Information Notice No. 92-45: Incorrect Relay Used in Emergency Diesel Generator Output Breaker Control Circuitry

                              UNITED STATES
                         WASHINGTON, D.C. 20555

                              June 22, 1992



All holders of operating licenses or construction permits for nuclear
power reactors.


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a potential problem that may exist at their
facilities resulting from the failure of a relay in the control circuitry
for an emergency diesel generator (EDG) output breaker to perform its
intended design function.  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 April 3, 1992, during the weekly idle start test of an EDG at the
St. Lucie Plant, Unit 1, of the Florida Power and Light Company (the
licensee), the control room operator was unable to close the output
circuit breaker after the EDG had started, warmed up, and achieved rated
speed.  The licensee analyzed the control circuitry and found that a relay
was not functioning properly for an unknown reason.  The licensee sent the
relay to Westinghouse for failure analysis.  This relay is intended to
prevent the output circuit breaker from closing when the generator is
operating at a frequency below 54 Hz.  However, the licensee found
independent of the specific cause of the failure which occurred on
April 3, 1992, that the relay had not been properly selected and was not
able to perform its intended function.  When operating as actually
designed, the relay would permit the circuit breaker to close at all
frequencies up to 62.5Hz.  The licensee had not noticed this discrepancy
during previous testing because the EDG usually achieves 90 percent of
full voltage (the circuitry also includes an under-voltage permissive
relay function) at almost the same time as it achieves 90 percent of full
frequency.  The licensee determined that the relay had been incorrectly
selected because of an ambiguity in the Westinghouse catalog description
for this particular relay (Style 177C717G03, 60 Hz/120 V).  Westinghouse
has since corrected the catalog description.  The licensee corrected the
error in relay selection by replacing the relay with another relay that
properly performs the under-frequency permissive function.

9206060071.                                                            IN 92-45
                                                            June 22, 1992
                                                            Page 2 of 2


The EDG control circuitry is designed to prevent the output circuit
breaker from closing prematurely.  This circuitry includes under-
frequency and under-voltage permissive relays to ensure that the EDG
attains both 90 percent of rated frequency and 90 percent of rated voltage
before the output circuit breaker closes.  Under emergency conditions
(e.g., accident conditions with loss of offsite power), use of the
incorrect relay could allow the output circuit breaker to close before the
EDG reaches the necessary operating conditions possibly resulting in
shutdown of the diesel engine and/or damage to the safety-related loads.

This information notice requires no specific action or written response. 
If you have any questions about the information in this notice, please
contact one of the technical contacts 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 contacts:  Walter P. Haass, NRR
                     (301) 504-3219

                     E. Nick Fields, NRR
                     (301) 504-1173

Attachment:  List of Recently Issued NRC Information Notices


Page Last Reviewed/Updated Wednesday, March 24, 2021