ML20042G779

LER 90-004-00:on 900404,single Failure Potential in Safeguards Switchgear B03/B04 Tie Breaker Discovered & Could Have Resulted in Failure of Diesel Generator.Control Power Fuses for Tie Breakers Removed

ML20042G779
Person / Time
Site:	Point Beach
Issue date:	05/08/1990
From:	Fay C WISCONSIN ELECTRIC POWER CO.
To:
Shared Package
ML20042G737	List: ML20042G736 ML20042G779
References
LER-90-004, LER-90-4, NUDOCS 9005160079
Download: ML20042G779 (4)

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ABST_RACT During an NRC Electrical Distribution System Function Inspection, a potential single f ailure was discovered which could cause the B03/B04 bus tie breaker to close. This breaker closure could parallel both emergency diesel generators (EDG) out of phase via the 480 volt safe-i guards busses.1B03/1B04. This, in turn, could result in failure of the diesel generator and loss of on-site ac.

l l The de control power fuses for the tie breakers were removed, thus preventing the unwanted operation of the breakers. This event is reported pursuant to 10 CFR 50.7 3 (a) (2) (v) and (vi).

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010 0 12 mia . w. - mic,.,,,,3su unn EVENT DESCRIPTION During the course of a NRC Electrical Distribution System Function Inspection, a potential single failure was identified that could cause the B03/B04 tie breaker to close. Closure of this breaker could result in the Emergency Diesel Generators being paralleled-out of phase to the 480 VAC safeguards buses, 1B03/lB04 or 2B03/

2B04. The single failure is a hypothetical "DC+" short to the control circuit on the (+) side of the "X" relay. This could cause the relay to pick up, in turn energizing the "CC" (closing '

coil) causing the B03/B04 tie breaker to close. r It is postulated that, following a fast start of both diesel generators without synchronization, the closure of the tie breaker would cause a failure in this system. The busses would be stripped from their normal power supplies and one bus would be powered from one emergency generator and the other bus would be powered from the other emergency generator. If the tie breaker would close and tie together the two unsynchronized systems, a failure of one or both EDG's is postulated.-  ;

An additional concern is whether the presence of an undetected ground on the circuit could provide another path which could energize relays with a "DC+" short to the control circuit. Since the PBNP battery chargers have ground detection capable of detecting grounds of 500 ohms or less, a significant ground fault would be quickly identified and corrected. Assuming a ground present on the bus of 500 ohms (undetected) coincident with the "DC+" short to the control circuit, current may flow in the circuit.. The "X" relay (based on a sampling of direct measurements) has.a coil resistance of approximately 1500 ohms (maximum). This 1500 ohms in series with the 500 ohm ground may not provide enough current limiting resistance to prevent,the'"X" coil from operating. Thus for the purpose of analysis we will assume that this relay operates, closing its "X" contacts and providing-a current path to the closing coil.

l l Based on the equipment manual, the "CC" current for operation is 32 amperes. At 125 VDC the coil resistance calculates to about 4 ohms. The parallel arrangement of the 1500 ohm "X" coil with the "CC" in series with the 500 ohm ground does however provide adequate current limiting resistance to prevent the "CC" relay from operating. The addition of the 500 ohms ground fault to the "CC" circuit will limit the current to 0.25 amperes and the relay will not operate, thus the breaker will not operate.

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! rsxT u, . w. e cs m m nn SYSTEM DESCRIPTION The tie breaker provides a means to tie together safeguards-busses 1-B03 with 1-B04 and 2-B03 with 2-B04 (Ref. FSAR Figure 8.2 -9) .  !

These busses supply 480 VAC to train "A" and train "B" safeguards loads. Power is normally supplied to these busses from the station auxiliary transformers via A03 and A04. In the event of a loss of normal supply with an EDG fast start, these busses are supplied by.

the EDG via AOS and A06. The tie breaker between B03 and B04 is- 1 provided for_use during unit shutdowns to facilitate maintenance on the normal bus supply breakers. ,

_RE; PORTABILITY This License Event Report is provided. pursuant to 10 CFR 50.73(a)(2)(v) and (vi): "Any event or condition that alone could have prevented the I fulfillment of the safety function of systems need to ...(D) mitigate l the consequences of an accident.": Events include discovery of l design inadequacies.

The Energy Industry Identification System component. function identifier for the r.afeguards switch gear is SWGR. The~ system designation for the safeguards busses is EB.

SAFETY ASSESSMENT The B03/B04 bus tie breaker is not operated during nomal operations.

The tie breaker is provided to facilitate maintenance during unit shutdown. In the event of a design basis accident coincident with a loss of offsite power and this single. failure,'the Emergency .

Diesel Generators (EDG) could operate in parallel without synchroni-zation and it is likely that a failure of one or both EDG would-ensue. Such a combination of events and failures could result in consequences more severe than analyzed in the safety analysis report. The probability of the combination of events necessary to result in this failure-is extremely small. The corrective action taken will further ensure that the safety of the plant and the-health and safety of the public and plant employees are not jeopardized.

CORRECTIVE ACTION The de control power fuses for the B03-B04 tie breakers were removed. This corrective action eliminates the possibility of the sequence of failures and events as described in the Safety Assessment above.

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o ls lo lo jo l2l6l6 9l 0 -- 0l0j4 .- 0j0 0 l4 or0l4 rext <n . , ., -wc w asia .iim The breakers are administratively controlled by an operating instruction (OI-35) and routine maintenance procedures (RMP-23A and RMP-23B for units 1 and 2, respectively.) OI-35 has been revised to further describe normal operation and authorized use of the B03 and B04 tie breakers. RMP-23A has been revised to address installation and removal of the control power fuses for Unit 1. A similar-revision will be made to RMP-23B by June 1990 or prior to the procedure's next use.

SIMILAR OCCURRENCES In 1988, a similar chain of potential events was identified and documented in Unit 1 LER 88-001. Although the chain of potential events in this LER is similar to those in LER 88-001, the initiating events necessary for failure are of a different nature, I

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