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, 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 then sent to the display to display the measurement The current and resistance value. And when the current test loop is disconnected or poorly connected, the loop resistance tester will judge the current loop to be poorly connected or open according to the voltage on the current shunt.

1. Possible problems in the field test

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 Condition:

(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 test line of the voltage loop of the instrument is abnormal. After the abnormality of the test line of the voltage loop of the instrument is eliminated, the final correct test result can be obtained;

(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 wire clamp itself will also produce a certain contact resistance. When the contact resistance reaches the value equivalent to the internal resistance of the voltage sampling loop, it will have a serious impact on the test results.

(3) The voltage circuit is open or poorly contacted (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, due to interference, makes a differential mode voltage appear at both ends of the loop tester's voltage collection line.

2. Use the improved four-wire measurement method to solve the above problems

Based on the analysis of the above problems, the Hertz power loop resistance tester adopts power supply technology to test the sample through a large current constant current source, collects the value of voltage and current, calculates the resistance value of the sample, and overcomes the instantaneous current of the pulsed power supply. The disadvantages of the switch can effectively break down the oxide film of the switch contact, and solve the problems existing in the current traditional loop resistance tester.