limiting temperature,
either higher or lower, based on acceptable tests and/or service experience on the equipment in which it is
used.

A.4 Insulation service stresses

For the expected service life of the equipment, the electrical and mechanical properties of the insulation
system must not be impaired by the application of the limiting insulation temperature permitted by the
particular limiting temperature rise speciﬁed. The word “impaired” implies any change that would disqualify
the insulating material from performing its intended function whether creepage spacing, mechanical support,
or dielectric barrier action.

During the service life of the equipment, the insulation system may be exposed to stress factors that can ulti-
mately impair its ability to perform its intended function. Usually service stresses that degrade the insulation
result in a gradual deterioration over time. The following subclauses A.4.1–A.4.4 are brief comments on the
major service stresses.

A.4.1 Thermal aging

--``,-`-`,,`,,`,`,,`---
Thermal aging of insulating materials is the progressive deterioration in electrical and mechanical properties
as a result of prolonged exposure to high temperatures. The process of thermal aging is complex, and the
mechanisms vary with different materials and under different service conditions.

Typical mechanisms include the following:

a) Loss of volatile constituents

b) Oxidation leading to molecular changes and embrittlement
c) Molecular polymerization (softening, melting) leading to embrittlement
d) Hydrolytic degradation (moisture reacts with the insulation)
e) Chemical breakdown of constituents

Different insulating materials react in different ways to the various aging processes, and it is essentially
impossible to predict thermal performance from chemical composition.

A.4.2 Electrical stresses

The normal electrical stress results from exposure to high voltages. Insulation deterioration can result in
ﬂashovers. Arcing fault currents can cause immediate damage or destruction of the insulation system,
whether or not deterioration has occurred.

A.4.3 Mechanical stresses

Typical mechanical stresses result from supporting other components of the equipment, vibration from any
cause, and differential thermal expansion. Insulation deterioration can cause mechanical failure leading to an
electrical fault.

A.4.4 Environmental stresses

Environmental stresses result from exposure to oxygen, moisture, dust, dirt, and chemicals. Some insulating
materials will be more vulnerable to such exposure than others. The life of the equipment will be longer if
the insulation system is suitably protected than if it were freely exposed to industrial atmospheres or the
weather. The design and utilization of equipment should take appropriate account of the characteristics and
capabilities of the insulating materials used.

--``,-`-`,,`,,`,`,,`---
33
Copyright The Institute of Electrical and Electronics Engineers, Inc.
Provided by IHS under license with IEEE
No reproduction or networking permitted without license from IHS Not for Resale
IEEE
Std C57.12.44-2000 IEEE STANDARD REQUIREMENTS

Annex B
(informative)

Network fuse application

B.1 General

The purpose of this tutorial is to furnish the application engineer with some guidance for the selection of net-
work protector fuses.

The selection of a protector fuse is still an art. There are trade-offs in selecting a fuse which may make one
fuse better than another depending on how the application engineer views the trade-offs available. The expe-
rience each engineer has had may differ and may suggest different practices than those outlined here.

Reference should be made to IEEE Std C37.108-1989 for an overall system view of protection.

B.2 Protector fuse

The protector fuse is selected to allow for normal load current to ﬂow and to provide backup for the protector
in the case where the protector fails to open on reverse current. The fuse clearing time should be long enough
to allow for

a) Normal load current

b) Downstream limiter operation
c) Coordination with customer fuses
d) Coordination with protector breaker operation

The minimum melt curve of the protector fuse should fall to the right of

a) Normal load current

b) Total clear curve of the limiter
c) Total clear curve of the customer fuse
d) The total tripping time of the protector relay and breaker

The fuse clearing time should also be short enough to protect its associated transformer. The total clear curve
of the fuse should coordinate with the thermal damage curve of the transformer. The interrupting capability
--``,-`-`,,`,,`,`,,`---

of the fuses should be higher than the expected available fault. Some utilities use the maximum through cur-
rent as the maximum fault current that the fuse will have to interrupt. Some utilities, because of the remote
chance that the protector can fail internally, use the maximum fault current of the network as the interrupting
current that the fuse will have to interrupt.

