United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 83-17: Electrical Control Logic Problem Resulting in Inoperable Auto-start of Emergency Diesel Generator Units

                                                            SSINS No.: 6835 
                                                            IN 83-17  

                                UNITED STATES
                           WASHINGTON, DC  20555 

                               March 31, 1983 

                              INOPERABLE AUTO-START OF EMERGENCY DIESEL 
                              GENERATOR UNITS 


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


This information notice is provided as a notification of a potential problem
in the control logic circuitry which could adversely affect the auto-start 
feature provided for diesel generators at nuclear facilities. It is expected
that recipients will review the information for applicability to their 
facilities. No specific action or reference is required. 

Description of Circumstances: 

Recently the licensee performed a loss of offsite power test at the North 
Anna Unit 2 Station. The test is conducted periodically and is a technical 
specification requirement. Specifically, this test is performed to verify 
the capability of the emergency diesel generators (EMDs) to load-shed and 
reload the essential emergency busses following loss of an EMD when offsite 
power is unavailable. 

Briefly, the selected emergency bus was "set up" to simultaneously receive 
electrical power from the offsite power source and the associated EMD. A 
safety injection test signal was then initiated and the diesel generator 
unit was manually shut down. These actions were followed by opening the 
offsite power supply feeder breaker to the selected emergency bus. Given 
these conditions, the associated EMD did not respond to an auto-start call 
upon resetting its shutdown relay. 

When the licensee investigated the event, he found that the periodic test 
procedure used to conduct this test did not specifically address a 60-second
time delay feature in the diesel generator's restart circuitry. The purpose 
of this time delay is to allow the diesel generator to come to a complete 
stop before attempting a restart. This time delay prevents fuel from. being 
supplied to the diesel generator for 60 seconds after the shutdown relay has 
been manually reset by a remote pushbutton located in the control room. 

Further review of this event determined that when the shutdown relay was 
manually reset (because of the presence of the safety injection signal 
mentioned above), the compressed starting air was admitted to start the 
diesel even though no fuel was admitted during the 60-second delay period. 
At the end of this delay, 


                                                           IN 83-17  
                                                           March 31, 1983 
                                                           Page 2 of 2  

fuel was admitted but the supply of compressed air used to rotate the engine
for a restart attempt had been consumed. Thus, the diesel unit effectively 
became inoperable until the air storage tanks were repressurized. 

This licensee's action taken to preclude this event from recurring included 
modifying the test procedures so as to require the control selector switch 
for the diesel generator being tested to be placed in the "local" position 
before the shutdown relay is reset. This action, in effect, blocks the 
auto-start diesel signal from activating the air start solenoid for the 
diesel generator being tested. Following this action, the shutdown relay may
be manually reset and the time delay permitted to run out. Subsequently, the
control selector switch may be returned to its normal "remote" position at 
which time the diesel generator unit being tested should auto-start and 
reload its associated emergency busses in accordance with design 

During an in-depth study of the control logic circuitry for the diesel 
generator units at Hatch Unit 1 Station, a potential control logic problem 
was identified which is identical to the problem at North Anna Unit 2 
Station. Subsequently, the licensee implemented a design change which now 
allows fuel to be supplied into the diesel unit concurrent with the logic 
signal calling for diesel generator "re-start."  

Since North Anna Unit 2 is a pressurized water reactor power plant and Hatch
Unit 1 is a boiling water reactor power plant, the control logic problem, as
discussed, has strong generic implications ,and may affect many of the 
addressees of this information notice. Licensees should be aware that since 
the local/remote control switch is located in the control room and is placed
in the "remote" position during normal plant operation, it will require 
direct immediate operator corrective action to preclude failure of the 
diesel generator to re-start under the set of conditions discussed above. On 
this basis, we suggest these addressees review this information for 
applicability to the control logic circuitry of diesel generator units in 
use at their plant. 

No written response to this notice is required. If you have questions 
regarding this matter, please contact the Regional Administrator of the 
appropriate NRC Regional Office, or this office. 

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

CONTACT:  V. D. Thomas 

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