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Status Indication of Control Power for Circuit Breakers Used In Safety-Related Applications
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 November 28, 1991 NRC INFORMATION NOTICE 91-78: STATUS INDICATION OF CONTROL POWER FOR CIRCUIT BREAKERS USED IN SAFETY-RELATED APPLICATIONS 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 with control logic circuitry for circuit breakers which have fuses for closing coils or closing actuating relays that are wired separately from the remainder of the control circuitry. This separate circuitry allows blown fuses or improperly installed fuses to remain unnoticed because there would be no alarm or loss of local or control room indication. 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 23, 1991, personnel at the Palisades Nuclear Power Station were performing routine surveillance testing on the containment spray pumps. The "C" containment spray pump failed to start locally on two attempts. Personnel confirmed that the control power lights, both in the control room and locally at the circuit breaker, were lit. An auxiliary operator removed the closing coil fuses, found them to be adequate, and reinstalled them. On the third attempt, the pump started (LER 50-255/91-016). The fuse holder fingers, which connect the fuses to the circuit, had become sufficiently deformed such that poor or no contact was made in the closing coil circuit. The NRC questioned how control power lights could be energized with no power available to the closing coil circuit. In addition, the NRC inspector questioned the "as found" operability of the pump (NRC Inspection Report 50-255/91012). The licensee and the inspector reviewed wiring schematic 9111220068 . IN 91-78 November 28, 1991 Page 2 of 3 diagrams and found that the closing coil circuits were wired and fused sepa- rately from the remainder of the control power circuits for most of the plant's 2400 volt and 4160 volt circuit breakers. Thus, the closing coil fuses could be blown, removed, or improperly installed, and there would be no indication or alarm that the breaker could not be closed when needed. At Palisades, this situation existed for both the emergency diesel generator (EDG) tie breakers, the breakers that supply power to several station power transformers, and to the following pumps: primary coolant, condensate, service water, auxiliary feedwater, component cooling water, low pressure safety injection, high pressure safety injection, and containment spray. The licensee decided to change this wiring arrangement for all the safety-related circuit breakers above during the next refueling outage. The licensee has scheduled changes of the wiring arrangement for the non-safety-related circuit breakers during the following refueling outage. This lack of indication of a loss of control power (closing coil power) could prevent personnel from recognizing when required safety equipment becomes inoperable. Under these conditions, the equipment may remain inoperable for an extended period until its condition is discovered through testing or initiation of an actuation signal (NRC Inspection Reports 50-255/91017 and 50-255/91020). Figures 1 and 2 are simplified schematic diagrams of control circuits for circuit breakers that illustrate these circumstances. These diagrams represent circuit breakers used at Palisades in 2400 and 4160 volt applications and at the Ft. Calhoun Station in 4160 and 480 volt applications. Discussion The common purpose of a circuit breaker is to open an energized circuit for a piece of equipment upon a fault condition. Designers sometimes ensure this action by separately fusing the trip coil circuit in the circuit breaker so the circuit breaker can always be tripped (to open the circuit) even if a malfunction occurs in the control circuit that energizes the closing circuit. However, when the function of the equipment is important, designers also provide a means to indicate a loss of control power to the closing circuit. The circuitry providing this indication also ensures that if the fuse is blown, improperly installed, or even removed, then either the lights indicating the breaker position are no longer illuminated or an alarm is energized. . IN 91-78 November 28, 1991 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 one of the technical contacts listed below or the appropriate Office of Nuclear Reactor Regulation project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical contacts: Edward R. Schweibinz, RIII (708) 790-5542 Vern Hodge, NRR (301) 492-1861 Attachments: 1. Figures 1 and 2. Typical Simplified Electrically Operated Circuit Breaker Control Circuit at Palisades and Ft. Calhoun 2. List of Recently Issued NRC Information Notices .
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