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PrintHome > NRC Library > Document Collections > Generic Communications > Information Notices > 1993 > IN 93-65 Information Notice No. 93-65: Reactor Trips Caused by Breaker Testing with Fault Protection Bypassed                                 UNITED STATES                         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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 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generators.  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 9308090187.                                                            IN 93-65                                                            August 13, 1993                                                            Page 2 of 3   dielectric gas.  The electrical fault in the breaker is believed to haveresulted from breakdown of the dielectric gas due to contamination in thebreaker.Discussion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.  .                                                                       IN 93-65                                                            August 13, 1993                                                            Page 3 of 3  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included: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such challenges.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                      (404) 331-5576                      Milton B. Shymlock, RII                      (404) 331-5596Attachment:List of Recently Issued NRC Information Notices.Page Last Reviewed/Updated Tuesday, November 12, 2013Home- News   Releases
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