NEMA Standards Publication ICS 10-2005

Industrial Control and Systems

Part 2: Static AC Transfer Equipment

Published by:

National Electrical Manufacturers Association

1300 North 17th Street, Suite 1847
Rosslyn, Virginia 22209

www.nema.org

© Copyright 2005 by the National Electrical Manufacturers Association. All rights, including
translation into other languages, reserved under the Universal Copyright Convention, the Berne
Convention for the Protection of Literary and Artistic Works, and the International and Pan
American Copyright Conventions.
 NOTICE AND DISCLAIMER

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of which the document contained herein is one, are developed through a voluntary consensus
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CONTENTS

FOREWORD ....................................................................................................................... iii

Part 2 STATIC AC TRANSFER EQUIPMENT ........................................................................ 1
1 SCOPE .......................................................................................................................... 1
1.1 General................................................................................................................. 1
1.2 Normative References ........................................................................................... 1
2 DEFINITIONS ................................................................................................................ 2
2.1 Transfer Equipment............................................................................................... 2
2.2 Operation of Static Transfer Equipment ................................................................. 2
3 CLASSIFICATIONS........................................................................................................ 3
4 CHARACTERISTICS AND RATINGS .............................................................................. 3
4.1 Rated and Limiting Values for the Main (Power) Circuit.......................................... 3
4.1.1 System Voltage Ratings............................................................................. 3
4.1.2 Continuous Current Rating ........................................................................ 3
4.1.3 Rating Based on Load Characteristics ....................................................... 3
4.1.4 Interrupting Rating..................................................................................... 4
4.1.5 Motor Starter Rating .................................................................................. 4
4.1.6 Withstand and Closing Ratings .................................................................. 4
4.1.7 Overcurrent Protection .............................................................................. 4
5 PRODUCT MARKING, INSTALLATION, AND MAINTENANCE INFORMATION................ 5
5.1 Markings ............................................................................................................... 5
5.1.1 Transfer Equipment ................................................................................... 5
5.1.2 Bypass Isolation Switch Equipment ............................................................ 6
5.1.3 Control Circuits ......................................................................................... 6
5.2 Maintenance of Transfer Equipment ...................................................................... 6
6 SERVICE AND STORAGE CONDITIONS ....................................................................... 6
7 CONSTRUCTION........................................................................................................... 6
7.1 Spacings............................................................................................................... 6
7.2 Test Switch ........................................................................................................... 7
7.3 Interlocks and Protective Circuits .......................................................................... 7
7.3.1 Protection from Cross-Connection ............................................................. 7
7.3.2 Bypass Isolation Switch Interlock ............................................................... 7
7.4 Service Equipment ................................................................................................ 7
7.5 Emergency Power Circuit Service.......................................................................... 7
7.6 Control Circuits ..................................................................................................... 7
8 PERFORMANCE REQUIREMENTS AND TESTS ............................................................ 9
8.1 Performance Requirements ................................................................................... 9
8.1.1 Control Circuits ......................................................................................... 9
8.1.2 Ground-Fault Protection ............................................................................ 9
8.1.3 Undervoltage Monitoring Characteristics .................................................... 9
8.2 Design Tests ......................................................................................................... 9

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ICS 10-2005, Part 2 Static AC Transfer Equipment
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8.2.1 Required Testing ....................................................................................... 9

8.2.2 Optional Performance Verification............................................................ 10
9 APPLICATION ............................................................................................................. 11
9.1 AC Transfer Equipment Specification .................................................................. 11
9.2 Power Quality ..................................................................................................... 11
9.3 Motor Transfer .................................................................................................... 14
9.4 Features for Particular Applications ..................................................................... 14
9.5 Considerations for Non-Linear Load Applications................................................. 16
9.6 Graphical Symbol for Static Transfer Equipment .................................................. 16
ANNEX A Non-Linear Loads ............................................................................................... 18
A.1 General............................................................................................................... 18
A.2 Non-Linear Electrical Load .................................................................................. 18
A.3 Crest Factor (CF) of a System ............................................................................. 18
Annex B Short Time Rating ................................................................................................ 20
Annex C Neutral Conductors in Power Transfer Systems .................................................... 22
C.1 Separately Derived Sources ................................................................................ 22
C.2 Non-Separately Derived Sources......................................................................... 23

© Copyright 2005 by the National Electrical Manufacturers Association.

 ICS 10-2005, Part 2 Static AC Transfer Equipment
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FOREWORD

This Standards Publication was prepared by a technical committee of the NEMA Industrial
Control and Systems Section. It was approved in accordance with the bylaws of NEMA.

ICS 10-1999, Part 2 is a continuation of ICS 10-1993 which superseded Part ICS 2-447 of NEMA
Publication ICS 2-1988.

This Standards Publication provides practical information concerning ratings, construction, test,
performance and manufacture of industrial control equipment. These standards are used by the
electrical industry to provide guidelines for the manufacture and proper application of reliable
products and equipment and to promote the benefits of repetitive manufacturing and widespread
product availability.

NEMA Standards represent the result of many years of research, investigation and experience
by the members of NEMA, its predecessors, its Sections and Committees. They have been
developed through continuing consultation among manufacturers, users and national
engineering societies and have resulted in improved serviceability of electrical products with
economies to manufacturers and users.

One of the primary purposes of this Standards Publication is to encourage the production of
reliable control equipment which, in itself, functions in accordance with these accepted
standards. Some portions of these standards, such as electrical spacings and interrupting
ratings, have a direct bearing on safety; almost all of the items in this publication, when applied
properly, contribute to safety in one way or another.

