Information Notice No. 92-40: Inadequate Testing of Emergency Bus Undervoltage Logic Circuitry

                                UNITED STATES
                           WASHINGTON, D.C  20555

                                May 27, 1992

                               UNDERVOLTAGE LOGIC CIRCUITRY


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 a safety problem that could result from the 
use of an undervoltage logic test method that fails to verify the 
de-energization of the emergency safety busses.  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

In November 1991, an NRC electrical distribution functional inspection team 
at the LaSalle County Nuclear Station discovered that the surveillance 
testing method used for the loss of offsite power test was not testing the 
capability of the undervoltage logic circuitry to de-energize the emergency 
bus.  This deficiency affected all six of the electrical divisions for both 
nuclear power units.  The undervoltage circuitry automatically opens the 
emergency bus feed breakers from the offsite power system to isolate the 
emergency bus when a loss of voltage is sensed.  This circuitry also 
initiates load shedding, starts the emergency diesel generator (EDG), 
re-energizes the emergency bus, and sequences the necessary emergency loads 
onto the emergency bus.

This test deficiency is significant because the failure of the normally 
closed feed breaker to open automatically in response to a loss of offsite 
power would prevent the emergency diesel from automatically re-energizing 
the emergency bus and also would result in the emergency bus remaining 
connected to a degraded offsite power source.

In conducting an internal lessons learned program for the Byron and 
Braidwood stations, the Commonwealth Edison Company (the licensee) 
determined that it was performing inadequate tests of its undervoltage logic 
circuitry in a manner similar to the LaSalle Station tests.


                                                            IN 92-40
                                                            May 27, 1992
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The technical specifications for the LaSalle County Nuclear Station require 
the licensee to simulate a loss of offsite power and to verify the 
de-energization of the emergency busses.  The licensee was simulating a loss 
of offsite power by manually opening the emergency bus main feed breaker.  
By simulating a loss of offsite power in this manner, the licensee did not 
demonstrate that the undervoltage logic circuitry would automatically 
de-energize the emergency bus in response to an undervoltage condition.  
Thus, the licensee did not test the capability of two undervoltage relay 
contacts to trip the normal bus feed breaker from the main station 
transformer and the unit tie breaker.  These breakers provide an EDG output 
breaker closure permissive.  If a feed breaker failed to trip open 
automatically, it would prevent the EDG output breaker from closing 
automatically and also result in the emergency bus remaining connected to a 
degraded offsite power source.

After discovering this problem, LaSalle personnel successfully tested all of 
the affected undervoltage logic circuits.  The licensee simulated the loss 
of offsite power by opening the undervoltage relay potential transformer 
test switches and verifying that the appropriate breakers had tripped.  The 
licensee satisfactorily tested the capability of the logic circuits to shed 
loads from the emergency busses, the capability of the EDGs to start 
automatically, and the capability to energize the emergency safety loads 
with the emergency busses.

The Byron and Braidwood staff also revised their testing method and success-
fully tested the capability of their undervoltage logic circuitry to auto-
matically de-energize the emergency busses.                                     

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:  David S. Butler, RIII
                    (708) 790-5796

Attachment:  List of Recently Issued NRC Information Notices

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