Information Notice No. 85-73: Emergency Diesel Generator Control Circuit Logic Design Error

                                                         SSINS No.: 6835 
                                                            IN 85-73       

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                               August 23, 1985

                                   CIRCUIT LOGIC DESIGN ERROR 


All nuclear power reactor facilities holding an operating license (OL) or a 
construction permit (CP). 


This information notice is to alert recipients of a potentially significant 
emergency diesel generator (EDG) control logic error that could prevent 
transfer to the emergency bus while the EDG is in the "maintenance shutdown"
mode. It is expected that recipients will review the information for 
applicability to their facilities and consider actions, if appropriate, to 
preclude a similar problem occurring at their facilities. However, 
suggestions contained in this information notice do not constitute NRC 
requirements; therefore, no specific action or written response is required.

Description Of Circumstances: 

According to the design, the EDGs at Rancho Seco Nuclear Power Generating 
Station enter the maintenance shutdown control mode whenever they are 
normally shut down from the control room or the remote EDG control panel. On
June 1, 1985, the plant was shut down for refueling, an EDG was in the 
maintenance shutdown control mode after being secured from an operational 
condition (idling at 600 rpm with the output breaker open), when an 
emergency bus was de-energized for planned work on a parallel bus. This 
created an undervoltage condition equivalent to a loss of offsite power 
(LOOP) on the emergency bus. The diesel generator sped up to the design 
speed but the EDG output breaker continuously cycled open and closed, 
thereby rendering the EDG set inoperable. Investigation by the licensee 
indicates that the cycling of the EDG output, breaker was the result of a 
design error in the EDG control circuit logic. According to the licensee, 
the design "deficiency affects proper response of the EDG set when it is 
operating in the maintenance shutdown control mode. Normal surveillance 
testing would not discover the control circuit design error because 
surveillance is not done in the maintenance shutdown control mode. The June 
1, 1985 event at Rancho Seco represents the first time in the life of the 
plant that an undervoltage signal occurred with an EDG in the maintenance 
shutdown control mode. 


                                                            IN 85-73       
                                                            August 23, 1985 
                                                            Page 2 of 2    

When an EDG is secured from operation, the control circuit logic places it 
in the maintenance shutdown control mode. In this mode, the control logic 
opens its output breaker and reduces its speed from,900 to 600 rpm. The EDG 
then idles at 600 rpm for 15 minutes before coasting down to rest. 

If a LOOP should occur while an EDG is in the maintenance shutdown control 
mode, the undervoltage signal causes it to speed back up to 900 rpm and to 
close its output breaker. This would cause the undervoltage signal to drop 

However, the maintenance shutdown control mode does not drop out for 30 
seconds after the receipt of the undervoltage signal because of the control 
circuit design error. Thus, the maintenance shutdown control logic senses 
that the EDG output breaker is closed, opens the breaker, and resets the 
15-minute timer for the maintenance shutdown control mode. As soon as the 
EDG output breaker opens, the undervoltage signal recurs and the EDG output 
breaker closes in response to the LOOP. The EDG output breaker continues to 
cycle open and closed as this process repeats itself. At Rancho Seco, this 
control circuit logic design error has been corrected by installing a relay 
to de-energize the maintenance shutdown control logic immediately upon 
receipt of an undervoltage signal. The Rancho Seco plant utilizes General 
Motors (GM) Model 20-465-E4 diesel generators with a 2750 kw nameplate 
rating. According to the licensee, the design error was in the interface 
provided by the Architect-Engineer (Bechtel) to the shutdown control logic 
provided by GM. Bechtel has advised the NRC that the Rancho Seco diesel 
generator control logic is unique and other plants designed by them are not 

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. 

                                   Edward L. Jordan Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  W. Swenson, NRR
                    (301) 492-7876

                    R. Singh, IE
                    (301) 492-8985

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