05000317/LER-1990-019

LER 90-019-00:on 900622,determined That Inadequate Breaker Coordination Resulted in Condition Outside Design Basis. Root Cause of Improperly Sized Breaker Not Determined. W/900720 Ltr

ML20055G779
Person / Time
Site:	Calvert Cliffs
Issue date:	07/20/1990
From:	Denton R, Gross K BALTIMORE GAS & ELECTRIC CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-019, LER-90-19, NUDOCS 9007240166
Download: ML20055G779 (5)

LER-2090-019,

Event date:
Report date:
3172090019R00 - NRC Website

text

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• BALTIMORE i OAS AND i i ELECTRIC l CHARLES CENTER e P.O. BOX 1475 e BALTIMORE, MARYLAND 21203 1475 l R E. DENTON d

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July 20, 1990 f i

U.S. Nuclear Regulatory Commission Docket No. 50 317 t .- Document Control Desk License No. DPR 53 >

Washington, D.C. 20555

Dear Sirs:

The attached LER 90 019, Revision 0, is being sent to you as required under 10 CFR 50.73 guidelines.

Should you have any questions regarding this report, we would be pleased to discuss them with you.

Very truly yours, ,

/r R. E. Denton i Manager i

KWG/bjd r Attachment 1

c c '. - Mr. T. T. Martin l Director, Office of Management Information l- and Program Control dessrs: C. C. Creel C. H. Cruse R. E. Denton R. P. Heibel J. R. Lemons L. B. Russell ,

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] no gg2 3;3 9;o nestiact <t a.= = s.= ... . . ,, . . ,, , o ei On June 22, 1990 during a review of a design discrepancy, it was octermined that certain breakers supplying power to non safety related loads from 1E safety related buses were not properly coordinated with the motor control center supply breaker. Under certain circumstances this condition could have resulted in an adverse affect on the safety related loads of the motor control center (MCC). If a ground fault had existed on the non safety related load circuits, the MCC supply breaker may have opened causing a loss of power to the safety and non safety related loads supplied by the MCC. In addition, one breaker was identified which was of improper current interruption capability for its application, which could have also caused the MCC feeder breaker to open. This condition was limited to thermal magnetic breakers without ground fault protection which were added to safety related MCOs.

The root cause of the lack of breaker ground fault coordination was a design error due to use of a MCC breaker design which was not addressed in tht existing MCC coordination calculation. The root cause of the use of em improperly sized breaker has not been determined.

Since the improperly coordinated breakers were installed, design unit checklists have been revised to specifically document consideration of circuit coordination. Circuits powered from safety related MCCs have been reviewed to identify similar coordination concerns. Corrective actions will also include replacement and/or roodification ci the MCC breakers to provido proper coordination, and breaker size verification through walkdowns, once - se. = =

UCENSEE EVENT REPORT (LER) TEXT CONTINUATION menm uw oocatnumeen tan mumun uw Calvert Cliffs, Unit 1 05000317 90 019 00 0 2 0F 0 4 '

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I. DESCRIPTION OF CONDITION on June 22, 1990 during a review of a design discrepancy, it was determined that certain non safety related loads supplied from the Class 1E 480 Volt power system motor control cen'.ers (MCC) could, under certain conditions, cause the associated MCC supply breakers to open, de energizing the entire ,

MCC. In addition, one breaker was inappropriately sized and did not have the required maximum fault current interruption capability. This is a condition outside the design bases of the Calvert Cliffs Nuclear Power Plant (CCNPP).  ;

At the time of discovery, Unit I was in Mode 5, Cold fhutdown at etmospheric '

pressure and approximately 140 F, and Unit 2 was defueled.

The CCNPP 480 Volt power system includes four safety related 1E motor control centers, two for each unit. Two of the safety related 1E MCCs, one on each unit, were determined to supply power to three non safety related loads (2 on one MCC in unit 1, 1 on one MCC in unit 2), which could adversely affect the ,

ability of these two MCCs to supply power to safety related loads.

Specifically, the three thermal magnetic MCC circuit breakers which supply the '

non safety related loads were not fully coordinated with the MCC feeder breaker. These breakers did not include ground fault protection, and for a limited range of ground fault currents the MCC feeder breaker may have opened in response to a ground fault on the non safety related portion of the three loads. The MCC breakers should have been coordinated in a way which isolated a postulated fault frem the MCC, allowing the safety related portions of the 1

system to continue to perform their required functions.

