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 1 U.S. NUCLEAR REGULATORY COMMISSION January 1975

• REGULATORY GUIDE

REGULATORY GUIDE 1.75 PHYSICAL INDEPENDENCE OF ELECTRIC SYSTEMS

A. INTRODUCTION

auxiliary or supporting systems that must be operable for the protection system and the systemls it actLuaeCs to Section 505.5a. "Codes and Standards," of l0 CFR perlform their saf'ety-related I'ulctions. This guide a pplics Part 50. "Licensing of Production arid Utiliz.ation to all types of niclear power plants.

Facilities," requires in paragraph (hi) that protectihon systems incel (tie requirenments set forth in tihe Institute

B. DISCUSSION

ofl Electrical and Electronics Engineers Standard,

"'Criteria for Protectiun Systemis fur Nuclear Power Draft II'I"E Standard. "'Criteria t or Separation ot Generating StatiolnS." (li'lE 271W . Section 4.6 of IEEE Class IEIEquipnment and Circuits," dated July 20. 1973.

Std 279-1971 (also designated ANSI N42.7-1972) was prepared by Ad 1l0c Subcomnmnittee 0 of the Nudear requires. in part. that channels that provide signals for Power Enghieering Comlmittee (NPE(") of the lInstitute the same protective function be independent arid of Electrical and Electronics Enilneers. The dralft was physically separated. General Design Criterion 3. "Fire subsequently miodified by NPEC in August 1973 Protection.'" it' Appendix A. "(Gencinal DXsiwi Criteria incident to the normnal process of' developing its fur Nuclear Power Plants." to 10 CFR Part 50 requires. technical content. The modified draft standard provided in part. that structuress, systems. and components criteria for the separation of redundant Class IFI

important to safety be desitgned anid located to equipment and circuits installed at nuclear power plants.

IminiriZe , consistent with other safety requiremen ts, the Inasnmuch as there was an urgent need for explicit probability and effect of fires. General Design Criterion guidance in the area of physical independence of electtic

1 7. "Electric Power Systeins," requires. in part. that the systems and in view of the considerable guidance already onsite electric power supplies, including the batteries. available from tile modified IE-E draf't standard, the and the onsite electric distribution system have Regulatory staff prepared a document entitled, "Appen- sufficient independence to performn their safety flunc- dix I to Regulatory Guide 1.75--Physical Independence tions assuming a single failure. General Design Criterion of Electric Systenms." This Appendix, which was

21, "Protection System Reliability and Testability," essentially the nmodified IEEE draft standard further requires, in part. that independence desiLued into modified to (a) address acceptably those portions of the protection systems be sufficient to ensure that no single standard on which there was not complete agreement.

failure results in loss of the protection function. This (b) describe logical extensions of the standard's guide describes a method acceptable to the Regulatory provisions that were acceptable to tile Regulatory staff.

staff of complying with IEEE Std 279-1971 and Criteria and (c) provide clarification where necessary. was

3. 17. and 21 of Appendix A to 10 CFR Part 50 with endorsed by the February 1974 version of this guide.

respect to thie physical independence of the circuits and Subsequent to the issuance of the February 1974 electric equipment comprising or associated with the version of this guide, the modified IEEE draft standard Class IE power system, the protection system, systems upon which the guide and its Appendix were based actuated or controlled by the protection system, and 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

'Copies may be obtained froemthe Institute ot' Electrical and Circuits," (also designated ANSI N41.14). IEEE Sid Electronics Engineers, United Engineering Center, 345 East 47th 384-1974 has undergone balloting within Ad Hoc Street, New York. New York 10017. 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-inistIn method% acceptable to the NRC staff of impfrrntenling specific parts of Iliae nitptoven*nI in these guides are encoutraged and *shaltd hIe sent to the.

