United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 87-42: Diesel Generator Fuse Contacts

                                                    SSINS No.:  6835
                                                       IN 87-42

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                                September 4, 1987

Information Notice No. 87-42:  DIESEL GENERATOR FUSE CONTACTS


All nuclear power reactor facilities holding an operating license or a con-
struction permit.


This information notice is being provided to alert recipients to potential 
failures of emergency electrical power supplies resulting from misalignment 
and/or degradation of fuse contacts.  It is expected that recipients will 
review the information for applicability to their facilities and consider 
actions, if appropriate, to avoid similar problems.  However, suggestions 
contained in this notice do not constitute NRC requirements; therefore, no 
specific action or written response is required.  

Description of Circumstances:

At Browns Ferry Nuclear Plant, Unit 3, during routine surveillance testing of 
an emergency diesel generator on April 20, 1987, an explosion occurred in the 
electrical control cabinet.  The explosion resulted from a phase-to-phase 
short in a cable bundle in the potential transformer (PT) fuse compartment.  

The electrical control cabinet was supplied by Power Systems Division of 
General Motors, the diesel generator manufacturer.  The licensee's investiga-
tion revealed that the cables routing power from the PT fuses to the trans-
formers shorted after their insulation failed because of an overtemperature 
condition.  This condition is believed to have been caused by an excessive air 
gap and/or poor contact between the spring finger contact arrangement of the 
PT fuses.  The spring finger contacts were manufactured by Allis-Chalmers.  
The moveable contacts are mounted to the PT fuse compartment door so that when 
the door is opened, the contacts disconnect, thereby preventing anyone from 
pulling the fuse while it is carrying current.  When the door is closed, the 
moveable contact finger connects with the stationary contact.  Although the 
contacts were destroyed by the event, the licensee concluded that an air gap 
due to misalignment or contact degradation from corrosion, pitting, or 
burning, caused arcing and eventual failure.  Although the cause has been 
attributed to inadequate maintenance of the contact, the licensee is 
considering design changes as a permanent solution.  

.                                                            IN 87-42
                                                            September 4, 1987
                                                            Page 2 of 2


The licensee inspected the other seven diesel generator control cabinets and 
found three other instances of poor contact.  Visual inspection for burning, 
pitting, corrosion and improper alignment and mating of contact surfaces were 
performed as well as micro-ohmmeter checks.  Since 1973, four previous onsite 
events involving this type of pressure contact arrangement have occurred on 
non-safety-related electrical boards designed by General Electric Co.  Because 
of this type of problem, General Electric modified similar contacts on its 
electrical boards to knife switch contacts.  The licensee considers the 
problem to be generic and has initiated a visual inspection of all PT fuse 

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the Regional 
Administrator of the appropriate regional office or this office. 

                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contacts:  C. Brooks, RII
                     (205) 729-6196

                     Samuel D. MacKay, NRR
                     (301) 492-8394

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