Information Notice No. 92-19: Misapplication of Potter & Brumfield MDR Rotary Relays

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                                March 2, 1992

                               ROTARY RELAYS 


All holders of operating licenses or construction permits for nuclear power 


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. 


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 


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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. 

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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

1.  P&B's Comments On Switching Direct Current and Low Level Loads
2.  List of Recently Issued NRC Information Notices

Page Last Reviewed/Updated Friday, May 22, 2015