Regulatory Guide 1.75

Physical Independence of Electric Systems

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

Revision 1 January 1975 U.S. NUCLEAR REGULATORY

COMMISSION

• REGULATORY

GUIDE REGULATORY

GUIDE 1.75 PHYSICAL INDEPENDENCE

OF ELECTRIC SYSTEMS

A. INTRODUCTION

Section 505.5a. "Codes and Standards," of l0 CFR Part 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 Power Generating StatiolnS." (li'lE 271W .Section 4.6 of IEEE Std 279-1971 (also designated ANSI N42.7-1972)

requires.

in part. that channels that provide signals for the same protective function be independent arid physically separated.

General Design Criterion 3. "Fire Protection.'" it' Appendix A. "(Gencinal DXsiwi Criteria fur Nuclear Power Plants." to 10 CFR Part 50 requires.in part. that structuress, systems. and components important to safety be desitgned anid located to I miniriZe , consistent with other safety requiremen ts, the probability and effect of fires. General Design Criterion 1 7. "Electric Power Systeins," requires.

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

and the onsite electric distribution system have sufficient 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 into protection systems be sufficient to ensure that no single failure results in loss of the protection function.

This guide describes a method acceptable to the Regulatory staff of complying with IEEE Std 279-1971 and Criteria 3. 17. and 21 of Appendix A to 10 CFR Part 50 with respect to thie physical independence of the circuits and electric equipment comprising or associated with the Class IE power system, the protection system, systems actuated or controlled by the protection system, and'Copies may be obtained froemthe Institute ot' Electrical and Electronics Engineers, United Engineering Center, 345 East 47th Street, New York. New York 10017.auxiliary or supporting systems that must be operable for the protection system and the systemls it actLuaeCs to perlform their saf'ety-related I'ulctions.

This guide a pplics to all types of niclear power plants.

B. DISCUSSION

Draft II'I"E Standard.

"'Criteria t or Separation ot Class IE IEquipnment and Circuits," dated July 20. 1973.was prepared by Ad 1l0c Subcomnmnittee

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

The dralft was subsequently miodified by NPEC in August 1973 incident to the normnal process of' developing its technical content. The modified draft standard provided criteria for the separation of redundant Class IFI equipment and circuits installed at nuclear power plants.Inasnmuch as there was an urgent need for explicit guidance in the area of physical independence of electtic systems and in view of the considerable guidance already available from tile modified IE-E draf't standard, the Regulatory staff prepared a document entitled, "Appen-dix I to Regulatory Guide 1.75--Physical Independence of Electric Systenms." This Appendix, which was essentially the nmodified IEEE draft standard further modified to (a) address acceptably those portions of the standard 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.

was endorsed by the February 1974 version of this guide.Subsequent to the issuance of the February 1974 version of this guide, the modified IEEE draft standard upon which the guide and its Appendix were based evolved, in the normal course of standard developnment.

into IEEE Std 384-1974, "IEEE Trial-Use Standard Criteria for Separation of Class IE Equipment atid Circuits," (also designated ANSI N41.14). IEEE Sid 384-1974 has undergone balloting within Ad Hoc Subcommittee

6 arid NPEC and was approved by tile USNRC 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 tIn method% 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 Section applicants 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 to the issuance or continuance of A permit or license by the Commission. "i Power Reactots 6. Products 2. Reearch and Toes Reacturs

7. rtiansporinaort

3. Fuels and Materiail lactlilies

8 Occupational Health Published guides will be tevised periodicitlly.

as appropriate.

to accommodale

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

5. Materials and Plant Protection

10. Genteral IEEE Standards Bfoard on February 28, 1974. This revision to tile guide endorses, with certain exceptions.

IEEE Std 384-1974.The Regulatory staff does not agree with certain proisions of the trial-use standard such as those pertaining to the definition of "raceway." the routing of power cables through the cable spreading area(s) and control room, and the status of non-Class It: circuits that are 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 are logical extensions of the Standard's provisions and reflect current Regulatory staff review practice.

For example, a provision of the standard which addresses the"'degree of separation commensurate with the damage potential of the hai.ard" does not specifically cover cable tunnels which, in the event of a fire. may not effectively separate redundant circuits or equipment.

As another example. the standard requires that methods of identification distinguish between redundant Class IE systems, 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 be provided.I.)tailed bases are included herein for those regulatory positions that are significantly at variance with the standard's provisions.

The remaining regulatory positions are logical extensions.

or clarifications.

of the standard's provisions.

C. REGULATORY

POSITION IEEE Std 384-1974 sets forth criteria for the separation of circuits and equipment that are redundant.

