Regulatory Guide 1.75

Physical Independence of Electric Systems

ML13350A340
Person / Time
Issue date:	01/31/1975
From:	Office of Nuclear Regulatory Research
To:
References
RG-1.075, Rev. 1
Download: ML13350A340 (3)

Revision

1January 1975U.S. NUCLEAR REGULATORY

COMMISSION

• REGULATORY

GUIDEREGULATORY

GUIDE 1.75PHYSICAL

INDEPENDENCE

OF ELECTRIC

SYSTEMS

A. INTRODUCTION

Section 505.5a. "Codes and Standards,"

of l0 CFRPart 50. "Licensing of Production arid Utiliz.ation Facilities,"

requires in paragraph (hi) that protectihon systems incel (tie requirenments set forth in tihe Institute ofl Electrical and Electronics Engineers Standard,

"'Criteria for Protectiun Systemis fur Nuclear PowerGenerating StatiolnS."

(li'lE 271W .Section 4.6 of IEEEStd 279-1971 (also designated ANSI N42.7-1972)

requires.

in part. that channels that provide signals forthe same protective function be independent aridphysically separated.

General Design Criterion 3. "FireProtection.'"

it' Appendix A. "(Gencinal DXsiwi Criteriafur Nuclear Power Plants."

to 10 CFR Part 50 requires.

in part. that structuress, systems.

and components important to safety be desitgned anid located toI miniriZe

, consistent with other safety requiremen ts, theprobability and effect of fires. General Design Criterion 1 7. "Electric Power Systeins,"

requires.

in part. that theonsite electric power supplies, including the batteries.

and the onsite electric distribution system havesufficient independence to performn their safety flunc-tions assuming a single failure.

General Design Criterion 21, "Protection System Reliability and Testability,"

requires, in part. that independence desiLued intoprotection systems be sufficient to ensure that no singlefailure results in loss of the protection function.

Thisguide describes a method acceptable to the Regulatory staff of complying with IEEE Std 279-1971 and Criteria3. 17. and 21 of Appendix A to 10 CFR Part 50 withrespect to thie physical independence of the circuits andelectric equipment comprising or associated with theClass IE power system, the protection system, systemsactuated or controlled by the protection system, and'Copies may be obtained froemthe Institute ot' Electrical andElectronics Engineers, United Engineering Center, 345 East 47thStreet, New York. New York 10017.auxiliary or supporting systems that must be operablefor the protection system and the systemls it actLuaeCs toperlform their saf'ety-related I'ulctions.

This guide a pplicsto all types of niclear power plants.

B. DISCUSSION

Draft II'I"E Standard.

"'Criteria t or Separation otClass IE IEquipnment and Circuits,"

dated July 20. 1973.was prepared by Ad 1l0c Subcomnmnittee

0 of the NudearPower Enghieering Comlmittee (NPE(") of the lInstitute of Electrical and Electronics Enilneers.

The dralft wassubsequently miodified by NPEC in August 1973incident to the normnal process of' developing itstechnical content.

The modified draft standard providedcriteria for the separation of redundant Class IFIequipment and circuits installed at nuclear power plants.Inasnmuch as there was an urgent need for explicitguidance in the area of physical independence of electticsystems and in view of the considerable guidance alreadyavailable from tile modified IE-E draf't standard, theRegulatory staff prepared a document entitled,

"Appen-dix I to Regulatory Guide 1.75--Physical Independence of Electric Systenms."

This Appendix, which wasessentially the nmodified IEEE draft standard furthermodified to (a) address acceptably those portions of thestandard on which there was not complete agreement.

(b) describe logical extensions of the standard's provisions that were acceptable to tile Regulatory staff.and (c) provide clarification where necessary.

wasendorsed by the February

1974 version of this guide.Subsequent to the issuance of the February

1974version of this guide, the modified IEEE draft standardupon which the guide and its Appendix were basedevolved, in the normal course of standard developnment.

into IEEE Std 384-1974,

"IEEE Trial-Use StandardCriteria for Separation of Class IE Equipment atidCircuits,"

(also designated ANSI N41.14).

IEEE Sid384-1974 has undergone balloting within Ad HocSubcommittee

6 arid NPEC and was approved by tileUSNRC REGULATORY

GUIDES Copies of published quides may be obtained byV equest indicating the divisions fdett ited to the U.S, Nucle,,,

Reguatinty Commiiveon.

Washir.glot,.

0 C 20b55.Regulatory Guides ae kt~ild Io des~cribe and matke available to the public Altelmn Director of Standards Oeveelopme.nt Commentts and %utgeest-inis tInmethod% acceptable to the NRC staff of impfrrntenling specific parts of Iliae in these guides are encoutraged and hIe sent to the.Commisslion's regutaiotr,,

to detinatet ftchn

i. hitueS

ust.t1 by the staff in srcetlatv of the? Comlmiisin.

U.S Nucleart Regulatorv Comtnlitssll.

evaluating specific ptoblemls or postulated accidents.

or to provide guidance to Washingtotn.

