United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 89-87: Disabling of Emergency Diesel Generators by their Neutral Ground-Fault Protection Circuitry

                                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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