The testing method of using a multimeter to check whether the circuit is short-circuited or grounded.
If you want to check whether the circuit is short-circuited, first cut off the power supply of the circuit, then open each load switch, and use the ohm gear of the multimeter to measure the resistance between every two wires. Under normal circumstances, the greater the resistance, the better. If you want to determine whether the circuit is grounded, you can also use the ohm gear of the multimeter to measure the resistance of each wire to the ground. Again, the greater the resistance, the better. It should be noted that using a multimeter to measure whether the circuit is short-circuited or grounded is inaccurate and inappropriate. If the grounding or short-circuit resistance is very small, it can be detected by a multimeter. However, if the resistance is slightly larger, the multimeter cannot detect it. In a 380V low-voltage circuit, a 500V insulation resistance tester (megger) should be used for measurement. Whether it is between the lines or to the ground, the resistance should be above 0.38 megohms. Otherwise, it is unqualified.
The method of using a multimeter to detect a short circuit in the circuit: First, cut off the power supply of the measured circuit. Then, remove one end of the control transformer, indicator light, and voltage transformer in the circuit, because these components are directly connected across the circuit. If not removed, their impedance may affect the measurement. In some circuits, there are also three-phase loads such as frequency converters and rectifier bridges, and these devices should also be disconnected from the circuit loop. Then, set the multimeter to the diode gear (buzzer gear), and connect the conductive parts of the two wires under test to the red and black test leads of the multimeter. At this time, if the multimeter beeps or shows impedance, it indicates that there is a problem with the measured circuit. Normally, there should be no beeping sound, and the display of the multimeter should have no digital change. The method of detecting a ground fault in the circuit is the same. This is just a conventional detection method. For foolproof results, it is more reliable to use an insulation resistance tester, and the resistance should be at least above 0.5 megohms.
1. Suppose you want to measure whether there is a short circuit between wire A and wire B. There may be a voltage between wire A, wire B and the neutral wire (for example, 220 volts). The electric potentials on these wires are potential A and potential B respectively. Many people may think that if you directly measure with the resistance gear, you must cut off the power supply on wire A and wire B respectively before measurement. This idea is not wrong, but it is too conservative.
2. Directly set the multimeter to the AC voltage gear and select the highest range, such as AC1000 volts, etc. Then use the AC voltage gear of the multimeter to measure wire A and wire B. If there is a relatively high voltage between the two wires (for example, 200 volts), it can be proved that potential A and potential B are not equal, that is, there is a voltage difference between potential A and potential B, and these two wires are not short-circuited together.
3. If there is no voltage when measuring between wire A and wire B with the AC voltage gear, for the sake of certainty, select the DC voltage gear, such as the 1000-volt gear, to measure between them and confirm that there is no DC voltage. This can prove that potential A and potential B are equal. Note that equality does not mean that there is no voltage between them and the neutral wire N. For example, both wire A and wire B have a voltage of 220 volts to the neutral wire N, but the voltage between them is 0 volts. At this time, you can use the minimum resistance gear to measure the resistance between these two wires. If it is close to 0 ohms, it means that these two wires are short-circuited together.
4. As for measuring whether it is grounded, you can simply use the above method for measurement, with the idea of treating the ground wire as an ordinary wire. However, generally, to measure whether it is in contact with the ground, an insulation resistance tester can be used to measure the insulation resistance (usually the insulation resistance is 5 megohms), and the power should be cut off during the measurement.
