05000346/LER-1979-067-01, /01T-0 on 790628:short Circuit Caused Loss of Essential Bus Y4 Sypplying Power to Safety Feature Actuation Sys.Blown Inverter Fuse Was Replaced & Y4 Bus Returned to Normal Supply

/01T-0 on 790628:short Circuit Caused Loss of Essential Bus Y4 Sypplying Power to Safety Feature Actuation Sys.Blown Inverter Fuse Was Replaced & Y4 Bus Returned to Normal Supply

ML19224D782
Person / Time
Site:	Davis Besse
Issue date:	07/10/1979
From:	Beeler T TOLEDO EDISON CO.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
Shared Package
ML19224D777	List: 05000346/LER-1979-067, Forwards LER 79-067/01T-0 05000346/LER-1979-067-01, /01T-0 on 790628:short Circuit Caused Loss of Essential Bus Y4 Sypplying Power to Safety Feature Actuation Sys.Blown Inverter Fuse Was Replaced & Y4 Bus Returned to Normal Supply
References
LER-79-067-01T, LER-79-67-1T, NUDOCS 7907170034
Download: ML19224D782 (3)

LER-1979-067, /01T-0 on 790628:short Circuit Caused Loss of Essential Bus Y4 Sypplying Power to Safety Feature Actuation Sys.Blown Inverter Fuse Was Replaced & Y4 Bus Returned to Normal Supply

Event date:
Report date:
3461979067R01 - NRC Website

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U. S. NUCLE AR REGUL ATORY COM,'.ilSSION NRC FORM 3GG [7 77)

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[ At 0157 hours0.00182 days <br />0.0436 hours <br />2.595899e-4 weeks <br />5.97385e-5 months <br /> on June 28, 1979, an accidental short circuit causea the loss of essen-)

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tial bus Y4 which supplies power to Safety Features Actuation dystem (STAS) Channel 4 4 fo 3

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DH 11 was reopened and the 2500J The blown inverter fuse was replaced and the j gpm DH flow re-established by 0215 hours0.00249 days <br />0.0597 hours <br />3.554894e-4 weeks <br />8.18075e-5 months <br />.

[ pgl l Y4 bus returaed to its normal supply.

Facility Change Request 79-273 was prepared tol 11 [.[ [ he l ;, prevent accidental shOrt circuitin:: of the 12 voit AC sources.

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TOLEDO EDISON COMPANY DAVIS-BESSE NUCLEAR POWER STATION U'iIT ONE SUPPLEMENTAL INFOR't\\ TION FOR LER NP-32-79-09 DATE OF EVENT:

June 28, 1979 FACILITY: Davis-Besse Unit 1 IDENTIFICATION OF OCCURRENCE: Loss of Safety ' Features Actuation System (SFAS)

Channel 4 due to loss o1 Y4 Conditions Prior to Occurrence: The unit was in Mode 5, with Power (MWT) = 0, and Load (Gross MWE) = 0.

, Description of Occurrence: At 0157 hours0.00182 days <br />0.0436 hours <br />2.595899e-4 weeks <br />5.97385e-5 months <br /> on June 28, 1979, while conducting surveil-lance testing on the Reactor Protection System (RPS) Channel 4, an alligator clip slipped off a terminal screw and shorted across another terminal.

The 120 volt AC power supply to RPS Channel 4, Y4 was lost causing a loss of power to the SFAS Chan-ne l 4.

The SFAS Channel 4 bistables tripped as designed upon a loss of power.

The trip of one SFAS channel does not actuate any SFAS incident level but the trip of SFAS Channel 4 does actuate the bistable which closed the Decay Heat (DH) Normal Cooldown Isolation Valve DH 11.

The closure of DH 11 isolated the DH System and since tiiet e were av Keatuut Cvulant Pumps in operation, placed the unit in violation of Technical Specification 3.4.1, which requires in Moda 5 at least one Reactor Coolant Pump or Decay Heat Pump be in op era t io n. The Decsy Heat Pump uas manually shutdoun before all DH flow was lost to pro te c t the pump.

Designation of Annarent Cause of Occurrence: The shorting of terminals with an alli-gator clip was due to not having large enouSh termination screws to properly retain an alligator clip. However, the loss of Essential Instrumentation Panel Y4 and sub-sequent trip of SFAS Channel 4 and RPS Channel 4 can be attributed to a characteris-tic of inverter YV4, which causes its fuses to blow on a short circuit.

Analysis of Occurrence:

There was no danger to the health and safety of the public or to station personnel.

Since decay heat generation at the time of the occurrence was low, the flow was not required for decay heat removal. The RPS and SFAS channels fail to the actuated (safe) position upon a loss of power and were not rendered inop-erable by the loss of power.

The three remaining SFAS channels were operable.

Corrective Action

Panel Y4 was supplied by its alternate source YBR.

SFAS and RPS Channels 4 were re-energized and reset.

Operations personnel then reopened valve DH 11, restarted Decay Heat Pump 1-2 and re-established greater than 2800 gpm flow to Reac tor Coolant Sys ten by 0215 hours0.00249 days <br />0.0597 hours <br />3.554894e-4 weeks <br />8.18075e-5 months <br />, June 28, 1979. Under Maintenance Work Order LER #79-067

TOLEDO EDISON COMPANY DAVIS-BESSE Ml' CLEAR P0;ER STATION UNIT ONE PAGE 2 SUPPLEMENTAL INFOR"\\ TION FOR LER NP-32-79-09 79-2405, the blown fuse was replaced in YV4.

At 1430 hours0.0166 days <br />0.397 hours <br />0.00236 weeks <br />5.44115e-4 months <br /> on June 28, 1979, YV4 was returned to service and operations personnel re-energized panel Y4 to normal.

Previoas invastigation into the loss of an inverter by Power Engineering and the ven-dor have deteruined it would be more detrinental to modify the inverter to prevent recurrence than to leave the inverter in its initial state.

Facility Change Request 79-273 was prepared to prevent shorting of the 120 volt AC sources to the RPS cabi-nets by nodifying the terninals.

Failure Data:

Previous f ailures fer loss of essential 120 volt aC power supply have been reported in Licensee Event Reports NP-33-77-05, NP-33-77-10, NP-33-77-14,

' and NP-33-79-13.

LER #79-067 s