Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
December 19, 1989
Information Notice No. 89-87: DISABLING OF EMERGENCY DIESEL GENERATORS
BY THEIR NEUTRAL GROUND-FAULT PROTECTION
CIRCUITRY
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is intended to alert addressees to possible
unconsidered failure modes in which emergency diesel generators could be
rendered inoperable as a result of their neutral ground-fault protection
circuitry. 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 do not constitute NRC requirements; therefore, no specific action or
written response is required.
Description of Circumstances:
On May 25, 1989, a plant engineering design review at Perry Unit 1 revealed
a design anomaly whereby ground faults on emergency diesel generator loads
coincident with loss of offsite power during a seismic event or fire could
lead to the inoperability of more than one emergency diesel generator. As a
result, the Perry staff declared several of the emergency diesel generators
inoperable until temporary modifications could be made to disable the
neutral ground-fault relay contacts that were designed to trip the emergency
diesel generators.
Discussion:
At Perry Unit 1, each emergency diesel generator was designed with a neutral
ground circuit consisting of a high impedance path from the neutral to
ground, which limits ground-fault current to no more than 2 amperes (see
typical network in Figure 1). The purpose of providing this grounding path
(in lieu of an ungrounded system) is to limit the buildup of high voltage
stress during certain ground-fault conditions that could ultimately result
in the breakdown of the insulation of such components as motors and cables.
It also provides a convenient means of detecting a ground in the system so
that a search can be made to eliminate the ground before a second ground
occurs and causes a phase-to-phase fault.
8912130473
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IN 89-87
December 19, 1989
Page 2 of 2
Ground faults are detected by sensing the voltage that is developed in the
emergency diesel generator grounding circuit whenever a ground fault exists
in the electrical distribution system supplied by the emergency diesel
generator. In the Perry design, a voltage sensing relay would initiate a
trip of the corresponding emergency diesel generator whenever this voltage
exceeded the relay's pickup value. This relay's contacts are bypassed by
the automatic response to a loss-of-coolant accident. For non-LOCA events,
however, a ground fault in any component, including non-Class 1E components,
would have the undesirable result of shutting down the emergency diesel
generator. This raises the concern that a seismic event or fire could have
resulted in simultaneous ground faults in non-safety components supplied by
all of the redundant emergency diesel generators. Action by the protection
circuitry at Perry could then have shut down all of the emergency diesel
generators, preventing them from performing their intended safety functions.
The Perry staff has temporarily disabled the neutral ground-fault relays to
prevent them from shutting down the emergency diesel generators. Permanent
modifications are planned to replace the ground-fault emergency diesel
generator trip function with ground-fault alarms in the control room. These
modifications will be supported by alarm response procedures requiring that
the operators determine the location and safety significance of ground
faults and take appropriate action.
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 NRR project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: F. Burrows, NRR
(301) 492-0833
Attachments:
1. Figure 1
2. List of Recently Issued NRC Information Notices
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