Properly constructed industrial control equipment is, however, only one factor in minimizing the
hazards which may be associated with the use of electricity. The reduction of hazard involves
the joint efforts of the various equipment manufacturers, the system designer, the installer and
the user. Information is provided herein to assist users and others in the proper selection of
control equipment.

The industrial control manufacturer has limited or not control over the following factors which are
vital to a safe installation:

1. Environmental conditions
2. System design
3. Equipment selection and application
4. Installation
5. Operation practices
6. Maintenance

This publication is not intended to instruct the user of control equipment with regard to these
factors except insofar as suitable equipment to meet needs can be recognized in this publication
and some application guidance is given.

This Standards Publication is necessarily confined to defining the construction requirements for
industrial control equipment and to providing recommendations for proper selection for use
under normal or certain specific conditions. Since any piece of industrial control equipment can
be installed, operated and maintained in such a manner that hazardous conditions may result,

© Copyright 2005 by the National Electrical Manufacturers Association.

ICS 10-2005, Part 2 Static AC Transfer Equipment
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conformance with this publication does not by itself assure a safe installation. When, however,
equipment conforming with these standards is properly selected and is installed in accordance
with the National Electrical Code and properly maintained, the hazards to persons and property
will be reduced.

U.S. customary units are gradually being supplemented by those of the modernized metric
system known as the International Systems of Units (SI). This transition involves no changes in
standard dimensions, tolerances, or performance specifications.

NEMA Standards Publications are subject to periodic review. They are revised frequently to
reflect user input and to meet changing conditions and technical progress. Users should secure
the latest editions.

Proposed additions to this Standards Publication should be submitted to:

Vice President, Technical Services Department

National Electrical Manufacturers Association
1300 North 17th Street
Rosslyn, Virginia 22209

This Standards Publication was developed by the Industrial Control and Systems Section. Section approval
of the standard does not necessarily imply that all section members voted for its approval or participated in its
development. At the time it was approved, the Section was composed of the following members:

ABB Control, Inc.—Wichita Falls, TX

ABB Automation Technologies – Raleigh, NC
ASCO Power Technologies—Florham Park, NJ
Automatic Switch Company – Florham Park, NJ
c3controls—Beaver, PA
California Linear Devices – Carlsbad, CA
CARLO GAVAZZI, INC.—Buffalo Grove, IL
Cooper Bussman – St. Louis, MO
Cummins, Inc.—Minneapolis, MN
Eaton Electrical, Inc.—Milwaukee, WI
Electro Switch Corporation—Weymouth, MA
Emerson Process Management—Austin, TX
GE Consumer & Industrial—Charlottesville, VA
Hubbell Incorporated—Wadsworth, OH
Hubbell Industrial Controls, Inc.—Archdale, NC
Joslyn Clark Controls, Inc.—Lancaster, SC
L-3 Communications/SPD Technologies – Anaheim, CA
Master Controls Systems, Inc.—Lake Bluff, IL
Metron, Inc.—Denver, CO
Mitsubishi Electric Automation, Inc.—Vernon Hills, IL
Moeller Electric Corporation—Houston, TX
Omron Electronics LLC—Schaumburg, IL
Peerless Electric—Warren, OH
Phoenix Contact, Inc.—Harrisburg, PA
Post Glover Resistors, Inc.—Erlanger, KY
Reliance Controls Corporation—Racine, WI
Rockwell Automation—Milwaukee, WI
Russelectric, Inc.—Hinngham, MA
Schneider Automation, Inc.—North Andover, MA
Schneider Electric North America/Square D Company—Raleigh, NC
Schneider North American Operation Division—Lexington, KY

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 ICS 10-2005, Part 2 Static AC Transfer Equipment
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SEW-Eurodrive, Inc.—Lyman, SC
Siemens Corporate Research—Princeton, NJ
Siemens Energy & Automation, Inc.—Norcross, GA
Siemens Shared Services LLC—Duluth, GA
Square D Company—Raleigh, NC
Torna Tech Inc.—Saint-Laurent, Canada
Toshiba International Corporation—Houston, TX
Total Control Products, Inc.—Terrace Park, OH
Tyco Electronics/AMP—Harrisburg, PA
WAGO Corporation—Germantown, WI
Weidmuller Inc.—Richmond, VA
Yaskawa Electric America, Inc.—Waukegan, IL

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© Copyright 2005 by the National Electrical Manufacturers Association.

 ICS 10-2005, Part 2 Static AC Transfer Equipment
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Part 2
STATIC AC TRANSFER EQUIPMENT

1 SCOPE
The standards in this part apply to static automatic and static non-automatic transfer equipment
without cross-connection of sources during transfer or retransfer, with or without bypass
isolation switches rated 600 volts AC or less, not exceeding 6000 amps, for use on single-phase
and polyphase AC circuits.

Static transfer equipment that does have cross-connection of sources during transfer or
retransfer is outside the scope of this standard.

1.1 General
The definitions and standards of ICS 1, except for clause 7 pertaining to spacings as indicated,
also apply to this standard.

These requirements cover solid state automatic transfer equipment intended for use in ordinary
locations to provide for lighting and power only in optional stand-by systems in accordance with
Article 702 of the National Electrical Code, ANSI/NFPA 70.

1.2 Normative References

In this NEMA Standards Publication, reference is made to the following standards listed below.
Copies are available from the indicated sources.