B.3 Types of fuses

The following are network protector fuse types:

Copper link fuses with or without covers: (See Table B.1.) Fuses of this type have a relatively steep time-
current characteristic, which makes this type of fuse hard to coordinate with cable limiters. These fuses are
normally used on 216 Y/125 V networks with covers, and on the 480 Y/277 V networks without covers (see
Figure B.1 for typical characteristic curves).

CAUTIONS:
1—Fuse characteristic curves vary with whether they are enclosed or external.
2—Fuse characteristic curves will vary with the manufacturer and the curves in this document are representative
only and should not be used for coordination.

Table B.1—Copper fuses

Protector rating
Fuse Cutler- Richards General
(open or Typical fuse style
curve Hammerb Mfg.b Electricb
submersible)a

#1 800 A Y-11 NPF-11-L NWP-7

#2 1200 A Y-15 NPF-15-L NWP-6

#3 1600 A & 1875 A Y-22.5 NPF-22-L NWP-5

#4 2000 A & 2250 A Y-25 NPF-25-L NWP-4

#5 2500 A & 2825 A Z-37.5 NPF-37.5-Q NWP-3

#6 3000 A & 3500 A Z-44, Z-50 6145290

aRatings are those of the manufacturer. IEEE does not represent or warrant the accuracy of these ratings.
bFuse and manufacturer designations are illustrative only of this type of fuse and do not constitute an endorsement by
the IEEE of these products.
--``,-`-`,,`,,`,`,,`---

NOTE—These are typical characteristic curves and should not be used for coordination.

Figure B.1—Copper fuse curves

Low loss fuses: (See Table B.2.) Fuses of this type have time-current characteristics similar to the copper
link fuses, except that the heat loss is approximately half of the losses in ordinary copper fuses. This allows
increased loading of the network protector under contingency conditions. They are for 216 Y/125 V network
applications only (see Figure B.2 for typical characteristic curves).

Table B.2—Low loss fuse

Protector rating
Fuse Chase Richards
(open or Typical fuse style
curve Shawmutb Mfg.b
submersible)a

#1 1875 A S 2F LLLF2SKA

#2 2000 A S2 LLF2KA

#3 2250 A S 2.25 LLF2.25KA

#4 4000 A S4 LLF4KA

#5 5000 A S5 LLF5KA
--``,-`-`,,`,,`,`,,`---

NOTE—These are typical characteristic curves and should not be used for coordination.

Figure B.2—Low loss fuse curves

Alloy fuses: (See Table B.3.) Fuses of this type have a much longer time delay at higher currents and allow
more time to burn secondary faults clear. They have a low melting temperature, making them more sensitive
to unwanted fuse blowing where high ambient temperatures occur. Because of their longer time delay on
higher currents, they will coordinate well with cable limiters and protectors (see Figure B.3 for typical char-
acteristic curves).

Table B.3—Alloy fuse

--``,-`-`,,`,,`,`,,`---

Open Submersible

Protector Cutler- Protector Cutler-

Fuse Fuse
ampere Hammer ampere Hammer Typical fuse style
curve curve
ratinga fuseb ratingb fuseb

#1 800 A 1173006 #1 800 A 1173007

1173007
#2 1200 A 1173007 #2 1200 A 1173008

#3 1600 A 1173008 #3 1600 A 1173010

1875 A 1173011
#4 1875 A 1173009 #4

#4 2000 A 1346880 #4 2000 A 1346881 1346881

#5 2250 A 1346881 #5 2250 A 2A9867G06

#5 2500 A 1346917 #5 2500 A 1247325

1247325
#6 2825 A 1247325 #6 2825 A 1291274

#6 3000 A 1247325 #6 3000 A 12A3822G07

#7 3500 A 1291274 #7 3500 A 12A3822G07

Protector ampere rating G.E. fusea Typical fuse style

800 A NF2, NF3

1200 A NF3, NF4

1600 A, 1875 A NF5, NF6

NF3

2500 A NF7

3000 A NF10

--``,-`-`,,`,,`,`,,`---

NOTE—These are typical characteristic curves and should not be used for coordination.

