Information Notice No. 93-65: Reactor Trips Caused by Breaker Testing with Fault Protection Bypassed
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
August 13, 1993
INFORMATION NOTICE 93-65: REACTOR TRIPS CAUSED BY BREAKER TESTING WITH
FAULT PROTECTION BYPASSED
All holders of operating licenses or construction permits for nuclear power
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to problems caused by high-voltage breaker testing
with inadequate fault protection. 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 December 31, 1992, Tennessee Valley Authority workers were conducting
differential relay phasing checks on a newly installed circuit breaker in the
Sequoyah Nuclear Plant 500-kV switchyard when a 20,000 ampere phase-to-ground
fault occurred in the breaker. Because all of the primary fault protection
relays had been bypassed for the duration of these checks, the fault continued
for about 1.5 seconds (90 cycles) before it was cleared by backup fault
protection. The faulted breaker connected the 500-kV switchyard through
inter-tie transformers to a 161-kV switchyard which provided power to both of
the Sequoyah units through station service transformers. The fault caused a
61-kV drop in the 161-kV switchyard, which cascaded through the on-site power
systems of both units, and affected the power to the reactor coolant pumps.
The resulting pump undervoltage signals caused reactor trips for both units.
The low voltage also affected the 6.9-kV shutdown boards, where it was sensed
by the under voltage relays. These actuated, causing the safety-related buses
to separate from the grid, and starting up and loading the emergency diesel
The new breaker is an ASEA Brown Boveri power circuit breaker which contains
sulfurhexafluoride (SF6) dielectric gas. This breaker is normally provided
with a 20-cycle time delay between opening and closing by a closing delay
relay. However, this delay relay was bypassed during the timing tests of the
breaker to improve the accuracy of the timing checks. Consequently this
allowed the breaker to rapidly cycle open and close, thus mechanically
damaging the breaker and ejecting particulate contamination into the
August 13, 1993
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dielectric gas. The electrical fault in the breaker is believed to have
resulted from breakdown of the dielectric gas due to contamination in the
The Sequoyah customer group personnel performed most of the vendor recommended
post-installation tests on the new breaker with the breaker de-energized.
However, the differential relay phasing checks had to be made with the breaker
energized. The switching orders, which had been developed by the licensee for
these checks, called for all primary protective relays to be disabled during
the phasing checks by opening their trip cutouts. This was done because the
licensee was concerned that operating the primary fault protection relays
before they could properly verify the phasing of the differential relays would
increase the chance of spurious trips. Had any of the primary protective
relays remained in service, the fault would have cleared in about 3.5 cycles
(instead of 90) and would most likely not have caused either unit to trip.
The licensee assumed that the necessary fault protection would be provided by
the backup protective relays. The operations personnel performing the tests
did not question the switching orders, since the practice of disabling the
protective relays was described as common practice by the licensee customer
group and they were not aware that an electrical fault under these conditions
could lead to the reactor trips.
The licensee strengthened its controls over customer group work activities
that could have a significant effect on safety. These included:
(1)Requiring evaluation and approval of non-routine work in the switchyard to
minimize the removal of protective devices and minimize the length of time
that these devices are out of service.
(2)Requiring approval by operations and maintenance managers in order to
energize switchyard equipment with any protective relay trips disabled.
(3)Requiring consultation with the manufacturer for testing beyond the
manufacturer recommended tests.
(4)Increasing the sensitivity of backup relay protection so as to maximize
protection while phasing is in progress.
August 13, 1993
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In order to complete the testing of the new breaker, the licensee changed the
test procedures to prevent a recurrence of the initial problems. Changes
1. Leaving the anti-pump relay in its normal configuration. The delay
induced by this relay was determined and subtracted from the total times
measured during the timing checks.
2. Leaving at least one of the primary fault protection relays in its normal
configuration. These relays were sequentially bypassed as necessary.
3. Reducing the time delay of the backup fault protection.
The testing problems discussed in this notice resulted in challenges to safety
systems. Both the NRC and the industry have recognized the need to minimize
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 contact listed below or the appropriate project manager
in the Office of Nuclear Reactor Regulation.
/s/'d by CIGrimes/for
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: George MacDonald, RII
Milton B. Shymlock, RII
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