Talking about the Common Problems of Field Test of Loop Resistance Tester

The principle of most loop resistance testers on the market is to use a typical four-wire measurement method, which provides a test current greater than 100A from a high-frequency switching power supply. When the measurement key is pressed, the high-frequency switching power supply outputs a test current greater than 100A. The sampling circuit begins to sample, 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, processed by the microprocessor, and finally sent to the display to show the measurement. Current and resistance values. And when the current test circuit breaks or the contact is bad, the instrument will judge the current circuit bad contact or open circuit according to the voltage on the current shunt. DC resistance quick tester adopts new power supply technology, which has the characteristics of rapid measurement, compact size, easy use, and high measurement accuracy. It is an ideal device for measuring DC resistance of transformer windings and high-power inductive devices.

1, the existing test method problems

The loop resistance tester designed in accordance with the conventional design principle found in field tests that there is a common problem: When the tester's voltage wiring loop is poorly connected or opens, the tester will also display a numerical value. Happening:

(1) When the voltage loop is open, there is no strong electric field interference at the test site. In this case, the differential voltage of the amplifier input is basically 0, so the test value displayed by the instrument is close to 0. If the tester has enough on-site test experience, It can be judged that the instrument voltage loop test line is abnormal. After the abnormality of the instrument voltage loop test line is eliminated, the final correct test result can be obtained; if the tester does not have enough on-site test experience, it may misjudge the tester to have a problem and interrupt. Testing, replacing or repairing the instrument delays the power outage and brings unnecessary trouble to the test.

(2) Poor contact of the voltage circuit. In most cases, after the long-term operation of the terminal block of the circuit breaker, an oxide film or an oil film will be generated on the outer surface of the terminal block. When the voltage test clamp of the circuit resistance meter is clamped to such a terminal block, If the contact is poor, the voltage test wire clamp itself must also generate a certain contact resistance. When the contact resistance value reaches 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 (visual contact resistance R1 is infinite when open). There is strong electromagnetic interference at the test site. If the bus is energized, the live bus passes the air-based capacitor and interferes with the tester. In the voltage test line, differential mode voltage appears across the voltage tester line of the loop tester due to interference.

If the interference is large, the loop resistance tester will show a much larger value than the resistance value of the test object. At this time, if the tester has enough on-site test experience, it may be able to determine the abnormality of the test result, attract attention and eventually Get the correct test result. However, if the tester does not have sufficient experience, it may misjudge the switch circuit resistance value to exceed the standard. It may use the method of power failure maintenance to deal with the defects and cause unnecessary losses to the power production.

If the intensity of the interference is not great, the instrument display value is exactly within the acceptable resistance range of the circuit breaker. This situation is the same as the result of “(2) Poor contact of the voltage circuit”, which also causes misjudgment.
 

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

Based on the analysis of the above issues, an improved four-wire measurement method is proposed. Using the latest power supply technology, a large current can be continuously output for a long time, which overcomes the drawbacks of the pulsed power supply's instantaneous current and can effectively break down the contact oxide film of the switch. There are problems with conventional loop resistance testers.

Through the large current constant current source to test the test sample, collect the voltage and current values, and calculate the resistance value of the sample. The value measured at this time must be the true resistance value of the test sample, eliminating the occurrence of false data. Even if there is a strong electric field interference in the site, since the resistance in the voltage loop is very small, the voltage at the voltage test terminal is sensed as a common-mode voltage. This product will reduce this AC common-mode interference to a very low value through capacitive grounding, and will not test. Accuracy affects.

The above shows that the automatic judgment and prompting function greatly improves the test accuracy and improves the production efficiency. It is a reliable basis for field test personnel to quickly solve problems.