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PC57.13.3IEEE Guide for Grounding ofInstrument Transformer SecondaryCircuits and Cases

OUTLINEScopeReferencesNeed for grounding; WarningDefinition of Instrument transformersGrounding secondary circuits Grounding at a single location Conductor size for connecting to ground

ScopeThe scope of the guide includes the groundingpractices presently used and the practices thatwere not previously reported. Specifically, theissue of the grounding of cases of electronictransducers is addressed.

CoversThe practices described in this standard apply toall instrument transformers, including capacitivevoltage transformers and linear couplers,irrespective of primary voltage or whether theprimary windings are connected to, or are in,power circuits or are connected in the secondarycircuits of other transformers as auxiliary cts orvts.

Does not coverThis guide does not discuss the grounding of someapplications. For example, grounding of gas insulatedsubstations and metal clad switchgear is not discussed inthis guide; the reader will find these topics addressed inreference [9] listed in clause 2. The grounding of circuitsof 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 inthis guide is the practice of using separate safety andcontrol grounds. For discussion on this topic, the readeris 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 andPolarizing Relay Circuit Testing[4] C57.13.1 -1981 (Reaffirmed 1999), IEEE Guide forField Testing of Relaying Current Transformers[5] C37.92 -2004, IEEE Trial Use Standard for LowEnergy Analog Signal Inputs to Protective Relaying

References[6] C62.92 -2000, IEEE Guide for the Application ofNeutral Grounding in Electrical Utility Systems[7] Std. 80 -2000, IEEE Guide for Safety in ACSubstation Grounding[8] Std. 142 -1991, IEEE Recommended Practice forGrounding of Industrial and Commercial Power Systems[9] Std. 242 *2001, IEEE Recommended Practice forProtection and Coordination of Industrial and CommercialPower Systems[10] Std. 518 -1982 (R1996), IEEE Guide for theInstallation of Electrical Equipment to Minimize ElectricalNoise Inputs to Controllers from External Sources

References[11] Std. 525 -1992, IEEE Guide for the Design andInstallation of Cable Systems in Substations[12] Std. 665-1995, IEEE Guide for Generating StationGrounding[13] Std. 1050-1996, IEEE Guide for Instrumentation andControl Equipment Grounding in Generating Stations[14] Std. 1100 -1999, IEEE Recommended Practice forPowering and Grounding Electronic Equipment[15] Std. 1143 -1994 (Reaffirmed 1999), IEEE Guide onShielding Practice for Low Voltage Cables

Need for Grounding Secondary CircuitsTo protect equipment connected to the circuitsTo Protect personnel who might come intocontact with the equipmentHow the problem manifests When secondary circuit is not grounded When a case is not grounded Voltage due to charge accumulation on anungrounded secondary circuit or case

WarningGrounding of secondary circuits is an issue ofsafety of personnel and equipment connected tothe secondary circuits. The reader is urged toconsider it seriously and follow the stipulations ofrelevant standards and ensure that thestipulations of the National Electrical Code [1]and the National Electric Safety Code [2] arealways adhered to.

WarningArticle 90.2 (A) of National Electrical Code [1]lists the installations that are covered by theCode [1] and Article 90.2 (B) lists the installationsthat are not covered by the Code. For immediatereference of the readers, Article 90.2 of NationalElectrical Code [1] is reproduced in Annex C.

Grounding of Instrument TransformerSecondary CircuitsDefinition of a secondary circuit for the purposesof the guideIssues Grounding at a single location Minimum size of the grounding conductor

IssuesGrounding at a single location Voltage at different locations of the physicalground and circuits connected to ground isdifferent when fault currents flow Convenience of testing a secondary circuit Problem with forming a common neutral ofmore than one instrument transformers andthen grounding the neutral bus Isolation of all secondary circuits fromground

IssuesRecommended point of grounding switchboard or relay panelGrounding unused secondary windingsGrounding examples

Grounding 3 4 Wire VT Circuits

Grounding 3 3 Wire VT CircuitsPAOPENDELTAPBPCPACONNECTIONS TOGROUND BUS ATSWITCHBOARD(#12 AWG CUMINIMUM)PBWYEPCNEUTRAL NOT BROUGHTOUTGROUND ISOLATINGCONNECTION

Grounding unused CTs

ISSUESMinimum size of grounding conductor 12 AWG required by NESC

TYPICAL APPLICATIONSVT circuitsVoltage provided from distribution transformers

Using Voltage from a Distribution Transformer

Generator Neutral Grounding TransformerPGTO GENNEUTRALTO RELAYS,OSCILLOGRAPH,ETC.RPNGROUND ISOLATING CONNECTIONSTATION GROUNDMATSWITCHBOARDGROUND

Generator Neutral Grounding TransformerPGTO GENNEUTRALROPTIONALCONNECTIONTO RELAYS,OSCILLOGRAPH,ETC.PNSTATION GROUNDMATCENTER TAP GROUND SOMETIMESUSED SO THAT ACCIDENTAL GROUNDON PG WILL NOT COMPLETELY DISABLEGENERATOR GROUND PROTECTION

Generator Neutral Grounding Transformer

Generator Grounding TransformerRGROUNDINGTRANSFORMERTO RELAYS,OSCILLOGRAPH,ETC.

