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
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
July 31, 1986
Information Notice No. 86-62: POTENTIAL PROBLEMS IN WESTINGHOUSE MOLDED
CASE CIRCUIT BREAKERS EQUIPPED WITH A
SHUNT TRIP
Addressees:
All nuclear power reactor facilities holding an operating license or a
construction permit.
Purpose:
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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IN 86-62
July 31, 1986
Page 2 of 3
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.
Discussion:
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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IN 86-62
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
Attachment: List of Recently Issued IE Information Notices
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