0 C 20555. Attention Oockeling and Service Sectionapplicants Regulatery Guides sre ntof %ubslitutes fot tegllation$

and compliance with them is not tegqired.

Methods aid soluftons different from those set out irn The guides ate issued its the fallowing feti broad division%

the guides wtll be acceptable if Ihey provide a ba$si$i for the findings requisite tothe issuance or continuance of A permit or license by the Commission.

"i Power Reactots

6. Products2. Reearch and Toes Reacturs

7. rtiansporinaort

3. Fuels and Materiail lactlilies

8 Occupational HealthPublished guides will be tevised periodicitlly.

as appropriate.

to accommodale

4 Envitonmental and Silinit 9 Antitrust Reviewcomments and to reflect new informatlort or esperience

5. Materials and Plant Protection

10. Genteral IEEE Standards Bfoard on February

28, 1974. Thisrevision to tile guide endorses, with certain exceptions.

IEEE Std 384-1974.

The Regulatory staff does not agree with certainproisions of the trial-use standard such as thosepertaining to the definition of "raceway."

the routing ofpower cables through the cable spreading area(s) andcontrol room, and the status of non-Class It: circuits thatare not separated from associated circuits by acceptable distance or barriers.

This lack of agreement is reflected in Regulator)y lositions C.1. 2. 4. 6. 7, 9. 10. and 12;There are also several regulatory positions that arelogical extensions of the Standard's provisions andreflect current Regulatory staff review practice.

Forexample, a provision of the standard which addresses the"'degree of separation commensurate with the damagepotential of the hai.ard"

does not specifically cover cabletunnels which, in the event of a fire. may not effectively separate redundant circuits or equipment.

As anotherexample.

the standard requires that methods ofidentification distinguish between redundant Class IEsystems, associated circuits, and non-Class IE systems.By implication, associated circuits assiged to different redundant divisions should also be identified.

However.the provision is implicit.

An explicit provision should beprovided.

I.)tailed bases are included herein for thoseregulatory positions that are significantly at variancewith the standard's provisions.

The remaining regulatory positions are logical extensions.

or clarifications.

of thestandard's provisions.

C. REGULATORY

POSITIONIEEE Std 384-1974 sets forth criteria for theseparation of circuits and equipment that are redundant.

The determination of which circuits and equipment areredundant and the degree of redundancy required isoutside, the scope of this guide and the standard.

Thestandard also sets forth criteria relating to tests andanalyses for determining the flame-retardant character- istics of proposed cable installations.

The criteria areacceptable provided such tests and analyses are based onrealistic premises and are otherwise fully applicable tothe actual cable installations.

The guidance in IEEE Std 384-1974,

"IEEE Trial-Use Standard Criteria for Separation of Class IE Equipment and Circuits,"

dated Match 15, 1974, is generally acceptable to the Regulatory staff and provides anadequate basis for complying with IEEE Std 279-1971and the Commission's General Criteria

3, 17, and 21 ofAppendix A to 10 CFR Part 50 with respect to thephysical independence of the circuits and electricequipment comprising or associated with the Class IEpower system, the protection system, systems actuatedor controlled by the protection system, and auxiliary orsupporting systems that must be operable for theprotection system and the systems it actuates to perform.their safety-related functions, subject to the following:

I. Section 3, Isolation Device, should be supplemented as follows:

"(Interrupting devices actuated only by faultcurrent are not considered to be isolation devices withinthe context of this document.)"

Basis: Loigcal extension of the Standard

' prow isons.The standard defines "isolation device'"

in terms ofpreventing

,nalfinct ions in one section of a circuit fromcausing unzacceptable influences in other sections of thecircuit or other circuits.

Under the postulated conditions of a loss-of coolant accident, loss of offsite power, and acable tray' fire. the proximity of circuits energi2ed fromre'dundant Class IE power sources could lead toconcurrent high fault currents (e.g. short to ground)which, in turn, threaten the redundant main circuitbreakers.

Also. the susceptibilityv of non-Class I1:" loadsetrergized fromi redundant Class IE power sources todesign basis event (e.g., seismic events) could similarly'

threaten the redundant main circuit breakers.

7Tippingof the mttain circcuit breakers wouhld cause f/ie loss ofelercgen'c, pow'er to redundant

"divisionss"

of equip-ni'nf. It is rec-ognized that proper breaker or fiisecoordination would preclude such an event. However,because the main breakers are in series with the fault andcould experience monmentaryv currents above theirsetpoints, it is pnrdent to preclude the use of inter-rupqting devices actuated onl' by fault current asacceptable devices for isolating non-Class A1 circuitsfrom Class IE or A ssociated circuits.

Breakers that trip on receipt of a signal other thanone derived from the flhult crurrentt or its effects (e.g.. anaccident sigital)

are acceptable since the downstream circuits would already be isolated from their respective power sources under accident conditions and could posenro threat to these sources.2. Section 3, Raceway:

Interlocked armor enclosing cable should not be construed as a "raceway".