The non safety related loads supplied by the safety related buses listed above were installed as plant modifications between 1975 and the mid 80s. The breakers used for the non safety related loads were molded case, thermal magnetic without the ground fault features required to assure proper coordination. The breakers were apparently erroneously chosen based on misapplication of a breaker coordination calculation package for the associated MCCs.

Furthermore, one of the three breakers was rated for a maximum fault current ,

L of 14000 amperes when the design of the MCC specified use of a breaker capable of interrupting a current of 25000 amperes. Had a postulated fault existed on the non safety related circuit which exceeded this rating, the MCC supply breaker may have opened, isolating the entire MCC and its loads from their power supply.

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4 4 g N LICENSEl! EVENT REPORT (LER) TEXT CONTINUATION FAciUTY hatet DOCKtt leutet4R L8R NutettR PAat Colvert Cliffs, Unit 1 05000317 90 019 00 0 3 0F 0 4 Trxton wan w o .mi *im)

II. CAUSE OF EVENT 1 The root cause of the lack of breaker ground fault coordination was a design ,

error due to use of a MCC breaker design which was not addressed in the j existing MCC coordination calculation. The root cause of the use of an 1 improperly sized breaker hao not been determined.

III.- ANALYSIS OF EVENT  ;

This condition was reportable in accordance with the requirements of 10 CFR 50.73(a)(2)(ii)(B) in that the plant was operated in a condition outside its design bases.

Cround Fault Concern If a ground fault had occurrod on one of the three affected non safety related circuits, and the fault current was of a defined magnitude, the safety related portions of the' motor control center might have been adversely effected by the  ;

opening of the supply breaker, isolating the entire MCC and its loads from the power supply. ,

Current Interruption Capability Concern Had a postulated fault existed on the non safety related circuit with the ,

undersized fault interruption capability, the breaker may not have opened. If this had occurred, the MCC supply breaker would have opened, isolating the entire MCC and its loads from their power supply.  !

This condition did not significantly threaten the health and safety of the public, nor did it significantly increase the risk, or potential severity, of a postulated design bases accicent.

The reportable condition was limited to thermal magnetic breakers without ground fault protection which ware added to safety related MCCs. For the condition to have resulted in an actual event, a number of specific conditions would have to exist at the same time. For the ground fault concern to have resulted in an actual event, the fault would have to result in a current within a specific magnitude rante, (i.e. neither extremely high or very low resistance paths to ground.) Fcr the undersized fault interruption capability to have resulted in an actual ev+nt, the fault experienced by the circuit would have to be of low resistanae and high current carrying capability.

Finally, had a deficiency ruaulted in an actual failure, only a single MCC would have been affected, the refundant train of safety related equipment would not have been affected.

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LICENSEE EVENT REPORT (LER) TEXT CONTINUATION eacutt maan oocartmuseen tan muussa Paoc j C:1 vert Cliffs Unit 1 05000317 90 019 00 0 4 0F 0 4 i i

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IV. CORRECTIVE ACTIONS The effected breakers have been opened and tagged to administrative 1y assure their associated circuits are isolated from the safety related MCCs which normally power them. This assures that any postulated fault will not affect i s

safety related loads currently powered from the MCC. q Since the period during which these breakers were installed, the design unit checklists have been modified to specifically require consideration of circuit f coordination.

Circuits powered from safety related motor control centers have been reviewed to identify similar coordination concerns. Two breakers which supply power to the safety related Post Accident Sampling System were identified which could also result in a ground fault propagating to the associated load control '

center feeder breaker. All five (3 non. safety related, 2 safety related) occurrences will be resolved by equipment replacement or modification.

To enhance configuration management, a task has been organized to review breaker coordination in all MCC circuits and prepare separate coordination curves for each circuit. The review will assure proper coordination exists t and is properly documented.

The cause of the improporiv sized breaker has not yet been determined. Initial actions will verify that .11 other MCC breakers are properly sized. The need for additional corrective actions will be determined following completion of an investigation into the circumstances which led to the breakers installation. The results of this investigation and any additional corrective actions will be described in a supplement to this LER.

V. ~ ADDITIONAL INFORMATION L No similar events have been identified.

Equipment Affected Description EIIS System Code EIIS Component Code Breakers ED BKR l

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