Figure B.3—Alloy fuse curves

(Cutler-Hammer fuses illustrated)

Silver sand current-limiting fuses: (See Table B.4.) Fuses of this type are used primarily for 480Y/277 V
applications. They have steep time-current characteristic and are sensitive to changes in ambient
temperatures. They should be located where the vault ambient temperature does not exceed 32 °C and the
ambient inside the fuse enclosure does not exceed 15 °C rise over the vault ambient. Silver sand fuses are not
physically interchangeable with other types of fuses and must therefore, be located external to the network
protector. Silver sand fuses are not waterproof in themselves; therefore, a separate waterproof enclosure
must be supplied if the fuses are subjected to water. At low levels of overcurrent, these fuses may become
damaged without complete melting. Subsequent transformer loading can then cause them to overheat, open,
or catch ﬁre (see Figure B.4 for typical characteristic curves).

Table B.4—Silver-sand current limiting fuse

Fuse Protector Cutler-

Typical fuse style
curve ratinga Hammerb

#1 800 A NPL-800

#2 200 A NPL-1200

#3 1875 A NPL-1875

#4 2825 A NPL-2825

#5 3000 A NPL-3000

--``,-`-`,,`,,`,`,,`---

NOTE—These are typical characteristic curves and should not be used for coordination.

Figure B.4—Silver-sand current limiting fuse

--``,-`-`,,`,,`,`,,`---

B.4 History

Secondary networks ﬁrst came into existence as fused secondaries of distribution transformers. Networks
later became more sophisticated when special relayed circuit breakers replaced the fuse. Evidently, early
relays and circuit breakers did not have the desired degree of reliability. Therefore, fuses were used in series
with circuit breakers for secondary network service. The sole purpose of the fuse was to act as backup pro-
tection should the network relay or circuit breaker fail to operate. Even though present network equipment
has a high degree of reliability, users have been reluctant to eliminate fuses.

Before the 1940s, most network protectors were rated 216 Y/125 V. Since then, the growth of 480 Y/277 V
spot networks has been steadily increasing. At present, a large percentage of protectors sold are of the 480 Y/
277 V class. Fusing practice was generally perfected and coordinated for the 216 Y/125 V systems, so with
the introduction of higher rated networks, the same philosophy of fusing for 216 Y/125 V systems was trans-
ferred to the 480 Y/277 V systems.

The most common types of network fuses (alloy and copper) were tested up to 600 V. The 216 Y/125 V alloy
fuse had to have arc horns and ﬂash barriers added before it would successfully clear at 600 V. Before the
copper fuse would clear, the customary enclosure cover had to be removed. Another characteristic that
required investigation before applying network fuses to 480 V systems was the condensing of metal vapors
when a fuse clears. When either an alloy or copper fuse interrupts, a portion of the metal is vaporized and
condenses throughout the enclosed circuit breaker. The greater the current, the more vapor. From a
theoretical standpoint, it would not be good practice to disperse metal vapor over insulated parts and expect
those insulated parts to provide adequate insulation. In a 216Y/125 V system where electric arcs are self-
extinguishing, one would give little thought to metal vapors. However, 480Y/277 V arcs are typically self-
sustaining, so one should consider the possibility of metal vapors causing general arcing throughout an
enclosed protector.

