How to use a multimeter to check short circuit and grounding

　　Multimeters are generally used to test whether there is a short circuit in the electrical wiring at home or whether there will be leakage, or to test which wire has power.

　　If you want to check whether the line is short-circuited, first turn off the power of the line, then open each load switch, and use the ohmmeter of the multimeter to measure the resistance between the two wires. Under normal circumstances, the larger the resistance, the better. If you want to determine whether the line is grounded, you can use the ohmmeter of the multimeter to measure the resistance of each line to the ground. The larger the better. It should be pointed out that it is inaccurate to use a multimeter to measure whether the line is short-circuited and grounded. It should not be done. If the grounding or short-circuit resistance is very small, it can be detected by a multimeter. If the resistance is slightly larger, it cannot be detected by a multimeter. In a 380V low-voltage circuit, a 500V megohmmeter should be used for measurement, whether between lines or to the ground. It should be above 0.38 megohms. Otherwise, it is unqualified.

　　First, you need to separate the live wire and the neutral wire.

　　Ground wire: Set the multimeter to AC voltage, and the position should be higher than 220V. Insert the red test lead into the voltage hole, leave the black test lead unplugged, and then plug the red test lead into one of the sockets to observe the reading.

　　The one with the largest reading is the live wire, the one with the smaller reading is the neutral wire, and the one with basically no movement is the ground wire.

　　If there are two small readings and one large reading, it means that the ground wire is not grounded and the ground wire is also connected to the neutral wire. You do not need to measure in the second step.

　　Set the multimeter to the "short circuit" test function (if not available, you can set it to the resistance test), and connect the red and black test leads to the ground of the circuit and the ground of the AC power supply respectively. If the test result is a short circuit, or the resistance is very small, then the line is grounded, otherwise it is not.

　　Check leakage and grounding, and set the multimeter to 200M. For example, to measure the insulation of the equipment, connect one end of the test lead to the equipment housing or ground wire, and the other end of the test lead to the circuit. When measuring insulation, do not touch the test lead with your hand to prevent measurement errors.

　　Adjust the resistance range of the multimeter to 20K or 200K, turn off the main power supply and load power supply, connect one probe of the multimeter to the live wire and one probe to the ground wire, check the resistance, then connect one probe to the neutral wire and one probe to the ground wire, observe the resistance twice, if one of them is above 7.3, or above 14, it means that the live wire or neutral wire connected to the resistance is leaking.

　　Many electricians will tell you to cut off all power supplies, use the smallest resistance range (or diode range) to measure the resistance between the two wires to see if it is close to 0 ohms (or the diode range shows 0). If it is close to 0, it can basically be determined that the two wires are short-circuited together. Similarly, measure this line to the ground to see if the same situation occurs. If it proves that this line is grounded, this method is certainly feasible. It’s just that in actual work it is not convenient to cut off the power in many places. If the measurement is done without cutting off the power, is it feasible?

　　The essence of voltage is potential difference. As long as the voltage between two wires is 0, you can use the resistance range to measure:

　　1. Suppose you want to measure whether there is a short circuit between line A and line B. There may be a voltage between line A and line B relative to the neutral line (for example, 220 volts). The potentials on their lines are potential A and potential B respectively. Many people think of the first thing that if you use the resistance meter to measure directly, you must cut off the power supply on line A and line B respectively before measuring. This idea is not wrong, it can only be said to be too conservative.

　　2. Directly switch the multimeter to the AC voltage range and select the highest range, such as AC1000 volts, etc. 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 potential A and potential B are not equal, that is, there is a voltage difference between potential A and potential B, the two lines are not of equal potential, and the two lines are not short-circuited together.

　　3. If there is no voltage between line A and line B when measured using the AC voltage range, for the sake of safety, use the DC voltage range, such as the 1000 volt range, to measure between them and confirm that there is no DC voltage. This proves that potential A and potential B are equal. Note that being equal does not mean that they have no voltage to 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 volt. At this time, you can use the smallest resistance range to measure the resistance between the two lines. If it is close to 0 ohms, it means that the two lines are short-circuited together.

　　4. As for measuring whether it is grounded, you can also use the above method to measure it simply. The idea is to treat the ground wire as an ordinary wire. However, to measure whether it touches the ground, you can use an insulation resistance meter to measure the insulation resistance (usually 5 megohms). At this time, you need to turn off the power to measure.