National Electrical Manufacturers Association

1300 North 17th Street
Rosslyn, Virginia 22209

ICS 1-1993 Industrial Control and Systems

General Requirements

ICS 1.3-1986 (R1991) Preventive Maintenance of Industrial

Control and Systems Equipment

MG 1-1998 Motors and Generators

Underwriters Laboratories Inc.

333 Pfingsten Road
Northbrook, IL 60062

UL 869A Service Equipment

UL 1008 Transfer Switch Equipment

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ICS 10-2005, Part 2 Static AC Transfer Equipment
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National Fire Protection Association

Batterymarch Park
Quincy, MA 02269

NFPA 70 National Electrical Code (NEC)

NFPA 99 Health Care Facilities

2 DEFINITIONS
For the purposes of this NEMA Standards Publication, the following definitions apply.

2.1 Transfer Equipment

bypass isolation switch: A manually operated device used in conjunction with transfer
equipment to provide a means of directly connecting load conductors to a power source and of
disconnecting the transfer equipment.

A bypass isolation switch may be supplied with an automatic or non-automatic transfer

equipment.

static automatic transfer equipment: Self-acting equipment using electronic power devices
for transferring one or more load conductor connections from one power source to another.

Static automatic transfer equipment may include logic to inhibit automatic operation in either or
both directions, provided the equipment reverts to automatic operation upon loss of power to the
load.

Static automatic transfer equipment may be supplied with or without a bypass isolation switch.

static nonautomatic transfer equipment: Manually controlled equipment using electronic

power devices for transferring one or more load conductor connections from one power source
to another.

Nonautomatic transfer equipment may be supplied with or without a bypass isolation switch.

2.2 Operation of Static Transfer Equipment

conditional short-circuit rating: The available rms symmetrical current that transfer
equipment can satisfactorily withstand for a total operating time of the overcurrent protective
device under specified conditions of use and behavior. (See clause 4.1.6)

cross connection of sources: The flow of current, excluding normal leakage currents,
between sources during transfer.

NOTE—Static transfer equipment that does have cross-connection of sources during transfer or retransfer is outside
the scope of this standard.
monitored source deviation: A variation in the power source being monitored that signals the
transfer equipment to operate.

For clarification of terms:

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 ICS 10-2005, Part 2 Static AC Transfer Equipment
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a. Typical variations that may be detected are changes in voltage and frequency.
b. “Transfer Signal” is a term used to describe the signal that is initiated by the monitored
source deviation and that signals the transfer equipment to operate.

total system transfer time: The time interval, including any purposely introduced time delays
and engine-generator start-up times, during which the power to the load deviates from the
allowable tolerances. “Total System Transfer Time” is a parameter of the total system and not
solely of the transfer equipment.

transfer time: The time interval during transfer in which the power to the load deviates from the
allowable tolerances.

3 CLASSIFICATIONS
Transfer equipment is classified as Type A or Type B as follows:

Type A: Transfer equipment that is not intended to provide required overcurrent (overload and
short-circuit) protection.

Type B: Transfer equipment that is intended to automatically provide required overcurrent

(overload and short-circuit) protection.

4 CHARACTERISTICS AND RATINGS

4.1 Rated and Limiting Values for the Main (Power) Circuit

4.1.1 System Voltage Ratings

The system voltage rating of transfer equipment shall be 120, 208, 220, 240, 400, 415, 480, or
600 volts AC, single phase or polyphase.

4.1.2 Continuous Current Rating

The continuous current rating of transfer equipment shall be based on an ambient temperature
of 40°C.

4.1.3 Rating Based on Load Characteristics

Transfer equipment shall be rated for one or more of the following types of loads:

a. Total system load consisting of any combination of motors, electric discharge lamps, electric
heating (resistive) loads, and tungsten lamp loads, provided the latter does not exceed 30% of
the continuous current rating of the transfer equipment.
b. Tungsten lamp load consisting entirely of tungsten lamps.
c. Electric discharge lamp load consisting entirely of electric discharge lamps, including
fluorescent lamps.
d. Resistive load consisting of heater and other non-inductive loads in which the inrush current
does not exceed 150% of the continuous current rating of the equipment.

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ICS 10-2005, Part 2 Static AC Transfer Equipment
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4.1.4 Interrupting Rating

Current interrupting ratings shall be expressed in rms symmetrical amperes.

4.1.5 Motor Starter Rating

Transfer equipment intended for a load consisting of a single motor may include overload relays
and may be rated in horsepower or in full-load and locked-rotor current.

4.1.6 Withstand and Closing Ratings

Transfer equipment shall have withstand and closing ratings, expressed in maximum available
fault current (rms symmetrical amperes) at maximum rated voltage, and marked in accordance
with UL 1008 and this standard. The type and rating of any associated protective devices
necessary to meet the requirements of UL 1008 shall also be marked on the transfer equipment.

The withstand and closing current rating shall be one of the values specified in Column 1 of
Table 4-1.

Standard withstand and closing ratings are based on transfer equipment testing conducted at
maximum allowable test circuit power factors and corresponding minimum X/R ratios specified in
Table 4-1. When applying transfer equipment, consideration should be given to both available
fault current and circuit X/R ratio at the point of application.

4.1.7 Overcurrent Protection

Each service, feeder, and branch circuit should have the appropriate short-circuit, ground-fault,
and overload protection, provided either externally or as an integral part of the transfer
equipment.

