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Principle test of loop resistance tester

time：2021/4/8   source：华天电力  reading：1130 time

The loop resistance tester adopts the four-terminal method to test. The high-frequency switching power supply generates 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. Together, the sampling circuit opens the end to the voltage input end. Sampling with the internal current shunt voltage, 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 finally sent to the displayer The current and resistance values measured this time are displayed.

The above principles work well in normal laboratory experiments, and when the current test loop is broken or has a bad touch, the instrument will judge the current loop to be open based on the voltage on the current shunt. However, when the voltage wiring circuit shows a bad touch or an open circuit, the tester will also show a value. At this moment, it will show the following conditions:

The voltage loop is open or the touch is bad, and there is no strong electric field disturbance at the test site. R1 indicates the resistance in the voltage line loop of the loop resistance tester. When the voltage line is disconnected, the resistance of R1 is infinite, and sometimes there will be an oxide film or an oil film on the wiring block of the circuit breaker.

When the loop resistance voltage test clamp is connected to such a wiring block, poor touch may occur. The test touch resistance is not infinite, nor is it small enough to be negligent (about the input resistance of voltage sampling). In this situation, the relationship between the data displayed by the loop resistance tester and the resistance value data of the practical sample is as follows:

Suppose the resistance of the test product is Rx, the input internal resistance of the voltage sampling loop is Ri, and the current sampling is not affected. The value displayed by the instrument = Rx*Ri/(R1 Ri), which is less obstructive than the practical test. If the resistance value Rx of the test product at this moment has exceeded the safe scale, the value displayed by the instrument may be within the qualified scale. , At this moment, it simply causes the experimenters to make a misjudgment and a safety hazard.

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