How to use a multimeter to check if the circuit is short-circuited or grounded
If you want to check for a short circuit in the circuit. First, cut off the power supply to the line, then open the load switches and use the ohm range of the multimeter to measure the resistance between the two wires. Under normal circumstances, a higher resistance is better. If it is determined whether the circuit is grounded, the ohm range of the multimeter can be used. Measure the resistance of each circuit to ground. The bigger the better. It should be pointed out that using a multimeter to measure for short circuits and grounding in a circuit is inaccurate. It is also not appropriate. If the grounding or short-circuit resistance is very small, it can be detected with a multimeter. If the resistance is slightly larger, it should not be detected. A multimeter cannot detect it in a low voltage circuit of 380V. A 500V shaker should be used for measurement, whether between lines or to ground. All should be above 0.38 megaohms. Otherwise, it is unqualified.
The method of using a multimeter to detect short circuits in a circuit. Firstly, cut off the power supply of the tested line. Then remove one end of the control transformer, indicator light, and voltage transformer in the circuit, as these components are directly connected across the circuit. If not removed, the impedance of these components may affect your measurement. Some circuits also have three-phase loads such as frequency converters and rectifier bridges, and these devices need to be disconnected from the circuit. Then turn the multimeter to the diode mode (buzzer mode) and measure the conductivity of the two wires being tested
If the red and black probes of the multimeter overlap, and there is a buzzing sound or impedance displayed on the multimeter, it indicates that there is a problem with the tested circuit. Normally, there is no sound and the display screen of the multimeter does not show any numerical changes. The grounding detection method for the line is also the same. This is just a conventional detection method. To be foolproof, it is more reliable to use a shaking table for detection, which should be at least greater than 0.5 megohms.
The essence of voltage is potential difference. As long as the voltage between two lines is 0, it can be measured using resistance mode:
1. Assuming there is a short circuit between line A and line B to be measured, there may be a voltage (such as 220 volts) between line A and line B to the neutral line, and the potentials on their lines are potential A and potential B. Many people first think that if they measure directly in resistance mode, they must disconnect the power supply on line A and line B separately before measuring. This idea is not wrong, but rather conservative.
2. Directly set the multimeter to the AC voltage range, select the highest range, such as AC1000 volts, and then use the AC voltage range of the multimeter to measure line A and line B. If there is a relatively high voltage (such as 200 volts) between the two lines, it can prove that potential A and potential B are not equal, that is, there is a voltage difference between potential A and potential B. These two lines are not equipotential and are not short circuited together.
3. If there is no voltage measured between line A and line B using the AC voltage range, for the sake of assurance, a DC voltage range such as 1000 volts can be selected to measure between them, and it can be confirmed that there is no DC voltage. This can prove that the potential A and potential B are equal. Note that being equal does not mean that they have no voltage on the neutral line N. For example, both line A and line B have a voltage of 220 volts on the neutral line N, but the voltage between them is also 0 volts. At this time, the smallest resistance range can be used to measure the resistance between these two lines. If it is close to 0 ohms, it means that these two lines are short circuited together.
4. As for whether the measurement is grounded, the above method can also be used for simple measurement. The idea is to understand the ground wire as a regular wire. But in general, to measure whether it touches the ground, a megohmmeter can be used to measure the insulation resistance (usually 5 megohms for insulation), and at this time, a power outage is required for measurement.
