PC57.13.
IEEE Guide for Grounding of
Instrument Transformer Secondary
Circuits and Cases
OUTLINE
Scope
References
Need for grounding; Warning
Definition of Instrument transformers
Grounding secondary circuits
Grounding at a single location
Conductor size for connecting to ground
Scope
The scope of the guide includes the grounding
practices presently used and the practices that
were not previously reported. Specifically, the
issue of the grounding of cases of electronic
transducers is addressed.
Covers
The practices described in this standard apply to
all instrument transformers, including capacitive
voltage transformers and linear couplers,
irrespective of primary voltage or whether the
primary windings are connected to, or are in,
power circuits or are connected in the secondary
circuits of other transformers as auxiliary cts or
vts.
Does not cover
This guide does not discuss the grounding of some
applications. For example, grounding of gas insulated
substations and metal clad switchgear is not discussed in
this guide; the reader will find these topics addressed in
reference [9] listed in clause 2. The grounding of circuits
of core-balance CTs is also not discussed in this guide.
The reader can find this information also in reference [9]
listed in clause 2. Another issue that is not discussed in
this guide is the practice of using separate safety and
control grounds. For discussion on this topic, the reader
is directed to reference [12] listed in clause 2.
References
[1] NFPA 70-2002, National Electricity Code® (NESC®)
[2] ANSI C2-2002, National Electrical Safety Code®
(NESC®)
[3] C37.103™-2004, IEEE Guide for Differential and
Polarizing Relay Circuit Testing
[4] C57.13.1™-1981 (Reaffirmed 1999), IEEE Guide for
Field Testing of Relaying Current Transformers
[5] C37.92™-2004, IEEE Trial Use Standard for Low
Energy Analog Signal Inputs to Protective Relaying
References
[6] C62.92™-2000, IEEE Guide for the Application of
Neutral Grounding in Electrical Utility Systems
[7] Std. 80™-2000, IEEE Guide for Safety in AC
Substation Grounding
[8] Std. 142™-1991, IEEE Recommended Practice for
Grounding of Industrial and Commercial Power Systems
[9] Std. 242™*2001, IEEE Recommended Practice for
Protection and Coordination of Industrial and Commercial
Power Systems
[10] Std. 518™-1982 (R1996), IEEE Guide for the
Installation of Electrical Equipment to Minimize Electrical
Noise Inputs to Controllers from External Sources
References
[11] Std. 525™-1992, IEEE Guide for the Design and
Installation of Cable Systems in Substations
[12] Std. 665-1995, IEEE Guide for Generating Station
Grounding
[13] Std. 1050-1996, IEEE Guide for Instrumentation and
Control Equipment Grounding in Generating Stations
[14] Std. 1100™-1999, IEEE Recommended Practice for
Powering and Grounding Electronic Equipment
[15] Std. 1143™-1994 (Reaffirmed 1999), IEEE Guide on
Shielding Practice for Low Voltage Cables
Need for Grounding Secondary Circuits
To protect equipment connected to the circuits
To Protect personnel who might come into
contact with the equipment
How the problem manifests
When secondary circuit is not grounded
When a case is not grounded
Voltage due to charge accumulation on an
ungrounded secondary circuit or case
Warning
Grounding of secondary circuits is an issue of
safety of personnel and equipment connected to
the secondary circuits. The reader is urged to
consider it seriously and follow the stipulations of
relevant standards and ensure that the
stipulations of the National Electrical Code [1]
and the National Electric Safety Code [2] are
always adhered to.
Warning
Article 90.2 (A) of National Electrical Code [1]
lists the installations that are covered by the
Code [1] and Article 90.2 (B) lists the installations
that are not covered by the Code. For immediate
reference of the readers, Article 90.2 of National
Electrical Code [1] is reproduced in Annex C.
Grounding of Instrument Transformer
Secondary Circuits
Definition of a secondary circuit for the purposes
of the guide
Issues
Grounding at a single location
Minimum size of the grounding conductor
Issues
Grounding at a single location
Voltage at different locations of the physical
ground and circuits connected to ground is
different when fault currents flow
Convenience of testing a secondary circuit
Problem with forming a common neutral of
more than one instrument transformers and
then grounding the neutral bus
Isolation of all secondary circuits from
ground
Issues
Recommended point of grounding
switchboard or relay panel
Grounding unused secondary windings
Grounding examples
Grounding 3 4 Wire VT Circuits
Grounding 3 3 Wire VT Circuits
PA
OPEN PB
DELTA
PC
CONNECTIONS TO
GROUND BUS AT
SWITCHBOARD
PA (#12 AWG CU
MINIMUM)
PB
WYE PC
NEUTRAL NOT BROUGHT
GROUND ISOLATING
OUT
CONNECTION
Grounding unused CTs
ISSUES
Minimum size of grounding conductor
12 AWG required by NESC©
TYPICAL APPLICATIONS
VT circuits
Voltage provided from distribution transformers
Using Voltage from a Distribution Transformer
Generator Neutral Grounding Transformer
PG
TO GEN
NEUTRAL
TO RELAYS,
R OSCILLOGRAPH,
ETC.
PN
GROUND ISOLATING CONNECTION
SWITCHBOARD
STATION GROUND GROUND
MAT
Generator Neutral Grounding Transformer
PG
TO GEN
NEUTRAL
OPTIONAL
R CONNECTION TO RELAYS,
OSCILLOGRAPH,
ETC.
PN
CENTER TAP GROUND SOMETIMES
STATION GROUND USED SO THAT ACCIDENTAL GROUND
MAT ON PG WILL NOT COMPLETELY DISABLE
GENERATOR GROUND PROTECTION
Generator Neutral Grounding Transformer
Generator Grounding Transformer
TO RELAYS,
R OSCILLOGRAPH,
ETC.
