The Dangers of Bypassing Thermal Overload Relays in Nuclear Power Plant Motor Operated Valve Circuits

A situation may exist where a control circuit modification designed to enhance safety and reliability may actually jeopardize the integrity of the very system it is trying to protect. One such situation involves ac and dc Motor Operated Valve circuits in nuclear power plants which have been designed with their thermal overload protection bypassed or altogether eliminated. The purpose of this paper is to: (a) Point out the dangers of this design philosophy. (b) Show that the thermal overload relay can play an important part in improving overall nuclear plant safety. (c) Provide recommendations on the selection of thermal overload relays.