Commisslion's regutaiotr,, to detinatet ftchni.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 %ubslitutesfot tegllation$ andcompliance with them is not tegqired. Methods aid soluftons different from those set out irn The guides ateissued 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 1

0. Genteral

IEEE Standards Bfoard on February 28, 1974. This I. Section 3, Isolation Device, should be supplemented revision to tile guide endorses, with certain exceptions. as follows: "(Interrupting devices actuated only by fault IEEE Std 384-1974. current are not considered to be isolation devices within The Regulatory staff does not agree with certain the context of this document.)"

proisions of the trial-use standard such as those Basis: Loigcal extension of the Standard' prowisons.

pertaining to the definition of "raceway." the routing of The standard defines "isolation device'" in terms of power cables through the cable spreading area(s) and preventing ,nalfinct ions in one section of a circuitfrom control room, and the status of non-Class It: circuits that causing unzacceptable influences in other sections of the are not separated from associated circuits by acceptable circuit or other circuits. Under the postulated conditions distance or barriers. This lack of agreement is reflected of a loss-of coolant accident, loss of offsite power, and a in Regulator)y lositions C.1. 2. 4. 6. 7, 9. 10. and 12; cable tray'fire. the proximity of circuits energi2ed from There are also several regulatory positions that are re'dundant Class IE power sources could lead to logical extensions of the Standard's provisions and concurrent high fault currents (e.g. short to ground)

reflect current Regulatory staff review practice. For which, in turn, threaten the redundant main circuit example, a provision of the standard which addresses the breakers. Also. the susceptibilityv of non-Class I1:" loads

"'degree of separation commensurate with the damage etrergized fromi redundant Class IE power sources to potential of the hai.ard" does not specifically cover cable design basis event (e.g., seismic events) could similarly'

tunnels which, in the event of a fire. may not effectively threaten the redundant main circuit breakers. 7Tipping separate redundant circuits or equipment. As another of the mttain circcuit breakers wouhld cause f/ie loss of example. the standard requires that methods of elercgen'c, pow'er to redundant "divisionss" of equip- identification distinguish between redundant Class IE ni'nf. It is rec-ognized that proper breaker or fiise systems, associated circuits, and non-Class IE systems. coordination would preclude such an event. However, By implication, associated circuits assiged to different because the main breakers are in series with the fault and redundant divisions should also be identified. However. could experience monmentaryv currents above their the provision is implicit. An explicit provision should be setpoints, it is pnrdent to preclude the use of inter- provided. rupqting devices actuated onl' by fault current as I.)tailed bases are included herein for those acceptable devices for isolating non-Class A1 circuits regulatory positions that are significantly at variance from Class IE or A ssociated circuits.

with the standard's provisions. The remaining regulatory Breakers that trip on receipt of a signal other than positions are logical extensions. or clarifications. of the one derived from the flhult crurrentt or its effects (e.g.. an standard's provisions. accident sigital) are acceptable since the downstream circuits would already be isolated from their respective

C. REGULATORY POSITION

power sources under accident conditions and could pose nro threat to these sources.

IEEE Std 384-1974 sets forth criteria for the separation of circuits and equipment that are redundant. 2. Section 3, Raceway: Interlocked armor enclosing The determination of which circuits and equipment are cable should not be construed as a "raceway".

redundant and the degree of redundancy required is Basis: There is no precedent or other ktrown valid outside, the scope of this guide and the standard. The reason for consideritig such cable to be a "raceway"

standard also sets forth criteria relating to tests and 77Tis regulatory position is consistent with current analyses for determining the flame-retardant character- industry practice includinig the provisions of the istics of proposed cable installations. The criteria are NationalElectric Code.

acceptable provided such tests and analyses are based on realistic premises and are otherwise fully applicable to 3. Section 4.3 should be supplemented as follows: "In the actual cable installations. general, locating redundant circuits and equipment in The guidance in IEEE Std 384-1974, "IEEE Trial-Use separate safety class structures affords a greater degree Standard Criteria for Separation of Class IE Equipment of assurance that a single event will not affect redundant and Circuits," dated Match 15, 1974, is generally systems. This method of separation should be used acceptable to the Regulatory staff and provides an whenever practicable and where its use does not conflict adequate basis for complying with IEEE Std 279-1971 with other safety objectives."

and the Commission's General Criteria 3, 17, and 21 of Appendix A to 10 CFR Part 50 with respect to the 4. Associated circuits installed in accordance with physical independence of the circuits and electric Section 4.5(1) should be subject to all requirements equipment comprising or associated with the Class IE placed on Class IE circuits such as cable derating, power system, the protection system, systems actuated environmental qualification, flame retardance, splicing or controlled by the protection system, and auxiliary or restrictions, and raceway fill unless it can be supporting systems that must be operable for the demonstiated that the absence of such requirements protection system and the systems it actuates to perform. could not significantly reduce the availability c,f the their safety-related functions, subject to the following: Class IE circuits.

