1. The DC voltage is open circuit, and the test site has no strong electric field interference. In this case, the test value displayed by the instrument is close to 0 because the differential mode voltage at the amplifier input is basically 0. If the tester has satisfactory field test experience, it can be judged that the voltage of the DC test line is abnormal. If the DC test line with abnormal voltage of the instrument is excluded, the correct test result can be obtained; if the tester does not meet the field test experience, the test may be misjudged. The problem of the instrument, suspend the test, replace or repair the instrument, delay the power outage time, and bring unnecessary trouble to the test work.

2. If the DC voltage is open circuit or bad contact, there is strong electromagnetic interference at the test site. If the bus is charged, the charged bus will interfere with the two voltage test lines of the tester through the capacitance of the air as the medium. Due to interference, differential mode voltage will appear at both ends of the DC tester's voltage collection line.

3. The voltage DC touch is not good. In most cases, after long-term operation of circuit breaker terminals, oxide film or oil film will appear on the outer surface of the terminal block. When connecting the voltage test clips of the DC resistance meter to such terminal blocks, poor contact may occur. A certain contact resistance not only occurs in the voltage test clamp itself, when the contact resistance reaches the same value as the DC internal resistance of the voltage sampling, it will have a serious impact on the test results.