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Information Notice No. 92-12: Effects of Cable Leakage Currents on Instrument Settings and Indications
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 February 10, 1992 NRC INFORMATION NOTICE 92-12: EFFECTS OF CABLE LEAKAGE CURRENTS ON INSTRUMENT SETTINGS AND INDICATIONS 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 safety problem that could result from inaccuracies introduced into safety-related instrument loops because of increased leakage currents from instrument cables when subjected to a harsh environment. 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 15, 1989, while reviewing instrument loop accuracies, test technicians of the Virginia Electric and Power Company (VEPCO), the licensee for the Surry Power Station, determined that during a harsh environmental condition, the leakage currents in cables could prevent performance of safety functions (Licensee Event Report 50-280/89-20). These potential failures would be caused by reduction in the cables' insulation resistance (IR) in the harsh environment. Such a harsh environment could be produced inside containment by a loss of coolant accident (LOCA) or by a high energy line break (HELB) event. The instrument cables installed at the Surry plant were environmentally qualified, but the previous safety system calculations for the accuracy of the instrument loops and for trip setpoints did not account for the additional uncertainties that could be introduced by the reduced IR values. In particular, the licensee noted that this phenomenon could potentially mask the reactor trip signal for steam generator low level and the safety injection signal for pressurizer low pressure and, thus, prevent the required protective actions. Furthermore, the licensee also noted that the pressurizer level and reactor coolant system wide range pressure instrument systems could be adversely affected by leakage currents. The licensee reviewed all safety-related instrument loops and replaced, where appropriate, affected cables in both units with new cables having a higher IR value. The licensee reviewed setpoint calculations and verified the margins 9202040079 . IN 92-12 February 10, 1992 Page 2 of 2 taking into consideration errors caused by cable leakage currents. The licensee also reviewed the loop accuracy calculations for indication loops and revised affected emergency operating procedures to address errors in indications caused by leakage currents. The NRC is aware that many licensees are revising instrument setpoints using the latest industry standards and are assessing the effects of leakage currents. However, since most licensees for operating plants may not have addressed these effects in their original design calculations, the problem described above for Surry may be generic. Discussion Under conditions of high humidity and temperature associated with either a LOCA or a HELB, the IR may decrease in components of the instrument loop such as cables, splices, connectors, terminal blocks, and containment penetrations. Consequently, leakage currents increase and measurement of process variables becomes more uncertain. In a normal environment, however, leakage currents are small enough to be essentially calibrated out of consideration. The instruments of a safety-related system provide monitoring and control to ensure the system will perform its intended safety function. The decreased IR of the instrument loop components may disable such monitoring and control. In June 1984, the NRC issued Information Notice (IN) 84-47, "Environmental Qualification Tests of Electrical Termination Blocks." In this information notice, the staff identified the potential for errors caused by leakage cur- rents at terminal blocks when these blocks are subjected to a harsh environment. 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 (NRR) project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical contacts: S. V. Athavale, NRR (301) 504-2974 Jerry L. Mauck, NRR (301) 504-3248 Attachment: List of Recently Issued NRC Information Notices .
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