1. The voltage wiring circuit is open or poorly connected (the contact resistance is unlimited when the wiring is disconnected), when there is strong electromagnetic interference around the test. For example, the bus is charged. At this time, the live bus uses air as a medium, which will interfere with the capacitance of the tester. Due to the interference, the differential mode voltage of the two voltage test lines appears at both ends of the voltage acquisition line of the loop tester.

If the external interference is large, the loop resistance tester will display a value much larger than the resistance value of the tested product. At this time, if the tester has sufficient field test experience, the test result can be judged abnormal and the problem can be solved , Can attract attention and finally get the correct test results. However, if the tester is inexperienced, he may misjudge the resistance value of the switching circuit and may use power failure to deal with defects, which will cause unnecessary losses to future power generation. If the interference intensity is not great, the value displayed by the instrument is only within the qualified resistance range of the circuit breaker. This situation is the same as the result of "Poor voltage circuit contact", and may also lead to misjudgment.

2. The voltage wiring circuit is disconnected, the loop resistance tester is turned on, and there is no strong electric field interference at the test site. In this case, because there is basically no differential mode voltage input by the amplifier, the test parameters displayed by the instrument are close to zero. If the tester has sufficient field test experience, it can be determined 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. Otherwise, the tester may be mistaken for a problem and the test will be interrupted. The replacement or refurbishment of the instrument will delay the power-off time and bring unnecessary trouble to the test work.

3. The connection of the voltage circuit is poor. In most cases, after long-term operation, the terminal of the circuit breaker will produce an oxide film or oil film on the outer surface of the terminal block, and poor contact may occur. In addition, the voltage test clamp itself must produce a certain contact resistance, which will seriously affect the test results.