Un-powered thermal memory protection for circuit breakers

Abstract

The National Fire Protection Association (NFPA) 70, National Electrical Code contains extensive information and requirements for overcurrent protection and temperature limitations of conductors. Thermal-magnetic trip units in circuit breakers provide protection for overloads and short circuit faults in electrical distribution systems. The magnetic element protects against short circuit faults and a bimetal element protects against overloaded thermal conditions. Electronic trip units use microprocessors, electronic devices and software algorithms to provide the equivalent bi-metallic response of thermal-magnetic trip units. If the electronic trip unit does not contain unpowered thermal memory circuitry the protection is then not equivalent to the bimetal trip unit.