Information Notice No. 90-51: Supplement 1:Failures of Voltage-Dropping Resistors in the Power Supply Circuitry of Electric Governor Systems

                                UNITED STATES
                          WASHINGTON, D.C.  20555 

                              October 24, 1991

                                             RESISTORS IN THE POWER SUPPLY
                                             CIRCUITRY OF ELECTRIC GOVERNOR


All holders of operating licenses or construction permits for nuclear power 


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice supplement to alert addressees to a recent failure of a 
voltage-dropping resistor in the power supply for the electronic control 
module of an emergency diesel generator (EDG) governor system at the 
Waterford Steam Electric Station.  The resistor had been in service for 
approximately 15 months. The resistor failure resulted in the inoperability 
of the EDG.  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 August 20, 1991, at the Waterford Steam Electric Station (Waterford), EDG 
"A" failed to maintain rated speed following a successful start during a 
surveillance test.  After reaching rated speed of 600 rpm, engine speed 
began to fluctuate, dropping to a low of 280 rpm.  This prompted the 
licensee to shut down the EDG and declare it inoperable.  The licensee 
performed an investigation and determined that the speed anomaly resulted 
from the failure of one of the two voltage-dropping resistors in the power 
supply for the electronic control module for the governor system on the EDG.  
The NRC staff described this failure mode in NRC Information Notice (IN) 
90-51, issued August 8, 1990.

In IN 90-51, the staff discussed failures of voltage-dropping resistors in 
the power supplies for the electronic control modules for EDGs at the 
Commonwealth Edison Company's (CEC's) Braidwood and Byron Stations.  CEC 
conducted a root cause analysis and concluded that normal "aging" was the 
most likely cause of the resistor degradation or failure.  CEC subsequently 
began a preventive maintenance program that included periodic replacement of 
the voltage-dropping resistors.  After reviewing the original information 
notice, the Waterford licensee instituted a preventive maintenance program 
to replace the resistors every 18 months.  However, the resistor that 
recently failed at the 


                                                  IN 90-51, Supplement 1
                                                  October 24, 1991
                                                  Page 2 of 2 

Waterford Station had been in service for only about 15 months after having 
been replaced as part of the newly adopted preventive maintenance program.  
Although this is the only early failure of these voltage-dropping resistors 
of which the NRC staff is aware, it appears that these resistors can 
prematurely fail even before 18 months and that periodic replacement may not 
ensure adequate availability of the EDGs to perform their intended function.  

Following the publication of IN 90-51, Woodward Governor Company (Woodward) 
stated, in a letter to the NRC, that they did not concur with using periodic 
replacement of the two resistors, of the original design, as an acceptable 
preventive maintenance procedure.  Woodward stated that whenever the battery 
charging system is in operation, system voltage approached the design 
capacity of the dual resistor assembly.  The higher voltage developed by the 
charging system caused more current to flow through the resistors, resulting 
in more heat generation.  The increased heat generated at the higher voltage 
level would tend to reduce the life expectancy of the assembly.  

The letter from Woodward described the design of the governor control system 
and stated that several product improvements had been made that offer 
certain advantages over the original design.  Specifically, the letter 
stated that a replacement assembly for the dual voltage-dropping resistor 
configuration had been developed.  The replacement assembly substitutes a 
single resistor for the dual resistor design.   Besides the fact that the 
single resistor assembly is of greater thermal capacity than the dual 
resistor assembly, the letter states that, because of the design of the 
governor system, use of the single resistor has distinct advantages over the 
dual resistor configuration.   

With the single voltage-dropping resistor of the replacement assembly, a 
resistor failure would result in a backup mechanical governor taking control 
of speed.  The EDG would then continue to operate at rated speed.  The 
licensee for Waterford has concluded that the new design has advantages over 
the dual resistor design and has installed the new resistor assembly in each 
of its EDG governor systems. 

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 Office of 
Nuclear Reactor Regulation (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

                     O. Chopra, NRR
                     (301) 492-3265

Attachment:  List of Recently Issued NRC Information Notices

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