United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-62: Potential Problems in Westinghouse Molded Case Circuit Breakers Equipped with a Shunt Trip

                                                             SSINS NO: 6835 
                                                             IN 86-62      

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                                July 31, 1986

                                   CASE CIRCUIT BREAKERS EQUIPPED WITH A 
                                   SHUNT TRIP 


All nuclear power reactor facilities holding an operating license or a 
construction permit. 


This notice is to alert recipients to a potentially significant problem 
involving the failure of shunt trip coils in Westinghouse molded case 
circuit breakers (breakers) type LBB 22250 MW. Breaker types LB 22250, HLB 
22250, and DA 22250 have similar operating mechanisms which can affect the 
operation of the shunt trip coils. However, to date no failures of the shunt 
trip coil have been reported for these breakers. It is expected that 
recipients will review this information for applicability to their 
facilities and consider actions, if appropriate, to preclude a similar 
problem from 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: 

On February 6, 1986, an open occurred or was discovered in the shunt trip 
coil (STC) circuitry which could have resulted in the failure of the 
associated breaker to trip open on an abnormal voltage condition at the 
Peach Bottom Reactor. The breaker is identified as an LBB 22250 MW type 
molded case circuit breaker manufactured by Westinghouse. The breaker is 
installed in a transfer panel to protect the 120-V 60 Hertz power supply to 
the reactor protection system (RPS) from undervoltage, overvoltage and 
underfrequency conditions. In RPS applications, loss of the STC prevents the 
breakers from tripping automatically on abnormal voltage conditions. 
Tripping of this breaker on abnormal voltage conditions is essential because 
it interrupts a potentially damaging abnormal voltage supply to the RPS 
relays, scram solenoids, and other safety-related electronic devices. 
However, failures of the STC does not, by itself, prevent a reactor scram. 
Also, the breaker is equipped with a magnetic overcurrent trip device that 
protects the circuits in the event of a fault. The STC in series with a 
contact is activated through a toggle linkage by the breaker's moving main 
contact arms. The failure of the STC occurred when the contact did not open 
when the breaker tripped resulting in the overheating of 


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the STC and ultimately in an open circuit in the shunt trip circuit. If the 
breaker had been subjected to an abnormal voltage condition after reset the 
open shunt trip circuit would have prevented breaker trip. 

On January 16 and 24, 1986 the same types of problems were identified in 
similar breakers at the Peach Bottom Unit 3 plant. On November 9, 1984, 
Limerick 1 reported an identical failure. It is essential that the STC 
remain operable to assure the circuit breaker trips in the event of an 
abnormal voltage condition. Administrative measures have been established to
periodically test and verify the operability of the STC at the Peach Bottom 
and Limerick nuclear power plants. 


The failures described above have occurred in LBB 22250 MW type breakers 
with a STC. Westinghouse supplied the breakers to ASCO Electrical Products, 
Incorporated who installed them in safety-related RPS power supply 
monitoring panels required for boiling water reactors (BWRs). LB 22250, HLB 
22250, and DA 22250 type breakers, which have similar operating mechanisms 
to LBB 22250, may have been supplied to other manufacturers for use in 
safety-related applications. 

The performance of the breaker is affected only when used with a STC. The 
STC is energized by the closure of a normally open contact which is actuated
through a toggle linkage by the breaker's moving main contact arms. 
Westinghouse stated that the moving contact arms may be impeded from being 
fully displaced to the "open" position by excessive material in, the rivet 
which holds the handle post to the operating mechanism. 

The 2-pole molded case circuit breaker is equipped with an A contact that is
operated by toggle operating links controlled by the breaker operating 
handle. The A contact is an auxiliary switch which is open when the breaker 
is in the open or tripped position and is closed when the breaker is closed 
("ON" position). This A contact is in series with the 125-V dc STC. When the
breaker is closed, the auxiliary A contact is closed and the STC is ready to
receive a signal to trip the breaker if any one of the RPS power monitoring 
relays senses an abnormal voltage condition. The STC is not rated for 
continuous duty and will overheat and be damaged if subjected to full 
voltage for more than a few seconds. 

The STC can be made to fail, after the breaker has tripped by either of the 
following actions or conditions: 

1.   If the operating handle is pushed to the "ON" position without 
     resetting the breaker, when a trip signal is present. This will not, 
     close the breaker, but will close the A contact, thus continuously 
     energizing the STC. 

2.   If the previously described interference between the operating handle 
     and the toggle operating links exist the act of resetting the breaker 
     and, before clearing the trip signal, attempting to close the breaker. 
     The toggle operating links may jam in the "ON" position, unless the 
     operating handle is pushed toward the "OFF" or "RESET" position, again 
     causing the STC to be continuously energized. 

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                                                              July 31, 1986 
                                                              Page 3 of 3  

     The breaker will trip even if the operating handle is forcibly kept in 
     the "ON" position when a trip signal is received or present if the STC 
     is operable. However, because the operating handle is in the "ON" 
     position, the contact will not open to disconnect the power supply to 
     the STC. 

Westinghouse recommends the following tests to ascertain the operability of 
the breaker. 

1.   Perform a continuity check on the STC after each breaker operation via 
     the two leads exiting the breaker. 

2.   Operability of the STC protection can be determined by completing the 
     following tests: 

     a.   Manually close the breaker and continue to hold the operating 
          handle in the "ON" position. 

     b.   Apply the shunt trip rated voltage through the two leads exiting 
          the breaker until the breaker trips or one second elapses. 

     c.   While continuing to hold the handle in the "ON" position, perform 
          a continuity check of the shunt trip circuit. Continuity indicates
          a malfunction in the contact. An open circuit indicates that the 
          contact has performed its intended function. 

Westinghouse revised the circuit breaker mechanism assembly drawing to 
specify the removal of excessive rivet material and prevent recurrence of 
this problem. 

No specific action or written response is required by this notice. If you 
have any questions regarding this notice, 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 

Technical Contact:  K. R. Naidu, IE
                    (301) 492-4179

                    James C. Stewart, IE
                    (301) 492-9061

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