By hvhipot 
Three common problems and solutions for loop resistance testers
1. The loop resistance tester usually tests the current, and the Z value of the micro-ohm value is displayed as “1”.
Cause: The voltage clamp is in the wrong position. The measured loop resistance value exceeds 2000. The voltage signal wire is disconnected or not opened.
Treatment: Process the voltage signal circuit, clamp and clamp the voltage clamp. When over-range: Use a multimeter to measure the voltage values of P1 and P2, and the resistance value = voltage value/current value.
2. Press the “test switch” of the loop resistance tester, the ammeter has no current, and the Z height of the miniature ohmmeter displays “1”.
Reason: The 100A current loop is not connected. The test switch is not closed.
Treatment: Check the test leads, reconnect, re-clamp, and then close the switch.
3. The loop resistance test is connected to a 220V AC power supply. The fan is not running, then press the “test switch”. Ammeters and miniature western instruments are not shown.
Reason: 220V AC power supply is unavailable, and the fuse is not installed or blown.
Treatment: check and exclude. (100A instrument insurance is not less than 6A, then disconnect from the factory to solve the problem. (Do not connect DC or 380V AC power)