United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-75: Disabling of Diesel Generator Output Circuit Breakers by Anti-Pump Circuitry

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

                               September 16, 1988


Information Notice No. 88-75:  DISABLING OF DIESEL GENERATOR OUTPUT 
                                   CIRCUIT BREAKERS BY ANTI-PUMP CIRCUITRY 

Addressees: 

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

Purpose: 

This information notice is being provided to alert addressees to potential 
problems where the capability to either automatically or manually close diesel
generator circuit breakers from the control room may be lost.  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 June 5, 1988, operators at Browns Ferry Unit 3 were unable to reclose the 
diesel generator output breakers to the 4-kV shutdown boards from the control 
room during a loss-of-power/loss-of-coolant-accident (LOP/LOCA) test.  During 
the test, the output breakers connected the diesel generators to their respec-
tive 4-kV shutdown boards after the LOP signal was generated; however, the 
breakers tripped and remained open after receiving the LOCA signal.  Operators
diagnosed the output breaker lockout problem using system electrical configu-
ration drawings.  An operator was sent to the 4-kV shutdown board to manually 
transfer output breaker control power to its alternate source.  This 
momentarily removed power, enabling the breaker to close when the power was 
restored.  

On October 14, 1987, an operator at Wolf Creek Generating Station manually 
tripped the output circuit breaker of the emergency diesel generator (EDG) 
from the control room.  At the time, the EDG was supplying a vital bus.  When 
the operators tried to reenergize the vital bus from the still-operating EDG, 
they found that they could not close the EDG output breaker from the control 
room.  The vital bus was finally reenergized from the offsite power supply.  
Through examination of the breaker control schematics the licensee later found
that the EDG circuit breaker could be closed by cycling the EDG mode switch at
the EDG local control station. 





8809120197
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Discussion: 

The anti-pump circuit configuration will protect large breakers from rapid 
cycling and, under certain circumstances, will prevent breaker closure. At 
Browns Ferry Unit 3, a unique sequence of events, a LOP signal followed within
6 seconds by a LOCA signal, led to the discovery of a design deficiency of 
this circuit configuration.  Contacts from the undervoltage relay will seal in
the breaker anti-pump relay until the undervoltage condition on the 4-kV 
boards clears.   

The circuit is designed such that following a LOP, the undervoltage condition 
must exist for at least 5 seconds and the diesel must reach rated speed before
the diesel generator output breaker will close on the bus.  Once the breaker 
has closed, the closure spring recharge motor and the breaker anti-pump relay 
will be energized.  A fully discharged closure spring requires 2 seconds for 
the spring to be fully recharged.  During this 2 second window the anti-pump 
coil will remain energized via contacts sensing spring position, and if an 
undervoltage condition exists on the 4-kV bus it will seal in and lock open 
the breaker. 

During the Browns Ferry event, the EDG output breaker closed 5 seconds after 
the LOP signal, the undervoltage condition was eliminated, and the 
undervoltage relay began its 5 second cycle to reset from the undervoltage 
condition. About 1.5 seconds later, the LOCA signal retripped the breaker and 
created another undervoltage condition on the 4-kV bus.  At this point, 
because the undervoltage relay had not completed its reset cycle, the 
undervoltage relay remained in its undervoltage state.  Therefore, the 
undervoltage relay sealed in the still-energized anti-pump relay because the 
undervoltage condition occurred before the breaker charging spring was fully 
charged.  Thus, the breaker could not be closed from the control room either 
manually or automatically until control power was removed, which deenergized 
the anti-pump relay. 

The Browns Ferry licensee modified the breaker control logic to prevent the 
anti-pump relay from sealing in during a LOCA condition by adding a time-delay 
relay in the breaker trip coil circuitry.  This relay will be energized by a 
LOCA signal and its contact in the anti-pump coil seal-in path will open after
a 2-to 5-second delay to prevent anti-pump coil seal-in and breaker lockout.  

The Wolf Creek EDG output circuit breaker has automatic closing logic to close
the circuit breaker when the following five permissives are satisfied: 

     (1) Both offsite circuit breakers are open. 
     (2) The EDG mode switch is in the automatic mode. 
     (3) Lockout relays are deenergized. 
     (4) A 3-second time delay has elapsed. 
     (5) The EDG has reached operating speed and voltage. 

