Deteriorated insulators among strings potentially threaten the safety of power grid. Consequently

taking porcelain insulators as objectives

and utilizing infrared thermograph

we studied the influences of defective insulators of different impedance located on different positions of the strings under various relative humidity on detection. The thermal statics of insulator strings were subtracted and then plotted

heating patterns of defective strings were studied

and the simulation of the above was made by a finite element analysis software Comsol. The experimental results show that the deteriorated insulators at the high voltage position of the string are more easy to be detected

and deteriorated insulators of large impedance on zero voltage position or middle of the strings may lead to false examining. The relationship between temperature rise and its impedance is nonlinear

and the rise reaches its maximum when the impedance is 30 M?. Under Icontamination level

temperature rise will reach maximum when relative humidity variesfrom 70% to 80%

and temperature rise of the deteriorated insulators will decrease when relative humidity ascends to 90%

which may cause error in detecting.