By hvhipot 
Common problems in field test of loop resistance tester:
1. The voltage wiring circuit is open or poorly connected (when the wiring is open, the contact resistance is infinite). When there is strong electromagnetic interference around the test, such as the bus bar is charged, the charged bus bar passes through the air as the medium of the capacitor, which interferes with the tester. For the two voltage test lines, the differential mode voltage appears at both ends of the voltage acquisition line of the loop tester due to interference.
If the external 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 judge the abnormality of the test result and solve this problem. The problem can attract attention and finally get the correct test result. However, if the tester does not have sufficient experience, he may misjudge the resistance of the switch circuit to exceed the standard, and may use power outage repair methods to deal with the defects, causing unnecessary losses to future power production.
If the intensity of the interference is not too great, the loop resistance tester’s display value is just within the qualified resistance range of the circuit breaker. This situation is the same as the result of the situation “Poor voltage loop contact”, which will also cause misjudgment.
2. The voltage wiring loop is open, and the loop resistance tester is turned on. There is no strong electric field interference at the test site. In this case, since the differential mode voltage input by the amplifier basically does not exist, the test parameter displayed by the instrument is close to 0. If the tester has Sufficient field test experience can determine 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; otherwise, it may be misjudged that the loop resistance tester has a problem and is interrupted. Test, replace or repair the instrument, delay the power outage time, and bring unnecessary trouble to the test work.
3. The voltage circuit wiring is in poor contact. In most cases, the circuit breaker’s wiring terminals will have an oxide film or oil film on the outer surface of the terminal block after long-term operation. When the voltage test clamp of the loop resistance tester is connected to such a terminal block, it will Poor contact may occur, and 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.