Regulatory Guide 1.75

Physical Independence of Electric Systems

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Issue date:	09/30/1978
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RG-1.75, Rev 2
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U.S. NUCLEAR REGULATORY COMMISSION

Revision 2 Septsmber 1978 REGULATORY GUIDE

OFFICE OF STANDARDS DEVELOPMENT

REGULATORY GUIDE 1.75 PHYSICAL INDEPENDENCE OF ELECTRIC SYSTEMS

A.

INTRODUCTION

Section 50.55a, "Codes and Standards," of 10

CFR Part 50, "Domestic Licensing of Production and Utilization Facilities,"

requires in paragraph (h) that protection systems meet the requirements set forth in the Institute of Electrical and Electronics Engineers Standard,

"Criteria for Protection Systems for Nuclear Power Generating Stations" (IEEE 279).1 Sec tion 4.6 of IEEE Std 279-1971 (also designated ANSI N42.7-1972) requires, in part, that chan nels that provide signals for the same protec tive function be independent and physically separated. General Design Criterion 3, "Fire Protection," of Appendix A,

"General Design Criteria for Nuclear Power Plants," to I0 CFR

Part 50 requires, in part, that structures, systems, and components important to safety be designed and located to minimize, consistent with other safety requirements, the probability and effect of fires. General Design Criterion 17,

"Electric Power Systems," requires, in part, that the onsite electric power supplies, including the batteries, and the onsite electric distribution system have sufficient in dependence to perform their safety functions assuming a single failure. General Design Cri terion 21,

"Protection System Reliability and Testability," reqxiires, in part, that independ ence designed into protection systems be suffi cient to ensure that no single failure results in loss of the protective function.

This guide describes a method acceptable to the NRC 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 the physical independence of the circuits and electric equipment comprising or associanted with the

'Copies may be obtained from the Institute of Eectrical and Electronics Engineers.

United Engintering Center. 345 East

47th Street. New York, New York 10017.

Class 1E power system, the protection system, systems actuated or controlled by the protec tion system, and auxiliary or supporting sys tems that must be operable for the protection system and the systems it actuates to perform their safety-related functions. This guide ap plies to all types of nuclear power plants.

This guide addresses only some aspects of defense against the effects of fires. Additional criteria for protection against the effects of fires are provided in Regulatory Guide 1-120,

"Fire Protection Guidelines for Nuclear Power Plants.,

B.

DISCUSSION

Draft IEEE Standard, "Criteria for Separa tion of Class

1E Equipment and Circuits,"

dated July 20, 1973, was prepared by Ad Hoc Subcommittee

6 of the Nuclear Power Engineering Committee (NPEC) of the Institute of Electrical and Electronics Engineers. The draft was -subsequently modified by NPEC in August. 1973 incident to the normal process of developing its technical content. The modified draft standard provided criteria for the separation of redundant Class IE equipment and circuits installed at nuclear power plants.

Inasmuch as there was an urgent need for explicit guidance in the area of physical inde pendence of electric systems and in view of the considerable guidance already available from the modified -IEEE draft standard, the staff prepared a document entitled "Appendix 1 to Regulatory Guide 1.75 - Physical Independence of Electric Systems." This- Appendix, which was essentially the modified IEEE

draft standardi further modified to (a) address ac ceptably those portions of the standard on which there was not complete agreement, (b)

• Lines indicate substantive changes from previous issue.

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describe logical extensions of the standard's.

provisions that were acceptable to the staff, and (a) 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 MEEE

draft standard upon which the guide and its Appendix were based evolved, in the normal course of standard development, into IEEE Std 384-1974,

"IEEE Trial-Use Standard Criteria for Separation of Cuss JR Xupmen-

-Cir- cults" (also designated ANSI N41.14). IEEE Std 384-1974 has undergone balloting within Ad Hoc Subcommittee .6 and NPEC and was approved by the IEEE Standards' Board on February 28,

1974. This revision to the guide endorses, with certain exceptions, IEEE Ptd 384-1974.

The NRC staff does not agree with certain"

provisions 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 tie status of non-Class-lI circuits that are. not separated from associated circuits by accept able distance or bairiers. This lack of agree ment is reflected in Regulatory Positions 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 staff review practice.

For example, a provision of the standard that addresses the

"degree of separation commensurate with the damage

3otential of the hazard" does not specifically zover 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

1E

systems, associated circuits, and non-Class-IX

systems.

By implication, associated circuits assigned to different redundant divisions should also be identified.

However, the provision is implicit.

An explicit provision should be provided.

Detailid bases are included herein for those regulatory positions that are significantly at iariance with the standard's provisions. The remaining regulatory positions are logical ex tensions or clarifications of the standard's pro visions.

