NRC Information Notice No. 92-04: Potter and Brumfield model MDR rotary relay failures

On 14 Jan 1986, 17 Sep 1987, and 8 Dec 1987, an emergency diesel generator (EDG) failed an operability surveillance test at the LaSalle County Station, Units 1 and 2. In each case, while the Commonwealth Edison Company (CECO) attempted to synchronize the EDG to its bus, the EDG output breaker would not close. CECO replaced all P ampersand B MDR relays in the output breaker closing circuits with GE HFA relays. The NRC staff has received no reports of relay failures at LaSalle affecting EDGs since these were replaced. On 10 Oct 1988, the Arizona Public Service Company (APSC), the licensee for the Palo Verde Nuclear Generating Station, submitted a report which documented 18 instances over a 2-year period in which P ampersand B MDR relays failed to change position. APSC detected these failures during either routine surveillance testing or actuation of the engineered safety features (ESF) actuation system or the reactor trip switchgear. After replacing all P ampersand B MDR series relays, APSC experienced only two failures; improperly sized coils or contamination in the insulating material of the switch caused these two failures. The principal failure mechanism of P ampersand B MDR rotary relays appears to be mechanical binding of the rotor caused by deposits from coil varnish outgassing and chlorine corrosion products. A secondary failure mechanism appears to be the intermittent continuity of electrical contacts. A number of variables contribute to these failure mechanisms and caused the relays to fail at random mostly within 2--5 years of the in-service date. Failures may occur regardless of current or wattage, the use of ac or dc power, or whether normally energized or de-energized. It is also important to note that a relay rotor can bind immediately after a surveillance test. Attachment 1 provides a detailed description of the failure mechanisms, contributing causes, and failure investigations and also discusses modifications made to P ampersand B MDR series relays by the manufacturer