How to use a multimeter to check whether the line is shorted or grounded?
If you want to check whether there is a short circuit in the line. First of all, the line should be powered off, and then each load switch should be opened, and the resistance between the two lines should be blocked by the ohm of the multimeter. Normally, the higher the resistance, the better. If you judge whether the line is grounded or not, you can use the ohm block of the multimeter. To measure the resistance of each line to ground. Also the bigger the better. It should be pointed out that it is not accurate to use a multimeter to measure whether there is a short circuit and grounding in the line. It should not be, if the grounding or short circuit resistance is very small, it can be detected with a multimeter, if the resistance is slightly larger. A multimeter cannot check it out, it is in the 380V low-voltage circuit. Measurements should be made with a 500V megger, either between lines or to ground. Both should be above 0.38 megohms. Otherwise it is unqualified.
First of all, you need to separate the live wire and neutral wire.
Ground wire: Turn the multimeter to the AC voltage range, and the range is higher than 220V. Insert the red test lead into the voltage hole, leave the black test lead unplugged, then insert the red test lead into one of the jacks of the socket, and observe the reading.
The one with the largest reading is the live wire, the one with the smaller reading is the zero wire, and the one with basically no moving reading is the ground wire.
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.
Turn the multimeter to the "short circuit" test function (if there is no resistance test), connect the red and black test leads to the ground of the circuit and the ground of the mains respectively. If the test result is a short circuit or the resistance is extremely small, the line is grounded, otherwise it is not.
Check the leakage and grounding, and turn the multimeter to 200M. For example, to measure the insulation of equipment, connect one end of the test lead to the equipment shell or the ground wire, and the other end of the test lead to the line. When measuring insulation, hands should not touch the test leads to prevent measurement errors.
Adjust the resistance of the multimeter to 20K or 200K, turn off the main power supply and the load power supply, use the multimeter to connect one test lead to the live wire and one test lead to the ground wire to check the resistance value, then connect one test lead to the neutral wire and one test lead to the ground wire, and watch the resistance value twice. If there is a resistance value above 7.3 or 14. above, it means that the live wire or the neutral wire connected to the resistance value is leaking.
Many electricians will tell you to cut off all the power and measure the resistance between the two wires with the smallest resistance gear (or diode gear) to see if it is close to 0 ohms (or the diode gear shows 0).
The essence of voltage is the potential difference. As long as the voltage between the two lines is 0, it can be measured using resistance files:
1. Suppose you want to measure whether there is a short circuit between line A and line B. There may be a voltage (for example, 220 volts) between line A and line B to the neutral line. The potentials on their lines are potential A and potential B respectively. Many people think at the first time that if you directly use resistance gear to measure, you must cut off the power supply on line A and line B before measuring. This idea is not wrong, but too conservative.
2. Directly switch 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 between the two lines (such as 200 volts), it can be proved that the potential A and the potential B are not equal, that is to say, there is a voltage difference between the potential A and the potential B. The two lines are not equipotential, and the two lines are not shorted together.
3. If there is no voltage between line A and line B using the AC voltage range, in order to ensure that there is no DC voltage between them, use a DC voltage range such as 1000 volts to confirm 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, line A and line B are both 220 volts to the neutral line N, but the voltage between them is also 0 volts. At this time, you can use the smallest resistance range to measure the resistance between these two lines. The two wires are shorted together.
4. As for measuring whether it is grounded, you can simply use the above method to measure it. The idea is to understand the ground wire as an ordinary wire. But in general, you can use a megger to measure the insulation resistance (5 megohms for general insulation) to measure whether it touches the ground. At this time, you need to cut off the power to measure.