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 ICS 10-2005, Part 2 Static AC Transfer Equipment
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Table 4-1
AVAILABLE FAULT CURRENTS AND RESPECTIVE TEST POWER FACTORS AND
CORRESPONDING X/R RATIOS
Withstand and Closing Rating Maximum Test Minimum Corresponding
(rms symmetrical amperes) Power Factor* X/R Ratio*
5,000 0.50 1.73
7,500 0.50 1.73
10,000 0.50 1.73
14,000 0.30 3.18
18,000 0.30 3.18
22,000 0.20 4.90
25,000 0.20 4.90
30,000 0.20 4.90
35,000 0.20 4.90
42,000 0.20 4.90
50,000 0.20 4.90
65,000 0.20 4.90
85,000 0.20 4.90
100,000 0.20 4.90
125,000 0.20 4.90
150,000 0.20 4.90
200,000 0.20 4.90

5 PRODUCT MARKING, INSTALLATION, AND MAINTENANCE INFORMATION

5.1 Markings

5.1.1 Transfer Equipment

The nameplate markings for transfer equipment shall include:
a. Name of manufacturer and catalog number, or the equivalent serial number.
b. Voltage and frequency ratings.
c. Continuous current rating.
Additional markings shall be those required by UL 1008 and shall include the following, where
applicable:

a. Suitability for specific loads. See 4.3.5 of UL 1008.

b. Suitability for use as service equipment.
c. Interrupting or short-circuit ratings.
1. For Type A: Short-circuit rating or conditional short-circuit rating with the type and rating of
the additionally required short-circuit protective device.

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ICS 10-2005, Part 2 Static AC Transfer Equipment
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2. For Type B: Interrupting rating.

d. Cautions or warnings, or both.
e. Automatic or non-automatic.

5.1.2 Bypass Isolation Switch Equipment

The nameplate markings for bypass/isolation switches shall include:

a. Name of manufacturer and catalog number, or the equivalent serial number.

b. Voltage and frequency ratings.
c. Continuous current rating.

5.1.3 Control Circuits

Transfer equipment control circuits shall meet the marking requirements of UL 1008.

5.2 Maintenance of Transfer Equipment

A maintenance program and schedule should be established for each particular installation to
assure minimum down time. The program should include periodic testing, tightening of
connections, inspection for evidence of overheating and excessive contact erosion, removal of
dust and dirt, and replacement of contacts when required.

See NEMA Standards Publication ICS 1.3 for preventive maintenance instructions.

6 SERVICE AND STORAGE CONDITIONS

ICS 1, Clause 6 applies.

7 CONSTRUCTION

7.1 Spacings
Except as otherwise specified in the following paragraphs, spacings in automatic transfer
equipment and bypass isolation switches shall be not less than those shown in Table 7-1.

7.1.1 The spacings on printed wiring assemblies may be as small as 0.031 in. where the power
to the printed wiring assembly is limited and transient voltages are controlled as specified in ICS
1, Clause 7.

7.1.2 The spacings given in Table 7-1 do not apply to snap switches, lampholders, and similar
wiring devices that are used as a part of transfer equipment.

7.1.3 In a circuit involving potentials of not more than 50 VAC, the spacings at field-wiring
terminals may be 0.125 in. (3.2 mm) through air and 0.250 in. (6.4 mm) over the surface, and
the other spacings may be 0.053 in. (1.4 mm) through air and over the surface, provided that
insulation and clearances between the low-potential circuit and any high-potential circuit are in
accordance with the requirements that are applicable to the high-potential circuit.

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7.2 Test Switch

An external test switch or terminals for connection to an external test switch shall be provided.
Such a test switch shall be connected so that operation of the test switch simulates a failure of
the normal power source.

7.3 Interlocks and Protective Circuits

Transfer equipment shall meet the requirements of UL 1008 for interlocks and protective circuits.

7.3.1 Protection from Cross-Connection

The electronic circuitry of transfer equipment shall have provisions to reduce the possibility of
inadvertent cross-connections.

7.3.2 Bypass Isolation Switch Interlock

Bypass isolation switches shall be provided with necessary interlocks to prevent the
unintentional simultaneous connection of two power sources. In addition, bypass isolation
switches provided with overlapping contacts shall be provided with a means of interlocking to
prevent the opening of non-load-break contacts under load.

7.4 Service Equipment

Transfer equipment which is designed to be suitable for use as service equipment shall comply
with the requirements in UL 1008, UL 869A Service Equipment, and:

a. Have spacings in accordance with Table 7-1, Item 2.

b. Have accessible independent disconnecting means for both the normal and alternate power
sources
c. Be marked “suitable for use as service equipment.”
d. Where provided with a neutral, have provision for connection of the service grounding
conductor to the neutral terminal in accordance with Table 7-2.
e. Be provided with ground-fault protection where applied on solidly grounded wye electrical
services of more than 150 VAC to ground, but not exceeding 600 VAC phase-to-phase for each
service disconnect rated 1000 amperes or more.
f. The control circuit shall have short-circuit protection and a disconnecting means suitable for
the available current of the supply, where the control circuit is located ahead of the service
disconnecting means.
g. Be provided with overcurrent protection either immediately adjacent to, or as part of, the
transfer equipment disconnecting means.

7.5 Emergency Power Circuit Service

The alternate source for emergency systems shall not be required to have ground-fault
protection of equipment.

7.6 Control Circuits

Transfer equipment shall be exempted from the motor control circuit overcurrent protection
requirements of ICS 1, Clause 7.

