Information Notice No. 87-61: Failure of Westinghouse W-2-Type Circuit Breaker Cell Switches
IN 87-61
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
December 7, 1987
Information Notice No. 87-61: FAILURE OF WESTINGHOUSE W-2-TYPE
CIRCUIT BREAKER CELL SWITCHES
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 resulting from the failure of Westinghouse W-2-type circuit breaker
cell switches. 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 October 16, 1987, Westinghouse Electric Corporation notified the Nuclear
Regulatory Commission (NRC) staff, pursuant to the requirements of 10 CFR
Part 21, of the failure of a modified Westinghouse W-2-type switch that was
being utilized as a circuit breaker cell switch. The failure was identified
at Indian Point Station, Unit 3, on May 15, 1987, while the reactor was at
cold shutdown for a scheduled refueling outage. The output breaker for
emergency diesel generator (EDG) No. 31 was prevented from re-energizing the
480-volt bus 2A after plant personnel had inadvertently de-energized 480-volt
buses 2A and 5A. Subsequent investigation by the licensee determined that an
erroneous input to the EDG logic system had prevented the EDG output breaker
from closing. The licensee identified deformation of the spring retainer in
the spring-return mechanism of the cell switch in the 52/2A breaker cell as
the root cause of the erroneous input.
The spring retainer is continuously under stress whenever the breaker is
racked in (which it is, except when the breaker is racked out for testing or
maintenance) and releases whenever the breaker is racked out. Its deformation
allowed a loss of spring tension that rendered the cell switch unable to
spring-return to the racked out position when normal supply breaker 52/2A was
racked out for maintenance. Thus, the EDG logic system received an erroneous
input indicating that breaker 52/2A was racked in and the main contacts were
still closed. This erroneous input prevented the EDG No. 31 output breaker
from closing automatically in response to a loss of power. During
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December 7, 1987
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subsequent inspections by Indian Point personnel, 35 of a total of 37 similar
W-2-type switch spring retainers in the 480-volt system breakers exhibited
some sign of deformation.
All the cell switches had been shipped to Indian Point in 1971-1972 and all
had been in service for close to 15 years. The licensee reported the results
of its investigation to the NRC in Licensee Event Report 87-009-00 on October
2, 1987.
Discussion:
Westinghouse has determined (1) that the deformation of the spring retainer in
the spring-return mechanism of the cell switches was related to the aging of
the component and (2) that the failure mechanism was the continuous stress it
experiences while the breaker is racked in. The W-2-type cell switches are
available as optional equipment for all Westinghouse DS switchgear cabinets.
Westinghouse has indicated that inspection or testing performed when the
breaker is racked out would determine if a failure has occurred. Where
inspections and/or testing have not been performed, the potential exists that
if the breaker is not racked in, a cell switch malfunction may prevent the
completion of safety-related functions dependent on cell switch indication of
the breaker being racked out or in the test position.
In the 10 CFR Part 21 notification submitted to the NRC, Westinghouse
recommends that proper cell switch operation be verified through periodic
inspections or testing, or whenever the breaker is racked out. Proper
operation of the spring retainer is only verifiable when the breaker is moved
from its racked in position. Visible inspection may be used to verify cell
switch operation. However, it will be necessary to move the breaker out on
the rails to observe whether the switch operating lever is in its proper
position (30 degrees off vertical for the W-2 cell switch). Any uncertainty
in this observation may be resolved by manually ensuring that the switch has
returned to the proper position. Persons performing this inspection should
use caution not to contact any energized terminals.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical
contact 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: K. R. Naidu, NRR
(301) 492-9656
Jaime Guillen, NRR
(301) 492-8933
Attachment: List of Recently Issued NRC Information Notices
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