In reality, every time a circuit breaker interrupts, a small amount of vapor is liberated. A good portion of it is
conﬁned in the arc chute, but over a period of time, insulated parts become contaminated. Fuses liberate a
great deal more metal vapor than breakers during interruption; thus, after each fuse operation, all insulated
parts should be cleaned before the protector is placed back into service.
--``,-`-`,,`,,`,`,,`---

Metal vapor from tin alloy fuses is essentially nonconductive. Insulation resistance of a protector before and
after fuse operation does not signiﬁcantly change. Conversely, copper vapor is conducting and will lower the
insulation resistance.

Silver sand current limiting fuses are heat generators and therefore, can only be used for applications where
they are external to the protector steel enclosure. Some types of silver sand fuses are difﬁcult to coordinate
with the transformer and network relay. Silver sand fuses are not of waterproof construction.

Alloy fuses are used for internal applications. They are the easiest to coordinate with the transformer.

Annex C
(informative)

Pumping

C.1 General

Pumping is deﬁned as the unintentional cyclical tripping and closing of a network protector. Pumping usu-
ally occurs because the network protector is improperly allowed to close when system conditions will cause
reverse current to ﬂow after closure. The reverse current immediately trips the protector, and the cycle is
repeated. Pumping may also occur because the network protector properly attempts to close, and a mechani-
cal defect causes immediate trip. Some of the typical causes of pumping are described below.

C.2 Causes

C.2.1 Incorrect relay setting

The most common case occurs when an electromechanical phasing relay setting allows closure with a lag-
ging phasing voltage. The ensuing power ﬂow is in the reverse direction and pumping is established. With
electromechanical relays, the phasing relay is in a separate package from the network master relay. With
solid-state relays, the phasing relay function is normally included as part of a single network relay package.
Thus, the solid-state phasing relay is less vulnerable to an incorrect setting.

C.2.2 Transient system conditions

One typical case occurs when nondedicated feeders from different substations supply a spot network. This
situation can result in the primary voltages being slightly different in both level and phase angle. With light
network load and cyclical nonnetwork load (e.g., frequent motor starting), the network protector can trip and
reclose many times over a short period. This condition results in excessive frequency of operation and in
extreme cases, can develop into pumping. A solution to this problem is to apply time delays and/or increase
the reverse current trip level.

C.2.3 Breaker mechanical failure

The most common condition occurs when the breaker mechanism has some defect that prevents successful
closure. This is breaker trip-free operation, where attempts to close result in immediate trip. In this case, sys-
tem voltage conditions may be correct for proper closing, but the breaker mechanism prevents successful
closure. Hence, pumping is established.

C.3 Effects of pumping

The net effect of pumping is to raise the temperature of the closing motor. Excessive pumping can raise the
temperature to the point where the insulation fails and a short circuit develops in the motor winding.

--``,-`-`,,`,,`,`,,`---

Annex D
(informative)

The unprotected zone in network protectors

The unprotected zone in a network protector is that section of bus from the secondary terminals of the net-
work transformer to, and including, the moving contact assembly of the network protector. On transformer
mounted protectors, this path would extend from the secondary terminals of the transformer via the trans-
former bus down the back of the network protector, through the lower disconnect point, and back up to the
lower network protector breaker stationary bus, which is connected to the moving contact assembly. This
path becomes longer on separately mounted network protectors.

This path is considered unprotected because the substation relays, both phase overcurrent and ground, are
typically set in such a fashion that they do not respond to faults that are initiated on the low voltage side of
the transformer. Faults that originate in the unprotected zone, in theory, would produce reverse currents suf-
ﬁcient to operate the master relay. However, because such faults are very likely to be high resistance arcing
faults, it may become a race as to whether the network protector will trip before the control wiring is
destroyed. Even if the network protector successfully opens its circuit, the fault is still supplied from the
transformer, and it will continue to supply fault current until the station breaker is operated remotely, or until
the fault has migrated far enough into the low-voltage windings to permit the ground or phase overcurrent
relays to operate.

