(1) The voltage loop of the loop resistance tester (also called the switch loop resistance tester) is open, and there is no strong electric field interference at the test site. In this case, since the differential-mode voltage at the amplifier input is essentially 0, the test value displayed by the instrument is close to 0. If the tester has enough field test experience, it can be judged that it is an abnormal test line of the voltage circuit of the instrument. If the tester does not have enough field test experience, it may misjudge the tester's 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 of the loop resistance tester has poor contact. In most cases, the terminal of the circuit breaker will develop an oxide film or oil film on the outer surface of the terminal after long-term operation. When a voltage test clamp for loop resistance is connected to such a terminal strip, it can make poor contact, and even the voltage test clamp itself should make some contact resistance. When the contact resistance is equal to the internal resistance of the voltage sampling circuit, the test results will be seriously affected.
(3) The voltage loop of the loop resistance tester is open or in poor contact (when the open circuit is open, the visible contact resistance R1 is infinite), and there is strong electromagnetic interference at the test site, such as a live bus. At this time, the live busbar interferes with the two voltage test lines of the tester through the capacitor with air as the medium. Differential mode voltage appears at both ends of the voltage collection line of the loop tester due to the effect of interference.
If the interference is large, the loop resistance tester will display a much larger value than the resistance of the product under test. At this time, if the tester has enough experience in the field test, he can judge the abnormality of the test result, so as to attract everyone's attention, and finally get the correct test result, but if the tester does not have enough experience, he may misjudge the switch If the loop resistance value exceeds the standard, the defect may be dealt with by means of power outage maintenance, causing unnecessary losses to power production.
If the interference intensity is not very large, the instrument indicates that the value is just within the range of the qualified resistance of the circuit breaker, which is the same as the result of "(2) Poor contact of the voltage circuit", which will also lead to misjudgment.