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Table 7-1
SPACINGS
Minimum Spacings (inches)
Nominal Operating Voltage

51-150 151-300 301-600

Volts Volts Volts

1. Power Circuits Rated Not More Than 400 Amperes and Control Circuits
a. Between any uninsulated live part and (1) an through 0.125 0.250 0.375
uninsulated live part of the opposite polarity and (2) an air or oil (3.2 mm) (6.4 mm) (9.5 mm)
exposed metal part.*
over 0.250 0.375 0.500
surface (6.4 mm) (9.5 mm) (12.7 mm)
b. Between any uninsulated live part and the walls of a shortest 0.500 0.500 0.500
metal enclosure, including fittings for conduit or armored distance (12.7mm) (12.7mm) (12.7 mm)
cable.**

2. Power Circuits Rated Over 400 Amperes and Service Equipment

a. Between any uninsulated live part and an uninsulated through 0.500 0.750 1.000*
live part of the opposite polarity. air or oil (12.7mm) (19.1mm) (25.4 mm)
over 0.750 1.250 2.000
surface (19.1mm) (31.8mm) (50.8 mm)
†
b. Between any uninsulated live part and an uninsulated through 0.500 0.500 1.000
grounded part, exposed metal part, or the walls of a air or oil (12.7mm) (12.7mm) (25.4 mm)
metal enclosure, including fittings for conduit or armored
cable.**

*The spacing between wiring terminals of opposite polarity and the spacing between a wiring terminal and a ground part shall be not
less than 0.250 in. (6.4 mm) if short-circuiting of grounding of such terminals may result from projecting strands of wire.
** For the purpose of this requirement, a metal piece attached to the enclosure is considered to be a part of the enclosure if
deformation of the enclosure is likely to reduce spacings between the metal piece and uninsulated live parts.

† The through-air spacing may be not less than 0.500 in. (12.7 mm) at the main terminals and also between grounded dead metal
and the neutral of 277/480-VAC, 3-phase, 4-wire transfer equipment.

Table 7-2
SIZE OF GROUNDING CONDUCTORS
Size of Largest Service Conductor or
Equivalent Size of Multiple-conductor Cables Size of Grounding Conductor

Copper Aluminum Copper Aluminum

2 AWG or smaller 0 AWG or smaller 8 AWG 6 AWG
1 or 2 AWG 2/0 or 3/0 AWG 6 AWG 4 AWG
2/0 or 3/0 AWG 4/0 AWG or 250 kcmil 4 AWG 2 AWG
Over 3/0 AWG to 350 kcmil Over 250 to 500 kcmil 2 AWG 0 AWG
Over 350 to 600 kcmil Over 500 to 900 kcmil 0 AWG 3/0 AWG
Over 600 to 1100 kcmil Over 900 to 1750 kcmil 2/0 AWG 4/0 AWG
Over 1100 kcmil Over 1750 kcmil 3/0 AWG 250 kcmil

*The equivalent size for multiple conductor cables shall be the sum of the circular-mil areas of the individual
conductors.

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8 PERFORMANCE REQUIREMENTS AND TESTS

8.1 Performance Requirements

8.1.1 Control Circuits

Transfer equipment control circuits shall meet the overcurrent and mechanical protection
requirements of UL 1008.

8.1.2 Ground-Fault Protection

Where ground-fault protection is provided, the performance requirements are as follows:

a. The maximum setting of the ground-fault protection shall be 1200 amperes.

b. The maximum time delay shall be one second for ground-fault currents equal to or greater
than 3000 amperes.
c. In health care facilities, the ground-fault protection on the service disconnect must be
coordinated with the ground-fault protection on the next downstream feeder to ensure 100%
selectivity such that the feeder device and not the service disconnect shall open on ground
faults on the load side of the feeder device. There shall be a minimum time separation of six
cycles between tripping bands.

8.1.3 Undervoltage Monitoring Characteristics

Unless otherwise specified, transfer equipment shall have provision for initiating the transfer of
load conductor connections to an alternate power source when the voltage on any phase of the
normal power source falls below 70% of rated voltage. Provision shall also be made for
transferring the connections back to the normal power source when the voltage on all phases of
the normal power source returns to at least 90% of rated voltage.

Voltage critical loads may require adjustable voltage-monitoring and control means.

8.2 Design Tests

8.2.1 Required Testing

Transfer equipment and bypass isolation switches shall be capable of meeting required
performance tests as specified in UL 1008 for static transfer equipment.

Static transfer equipment shall not permit current flow between sources under any normal
operating condition. Design of control circuits shall guard against current flow between sources
with single source failures. To test for cross currents in each phase, the instantaneous currents
I 1 , I 2 , and I L (see figure 8-1) shall be displayed or recorded and shown to be related as:

I1 + I 2 = I L

Inequality outside of measurement error indicates a cross current and failure of the test.

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Figure 8-1
CROSS CURRENT TEST CIRCUIT DIAGRAM

8.2.2 Optional Performance Verification

The following verifications are performance capabilities that may be declared by the
manufacturer.

8.2.2.1 Total System Transfer Time and Transfer Time – In Phase Transfer
a. Open circuit one source
b. Short-circuit one phase, one source
c. Short-circuit three phase, one source
d. Output short-circuit

8.2.2.2 Total System Transfer Time and Transfer Time – Out of Phase Transfer
a. Open circuit one source
b. Short-circuit one phase, one source
c. Short-circuit three phase, one source
d. Output short-circuit

8.2.2.3 Non-Linear Loads

See Annex A for a discussion on non-linear loads.

