Selection of setpoint for the degraded voltage relays at commercial nuclear power plants

Inappropriate setting of the degraded voltage relays at commercial nuclear power plants has subjected the class 1E loads to voltages below their operating range and caused spurious actuation of the undervoltage protection scheme as indicated in the referenced information notices. The authors present a methodology that can be successfully used to eliminate such concerns. A similar method has been used at Nine Mile Point Unit 2 (NMP2). This methodology is generic and can be effectively used in other nuclear power plants. Historically, electrical protection schemes do not use tolerances in accordance with the guidelines provided in Regulatory Guide 1.105. It is noted that application of these guidelines for selection of the degraded voltage relay setpoint and reset value will ensure adequate voltages for all class 1E loads and eliminate nuisance tripping