Basis: There is no precedent or other ktrown validreason for consideritig such cable to be a "raceway"

77Tis regulatory position is consistent with currentindustry practice includinig the provisions of theNational Electric Code.3. Section 4.3 should be supplemented as follows:

"Ingeneral, locating redundant circuits and equipment inseparate safety class structures affords a greater degreeof assurance that a single event will not affect redundant systems.

This method of separation should be usedwhenever practicable and where its use does not conflictwith other safety objectives."

4. Associated circuits installed in accordance withSection 4.5(1) should be subject to all requirements placed on Class IE circuits such as cable derating, environmental qualification, flame retardance, splicingrestrictions, and raceway fill unless it can bedemonstiated that the absence of such requirements could not significantly reduce the availability c,f theClass IE circuits.

1.75-2 Basis: This is a logical extension of the standard sprovisions.

The specified minimum acceptable separation distances for raceways carrying Class IE circuits arepredicated on assumptions related to flame r?,tardawce, cable derating.

etc. The placement of cables of lesserqualification in these raceways would nullify theseassumptions.

5. The "Note" following Section 4.5 should besupplemented as follows:

"This exemption is limited anddoes not extend to other requirements such as those ofGeneral Design Criterion

17."6. Analyses performed in accordance with Sections4.5(3), 4.6.2, and 5.1.1.2 should be submitted as part ofthe Safety Analysis Report and should identify thosecircuits installed in accordance with these sections.

Basis: Extension of Regulatory Guide 1. 70 to providethe information needed in order for the staff toindependently verify conformance to the standard.

7. Non-Class IE instrumentation and control circuitsshould not be exempted from the provisions of Section4.6.2.Basis: There is no firm technical basis for anunrestricted exemption of these circuits.

Exetnptions should be justified by analysis.

8. Section 5.1.1.1 should not be construed to imply thatadequate separation of redundant circuits can beachieved within a confined space such as a cable tunnelthat is effectively unventilated.

9. Section 5.1.1.3 should be supplemented as follows:"(4) Cable splices in raceways should be prohibited."

Basis: Splices have been identified as the initiating cause of several fires in raceways.

Even where theseparation distance is adequate to prevent a fire in theraceways of one division from affecting cables in aredundant division, all practicable means should be usedto prevent the occurrence of a fire. This position againstsplices in raceways is therefore prudent.

Splices are nrot,by themselves, unacceptable.

If they exist, the resulting design should be justified by analyses.

The analysesshould be submitted as part of the Safety AnalysisReport.10. Section 5.1.2, the phrase -at a sufficient number ofpoints" should be understood to mean at intervals not toexceed 5 ft throughout the entire cable length. Also thepreferred method of marking cable is color coding.Basis: 7This is a logical extension of the standard's provisions.

A 5 ft maximum marking distance isconsidered necessary to facilitate visual verification thatthe cable installation is in conformance with separation criteria.

I1. Section 5.1.2 should be supplemented as follows:"The method of identification used should be simpleand should preclude the need to consult any reference material to distinguish between Class IE and Non-Class IE circuits, between Non-Class IF circuits associated with different redundant Class-IE

systenms, and betweenredundant Class IE systems.'

12. Pending issuance of other acceptable criteria, thoseportions of Section 5.1.3 (exclusive of the NOTEfollowing the second paragraph)

that permvi the routingof power cables through the cable spreading area(s) and.by implication, the control room. should not heconstrued as accepiwble.

Also. Section 5.1.3 should besupplemented as follows:

"WWhere feasible, redundant cable spreading areas should be utilized.-

Basis: This is a pnident specific interpretation o" thcstandard's provisions in thi absncee sPcciic guidan.wc.

The Regulator'

staff recognizes that vibset,qent investigation may, prove that this approach is tooconservative, however, in the absence of supporti,:e evidence to the contrarY,

this cotisern'ative apprroach isdesirable.

The use of redundant cable sprcading art-as is a logicalextension of the standard's pr.'f)isions (relf Scction5.1.1.1).

13, No significance should be attached to the different tray widths illustrated in Figure 2.14. Section 5.2.1 should be supplemented as follows:"And should have independent air supplies."

15. Where ventilation is required, the separate safetyclass structures required by Section 5.3.1 should heserved by independent ventilation systems.16. The first paragraph of Section 5.7 should beaugmented as follows:

"The separation requirements

.of5.6 apply to instrumentation cabinets."

D. IMPLEMENTATION

The purpose of this section is to provide information to applicants and licensees regarding the Regulatory staffs plans for utilizing this regulatory guidu.This guide reflects current regulatory practice.

Therefore, except in those cases in which the applicant proposes an acceptable alternative method for comn-plying with specified portions of the Commission's regulations, this guide will be used by the Regulatory staff in evaluating all construction permit applications for which the issue date of the Safety Evaluation Reportis February I, 1974, or after.1.75.3