Analysis of Common Faults of Loop Resistance Tester

When testing loop resistance testers designed according to conventional design principles on site, it is found that there is a common problem: when the voltage wiring loop of the tester has poor contact or an open circuit, the tester will also display a value. At this time, the following situations will occur:

(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 of the amplifier input is basically 0, the test value displayed by the instrument is close to 0. If the tester has sufficient on-site testing experience, it can be determined that the instrument voltage loop test line is abnormal. After eliminating the abnormality of the instrument voltage loop test line, the final correct test result can be obtained; if the tester does not have sufficient on-site testing experience, it may misjudge that there is a problem with the tester, interrupt the test, replace or repair the instrument, delay the power outage time, and bring unnecessary trouble to the test work.

(2) Poor contact in the voltage circuit. In most cases, an oxide film or oil film will form on the outer surface of the circuit breaker terminal block after long-term operation. When the voltage test clamp of the loop resistance meter is connected to such a terminal block, poor contact may occur. The voltage test wire clamp itself will also produce a certain contact resistance. When the contact resistance value reaches the same value as the internal resistance value of the voltage sampling circuit, it will have a serious impact on the test results.

(3) The voltage loop is open or has poor contact (when open, the visible contact resistance R1 is infinite). There is strong electromagnetic interference at the test site. For example, if the bus is energized, the energized bus will interfere with the two voltage test lines of the tester through the capacitor with air as the medium. Due to the interference, differential mode voltage appears at both ends of the voltage collection line of the loop tester.

If the interference is large, the loop resistance tester will display a value much larger than the resistance of the test product. At this time, if the tester has sufficient on-site testing experience, he may be able to determine the abnormality of the test result, draw attention and finally obtain the correct test result. However, if the tester does not have enough experience, he may misjudge that the switch loop resistance value exceeds the standard, and may use the power outage maintenance method to deal with the defect, causing unnecessary losses to power production.

If the intensity of the interference is not very large, and the value displayed by the instrument is just within the acceptable resistance range of the circuit breaker, this situation will have the same result as the situation “(2) Poor contact of the voltage circuit” and will also cause a misjudgment.