This logic sends a constant close signal to the circuit breaker that keeps the
breaker's internal anti-pump relay energized as long as the logic permissives 
are satisfied.  The anti-pump relay prevents the circuit breaker from cycling 
if attempts are made to hold the breaker closed against a valid trip signal.  
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                                                            Page 3 of 3 


When the Wolf Creek operator manually tripped the EDG output breaker, the 
auto-matic closing logic permissives remained satisfied.  Therefore, the 
anti-pump relay remained energized, preventing reclosure of the circuit 
breaker.  Cycling the EDG mode control switch at the local control station 
allowed the circuit breaker to reclose by momentarily interrupting the 
automatic close signal, there-by resetting the anti-pump logic.  When the mode 
switch contact was reclosed by returning the switch to the "auto" position, 
the circuit breaker's automatic closing logic closed the breaker.  

The Wolf Creek licensee modified the EDG breaker control switch located in the
control room to enable the operator to reclose the EDG circuit breaker from 
the control room.  This switch was originally intended only as a means of 
paralleling the EDG with the offsite power supply.  With the current 
modification, the switch can be used to reset the anti-pump logic and allow 
the automatic circuit to re-close the breaker.  The modification added a 
contact that is closed in the "normal" position and open in the "trip" and 
"pull to lock" positions of the control switch.  When operators manually trip 
the EDG circuit breaker from this control switch, the contacts open to 
interrupt the close circuit and reset the circuit breaker anti-pump relay.  If
the operator wishes to keep the breaker open, he must put the switch in the 
"pull to lock" position.  Returning the switch to the "normal" position 
completes the automatic close circuit and the breaker recloses.  

It should be noted that although the above discussion has dealt only with EDG 
output circuit breakers, anti-pump circuit problems could also apply to other 
breakers that use automatic closing logic, such as load-sequencing breakers 
and offsite supply breakers to the emergency buses.  

The information herein is being provided as an early notification of a 
potentially significant matter that is still under consideration by the NRC 
staff.  If NRC evaluation so indicates, specific licensee actions may be 
requested.  

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




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

Technical Contacts:  James Lazevnick, NRR 
                     (301) 492-0814 

                     Carl Schulten, NRR 
                     (301) 492-1192 

                     Fred Burrows, NRR 
                     (301) 492-0783 

Attachment:  List of Recently Issued NRC Information Notices
.                                                            Attachment 
                                                            IN 88-75 
                                                            September 16, 1988
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                            NRC INFORMATION NOTICES 
_____________________________________________________________________________
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-74          Potentially Inadequate        9/14/88        All holders of OLs
               Performance of ECCS in                       or CPs for W and 
               PWRs During Recirculation                    B&W-designed 
               Operation Following a LOCA                   nuclear power 
                                                            reactors. 

88-73          Direction-Dependent Leak      9/8/88         All holders of OLs
               Characteristics of                           or CPs for nuclear
               Containment Purge Valves                     power reactors. 

88-72          Inadequacies in the Design    9/2/88         All holders of OLs
               of dc Motor-Operated Valves                  or CPs for nuclear
                                                            power reactors. 

88-71          Possible Environmental        9/1/88         All holders of OLs
               Effect of the Reentry                        or CPs for nuclear
               of COSMOS 1900 and                           power reactors, 
               Request for Collection                       fuel cycle 
               of Licensee Radioactivity                    licensees, and 
               Measurements Attributed                      Priority 1 
               to That Event                                material 
                                                            licensees. 

88-70          Check Valve Inservice         8/29/88        All holders of OLs
               Testing Program                              or CPs for nuclear
               Deficiencies                                 power reactors. 

88-69          Movable Contact Finger        8/19/88        All holders of OLs
               Binding in HFA Relays                        or CPs for nuclear
               Manufactured by General                      power reactors. 
               Electric (GE) 

88-48,         Licensee Report of Defective  8/24/88        All holders of OLs
Supplement 1   Refurbished Valves                           or CPs for nuclear
                                                            power reactors. 

88-68          Setpoint Testing of Pres-     8/22/88        All holders of OLs
               surizer Safety Valves with                   or CPs for nuclear
               Filled Loop Seals Using                      power reactors. 
               Hydraulic Assist Devices 

88-67          PWR Auxiliary Feedwater Pump  8/22/88        All holders of OLs
               Turbine Overspeed Trip                       or CPs for nuclear
               Failure                                      power reactors. 
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Tuesday, November 12, 2013