How to use a megohmmeter?

A megohmmeter is primarily used to measure the insulation performance of motors.

Use a 500V megohmmeter to measure the insulation resistance between the phases and the casing, as well as the conductivity between phases. The specific steps are as follows: (I) Preparatory work before use 1. Check whether the megohmmeter is functioning properly. Place the megohmmeter horizontally and turn its handle manually; the pointer should point to the correct position. At point o, slowly turn the handle to momentarily short-circuit the output wires of terminals L and E; the pointer should quickly point to zero. Be careful not to allow L and E to be short-circuited for too long while turning the handle, otherwise it will damage the megohmmeter. 2. Check the electrical equipment and circuits being tested to ensure that the power supply has been completely disconnected. It is absolutely not allowed to use a megohmmeter to make measurements on equipment and wires while they are under voltage. 3. Before making measurements, the equipment and wires should be discharged first, to prevent the discharge of their capacitance from posing a risk to personal safety or damaging the megohmmeter. This also helps to reduce measurement errors. Additionally, make sure to wipe the points to be tested clean. (II) Proper use: 1. The megohmmeter must be placed horizontally on a stable and solid surface, to avoid measurement errors caused by shaking or tilting during testing. 2. The wiring must be correct; the megohmmeter has three terminals: “E” (ground), “L” (line), and “G” (guard ring or shield terminal). The function of the guard ring is to eliminate the effects of leakage between the \"L\" and \"E\" terminals on the surface of the case, as well as leakage from the surface of the insulating material being tested. When measuring the insulation resistance of electrical equipment to ground, ‘L’ is connected to the part of the equipment to be tested using a single wire, while ‘E’ is connected to the equipment’s casing using a single wire ； When measuring the insulation resistance between two windings in an electrical device, connect “L” and “E” to the terminals of the respective windings ； When measuring the insulation resistance of a cable, in order to eliminate errors caused by surface leakage, the ‘L’ terminal is connected to the core, ‘E’ is connected to the outer shell, and ‘G’ is connected to the insulation layer between the core and the outer shell. “The connection wires between \"L\", \"E\", \"G\" and the device under test must be single wires with good insulation; they must not be twisted, and their surfaces must not come into contact with the device under test. 3. The rotation speed of the handle should be uniform, with a standard value of 120 revolutions per minute; a variation of ±20% is allowed, and it should not exceed ±25% in any case. Usually, it is necessary to shake it for a minute until the pointer stabilizes before taking a reading. If there is a capacitor in the circuit being tested, shake the meter continuously for a while to allow it to charge the capacitor; wait until the pointer stabilizes before taking a reading. After the measurement, disconnect the wires first, then stop shaking the meter. If it is detected during measurement that the pointer is at zero, stop turning the handle immediately.

Dude, based on your description, I think it’s not a grounding issue, so there’s no need for a megohmmeter. As you said, I have a 380V pump here; it trips just a few seconds after being turned on. This shows that there’s no problem with the power supply to the circuit. It’s likely an issue with the fuse. If there were a problem with grounding, the pump would trip right away as soon as it’s turned on, without waiting a few seconds.

The reply from upstairs is great; it’s indeed the case. We had a motor like that before, and it turned out that the release mechanism was too small

The last edit to this post was made by qugd on 2017-8-15 at 15:37. He instructed the poster on how to use the megohmmeter: wrap the two wires of the measuring electrodes around two fingers of one’s hand, and then shake the handle of the megohmmeter vigorously. You will feel as if you are that steadfast *member from those days. Give it a try; it’s really great.