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9 APPLICATION

9.1 AC Transfer Equipment Specification

The following characteristics should be specified in order to properly match the transfer
equipment to the system:

a. Voltage.
b. Number of phases.
c. Number of wires.
d. Frequency.
e. Number of switched poles.
f. Type of load as defined in 4.1.3.
g. Continuous current or horsepower, or both, requirements of the load.
h. Available fault current.
i. Whether it is necessary to disconnect the load from both power sources simultaneously.
j. Whether the switch is to be suitable for use as service equipment.
k. Whether the switch is to include integral overcurrent protection.
l. Whether it is necessary to provide a bypass isolation switch in conjunction with the transfer.
equipment. Where a bypass isolation switch is provided, it should be compatible with the
transfer equipment.
m. Number of sources.

9.2 Power Quality

The transfer equipment shall be capable of transferring sensitive loads, such as computers and
process controllers, between the preferred and alternate power sources without interrupting their
normal operation. Examples of total system transfer time criteria applicable for most power
quality applications are shown in figure 9-1 taken from IEEE 446-1995. The Information
Technology Industry Council (ITI) published another criteria applicable to 120-volt single-phase
load equipment in 1996 (see figure 9-2).

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Figure 9-1
TYPICAL DESIGN GOALS OF POWER-CONSCIOUS COMPUTER MANUFACTURERS

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Figure 9-2
ITI (CMEMA) CURVE (REVISED 1996)

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9.3 Motor Transfer

Motors rated approximately 50 horsepower and greater are susceptible to damage when, in an
ON-OFF-ON operation, they are switched before the residual motor voltage has decayed to a
safe level. The high abnormal currents that may result from the out-of-phase relationship
between the motor's residual voltage and the voltage of the reconnected power source may also
trip overcurrent protective devices and, therefore, should be avoided. Consideration should be
given to include controls in AC transfer equipment to minimize these abnormal currents.

For a more in depth treatment of motor transfer, see the section on bus transfer and reclosing in
NEMA Standard MG 1.

9.4 Features for Particular Applications

Some conditions or requirements of particular applications may make additional features
necessary. These may include the following:

a. Time delay to override monitored source deviation—A fixed or adjustable time delay which
delays all signals for operation. This time delay prevents starting of the engine or the transfer of
the load from the normal power source to the alternate power source during momentary voltage
dips or disturbances of the normal power source.

b.* Time delay before transfer to alternate power source—A fixed or adjustable time delay
which delays the transfer of the load to an available alternate power source. This delay allows
an engine-generator time to stabilize at rated voltage and frequency before accepting the load.

c. Time delay before transfer to normal power source—An adjustable time delay which inhibits
transfer of the load back to the normal power source. This delay allows the normal power
source time to stabilize before accepting the load. The time delay may be automatically nullified
if the alternate power source fails and power is available at the normal power source.

d.* Time delay before engine shutdown—A fixed or adjustable time delay which delays the
shutdown of the engine-generator (alternate power source) after the load is transferred back to
the normal power source. This delay allows the engine to run at no load and thereby minimizes
the possibility of shutdown due to an increase in the temperature of the cooling water. It also
permits the immediate transfer of the load back to the engine-generator in the event that the
normal power source has not fully stabilized.

e.* Time delay to limit cranking—A fixed or adjustable time delay to limit the cranking time of
the engine-generator if the engine fails to start when the normal power source fails.

f.* Engine exerciser—A programmable time switch to initiate the starting of the
engine-generator for a preset period of time at preset intervals without transferring the load from
the normal to the alternate power source.

Caution: No load operation may be detrimental to the engine, and the engine generator manufacturer should be
consulted. Paragraph (g) would be considered.

g.* Alternate system exerciser—A programmable time switch to initiate the starting of the
engine-generator and to transfer the load from the normal to the alternate power source for a

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preset period of time at preset intervals. The load is transferred back to the normal power
source at the end of the exercise period.

Provision should be made for the initiation of immediate retransfer to the normal source in the
event of an engine generator failure during the exercise period.

h. Auxiliary contact—A contact, other than the power circuit contacts, that is part of transfer
equipment and is available for connection by the user. The number of auxiliary contacts, mode
of operation, and function of each contact should be specified.

i*. Engine start contact—A contact that initiates cranking of the engine generator set when the
normal power source fails.

j. Alternate source monitor—A device(s) that monitors voltage or frequency, or both, of the
alternate power source, and inhibits transfer to the alternate source until the monitored
parameters reach specified levels.

k. Test switch—A switch to simulate failure of the normal power source, causing transfer of the
load to the alternate power source.

l. Close-differential protection—A device(s) that monitors all lines of the normal power source
and initiates transfer of the load from the normal power source to the alternate power source
when any line of the normal power source drops below a predetermined value of voltage. It
initiates transfer of the load back to the normal power source when all lines of the normal power
source return to within specified limits. This feature is generally used for installations where only
a limited reduction in voltage can be tolerated.

m. Time-delay-bypass switch—A momentary contact switch that initiates immediate transfer of

the load back to the normal power source, bypassing any optional time delays.

n. Manual return-to-normal switch—A momentary contact switch that initiates transfer from the
alternate to the normal power source where automatic transfer is not desired.

o. Overcurrent protection—The addition of elements to permit the transfer equipment to provide

running overcurrent protection for a single motor.

p. Service equipment—See 7.4.

q. In-phase monitor—A device that monitors the relative voltage and phase angle between the
power source to be transferred to and the power from which the transfer is to be made, and
initiates transfer when acceptable values of voltage and phase angle are present. Such a device
may function on the transfer from either power source.

r. Delayed transition—Provides a timed disconnection of the load from the power sources
during transfer, primarily to allow decay of motor residual voltage.

