United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 92-09: Overloading And Subsequent Lock Out Of Electrical Buses During Accident Conditions

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                              January 30, 1992


NRC INFORMATION NOTICE 92-09:  OVERLOADING AND SUBSEQUENT LOCK OUT OF 
                               ELECTRICAL BUSES DURING ACCIDENT CONDITIONS


Addressees

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

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice to alert addressees to a particular scenario identified by a licensee 
whereby electrical buses could be overloaded during accident conditions, 
deenergized, and prevented from being reenergized.  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 November 21, 1991, the Power Authority for the State of New York, the 
licensee for Indian Point Nuclear Generating Unit 3, reported that the vital 
buses could become overloaded during a loss-of-coolant-accident (LOCA) as a 
result of emergency operating procedures directing operators to restore non-
essential loads to the buses if offsite power is not lost.  The procedure 
would result in both emergency and non-emergency loads being powered 
concurrently from the same bus.  The licensee's recent calculations reveal 
that performing this procedure could have overloaded buses.  The resulting 
overcurrent condition could lock out a bus (i.e., deenergize the bus and 
prevent it from being reenergized from any source including the associated 
emergency diesel generator).

Discussion

A LOCA is generally analyzed as the most limiting condition in power demand 
for onsite electrical systems.  When non-essential loads are added to the 
power system under this most demanding condition, electrical buses and 
breakers could be overloaded.  The electrical protection system recognizes 
such an overloaded condition as an electrical fault on a bus and locks out 
the bus.  The dc-powered breaker controls retain the faulted condition, and 
the emergency diesel generator will not be able to energize the bus until 
protective relays are manually reset.  The scenario could result in 
disabling redundant trains of safety-related equipment.  


9201240235
.

                                                      IN 92-09 
                                                      January 30, 1992 
                                                      Page 2 of 2 


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:  Thomas Koshy
                    (301) 504-1176

Attachment:  List of Recently Issued NRC Information Notices
.
Page Last Reviewed/Updated Tuesday, November 12, 2013