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Information Notice No. 93-15: Failure to Verify the Continuity of Shunt Trip Attachment Contacts in Manual Safety Injection and Reactor Trip Switches
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 February 18, 1993 NRC INFORMATION NOTICE 93-15: FAILURE TO VERIFY THE CONTINUITY OF SHUNT TRIP ATTACHMENT CONTACTS IN MANUAL SAFETY INJECTION AND REACTOR TRIP SWITCHES 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 a surveillance test method that is not adequate to verify the operability of the contacts in manually operated safety injection and reactor trip 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 are not NRC requirements; therefore, no specific action or written response is required. Description of Circumstances On May 19, 1992, the Houston Lighting and Power Company, the licensee for the South Texas Project, issued a notification of an unusual event and began a technical specification-required shutdown of both units. The licensee had discovered that it was not conducting adequate testing of the switch contacts that directly actuate the shunt trip attachment of the reactor trip circuit breakers. These initiation contacts are actuated by either the manual safety injection or reactor trip switches and are the means by which operator action directly actuates the shunt trip, one of two means by which the breaker is tripped, the other being the undervoltage trip. The licensee had been performing surveillance tests of the safety injection switches and the reactor trip switches without opening the associated block switch (see attached simplified diagram of the manual reactor trip circuitry). If not opened, the block switch allows the same manual switch contacts that actuate the undervoltage attachment to indirectly actuate the shunt trip attachment. Therefore, the manual switch contacts which directly actuate the shunt trip attachment are not necessary for actuation of the shunt trip and, therefore, not independently verified as operable. This surveillance inadequacy has also been identified at the Callaway Plant, the Seabrook Nuclear Station, and the Wolf Creek Generating Station. The above mentioned licensees have since properly verified the operability of the manual switch contacts. 9302120058. IN 93-15 February 18, 1993 Page 2 of 3 Discussion There are two manually operated safety injection switches and two manually operated reactor trip switches. Each switch contains circuitry which is designed to initiate a reactor trip. The attached diagram illustrates the manner by which a reactor trip breaker is opened when any of the manual switches is engaged. When a switch is operated, moveable contacts in that switch change state. For example, when safety injection switch designated SI1 is engaged, the SI1 contacts in the 48 Vdc circuit open and SI1 contacts in the 125 Vdc circuit close. The opened 48 Vdc SI1 contacts interrupt power to the undervoltage trip coil (UV) of the undervoltage trip attachment. Concurrently, the closed SI1 contacts in the 125 Vdc circuit allow the shunt trip coil (STC) of the shunt trip attachment to become energized. De-energizing the UV causes the armature of the undervoltage trip attachment to make contact with and lift the trip lever of the associated reactor trip breaker. Energizing the STC causes the shunt trip attachment armature to also make contact with and lift the trip bar of the associated reactor trip breaker. As a result of either of these actions, the circuit breaker opens to initiate a reactor trip. Opening of the SI1 contacts in the 48 Vdc circuit also de-energizes the auto shunt trip relay (STA) causing the associated STA contacts in the 125 Vdc circuit to close. The closed STA contacts provide an electrical path by which the STC is energized, thus actuating the shunt trip attachment. This path is in parallel to the electrical path provided by the closed 125 Vdc SI1 contacts when the switches are actuated. The licensee corrected their procedure as follows: During continuity tests of the 125 Vdc SI1 contacts, the block switch that is shown in series with the STA contacts is held in the open position. This action assures that the voltage measured at the terminal block of the reactor trip breaker results solely from the closure of the 125 Vdc SI1 contacts and not from the closed STA contacts. If the "block switch" is not held in the open position, it is not possible to independently verify the continuity of the 125 Vdc SI1 contacts. Continuity checks of the 125 Vdc contacts in the other manually operated switches are made in the same way. Related Generic Communication On May 23, 1985, the NRC issued Generic Letter 85-09 which described an appropriate methodology for verifying the operability of moveable contacts in the manual switches. . IN 93-15 February 18, 1993 Page 3 of 3 This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact the technical contact listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager. ORIGINAL SIGNED BY Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contact: N. Fields, NRR (301) 504-1173 Attachments: 1. Auto/Manual Reactor Trip Circuit Typical Train 2. List of Recently Issued NRC Information Notices .
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