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s. Load disconnect device—A device that initiates opening of a pilot contact prior to transfer
from either source and then provides a timed closure after transfer. It is used primarily to allow
decay of motor voltage without affecting the speed of transfer.

t. Bypass isolation switch—A switch that may provide any of the following features or functions:
1. Load-break contacts.
2. Overlapping contacts.
3. Combination of 1 and 2.
4. Single-source bypass.
5. Dual source bypass.
6. Total isolation of the transfer equipment for purpose of maintenance, testing, modification, or
repair. A bypass isolation switch can function as an independent nonautomatic transfer
equipment.

u. Preferred Source Selectability—Allows the user to assign either power source to be the
preferred, or normal, source.

v. Automatic Restart—Allows the transfer equipment to automatically restart when any power
source becomes available after a simultaneous outage on all sources. This will prevent the load
from being without power for an indeterminate period of time by overriding the standard
operating mode which requires the user to manually restart the equipment.

w. Local/Remote Selector—Provides a means to allow the transfer equipment to be controlled

from a remote location.

x. Reverse Power Flow Sensing—Detection of reverse power flow to the source

*Applicable to installations where the alternate power source is an engine-generator set.

9.5 Considerations for Non-Linear Load Applications

Applications for non-linear loads may require special consideration. See Annex A for further
information.

9.6 Graphical Symbol for Static Transfer Equipment

Figure 9-3 shows the preferred graphical symbol for static transfer equipment.

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Source 1 Source 2

Critical Load

Figure 9-3
STATIC TRANSFER EQUIPMENT

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ANNEX A
Non-Linear Loads

A.1 General
Static AC Transfer Equipment may be used to perform high speed power transfers to protect
sensitive non-linear electrical loads from voltage sags, swells, and other power anomalies,
including outages. One form of non-linear load is analytically described by its nominal 3:1 Crest
Factor characteristic. The predominant example of a 3:1 Crest Factor load is the switch-mode
power supply commonly found in computers and other electronic data processing equipment.

A.2 Non-Linear Electrical Load

Non-linear electrical loads do not behave according to the simple linear relationship proposed by
Ohm’s law. They are generally described by:

I = F (V )

Where F = a non-linear mathematical function, or

i = f (t )

Where f = a function of time.

The current-voltage relationship for popular electronic devices such as diodes or bi-polar
junction transistors is well-known. However, when these solid-state devices are used to form
complex systems like converters/inverters and switch-mode power supplies, the current-voltage
relationship becomes difficult, at best, to express analytically. Fortunately, any rational function
can be expressed as a linear sum of sines and cosines. In other words, the steady-state current
wave form can, in general, be expressed as *:

i(t ) = A sin(ω t ) + B sin( 3ω t ) + C sin( 5ω t ) + D sin( 7ω t ) +K

In simple terms, this means that non-linear electrical loads draw currents that are (odd multiple)
harmonics of the intended (60 Hz) current.

The proliferation of electronic loads, such as computers and motor drives, has increased the
proportion of harmonic currents (as compared to the fundamental 60 Hz) that exist in the facility
electrical system.

A.3 Crest Factor (CF) of a System

A measure of the ratio of the peak to the root-mean-square (RMS) value of a wave form:

Peak Value
CF =
RMS Value

* For the advanced reader: This implies, without further explanation, that even-harmonic terms cancel in the Fourier
series expansion of the function f(t). Moreover, a nonlinear load draws current described by:

i(t ) = A sin(ωt )

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Where:
CFPure sine wave (linear load ) = 2 = 1.414

CFCurrent drawn by a computer ≅ 3

A Crest Factor > 2 of the current drawn by a load is one measure of the non-linearity of the
load.

Figure A-1 shows the current drawn by a switch mode power supply.

Figure A-1
TYPICAL NON-LINEAR CURRENT WAVEFORM

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Annex B
Short Time Rating

Section 4.1.6 of ICS 10 outlines withstand and closing ratings for transfer switch equipment. As
noted, the requirement parallels those of UL Standard 1008. With the introduction of circuit
breakers with electronic trips, the requirements for withstand and closing capability of transfer
switch equipment (TSE) is more complex. UL 1008 addresses this issue by providing the option
for “short time ratings.”

Traditional thought said that the transfer switch is a piece of wire, and that it was protected by
the common thermal/magnetic circuit breaker. When protected by a circuit breaker with the
familiar I 2 T (thermal) trip, and instantaneous trip, the transfer switch merely had to be able to
“withstand” a short-circuit fault long enough to allow the upstream device to clear the fault.
However, with a short time trip element, it is possible that the upstream breaker may not trip
during a fault. This means the transfer switch contacts must survive this event as defined by UL
1008.

The worst case condition is determined by the Instantaneous pickup and Short Time Delay
setting on the Upstream Protective Device. The Transfer Switch must “survive” a current that is
just below the instantaneous pickup until the short time delay expires. If the TSE contacts fail to
survive, and the upstream device does not trip, then loss of power to the load results.

