But in the engineering application,this paper finds that the thermal inertia of main bimetal heat expansion of thermal relay,and the inertia causes of action agencies sending signals will result in phase break protection to lack enough sensitivity and slow response to fault current.

Proper selection and use of thermal relay are over expounded from six aspects.

This paper analyses the matters needing attention in selection and usage of thermal relay, and expounds the importance of rational selection and usage of thermal relay.

A heat relay is designed on the basis of single chip PIC16F876.

This paper introduces the selection of the heat relay s type and current its reset mode, points out that connection wire section of rated current of heat relay should be selected according to strict rules it can not take as the protective device of wire short circuit puts forward working environment and temperature of heat relay and matters needing attention of reset after operation.

The motors are effectively protected and the problems of the motor burnout are settled by the motor protector-thermorelay which is rationally selected,auurately adjusted,correctly maintained and regularly tested.

Through theoretical analysis and calculation current change relations of three common open-phase of a three-phase asynchonous motor are derived in the paper and discusses protective action of bimetallic trip thermorelay in open-phase running of three-phase asynchronous motor according to its basis.

This paper compares and analyzes the protection performance of thermorelay and intelligent integrated monitor,and introduces the practical application and application effect of intelligent integrated monitor in the motor protection of industrial enterprise.