Information Notice No. 92-40: Inadequate Testing of Emergency Bus Undervoltage Logic Circuitry
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C 20555
May 27, 1992
NRC INFORMATION NOTICE 92-40: INADEQUATE TESTING OF EMERGENCY BUS
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.
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
Attachment: List of Recently Issued NRC Information Notices
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