--``,-`-`,,`,,`,`,,`---

Annex E
(informative)

Contact surface oxidation

The continuous current rating of a network protector is based on maximum conductor temperatures, as con-
firmed in a thermal test. The matching contact surfaces of breaker contacts, disconnecting contacts, and
fuses are subject to oxidation over a period of time in service. This can increase contact resistance and result
in a decrease in the amount of current that can be carried without exceeding the rated temperature. There-
fore, the continuous current rating of a network protector is based on the assumption that maintenance will
be sufficient to keep the temperature rise within specified limits.

--``,-`-`,,`,,`,`,,`---

Annex F
(informative)

Reverse current sensitivity

F.1 General

For faults on a network primary feeder, with one exception, the power ﬂow, in terms of current, in the asso-
ciated network protectors will be relatively high and always in the reverse direction, after the primary feeder
breaker opens. Under these fault conditions, the reverse current trip setting of the network master relay is not
an issue, as the reverse power ﬂows are much higher than the available settings. This includes the case of a
single line to ground fault, with network transformers connected grounded wye-grounded wye.

The one exception to the above is a single line to ground fault on a network primary feeder, with network
transformers connected ∆-grounded Y. For this case, the reverse power ﬂow is of relatively low magnitude,
after the primary feeder breaker has opened. To assure that a protector will open for this single line to ground
fault, the reverse power ﬂow after feeder breaker opening must be greater than the reverse current trip set-

--``,-`-`,,`,,`,`,,`---
ting.

Similarly, when the primary feeder breaker is opened in the absence of a fault, the power ﬂows in the net-
work protectors will be relatively low. To assure that all network protectors on that feeder will open, the
reverse power ﬂow in at least one network protector must exceed its trip setting. The remaining protectors on
the feeder will then cascade open to clear the feeder.

Cascading occurs when a primary feeder is opened, because the network transformer units associated with
the protectors already tripped continue to be energized by the network units not yet tripped, via the primary
interconnections. Hence, the exciting current of the transformers with open protectors is added to the reverse
exciting current supplied by the closed protectors. The last protector to trip on any one feeder sees the sum
total of no-load losses on all transformers connected to that feeder.

F.2 Reverse power levels

When a primary feeder breaker is opened in the absence of a fault, there will be some variation in the voltage
at each network protector, in both magnitude and phase angle. This results from small differences in cable
impedances and transformer turns ratios. Hence, there will be a circulating current in the primary cable inter-
connections. Just after the primary feeder breaker opens, there will be “circulating” reverse power flows in
some protectors and forward power flows in others. A phase angle difference of 0.5 electrical degrees on half
of the transformers can produce a power flow in each protector as high as 7% of the protector rating. A volt-
age difference of ±0.5% of nominal on all transformers can produce a power flow in each protector as high
as 1% of the protector rating.

There are also I2R losses in the network transformer due to the circulating current and I2R losses in the
transformer due to the ﬂow of primary cable charging current. In most cases, these losses are small in com-
parison to transformer no-load losses and the circulating power.

In summary, the reverse power through at least some of the network protectors on an open primary feeder
consists of the following:

a) Transformer no-load losses

b) Reverse circulating power due to voltage differences
c) Transformer I2R losses due to circulating current and primary cable charging current

Another factor that will result in higher reverse watt ﬂow in the network protector when the primary feeder
breaker is open is the effect of voltage rise on the no-load transformer loss. Under many backfeed condi-
tions, there will be voltage rises due to primary cable capacitance. With a 10% voltage rise, no-load loss in
the back feeding network transformer will increase 15–30%.

Reverse power ﬂows with open primary breaker, single line to ground fault, and ∆-Y network transformers
will be relatively low. If the primary feeder cables are very short, the power ﬂows in each network protector
at the instant of breaker opening will be almost the same as if the fault were not present.

F.3 Relay settings

The network master relay is calibrated at rated three-phase voltage in terms of balanced three phase current,
leading network line to ground voltage by 180°. The relay responds to the direction and magnitude of net
(equivalent) three-phase power ﬂow.

