05000280/LER-2008-002
| Document Number05 17 2008 2008 -- 002 -- 00 07 08 2008 05000 | |
| Event date: | 05-17-2008 |
|---|---|
| Report date: | 07-08-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2802008002R00 - NRC Website | |
1.0 DESCRIPTION OF THE EVENT On May 17, 2008, at 0904 hours0.0105 days <br />0.251 hours <br />0.00149 weeks <br />3.43972e-4 months <br />, with Unit 1 at 100% power and Unit 2 in cold shutdown for a scheduled refueling, the breaker failure lockout relay (LOR) [El IS-EA-RLY] for the switchyard Bus 5, 34.5KV offsite supply breaker inadvertently operated causing the loss of power to Bus 5. This resulted in the loss of off-site power to "A" and "B" Reserve Station Service Transformers (RSSTs) [EIIS-EA-XFMR] which de-energized the "D" and "E" Transfer Buses [EIIS-EA-BU] that supplies the 2H and 1J Emergency busses. An under voltage auto start signal was generated and the #2 and #3 Emergency Diesel Generators (EDG's) [EIIS-EK-DG] started and loaded as designed on the 2H and 1J emergency buses respectively.
The function of this breaker failure LOR is to send a signal to open a sequence of breakers to secure the entire bus when a breaker receives a trip signal but fails to open. Upon initial observation after the event, the LOR was found in the tripped position with no valid signal having been sent. It was identified that a mechanical failure had occurred in the LOR.
Offsite power was restored at 1211 hours0.014 days <br />0.336 hours <br />0.002 weeks <br />4.607855e-4 months <br /> and EDGs were unloaded, taken offline and placed back in auto. The LOR was replaced with a new relay.
At 1344 hours0.0156 days <br />0.373 hours <br />0.00222 weeks <br />5.11392e-4 months <br />, an event notification was made to the NRC for Surry Unit 1 and 2 EDG actuation in accordance with 10 CFR 50.72(b)(3)(iv)(A).
This report is also being submitted, pursuant to 10 CFR 50.73(a)(2)(iv)(A), for automatic actuation of the EDGs.
2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS This event resulted in a partial loss of offsite power to both units and caused two EDGs to start as designed. Although the standby Unit 2 Residual Heat Removal (RHR) pump lost power, shutdown cooling was maintained since the operating pump was served from a separate non-affected bus. An assessment of Unit 1 showed an increase in risk with the partial loss of offsite power; however, due to timely recovery of the offsite power, no risk management actions were required to be implemented. The shutdown safety assessment for Unit 2 indicated that while the offsite power defense in depth was reduced, the risk threshold remained the same as prior to the event. The above actuations and occurrences are within the station design and therefore had no significant nuclear impact. As a result, the health and safety of the public were not affected.
3.0 CAUSE The investigation revealed that the LOR coil had been replaced during a prior maintenance activity. A functional test was performed on the breaker failure LOR prior to placing it in service. No operational concerns were noted. The design of the LOR positions the roller arm slightly above the coil while the plunger is fully recessed in the coil. When activation occurs, the plunger will have enough driving force to push the roller arm up causing a fast trip.
The cause of the LOR failure was due to the coil housing within the lockout relay being bent which prevented a plunger from being in the proper position within the coil. It is not known when the coil housing became bent. The bent housing caused the plunger's nose to protrude through the coil in a higher than normal position preventing the roller arm from reaching the full reset position. In this condition, vibration could activate the LOR into the trip position. At the time of this event, jack hammer activities going on outside the control house in the switchyard and/or work in the control house could have created vibrations to trip the relay.
4.0 IMMEDIATE CORRECTIVE ACTION(S) Offsite power was restored to the 2H and 1J Emergency busses and EDGs were taken offline and placed back in auto. The defective LOR was replaced with a new relay. The new LOR plunger and roller arm gap was inspected for proper clearance and was found acceptable.
5.0 ADDITIONAL CORRECTIVE ACTIONS An Apparent Cause Evaluation was completed to investigate the event.
The remaining LOR relays in the 230/500KV houses were inspected for potential mechanical issues and no problems were found.
6.0 ACTIONS TO PREVENT RECURRENCE A technical bulletin was created and distributed to substation technicians informing them of this event and providing additional instruction if a coil is replaced. The instruction requires, upon coil replacement, the clearance between the plunger and the rollerarm to be measured with a feeler gauge and be a minimum of .025 inches.
7.0 SIMILAR EVENTS There were no similar events.
8.0 MANUFACTURER/MODEL NUMBER Electro Switch / 7807D 9.0 ADDITIONAL INFORMATION None.