NRC Information No. 88-42: Circuit breaker failures due to loose charging spring motor mounting bolts

There are two emergency diesel generators (EDGs) at North Anna Unit 2 (H and J). On May 19, 1988, following completion of scheduled preventive maintenance, the ''J'' EDG failed to pick up and carry the required load during preoperational testing. The licensee then shut down the ''J'' EDG and performed unplanned corrective maintenance. The licensee's technical specifications require that the operable EDG be tested within 24 hours if the inoperable EDG is inoperable due to any cause other than preplanned preventive maintenance or testing; thus, because of the unplanned maintenance on the ''J'' EDG the licensee tested the ''H'' EDG. During this test, the ''J'' EDG output circuit breaker (ITE Type 5HK) failed to close. The licensee's investigation determined that the ''H'' EDG output breaker closing spring had not been charged because the charging spring motor mounting bolts had become loose, allowing the motor to become detached from the frame. A similar occurrence was previously reported by Philadelphia Electric Company (the Limerick Units 1 and 2 licensee) on August 27, 1984. During an EDG preoperational test, the charging spring motor of a BBC Brown Boveri, Inc. 5HK breaker failed to perform its intended function. The licensee determined that three out of four horizontal charging spring motor mounting bolts had loosened, allowing the charging spring motor to rotate. Loose charging spring motor mounting bolts may be a generic concern. Because of changes in corporate name, mergers etc., circuit breakers similar to those discussed may indicate manufacture by ITE, ITE Imperial, Gould, Brown Boveri, or a combination thereof