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Information Notice No. 92-19: Misapplication of Potter & Brumfield MDR Rotary Relays
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 March 2, 1992 NRC INFORMATION NOTICE 92-19: MISAPPLICATION OF POTTER & BRUMFIELD MDR ROTARY RELAYS Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to failures caused by the misapplication of Potter & Brumfield (P&B) MDR rotary relays. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required. Description of Circumstances On September 11, 1989, operators at the Shearon Harris Nuclear Power Plant (Shearon Harris) performed a periodic test of an emergency load sequenc-er. When the operators pressed the "Test Stop" button, the sequencer did not properly reset and generated an inadvertent start signal to the emergency service water (ESW) pump. The control room operators noticed the inadvertent start of the pump and secured it. On June 15, 1990, the licensee for the Waterford Steam Electric Station (Waterford) informed the NRC senior resident inspector that P&B MDR 66-4, MDR-4076, and MDR-5061 rotary latching relays had been determined to have a high failure rate warranting a root cause analysis. Licensees for other nuclear power plants have reported to the NRC that P&B MDR relays have operated intermittently with failures that could not be duplicated during subsequent testing. These failures occurred in applications of MDR relays for energizing equipment status indication lights, energizing computer input and display lights, and switching low level loads. Discussion The Carolina Power and Light Company (CP&L), the licensee for Shearon Harris, investigated the inadvertent start of the ESW pump. CP&L found that a P&B MDR 137-8 relay, installed in the test circuit, failed to reset at the proper time and continued to supply power to the equipment actuation relays longer 9202250331 . IN 92-19 March 2, 1992 Page 2 of 3 than designed. This failure resulted in the inadvertent start of the ESW pump. When the failed relay was inspected, CP&L found that the contacts were burned and that the leaf spring contact had melted into the plastic armature. CP&L concluded that the relay's contacts had been overloaded. In reviewing the problem, CP&L found that there is a substantial difference between the current rating of contacts used with direct current and the rating of contacts used with alternating current. The circuit design for the failed relay had been based on only the resistive loads and not the inductive loads carried by the contacts. This was also the case for other circuits using P&B MDR latching relays and Agastat microswitches. CP&L modified the circuits by eliminating the microswitches and connecting two pairs of MDR relay contacts in series to switch the inductive loads. CP&L reviewed Licensee Event Reports 88-29 and 88-08, which documented similar failures at Shearon Harris, and concluded that inductive loads in those circuits may have damaged the MDR relay contacts and caused the relays to fail. Attachment 1 includes a discussion of comments from P&B on the use of relays for switching direct current loads. Entergy Operations, Incorporated (Entergy), the licensee for Waterford, per- formed a root cause analysis of the failures of P&B rotary latching relays at Waterford. The analysis found that the design of the electrical system used some of the relays' own contacts to de-energize the relays' operate and reset coils. However, minor variations in the amount of time for the contacts to operate did not allow the relays' mechanisms to consistently reposition and reset contacts for the next operation of the relays. Consequently, the relays would fail in an intermediate position. The licensee subsequently implemented a design change to bypass the internal contacts that were in series with the coils. P&B engineers informed the NRC that, if P&B had known the intended application of the relays at Waterford, it could have designed and manufactured relays that would have performed reliably. P&B stated that it would assign a specific drawing number for such relays and would expect recipients to reference the drawing number in subsequent purchase orders to ensure that P&B furnished identical relays as replacements. The NRC staff and P&B engineers investigated the intermittent nonrepeatable MDR relay failures experienced at other plants and concluded that the P&B MDR relays may have been misapplied to switch low level loads. Attachment 1 includes a discussion of comments from P&B on the use of P&B relays for switching low level loads. Because the use of relays for switching direct current or low level loads may require special features, licensees may wish to review the adequacy of their applications of P&B MDR relays and other similar relays and improve their communications with P&B or other manufacturers in order to accomplish special functions reliably. The NRC has issued the following related generic communication: NRC Information Notice IN 92-04, "Potter and Brumfield Model MDR Rotary Relay Failures," January 6, 1992. . IN 92-19 March 2, 1992 Page 3 of 3 This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact the technical contact listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical contact: Kamal R. Naidu, NRR (301) 504-2980 Attachments: 1. P&B's Comments On Switching Direct Current and Low Level Loads 2. List of Recently Issued NRC Information Notices .
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