In electrical systems it is highly desirable to make sure that a fault is cleared by the upstream
protective device that is closest to the fault. To achieve this, the application for a “short time” trip
rating on upstream circuit breakers has become a necessity. The short time trip characteristic
gives the user the ability to program the trip curve of a particular circuit breaker so that it will
intentionally wait for a downstream device to clear the fault. This means that a transfer switch
applied between this upstream breaker and a downstream device must then truly withstand the
fault current seen while the protective devices react. Failure to do so will make the transfer
switch the weak link in the system.

The short-time characteristic is usually a “flat response” type. This means that a timer in the
circuit breaker is started when the fault current exceeds the short time pickup setting. If the
current remains above the pickup point and the timer times out, then the circuit breaker will trip.
The designer should be aware that there may be I 2 T short time characteristics available in
certain trip units, and these offer some increased coordination possibilities.

In either case it is important to note that should the fault current continue to increase, and reach
the instantaneous pickup point for the trip, that the breaker would then trip instantaneously.
Therefore, the worst case scenario for the transfer switch is the case where the fault current
rises to just below the instantaneous pickup, and the short time has to time out. This is the
particular point the user must be aware of, and allow for in the application of the transfer switch.
In other words, it is important to make sure that the transfer switch will truly withstand a current
equal to the instantaneous current pickup for the maximum time of the short time delay. It is also
important to realize that the instantaneous pickup point, and the maximum interrupt rating of the
breaker are not synonymous.

Let us use a generic example that may explain the application. We will deal with a 3000A circuit
breaker having an adjustable short-time pick up settable between 2 and 6 times the nominal
rating, and an instantaneous pickup settable between 2 and 10 times the nominal rating. The
maximum short time delay setting is 0.5 seconds, or 30 cycles on a 60 Hz system. Given this
example, proper application of a transfer switch would require a short time rating (for the
transfer switch) of 30 kA for 0.5 seconds. This relates to setting both the instantaneous pickup

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and the short time delay of the upstream breaker at their maximums. Since various circuit
breakers offer different options relating to pickups and time ranges, responsibility to properly
apply the transfer switch must fall on the designer.

The designer should understand that the short time rating of a transfer switch is much different
than the short-circuit rating . The short-circuit rating is based on tripping of the upstream device
and the need to transfer to the alternate source. The short time rating deals with a non-tripping
condition of the upstream device for a period of time, and the fact that the primary source of
power will not be interrupted. The transfer switch will not see a need to transfer, and must
therefore remain in a state to handle load current from the uninterrupted source during, and after
the fault is cleared by the downstream device.

When applying a transfer switch with a short time rating, the designer should always apply the
transfer switch having short-time current ratings equal to or greater than the selected
instantaneous pickup setting and short time delay setting of the properly coordinated upstream
breaker.

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Annex C
Neutral Conductors in Power Transfer Systems

Many transfer switching systems involve three phase AC power sources with a neutral conductor
(normally referred to as “4-wire distribution”) or single-phase sources with neutral. Single-phase
sources can be either 2-wire (line-neutral) or 3-wire (line-neutral-line) circuits. The application
of transfer systems for switching neutral connected loads between such power sources requires
consideration of several issues to achieve satisfactory operation, including compliance with the
National Electrical Code and preserving the functionality of ground fault monitoring equipment.

The National Electrical Code defines when power sources are required to be grounded and also
addresses the required grounding practices. In broad terms there are two types of sources,
those that are “separately derived” and those that are “non-separately derived.” Separately
derived sources are those with no direct connection (including a neutral conductor) to another
source. A non-separately derived source is directly connected to another power source,
typically through solidly connected neutral conductors. The determination as to whether a
source is to be separately derived or non-separately derived is typically made by the system
designer and/or authority having jurisdiction.

C.1 Separately Derived Sources

If it is determined that a power system consists of separately derived sources, each neutral must
be bonded to ground at its respective source. When transfer equipment is used in this type
system the neutral conductor must be switched along with the phase (line) conductors to prevent
multiple connections between neutral and ground within the system. Multiple connections
between neutral and ground can result in current flow through the grounding system, violating
the requirement that the ground system is never allowed to carry current, except in the case of a
fault. Multiple connections may also defeat or cause false operation of ground fault detection by
allowing ground fault currents to bypass monitoring equipment, or by allowing normal neutral
currents to appear as fault currents.

When switching the neutral conductor, timing of the opening and closing of the phase (line) and
neutral poles is important. If the phase conductors are connected without connection of the
neutral conductors a transient over-voltage can occur at the load.

The current rating of the neutral pole in transfer equipment is another consideration.
Unbalanced loads can result in high neutral currents. Also, nonlinear electronic loads
connected to transfer equipment can create current harmonics that add in the neutral conductors
that might make it necessary to over size the neutral current capacity to prevent overheating of
conductors or the switch poles. Depending on the nature of loads, the neutral rating may need
to be up to 200% of the phase rating.

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C.2 Non-Separately Derived Sources

For systems consisting of non-separately derived sources it is not appropriate to switch the
neutral conductors. If a power system consists of non-separately derived sources, the neutral
conductors from the sources must be solidly connected together. This “shared neutral” must be
bonded to ground at only one source. There must not be any switching device between the
neutral conductors of the sources. This technique allows the neutral conductor of each source to
carry its respective neutral current, and the ground conductors to carry only ground fault current.
By not switching the neutral conductors the possibility of transient overvoltage from neutral
switching delays or failures is eliminated.

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