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 characteristics 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 IR

Equipment and Circuits,"

dated

,farch 15, 1974, is generally acceptable to the NRC staff and provides an adequate basis for complying with IEEE Std 279-1971 and the Commission's General Design Criteria 3, 17, an& 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 1E 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:

1. Section 3,

"Isolation Device," should be supplemented as follows:

"(Interrupting de vices actuated only by fault current are not considered to be isolation devices within the context of this document.)"

Basis: Logical extension of the standard's provisions. The standard defines "isolation device" in terms of preventing malfunctions in one section of a circuit from causing un acceptable influences in other sections of the circuit or other circuits. Under the postu lated conditions of a loss-of-coolant accident,.

loss of offsite power, and a cable tray fire, the proximity of circuits energized from re dundant Class 1E power sources could lead to concurrent high fault currents (e.g., short to ground) which, in turn, threaten the re dundant main circuit breakers.

Also, the susceptibility of non-Class-IE loads ener gized from redundant Class 1E power sources to design basis events (e.g., seismic events)

could similarly threaten the redundant main circuit breakers. Tripping of the main circuit breakers would cause the. loss of emergency power to redundant "divisions" of equipment.

It is recognized that proper breaker or fuse coordination would preclude such an event.

However, because the main breakers are in series with the fault and could experience momentary currents above their setpoints, it is prudent to preclude the use of inter rupting devices actuated only by fault cur rent as acceptable devices for isolating non Class-IE circuits from Class 1E or associated circuits.

Breakers that trip on receipt of a signal other than one derived from the fault current or its effects (e.g., an accident signal) are acceptable since the downstream circuits would. already be isolated from their respec tive power sources under accident conditions and could pose no threat to these sources.

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2. Section 3,

"Raceway". Interlocked armor

"enclosing cable should not be construed as a

"raceway".

Basis: There is no precedent or other known R

reason for considering such cable to be a "raceway". This regulatory position is con sistent with current industry practice, including the provisions of the National Elec tric Code.

3. Section 4.3 should be supplemented as.

follows: "In general, locating redundant cir cuits and equipment in separate safety class structures affords a greater degree of assur ance that a single event will not affect redun dant 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 re quirementa placed on Class lE circuits such as cable derating, environmental qualification, flame retardance, splicing restrictions, and raceway fill unless it can be demonstrated that the absence of such requirements could not significantly reduce the availability of the Class IE circuits.

Basis: This is a logical extension of the stanaard's provisions. The specified minimum acceptable separation distances for raceways carrying Class 1E circuits are predicated on assumptions related to flame retardance, 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 require ments such as those of General Design Criter ion 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,

'Sfhdard Format and Content of Safety Ana lysis Reports for Nuclear Power Plants,"

to provide the information needed in order for the staff to independently verify conformance to the standard.

7. Non-Class-lE instrumentation and control circuits should not be exempted from the pro visions of Section 4.6.2.

Basis: There is no firm technical basis for an unrestricted exemption of these circuits.

Exemptions 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 n--iUting cause of several fires in raceway$.

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 race ways is therefore prudent. Splices are not, 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: This is a logical extension of the nsn-ard'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.

11. 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 1E and Non-Class-lE circuits, between Non-Class-lE circuits associated with different redundant Class 1E systems, and between redundant Class lE systems."

12. Pending issuance of other acceptable criteria, those portions of Section 5.1.3 (ex clusive of the Note following the second para graph) that permit the routing of power cables through the cable spreading area(s) and, by implication, the control room, should not be construed as acceptable.

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

Basis: This is a prudent specific interpreta

-n--of the standard's provisions in the ab sence of specific guidance. The staff recog nizes that subsequent investigation may prove that this approach is too conservative;

however, in the absence of supporting evidence to the contrary, this conservative approach is desirable.

1.75-3

The use of redundant cable spreading areas Is aloic" extension of the standard's provi sions re. Section 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 sepa rate safety. class structures required by Sec tion 5.3.1 should be served by independent.

ventilation systems.

16. The first paragraph of Section 5.7 should be augmented as follows, "The separation re UNITED STATES

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quirements of 5.6 apply to instrumentation cabinets."

D.

IMPLEMENTATION

The purpose o this section is to provide in formation to applicants and licensees regarding the NRC staff's plans for using this regulatory guide. This guide reflects current NRC staff practice. Therefore, except in those cases in which the applicant proposes an acceptable alternative method for complying with specified portions of the Commission's regulations, this guide will he used by the NRC staff in evaluat ing all construction permit applications for which the issue date of the Safety Evaluation Report is February 1, 1974, or after.

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1.75-4