At present, most loop resistance testers on the market adopt a typical four-wire measurement method. The high-frequency switching power supply provides a test current greater than 100A. When the measurement button is pressed, the high-frequency switching power supply outputs a test current greater than 100A. At the same time, the sampling circuit starts sampling, the obtained signal is amplified by the amplifier, and the analog signal is converted into a digital signal by the A/D converter, and then the data is filtered, calculated and processed by the microprocessor, and finally sent to the display to display the measurement Current and resistance values. And when the current test circuit is disconnected or poorly connected, the instrument will judge the current circuit to be poorly connected or open according to the voltage on the current shunt.

1. 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 circuit 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 final correct test result can be obtained; if the tester does not have sufficient field test experience, it may be misjudged that the tester is faulty and interrupted 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. It may be possible 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 to interfere with the two parts of the tester. A voltage test line, due to interference, makes a differential mode voltage appear at both ends of the loop tester's voltage collection line.

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 determine the abnormality of the test result, which can attract attention and eventually Get the correct test result. 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 within the qualified resistance range of the circuit breaker, this situation is the same as the result of the situation "(2) Poor connection of the voltage circuit", which will also cause misjudgment.