Information Notice No. 92-12: Effects of Cable Leakage Currents on Instrument Settings and Indications

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                              February 10, 1992

                               INSTRUMENT SETTINGS AND INDICATIONS


All holders of operating licenses or construction permits for nuclear power 


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 


                                                       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. 


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 

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 

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 

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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