United States Nuclear Regulatory Commission - Protecting People and the Environment

Status Indication of Control Power for Circuit Breakers Used In Safety-Related Applications

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

                              November 28, 1991


NRC INFORMATION NOTICE 91-78:  STATUS INDICATION OF CONTROL POWER FOR 
                               CIRCUIT BREAKERS USED IN SAFETY-RELATED 
                               APPLICATIONS


Addressees  

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

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice to alert addressees to problems with control logic circuitry for 
circuit breakers which have fuses for closing coils or closing actuating 
relays that are wired separately from the remainder of the control 
circuitry.  This separate circuitry allows blown fuses or improperly 
installed fuses to remain unnoticed because there would be no alarm or loss 
of local or control room indication.  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 May 23, 1991, personnel at the Palisades Nuclear Power Station were 
performing routine surveillance testing on the containment spray pumps.  The 
"C" containment spray pump failed to start locally on two attempts.  
Personnel confirmed that the control power lights, both in the control room 
and locally at the circuit breaker, were lit.  An auxiliary operator removed 
the closing coil fuses, found them to be adequate, and reinstalled them.  On 
the third attempt, the pump started (LER 50-255/91-016).  

The fuse holder fingers, which connect the fuses to the circuit, had become 
sufficiently deformed such that poor or no contact was made in the closing 
coil circuit.  The NRC questioned how control power lights could be 
energized with no power available to the closing coil circuit.  In addition, 
the NRC inspector questioned the "as found" operability of the pump (NRC 
Inspection Report 50-255/91012).  The licensee and the inspector reviewed 
wiring schematic 


9111220068 
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                                                       IN 91-78
                                                       November 28, 1991
                                                       Page 2 of 3


diagrams and found that the closing coil circuits were wired and fused sepa-
rately from the remainder of the control power circuits for most of the 
plant's 2400 volt and 4160 volt circuit breakers.  Thus, the closing coil 
fuses could be blown, removed, or improperly installed, and there would be 
no indication or alarm that the breaker could not be closed when needed.  At 
Palisades, this situation existed for both the emergency diesel generator 
(EDG) tie breakers, the breakers that supply power to several station power 
transformers, and to the following pumps:  primary coolant, condensate, 
service water, auxiliary feedwater, component cooling water, low pressure 
safety injection, high pressure safety injection, and containment spray.  
The licensee decided to change this wiring arrangement for all the 
safety-related circuit breakers above during the next refueling outage.  The 
licensee has scheduled changes of the wiring arrangement for the 
non-safety-related circuit breakers during the following refueling outage.  

This lack of indication of a loss of control power (closing coil power) 
could prevent personnel from recognizing when required safety equipment 
becomes inoperable.  Under these conditions, the equipment may remain 
inoperable for an extended period until its condition is discovered through 
testing or initiation of an actuation signal (NRC Inspection Reports 
50-255/91017 and 50-255/91020).

Figures 1 and 2 are simplified schematic diagrams of control circuits for 
circuit breakers that illustrate these circumstances.  These diagrams 
represent circuit breakers used at Palisades in 2400 and 4160 volt 
applications and at the Ft. Calhoun Station in 4160 and 480 volt 
applications.  

Discussion  

The common purpose of a circuit breaker is to open an energized circuit for 
a piece of equipment upon a fault condition.  Designers sometimes ensure 
this action by separately fusing the trip coil circuit in the circuit 
breaker so the circuit breaker can always be tripped (to open the circuit) 
even if a malfunction occurs in the control circuit that energizes the 
closing circuit.  However, when the function of the equipment is important, 
designers also provide a means to indicate a loss of control power to the 
closing circuit.  The circuitry providing this indication also ensures that 
if the fuse is blown, improperly installed, or even removed, then either the 
lights indicating the breaker position are no longer illuminated or an alarm 
is energized.  

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                                                       IN 91-78
                                                       November 28, 1991
                                                       Page 3 of 3


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 project manager.  




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


Technical contacts:  Edward R. Schweibinz, RIII
                     (708) 790-5542

                     Vern Hodge, NRR
                     (301) 492-1861

Attachments:  
1.  Figures 1 and 2.  Typical Simplified Electrically 
      Operated Circuit Breaker Control Circuit at 
      Palisades and Ft. Calhoun
2.  List of Recently Issued NRC Information Notices
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