United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 79-11: Faulty Overcurrent Trip Device in Circuit Breakers For Engineered Safety Systems

                               UNITED STATES 
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555

                                May 22, 1979

                                                      IE Bulletin No. 79-11 

FAULTY OVERCURRENT TRIP DEVICE IN CIRCUIT BREAKERS FOR ENGINEERED SAFETY 
SYSTEMS 

Discussion: 

We have received information from Westinghouse and an NRC licensee relating 
to the potential failure of a circuit breaker in an engineered safety system
of a nuclear power plant. This circuit breaker had a defect in one of its 
three time delay dashpots which resulted in a reduced time delay for 
overcurrent protection. The defect was a small hairline crack in the end cap
of the dashpot. Further investigation by this licensee disclosed that 7 out 
of 17 spare dashpot end caps and 2 non-engineered safety feature breakers 
also had similar defects. The circuit breaker is a Westinghouse type DB-75. 
Westinghouse type DB-50 breakers also use the same type of dashpot and end 
cap. DB-50 and -75 breakers are used extensively in PWR's, and some BWR's 
may also have the same breakers. 

Similar make and model circuit breakers, when used for scram purposes, do 
not require the overcurrent trip feature and thus are not of concern. The 
end cap crack defect, if severe enough, could result in premature tripping 
of the circuit breaker because of insufficient time delay in overcurrent 
protection; i.e., the motor starting (inrush) current could cause the 
breaker to trip inadvertently and thus prevent the motor start. 

The defects reported by the licensee in April 1979, occurred in the 
replacement end caps which were provided to solve the problem described in 
IE Bulletin 73-1. The subject of Bulletin 73-1 was end caps made of a black 
phenolic material. As a result of that Bulletin, the black end caps were 
replaced with a new type made of fibre-filled polyester material called 
"navy-gray". Prior to the April 1979 report, there have been no reports of 
suspect "navy-gray" end caps either from scheduled testing or unusual 
behavior in service. The manufacturer of the "navy-gray" end caps believes 
the crack defects may be linked to a raw material batch problem. That is, 
the molding ingredient materials used may have neared the end of their shelf
life before use. It is not believed the end caps, after fabrication, have a 
significant shelf life limit, due to the low residual stress and low crack 
propagation probabilities. 
.

IE Bulletin No. 79-11                                       May 22, 1979 
                                                            Page 2 of 3 

Description of Event: 

The following information was obtained from the Licensee Event Report dated 
April 12, 1979 and a subsequent meeting with Westinghouse, the NRC staff and
the licensee. 

During the 1979 surveillance tests, and a review of the previous refueling 
surveillance test results on a Westinghouse type DB-75 breaker (used as a 
480v ESF bus supply breaker) a drift in the overcurrent trip time from the 
manufacturer's design minimum value of 6 seconds to 5.50 and 5.12 seconds 
was observed. The 1979 test results showed a deviation and inconsistency in 
the delayed trip timings among three consecutive tests. 

The overcurrent devices were removed from each of the three phases and a 
visual inspection indicated a hairline crack in the end cap of one of the 
devices. That cap was replaced (without checking for a possible crack in the
replacement cap), and the breaker was again tested. This test also showed 
deviations and inconsistency in the delayed trip timings. The subsequent 
inspection of the devices revealed a hairline crack in the end cap which had
just been installed. This prompted an inspection of the in-stock spare caps.
Seven out of 17 caps were found to have similar cracks. 

Action Required of all Holders of an Operating License or Construction 
Permit: 

1.   Determine whether circuit breakers of the above described manufacture; 
     and type with overcurrent trip devices are in safety-related Class IE 
     service or in spares at your facilities. 

2.   If the subject breakers are in service in safety-related systems: 
     within 30 days, review the existing test data for all overcurrent trip 
     device calibrations since plant startup or since replacement caps were 
     installed and tested in response to Bulletin 73-1, whichever is most 
     recent. Determine if any delay times are: (1) outside of the acceptance 
     band; (2) marginally acceptable - on the low side of the acceptance 
     band; or (3) if any significant change in delay time performance has 
     been observed. These breakers should be retested and end caps replaced 
     as necessary to assure no loss of safety function. 

3.   Inspect all end caps in spares for cracks using at least a 3x 
     magnifying glass. Caps having visible flaws should be discarded, or 
     prevented from use in Class IE applications. 

4.   Review test procedures and test schedules for all safety-related 
     circuit breakers to assure that all such breakers are tested at least 
     each refueling outage to confirm overcurrent time delay protection. 
.

IE Bulletin No. 79-11                                       May 22, 1979 
                                                            Page 3 of 3 

For facilities with an operating license, a written report of the above 
actions, including the date(s) when they will be completed shall be 
submitted within 45 days of receipt of this Bulletin. 

For facilities with a construction permit, a written report of the above 
actions, including the date(s) when they will be completed shall be 
submitted within 60 days of receipt of this Bulletin. 

Approved by GAO, B180225 (R0072), clearance expires 7/31/80. Approval was 
given under a blanket clearance specifically for identified generic 
problems. 
Page Last Reviewed/Updated Tuesday, July 23, 2013