Basic Synchroscope CircuitPB25RUNCKT. 2PN25SCOPEPBINCCKT. 1PNGROUNDING ISOLATIONCONNECTION

Synchroscope with Multiple Incoming Circuits

Synchroscope with Multiple Incoming and RunningCircuitsCKT. 4PB25-2PB25-1SCOPERUNCKT. 325-2PB25-1PBINCCKT. 1PN25-125-1PNPN25-225-2PNGROUNDS NOT NECESSARILYELECTRICALLY CLOSECKT. 2GROUNDING ISOLATIONCONNECTION

Synchroscope with Multiple Incoming and RunningCircuits using Isolating TransformersPBPBCKT. 4CKT. 325-225-2SCOPE25-1RUNPNPN25-1PBPBCKT. 1INCPNPNGROUNDING ISOLATIONCONNECTION1 - 1 ISOLATINGTRANSFORMERSCKT. 2

Synchroscope with Synchronizing LightsSYNCHRONIZING LIGHTSPB25CKT. 225RUNPNPBCKT. 1INC25SCOPE ORSYNC-CHECKRELAYGROUNDS SHOULD BEELECTRICALLYAS GROUNDINGCLOSE AS PRACTICALISOLATIONCONNECTIONTHIS CONTACT PERMITS THE TWO VOLTAGECIRCUITS TO BE SEPARATELY GROUNDEDPN

Grounding Distribution Station with one used forInstrumentationSWITCHBOARD TERMINALSPNGROUND ISOLATIONCONNECTIONG(N)NOTE: BECAUSE OF THE TWO GROUNDSON G(N), IT SHOULD BE SUFFICIENTLYHEAVY SO THAT STATION GROUNDFAULTS WILL NOT CAUSE DAMAGE TOG(N)

Grounding 1 CT CircuitCPBURDENCN

Grounding 1 CT CircuitCPBURDENCNGROUND ISOLATIONCONNECTION

Grounding 3 CT Circuit

Grounding 3 CT Circuit

Grounding V Connected CT Circuit

Delta-Delta Connected CTs in a DifferentialProtection Relay

Delta-Wye Connected CTs in a DifferentialProtection CBBCBACBAGROUND ISOLATIONCONNECTIONA

Percentage Bus Differential TO CTSON CKT. 3CD3CPHASE ACD3NCD2ACD2BTO CTSON CKT. 2PHASE BCD2CCD2NCKT. 1ABCCD1ACD1BCD1CCD1NGROUND ISOLATIONCONNECTIONPHASE C

Ring Bus or Breaker and a Half SchemeTO METERS,RELAYS, ETC.TERMINALSAT CT LOCATIONGROUNDISOLATINGCONNECTIONCONTROL HOUSEGROUND BUSTERMINALSAT CT LOCATIONSWITCHBOARD TERMINALS(MAY BE SLIDING LINKTYPETO FACILITATE TESTING)

51-C51-B51A51A51-B51-CMultiple use of CTs

Grounding of CasesGrounding metallic conductive casesInsulated cases with conductive internal partsUngrounded metallic cases or internal parts Protected by suitable barriers or elevated toprevent contact if operating voltage exceeds1,000 V

Exceptions to GroundingIf the primary windings of instrument transformercircuits are connected to circuits of less than1000 V with no live parts or wiring exposed oraccessible to other than qualified persons, thecircuits may not be grounded.

Exceptions to GroundingFor instrument transformer cases, the cases orframes of CTs may not be grounded if theprimary windings are not over 150 V to groundand are used exclusively to supply current tometers.

Exceptions to GroundingCases of instruments, meters, and relaysoperating at less than 1000 V on switchboardshaving exposed live parts on the front of panelsare not usually grounded. Mats of insulatingrubber or other suitable floor insulation isprovided where the voltage to ground exceeds150 V.

Exceptions to GroundingInstruments, meters, and relays, whose currentcarrying parts operate at voltages to ground of1000 V and over, are isolated by elevating themor protecting them by suitable barriers, groundedmetal or insulating covers, or guards. In suchsituations, the cases are not usually grounded.

Grounding of Low-Energy Transducers

Grounding of Low-Energy Transducers

Grounding of Low-Energy Transducers

Annex AGrounding of shielded cables To consider current carrying capacity of theshield Cables with spiral design shields are groundedat both ends Cables with drain wires are grounded at oneend

Annex ACTs installed over shielded cables

Annex BIEEE Standards reviewed by the WG

Annex CClause 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 DSurvey of Grounding Practices Approximately one-third of the respondents donot ground voltage transformers “at the firstpoint of use” as described in the Guide. TheWorking Group is of the opinion that grounding“at the first point of use” is better thangrounding at other locations. Almost all respondents indicated that currenttransformers are grounded “at the first point ofuse” as described in the Guide.

[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.