The degree of short-circuit protection afforded small low-voltage A-C and D-C starters by means of fuses and circuit breakers

AIEE technical paper 42–79, entitled “Thermal Co-ordination of Motors, Control, and Their Branch Circuits on Power Supplies of 600 Volts and Less,”¹ was presented in April 1942. The purpose of the article as stated was “to show what the characteristics of the protective devices should be in order that the power will be disconnected from the main feeders under all conditions of excess current before excess temperatures are attained.” After the overload protecting requirements were discussed, it was pointed out that “to protect the conductors in the branch circuit adequately, the control devices, and the motor winding, we must use a device that is both fast in opening the circuit and also has the necessary interrupting ability.” In discussing the speed and the interrupting abilities of circuit breakers and fuses which were used to protect the motors and the controls against short circuits, the following statement was made: