The loop resistance tester designed according to the conventional design principle found that there is a common problem in the field test: when the loop resistance tester's voltage wiring circuit has poor contact or open circuit, the tester will also display a value, and the following will appear at this time Several situations:

(1) The voltage loop is open and there is no strong electric field interference at the test site. In this case, since the differential mode voltage input by the amplifier is basically 0, the test value displayed by the instrument is close to 0. If the tester has sufficient field test experience, it can be judged that the voltage loop test line of the instrument is abnormal. After the abnormality of the voltage loop test line of the instrument is eliminated, the final correct test result can be obtained. If the tester does not have sufficient field test experience, he may misjudge that the tester has a problem, interrupt the test, replace or repair the instrument, delay the power outage time, and bring unnecessary trouble to the test work.

(2) The voltage circuit is in poor contact. In most cases, the wiring terminals of the circuit breaker will have an oxide film or oil film on the outer surface of the terminal block after long-term operation. When the voltage test clamp of the loop resistance meter is connected to such a terminal block, poor contact may occur, and the voltage test line clamp itself also has a certain contact resistance. When the contact resistance value reaches the same value as the internal resistance value of the voltage sampling circuit, it will have a serious impact on the test result.

(3) The voltage circuit is open or poorly connected (the contact resistance R1 is infinite when the circuit is open), and there is strong electromagnetic interference at the test site. If the busbar is charged, the charged busbar will interfere with the two voltage test lines of the tester through a capacitor with air as the medium at this time. Due to the effect of interference, a differential mode voltage will appear at both ends of the voltage acquisition line of the loop tester; if the interference is large, it will Make the loop resistance tester display a value much larger than the resistance value of the tested product. At this time, if the tester has enough field test experience, he may be able to judge the abnormality of the test result, attract attention and finally get the correct test result, but If the tester does not have enough experience, he may misjudge that the resistance value of the switch circuit exceeds the standard, and may use power outage repair methods to deal with the defects, causing unnecessary losses to the power production; if the intensity of the interference is not very large, the value displayed by the instrument is just in the open circuit Within the qualified resistance range of the device, this situation has the same result as the situation "(2) Poor contact of voltage circuit", which will also cause misjudgment.