1.75-2

Basis: This is a logical extension of the standard s I1. Section 5.1.2 should be supplemented as follows:

provisions. The specified minimum acceptable separation "The method of identification used should be simple distances for raceways carrying Class IE circuits are and should preclude the need to consult any reference predicated on assumptions related to flame r?,tardawce, material to distinguish between Class IE and Non-Class cable derating. etc. The placement of cables of lesser IE circuits, between Non-Class IF circuits associated qualification in these raceways would nullify these with different redundant Class-IE systenms, and between assumptions. redundant Class IE systems.'

5. The "Note" following Section 4.5 should be 12. Pending issuance of other acceptable criteria, those supplemented as follows: "This exemption is limited and portions of Section 5.1.3 (exclusive of the NOTE

does not extend to other requirements such as those of following the second paragraph) that permvi the routing General Design Criterion 17." of power cables through the cable spreading area(s) and.

by implication, the control roo

m. should not he

6. Analyses performed in accordance with Sections construed as accepiwble. Also. Section 5.1.3 should be

4.5(3), 4.6.2, and 5.1.1.2 should be submitted as part of supplemented as follows: "WWhere feasible, redundant the Safety Analysis Report and should identify those cable spreading areas should be utilized.-

circuits installed in accordance with these sections. Basis: This is a pnident specific interpretation o" thc Basis: Extension of Regulatory Guide 1. 70 to provide standard's provisions in thi absncee *j sPcciic guidan.wc.

the information needed in order for the staff to The Regulator' staff recognizes that vibset,qent independently verify conformance to the standard. investigation may, prove that this approach is too conservative, however, in the absence of supporti,:e

7. Non-Class IE instrumentation and control circuits evidence to the contrarY, this cotisern'ative apprroach is should not be exempted from the provisions of Section desirable.

4.6.2. The use of redundant cable sprcading art-as is a logical Basis: There is no firm technical basis for an extension of the standard's pr.'f)isions (relf Scction unrestricted exemption of these circuits. Exetnptions 5.1.1.1).

should be justified by analysis.

13, No significance should be attached to the different

8. Section 5.1.1.1 should not be construed to imply that tray widths illustrated in Figure 2.

adequate separation of redundant circuits can be achieved within a confined space such as a cable tunnel 14. Section 5.2.1 should be supplemented as follows:

that is effectively unventilated. "And should have independent air supplies."

9. Section 5.1.1.3 should be supplemented as follows: 15. Where ventilation is required, the separate safety

"(4) Cable splices in raceways should be prohibited." class structures required by Section 5.3.1 should he Basis: Splices have been identified as the initiating served by independent ventilation systems.

cause of several fires in raceways. Even where the separation distance is adequate to prevent a fire in the 16. The first paragraph of Section 5.7 should be raceways of one division from affecting cables in a augmented as follows: "The separation requirements .of redundantdivision, all practicablemeans should be used 5.6 apply to instrumentation cabinets."

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

D. IMPLEMENTATION

design should be justified by analyses. The analyses should be submitted as part of the Safety Analysis Report. The purpose of this section is to provide information to applicants and licensees regarding the Regulatory

10. Section 5.1.2, the phrase -at a sufficient number of staffs plans for utilizing this regulatory guidu.

points" should be understood to mean at intervals not to This guide reflects current regulatory practice.

exceed 5 ft throughout the entire cable length. Also the Therefore, except in those cases in which the applicant preferred method of marking cable is color coding. proposes an acceptable alternative method for comn- Basis: 7This is a logical extension of the standard's plying with specified portions of the Commission's provisions. A 5 ft maximum marking distance is regulations, this guide will be used by the Regulatory considered necessary to facilitate visual verification that staff in evaluating all construction permit applications the cable installation is in conformance with separation for which the issue date of the Safety Evaluation Report criteria. is February I, 1974, or after.

1.75.3