References to protector current rating herein should be considered as referring to protector current trans-
former rating. On some protectors, the current transformer (CT) primary rating is lower than the protector
current rating. For example, the 1875 A protector uses 1600-5 A CTs.

As seen from previous paragraphs, network protectors can trip on reverse current even though their relay set-
ting is above no-load transformer losses. This applies whether the reverse current setting is at a minimum
value of 0.05–0.1%, or the more commonly used values of 0.15–0.2%, of the protector rating. However, if
the reverse current settings are too high, i.e., above 2–3% of the protector rating, the network protectors may
not cascade open, when the primary feeder breaker is opened in the absence of a fault.

For many years, transformer no-load losses were at levels high enough such that network relay sensitivity
was not a major concern. For example, a typical 500 kVA 216 V transformer would have no-load losses in
the range of 1200–1500 W. For the normal 1600 A or 1875 A protector used with the 500 kVA transformer,
these no-load loss levels are 0.2–0.25% of the protector rating. Hence, a normal relay setting of 0.2% would
be acceptable.

Transformer losses have declined in recent years so that no-load losses in the 500 kVA transformers can now
be as low as 500–600 W. These loss levels are 0.08–0.1% of the protector rating (1600 A–216 V). When such
low loss transformers are added to existing grid networks, there is no need to revise previously established
acceptable network relay setting levels for these new units. Even though the relay setting may be above the
no-load losses for these units, they will still trip out because of the additional reverse power contributions
and the cascading effect previously described. The example described above is generally representative of
the higher network unit ratings as well.

The only cases where a network protector would be required to trip only on the no-load losses of its associ-
ated transformer are where a primary feeder supplies only one transformer or where the transformer has a
primary breaker or load break switch, which isolates the transformer from the rest of the primary circuit.
Under these conditions, the protector will trip when the primary breaker is opened only if the transformer
no-load losses are greater than the network relay setting. These conditions should rarely occur.

--``,-`-`,,`,,`,`,,`---

These rare cases may require a relay setting down near minimum levels of 0.05% or 0.1%, depending on
transformer losses. The electromechanical network relay is normally provided with a minimum setting value
of 0.1% of the protector rating. The newer solid state or microprocessor network relays are normally pro-
vided with a minimum setting value of 0.05% of the protector rating.

Network protector reverse current trip settings should be only as low as required to assure automatic opening
when the primary feeder breaker is opened. Settings of 0.15% of protector current transformer rating will
usually yield satisfactory performance, even with low loss network transformers. Extremely sensitive reverse
current trip settings can cause unnecessary protector operations due to transient reverse power ﬂows.

Occasionally, system conditions may occur where sensitive reverse current tripping must be time delayed.
Typically, this occurs on new construction sites, where the network load is light and elevators being lowered
act as generators to create a temporary reverse current. To prevent excessive and/or undesirable trip opera-
tions, a time delay is required for sensitive tripping, in the order of 1–5 min. When this time delay is intro-
duced, a three-phase instantaneous overcurrent element must also be provided to trip for reverse fault
currents. This element is typically set in the range of 50–250% of the protector current rating. For electrome-
chanical relays, the time delay and instantaneous overcurrent functions are normally combined in one pack-
age separate from the other network relaying functions. For solid state or microprocessor relays, all network
relaying functions, including the above, are normally combined in one package.

--``,-`-`,,`,,`,`,,`---

Annex G
(informative)

Network protector enclosures

All network protector enclosures, whether submersible or nonsubmersible, will withstand all design and pro-
duction tests. It is not to be inferred that the enclosure shall be capable of withstanding internal arcing faults,
without incurring structural damage and/or complete destruction of the network protector.
--``,-`-`,,`,,`,`,,`---

Copyright The Institute of Electrical and Electronics Engineers, Inc.
Provided by IHS under license with IEEE
No reproduction or networking permitted without license from IHS Not for Resale

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