ML20090E470

AO 50-237/75-43:on 750911,diesel Generator Cooling Water Pump Main Breaker Tripped.Caused by Excessive Heat Buildup within Breaker Housing.Breakers on Buses Replaced & Cover Plates Removed for Ventilation

ML20090E470
Person / Time
Site:	Dresden
Issue date:	09/19/1975
From:	Stephenson B COMMONWEALTH EDISON CO.
To:	James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
References
10146, 623-75, AO-50-237-75-43, NUDOCS 8303310661
Download: ML20090E470 (3)

Text

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2 (j/ Address R;oly Chicago, ta:60690 Illinois Post Offica Box 767 BBS Ltr. #623-75 Dresden Nuclear Power Station RR #1 f Morris, Illinois 60450 September 19, 1975 ,c- < ,

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Mr. James G. Keppler, Regional Director

] y a,n Directorate of Regulatory Operation-Region III / ,

U. S. Nuclear Regulatory Co=:ission Yf -,14V

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Glen Ellyn, Illinois 60137 SUBJET: REPORT OF AMORMAL OCCURRE?CE PER SECTION 6.6.A 0F THE TECHNICAL SPECIFICATIONS UNIT 2/3 DIES:-1, OENERATOR COOLING WATER PUMP TRIP AND FOLLO*J-UP LEPTER TO REFORT NU:GER 50-237/75-42

References:

1) Regulatory Guide 1.16 Rev.1 Appendix A

2) Notification of Region III of U. S'. Nuclear Regulatory Comission Telephone P. Johnson, 1530 hours0.0177 days <br />0.425 hours <br />0.00253 weeks <br />5.82165e-4 months <br /> on September 11, 1975 Telegram: J. Keppler, 1620 hours0.0188 days <br />0.45 hours <br />0.00268 weeks <br />6.1641e-4 months <br /> on September 11, 1975 .

3) Drawing Number M-355, 12E2351B

4) Letter from B. B. Stephenson to J. G. Keppler, Report Number 50-237/75-42 Report Number: 50-237/75-43 - .

Report Dates September 19, 1975 Occurrence Date: September 11, 1975 ,

Facility: Dresden Nuclear Power Station, Morris, Illinois .

IDDITIFICATION OF OCCURRD!CE The breaker feeding the Unit 2/3 diesel generator cooling water pump from bus 28-3 tripped.

CONDITIONS PRIOR TO OCCURRDICE Unit-2 was operating at a steady-state' power level of 2466 IGt and 812 IWe.

Unit-3 was in the cold shutdown mode. .

1 S1k-3 8303310661 750919 PDR ADOCK 05000237 8 PDR b d

.Mr. James G. K;pplsr ' S:ptember 19, 1975 l (m) e 4 it DESCRIPTION OF OCCURRD CE i

At approximately 0300 hours0.00347 days <br />0.0833 hours <br />4.960317e-4 weeks <br />1.1415e-4 months <br /> on September 11, 1975, the main breaker of the Unit 2/3 diesel generator cooling water pump was found tripped. The pump had been tested the day before in order to identify the problem which had caused the breaker to trip on two previous occasions (August 29 and September 5, 1975) ,

reported in A.O. 50-237/75-42. .

I DESIGNATION OF APPARENT CAUSE OF OCCURRDICE The breaker trips were apparently caused by excessive heat build-up within the breaker housing. Inadequate heat transfer would cause the breaker to trip at a lower than rated current.

The motor control center containing the breaker has gasketed covers which do not provide significant ventilation. In addition, the rather high ambient temperature of the breaker's physical location is believed to intensify the thermal transfer problems.

ANALYSIS OF OCCURRD:CE The diesel generator cooling water pumps would have operated from the reserve bus 38-3 had the generator started while breaker 28-3 was tripped. The Unit-2 diesel generator was also operable and capable of providing the necessary emergency power. The health and safety of plant personnel and the public were not , jeopardized by this occurrence. .

CORRECTIVE ACTION On September 11, 1975, the cooling water pump was re-started to monitor current drawn by the p p at the mcment a breaker trip occurred. The pump operated for two hours and fifty minutos before the breaker en bus 28-3 tripped. The feed was automatically transferred to bus 38-3 After two hours of pump operation the breaker on bus 38-3 tripped. The current drawn by the pump remained at its rating of 130 amps throughout the test, indicating that the breakers themselves *

. were at fault.

The breakers on both buses were replaced and the cover plates above the breakers were removed for ventilation. The pump was subsequeritly operated for 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> without incident.

The' original breakers were load-tested for four hours and exhibited no abnormal --

characteristics. Disassembly of the breakers revealed no loose connections or other internal discrepancies. The breakers will be returned to the manufacturer for further analysis.

The original motor control center covers will be replaced with louvered plates to allow adequate ventilation. Further corrective action will be dependent upon the discovery of other contributing factors, in which case a follow-up.

letter will be submitted.-

Mr. Jamea G. K!ppler . ->- S:ptember 19, 1975

.o a o o

. This letter is subnitted in lieu of the supplemental report designated in report no. 50-237//5-42, dated september 8, 1975 -

FAILURE DATA A sirailar failure of the Unit-3 diesel generator cooling water pumps was reported on December 21, 1973 A loose connection in the breaker resulted in themally.

induced trips.

The failed breakers are General Electric TFJ236160 breakers, rated at 480 volts and 150 amps.

B. B. Stephenson Superintendent BBS s=p File /NRC e.

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