How do I use a multimeter to check if the circuit is a short or a ground?
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.
Firstly, it is necessary to separate the live and neutral wires.
Ground wire: Set the multimeter to the AC voltage range, and the range should be higher than 220V. Insert the red probe into the voltage port, do not insert the black probe, and then insert the red probe into one of the sockets to observe the reading.
The line with the highest reading is the live line, the line with the lowest reading is the neutral line, and the line with almost no movement is the ground line.
If two readings are small and one reading is large, it means that the ground wire is not grounded and the ground wire is also connected to the neutral wire. The second step behind does not need to be measured.
Set the multimeter to the "short circuit" test function (if not available, it can be used for resistance testing). Connect the red and black probes to the ground of the circuit and the ground of the mains respectively. If the test result shows a short circuit or extremely low resistance, the circuit is grounded. Otherwise, it is not.
Check for leakage and grounding, and set the multimeter to 200M. For example, when measuring equipment insulation, connect one end of the probe to the equipment casing or ground wire, and the other end of the probe to the circuit. When measuring insulation, hands should not touch the probe to prevent measurement errors.
Set the resistance of the multimeter to 20K or 200K, turn off the main power and load power, use a multimeter to connect one probe to the live wire and one probe to the ground wire, check the resistance value, then connect one probe to the neutral wire and one probe to the ground wire, and observe the resistance value twice. If there is a resistance value of 7.3 or above, or 14 or above, it indicates that the live wire or neutral wire connected to the multimeter with a resistance value is leaking.
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.