GROUNDING
TRANSFORMER
Basic Synchroscope Circuit
PB 25 SCOPE 25 PB
CKT. 2 RUN INC CKT. 1
PN PN
GROUNDING ISOLATION
CONNECTION
Synchroscope with Multiple Incoming Circuits
Synchroscope with Multiple Incoming and Running
Circuits
PB 25-2 25-2 PB
PB 25-1 SCOPE 25-1 PB
CKT. 4 CKT. 3 RUN INC CKT. 1 CKT. 2
PN 25-1 25-1 PN
PN 25-2 25-2 PN
GROUNDS NOT NECESSARILY GROUNDING ISOLATION
ELECTRICALLY CLOSE CONNECTION
Synchroscope with Multiple Incoming and Running
Circuits using Isolating Transformers
PB 25-2 25-2
PB
PB 25-1 SCOPE 25-1
PB
CKT. 4 CKT. 3 RUN INC CKT. 1 CKT. 2
PN PN
PN PN
GROUNDING ISOLATION
CONNECTION
1 - 1 ISOLATING
TRANSFORMERS
Synchroscope with Synchronizing Lights
SYNCHRONIZING LIGHTS
25 25 PB
PB
CKT. 2 RUN INC CKT. 1
25 PN
PN
SCOPE OR
SYNC-CHECK
RELAY
GROUNDS SHOULD BE
ELECTRICALLY
AS GROUNDING
CLOSE AS PRACTICAL
ISOLATION
CONNECTION
THIS CONTACT PERMITS THE TWO VOLTAGE
CIRCUITS TO BE SEPARATELY GROUNDED
Grounding Distribution Station with one used for
Instrumentation
SWITCHBOARD TERMINALS
PN
GROUND ISOLATION
CONNECTION
G(N)
NOTE: BECAUSE OF THE TWO GROUNDS
ON G(N), IT SHOULD BE SUFFICIENTLY
HEAVY SO THAT STATION GROUND
FAULTS WILL NOT CAUSE DAMAGE TO
G(N)
Grounding 1 CT Circuit
CP
BURDEN
CN
Grounding 1 CT Circuit
CP
BURDEN
CN
GROUND ISOLATION
CONNECTION
Grounding 3 CT Circuit
Grounding 3 CT Circuit
Grounding V Connected CT Circuit
Delta-Delta Connected CTs in a Differential
Protection Relay
Delta-Wye Connected CTs in a Differential
Protection Relay
A B C C B A
SWITCHBOARD
TERMINALS
DIFF.
RELAYS
CBC CBC
CBB CBB
CBA CBA
GROUND ISOLATION
CONNECTION
Percentage Bus Differential Scheme
SWITCHBOARD
TERMINALS RESTRAIN
T
CD3A
OPERATE
CD3B
TO CTS
ON CKT. 3 CD3C PHASE A
CD3N
CD2A
CD2B
TO CTS PHASE B
ON CKT. 2 CD2C
CD2N
A CD1A
B CD1B PHASE C
CKT. 1
C CD1C
CD1N
GROUND ISOLATION
CONNECTION
Ring Bus or Breaker and a Half Scheme
TO METERS,
RELAYS, ETC.
TERMINALS TERMINALS
AT CT LOCATION AT CT LOCATION
GROUND
ISOLATING
CONNECTION
SWITCHBOARD TERMINALS
(MAY BE SLIDING LINK
CONTROL HOUSE
TYPE
GROUND BUS
TO FACILITATE TESTING)
51-
A
51-B
51-C
Multiple use of CTs
51-C
51-B
51-
A
Grounding of Cases
Grounding metallic conductive cases
Insulated cases with conductive internal parts
Ungrounded metallic cases or internal parts
Protected by suitable barriers or elevated to
prevent contact if operating voltage exceeds
1,000 V
Exceptions to Grounding
If the primary windings of instrument transformer
circuits are connected to circuits of less than
1000 V with no live parts or wiring exposed or
accessible to other than qualified persons, the
circuits may not be grounded.
Exceptions to Grounding
For instrument transformer cases, the cases or
frames of CTs may not be grounded if the
primary windings are not over 150 V to ground
and are used exclusively to supply current to
meters.
Exceptions to Grounding
Cases of instruments, meters, and relays
operating at less than 1000 V on switchboards
having exposed live parts on the front of panels
are not usually grounded. Mats of insulating
rubber or other suitable floor insulation is
provided where the voltage to ground exceeds
150 V.
Exceptions to Grounding
Instruments, meters, and relays, whose current-
carrying parts operate at voltages to ground of
1000 V and over, are isolated by elevating them
or protecting them by suitable barriers, grounded
metal or insulating covers, or guards. In such
situations, the cases are not usually grounded.
Grounding of Low-Energy Transducers
Grounding of Low-Energy Transducers
Grounding of Low-Energy Transducers
Annex A
Grounding of shielded cables
To consider current carrying capacity of the
shield
Cables with spiral design shields are grounded
at both ends
Cables with drain wires are grounded at one
end
Annex A
CTs installed over shielded cables
Annex B
IEEE Standards reviewed by the WG
Annex C
Clause 90.2, Scope of National Electricity Code
Type of installations covered by the Code
Type of installations not covered bye the Code
Special permission
ANNEX D
Survey of Grounding Practices
Approximately one-third of the respondents do
not ground voltage transformers “at the first
point of use” as described in the Guide. The
Working Group is of the opinion that grounding
“at the first point of use” is better than
grounding at other locations.
Almost all respondents indicated that current
transformers are grounded “at the first point of
use” as described in the Guide.
