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Information Notice No. 89-16: Excessive Voltage Drop in DC Systems
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 February 16, 1989 Information Notice No. 89-16: EXCESSIVE VOLTAGE DROP IN DC SYSTEMS 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 unexpected large voltage drops in cables between the batteries and circuit breakers. 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: Nine Mile Point Unit 1 was shut down in December 1987 for refueling. During this shutdown, the licensee made modifications to its 125-Vdc power system. A sub-sequent review of these modifications led the licensee to conclude that there was a voltage supply problem associated with the operation of battery charger (MG set) circuit breakers. The manufacturer of these circuit breakers stated that the minimum voltage for operation of the circuit breakers was 90 volts. The technical specifications require a minimum of 106 Vdc at the battery terminals for the batteries to be considered operable. However, studies showed that with 114 Vdc at the battery terminals, the calculated voltage at the breaker would be less than 90 Vdc because of line losses due to current demand during breaker closure. A battery voltage of less than 114 Vdc could exist at the time ac emergency power is required to be reconnected to the battery chargers. Thus, if a loss of site ac power were to occur, the dc voltage at the battery charger circuit breakers may not be adequate for the battery chargers to be reconnected to emergency ac power. This situation would cause the batteries to discharge. The decreasing dc voltage would lead to degraded dc control power and a subsequent trip of the emergency diesel generators. Therefore, a loss of normal ac power could lead to a loss of dc power and the subsequent loss of emergency ac power. Discussion: Problems with electrical systems at commercial power reactors have been identified with increasing frequency. These problems led to the issuance 8902100283 . IN 89-16 February 16, 1989 Page 2 of 2 of Generic Letter 88-15, "Electric Power Systems - Inadequate Control Over Design Processes," on September 12, 1988. The above-described event is a unique illustration of one of the types of problems presented in the generic letter. The Nine Mile Point Unit 1 licensee is not aware of any testing that could be used to establish the adequacy of the original design, nor has the licensee found documentation of the original design basis. The voltage drop could be attributed to an inadequate original estimate of load, hence cable sizing; load creep over the years without assessment of the consequences; or use today of a potentially more conservative power profile as determined by current practice for battery sizing. After identifying the problem, the licensee assessed the actual loads and plans to replace the undersized cable to improve the system's operability. No specific action or written response is required by this information notice. If you have any questions about this matter, please contact one of the technical contacts 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: J. Carter, NRR (301) 492-1194 J. Lazevnick, NRR (301) 492-0814 Attachment: List of Recently Issued NRC Information Notices . Attachment IN 89-16 February 16, 1989 Page 1 of 1 LIST OF RECENTLY ISSUED NRC INFORMATION NOTICES _____________________________________________________________________________ Information Date of Notice No._____Subject_______________________Issuance_______Issued to________ 89-15 Second Reactor Coolant Pump 2/16/89 All holders of OLs Shaft Failure at Crystal or CPs for nuclear River power reactors. 89-14 Inadequate Dedication 2/16/89 All holders of OLs Process for Commercial or CPs for nuclear Grade Components Which power reactors. Could Lead to Common Mode Failure of a Safety System 89-13 Alternative Waste Management 2/8/89 All holders of NRC Procedures in Case of Denial specific licenses. of Access to Low-Level Waste Disposal Sites 89-12 Dose Calibrator Quality 2/9/89 All NRC medical Control licensees. 89-11 Failure of DC Motor-Operated 2/2/89 All holders of OLs Valves to Develop Rated or CPs for nuclear Torque Because of Improper power reactors. Cable Sizing 89-10 Undetected Installation 1/27/89 All holders of OLs Errors In Main Steam Line or CPs for BWRs. Pipe Tunnel Differential Temperature-Sensing Elements at Boiling Water Reactors. 89-09 Credit for Control Rods 1/26/89 All holders of OLs Without Scram Capability or CPs for test and in the Calculation of the research reactors. Shutdown Margin 89-08 Pump Damage Caused by 1/26/89 All holders of OLs Low-Flow Operation or CPs for nuclear power reactors. _____________________________________________________________________________ OL = Operating License CP = Construction Permit ..
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