United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-51: Failures of Voltage-Dropping Resistors in the Power Supply Circuitry of Electric Governor Systems

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

                               August 8, 1990


Information Notice No. 90-51:  FAILURES OF VOLTAGE-DROPPING RESISTORS IN 
                                   THE POWER SUPPLY CIRCUITRY OF ELECTRIC 
                                   GOVERNOR SYSTEMS


Addressees:

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

Purpose:

This information notice is intended to alert addressees to potential 
problems involving the failure or degradation of voltage-dropping resistors 
in the power supply circuitry to the electric governor controls for 
emergency diesel generators (EDGs).  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 April 13, 1990, the Commonwealth Edison Company (licensee) notified the 
NRC that during a refueling outage, the Byron Station Unit 1 A EDG had 
failed a scheduled 4-hour surveillance test.  When the diesel generator was 
started, its speed oscillated between 280 rpm and 600 rpm.  The licensee's 
followup investigation revealed that one of the two voltage-dropping 
resistors in the electrical power supply to the EDG's electric governor 
speed control unit had failed.  The second resistor had a dry, discolored 
appearance.  Failure of the resistor resulted in the loss of automatic speed 
control rendering the EDG inoperable.  The licensee replaced the faulty 
resistor and successfully tested the EDG.  

The licensee subsequently inspected the power supply circuit of the Byron 
Unit 2 A EDG governor and discovered that one of the voltage-dropping 
resistors was also in an inoperable condition.  When one of the resistor's 
leads was pulled, the interior of the resistor moved in relation to its 
metal casing.  However, the resistor continued to function normally.  The 
licensee declared the A EDG to be inoperable.  

On April 16, 1990, the Braidwood Unit 1 B EDG also failed a surveillance 
test. When the EDG was started, its speed oscillated between 300 rpm and 600 
rpm.  A failure of a voltage-dropping resistor was determined to have been 
the cause 



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                                                            Page 2 of 3 


of the speed oscillations.  Failures of voltage-dropping resistors in the 
electrical circuitry of the governor speed control for the B EDG had 
occurred at Braidwood Unit 2 on April 4, 1990. 

Discussion:

The resistors in question are commercial-grade items that are manufactured 
by the Pacific Resistor Company as type No. 100CH-300 and are supplied to 
the Woodward Governor Company (Woodward Part No. 1646-167).  The resistors 
are 300-ohm, 70-watt, wire-wound units encased with a thermally conductive 
potting compound in an aluminum shell that acts as an integral heat sink.  
Two of these resistors are electrically connected in parallel and are 
mounted to finned aluminum heat sinks to form a resistor assembly.  The 
resistor assembly reduces the 125 Vdc system voltage to 24 Vdc for input to 
the Woodward Type 2301 electric governor.  This governor is used at the 
Byron and Braidwood sites for speed control of the EDGs which are 
manufactured by the Cooper Bessemer Reciprocating Company.  The power supply 
circuit is constantly energized except when the 125 Vdc power supply bus is 
de-energized; consequently the resistors are subjected to continuous 
service. 

The licensee's root cause evaluation has determined that normal "aging" is 
the most likely cause for the resistor degradation and failure.  The 
licensee has therefore committed to replacing each of the resistors every 18 
months as part of their routine maintenance program.

In December 1989, Nine Mile Point Unit 2 reported a similar failure of the 
governor speed control for its Division II EDG.  During a monthly 
surveillance test, the speed of the diesel could not be controlled from the 
control room.  The cause of the control problem was attributed to a 
"defective" voltage-dropping resistor.  The licensee for Nine Mile Point 
stated that the defect resulted from "normal wear".  The resistor was 
replaced and the EDG was satisfactorily tested.   

Several emergency diesel generator vendors have been contacted to determine 
if any of the diesels they have supplied to nuclear facilities employ the 
Woodward Type 2301 governor.  The Cooper Bessemer Reciprocating and Morrison 
Knudsen/Power Systems Division (General Motors diesel engines) companies 
have responded affirmatively.  Colt Industries/Fairbanks Morse Engine 
Division and Cooper Industries/Enterprise Engine Services have stated that 
the type 2301 governors are not installed at any of the sites that they have 
supplied.  However, EDG governor/speed control units are sometimes supplied 
by firms other than the EDG manufacturer.

Power supply circuitry employing voltage-dropping resistors may be used in 
governor control systems other than the Woodward Type 2301.  Also, 
voltage-dropping resistors are known to be used in applications other than 
EDG governor speed control.  

Failures of voltage-dropping resistors have been recorded at other sites 
where the governor, to which power was being supplied, controlled steam flow 
to turbine-driven pumps.  In November 1982, the reactor core isolation 
cooling 
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                                                            August 8, l990 
                                                            Page 3 of 3 


(RCIC) system at Pilgrim Unit 1 failed to operate properly when the 
voltage-dropping resistor in its governor control system failed.  In June 
1983, Peach Bottom Unit 2 reported the failure of its high pressure coolant 
injection (HPCI) system due to a failed voltage-dropping resistor, and in 
October 1984, Brunswick Unit 2 experienced the failure of voltage-dropping 
resistors in both its HPCI and RCIC systems, rendering the systems 
inoperable.    

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
one of the technical contacts listed below or the appropriate NRR project 
manager. 



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




Technical Contacts: N. Fields, NRR
                    (301) 492-1173

                    W. Haass, NRR
                    (301) 492-3219

                    D. Butler, RIII 
                    (708) 790-5796

Attachment:  List of Recently Issued NRC Information Notices
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