The determination of which circuits and equipment are redundant and the degree of redundancy required is outside, the scope of this guide and the standard.

The standard also sets forth criteria relating to tests and analyses for determining the flame-retardant character- istics of proposed cable installations.

The criteria are acceptable provided such tests and analyses are based on realistic premises and are otherwise fully applicable to the 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 an adequate basis for complying with IEEE Std 279-1971 and the Commission's General Criteria 3, 17, and 21 of Appendix A to 10 CFR Part 50 with respect to the physical independence of the circuits and electric equipment comprising or associated with the Class IE power system, the protection system, systems actuated or controlled by the protection system, and auxiliary or supporting systems that must be operable for the protection 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 fault current are not considered to be isolation devices within the context of this document.)" Basis: Loigcal extension of the Standard ' prow isons.The standard defines "isolation device'" in terms of preventing ,nalfinct ions in one section of a circuit from causing unzacceptable influences in other sections of the circuit or other circuits.

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

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

threaten the redundant main circuit breakers.

7Tipping of the mttain circcuit breakers wouhld cause f/ie loss of elercgen'c, pow'er to redundant "divisionss" of equip-ni'nf. It is rec-ognized that proper breaker or fiise coordination would preclude such an event. However, because the main breakers are in series with the fault and could experience monmentaryv currents above their setpoints, it is pnrdent to preclude the use of inter-rupqting devices actuated onl' by fault current as acceptable devices for isolating non-Class A1 circuits from Class IE or A ssociated circuits.Breakers that trip on receipt of a signal other than one derived from the flhult crurrentt or its effects (e.g.. an accident sigital) are acceptable since the downstream circuits would already be isolated from their respective power sources under accident conditions and could pose nro 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 valid reason for consideritig such cable to be a "raceway" 77Tis regulatory position is consistent with current industry practice includinig the provisions of the National Electric Code.3. Section 4.3 should be supplemented as follows: "In general, locating redundant circuits and equipment in separate safety class structures affords a greater degree of assurance that a single event will not affect redundant systems. This method of separation should be used whenever practicable and where its use does not conflict with other safety objectives." 4. Associated circuits installed in accordance with Section 4.5(1) should be subject to all requirements placed on Class IE circuits such as cable derating, environmental qualification, flame retardance, splicing restrictions, and raceway fill unless it can be demonstiated that the absence of such requirements could not significantly reduce the availability c,f the Class IE circuits.1.75-2 Basis: This is a logical extension of the standard s provisions.

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

etc. The placement of cables of lesser qualification in these raceways would nullify these assumptions.

5. The "Note" following Section 4.5 should be supplemented as follows: "This exemption is limited and does not extend to other requirements such as those of General Design Criterion 17." 6. Analyses performed in accordance with Sections 4.5(3), 4.6.2, and 5.1.1.2 should be submitted as part of the Safety Analysis Report and should identify those circuits installed in accordance with these sections.Basis: Extension of Regulatory Guide 1. 70 to provide the information needed in order for the staff to independently verify conformance to the standard.7. Non-Class IE instrumentation and control circuits should not be exempted from the provisions of Section 4.6.2.Basis: There is no firm technical basis for an unrestricted exemption of these circuits.

Exetnptions should be justified by analysis.8. Section 5.1.1.1 should not be construed to imply that adequate separation of redundant circuits can be achieved within a confined space such as a cable tunnel that 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 the separation distance is adequate to prevent a fire in the raceways of one division from affecting cables in a redundant division, all practicable means should be used to prevent the occurrence of a fire. This position against splices in raceways is therefore prudent. Splices are nrot, by themselves, unacceptable.

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

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

A 5 ft maximum marking distance is considered necessary to facilitate visual verification that the 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 simple and 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 between redundant Class IE systems.'12. Pending issuance of other acceptable criteria, those portions of Section 5.1.3 (exclusive of the NOTE following the second paragraph)

that permvi the routing of power cables through the cable spreading area(s) and.by implication, the control room. should not he construed as accepiwble.

Also. Section 5.1.3 should be supplemented as follows: "WWhere feasible, redundant cable spreading areas should be utilized.-

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

The Regulator'

staff recognizes that vibset,qent investigation may, prove that this approach is too conservative, however, in the absence of supporti,:e evidence to the contrarY, this cotisern'ative apprroach is desirable.

The use of redundant cable sprcading art-as is a logical extension of the standard's pr.'f)isions (relf Scction 5.1.1.1).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 safety class structures required by Section 5.3.1 should he served by independent ventilation systems.16. The first paragraph of Section 5.7 should be augmented as follows: "The separation requirements .of 5.6 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 Report is February I, 1974, or after.1.75.3