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According to the statistics of the fault composite insulators, most of the defects take place at the high voltage (HV) end of the insulators. At present, the minimum defect length detected possibly at HV end of the insulators is about 7 cm by using electric field mapping device, which could not indicate the defects located between the last shed and the HV electrode. Therefore, it is important to improve the measuring precision that is suitable for indicating the defects less than 7 cm in inspecting the fault composite insulators based on electric field mapping device. In order to enhance the measuring precision of the device, we analyzed the electric field distribution along with an insulator by using the commercial software ANSYS. We found that a 5 cm defect can be found if we collect two to three electric field data between the two sheds. Therefore, we added a photoelectric cell array to trigger the device for collecting more data between the two sheds. The tests were conducted in our laboratory by using our new device. The results from our experiments show that the sensitivity of detecting the defects is increased and our new device can indicate the defects less than 5 cm at the HV end without grading rings.