Problems with existing test methods

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

(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 instrument voltage loop test line is abnormal. After the instrument voltage loop test line abnormality is eliminated, the correct test result can be obtained; if the tester does not have sufficient field test experience, it may misjudge that the tester has a problem and 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. This may happen when the voltage test clamp of the loop resistance meter is connected to such a terminal block. In case of poor contact, both the voltage test line clamp itself will also produce a certain contact resistance. When the contact resistance reaches the same value as the internal resistance of the voltage sampling loop, it will have a serious impact on the test results.

(3) The voltage circuit is open circuit or poor contact (the contact resistance R1 is infinite when the circuit is open), and there is strong electromagnetic interference at the test site, such as the busbar is charged. At this time, the charged busbar passes through the capacitor with air as the medium and interferes with the two parts of the tester. A voltage test line has a differential mode voltage at both ends of the voltage collection line of the loop tester due to interference.

If the 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, he may be able to judge the abnormality of the test result, which can attract attention and obtain Correct test results. However, if the tester does not have enough experience, he may misjudge the resistance value of the switch circuit to exceed the standard, and may use the power outage repair method to deal with the defect, causing unnecessary losses to the power production.

If the intensity of the interference is not too great, and the value displayed by the instrument is just within the qualified resistance range of the circuit breaker, this situation is the same as the result produced by the situation "(2) Poor contact of the voltage circuit", which will also cause misjudgment.