What does the multimeter display when the circuit is grounded or short - circuited?
Measure the circuit in the case of power outage using the 10k gear of the multimeter and disconnect the load. For a three-phase four-wire circuit, the resistance value of the four wires displayed by the multimeter should be infinite. For a single-phase circuit, it should also display infinite resistance under normal circumstances. When there is a resistance value, it can be determined that there is a short circuit in the circuit. For the determination of grounding, use one test lead of the multimeter to touch one end of a wire, and use the other test lead to measure the resistance to the ground. If the resistance is infinite, it is normal; otherwise, there is a grounding phenomenon. Measure all the wires one by one using this method to find the problem. In addition, if it is a steel tape cable, place the other end of the test lead on the steel tape for measurement. To ensure the accuracy of the measurement, it is best to check whether there is an open circuit in the circuit first, and then use the multimeter to detect short circuits and grounding faults. In fact, to troubleshoot these two types of faults, tools such as a megohmmeter and a detector can also be used to make the maintenance faster and more accurate.
Take home appliance lighting as an example: Home appliance lighting usually consists of one live wire, one neutral wire, and one ground wire. If there is a short circuit between the live wire and the neutral wire, the air switch will trip. At this time, turn off all electrical appliances, remove the two output wire ends from the air switch (the live wire and neutral wire ends of the user line), and measure the resistance between the two wires using the ohm gear of the multimeter. The resistance should be infinite. If the resistance is zero or only a few ohms, it indicates a short circuit between the live wire and the neutral wire.
Turn on the air switch, do not install the two ends of the output wire, turn off all electrical appliances, and use the 250V AC gear of the multimeter to measure: Connect one test lead to the live wire end of the air switch and one test lead to the user wire end. If there is a voltage of around 220 volts, it proves that the circuit is grounded.
In China's low-voltage power system, it is specified that the neutral point must be grounded, and the neutral wire cannot be used as a ground wire. A dedicated protective ground wire must be provided separately. After entering the user's electrical box, the live wire and the neutral wire must be insulated from the ground to work properly. When the resistance between the live wire or the neutral wire and the ground wire is less than 0.5 megohms, the leakage protection will operate and trip to cut off the power.
After the trip occurs, a multimeter can be used to detect whether there is a short circuit or grounding in the circuit and load behind the circuit breaker. The method is as follows:
Disconnect the main power switch and disconnect the connection between the neutral wire and the power source. (For some circuit breakers, the neutral wire is not disconnected in the open position, and the neutral wire must be removed). Then, use the maximum resistance gear of the multimeter (the range must be greater than 1 megohm) to measure the resistance of the live wire and the neutral wire to the ground wire respectively. If the resistance is lower than 0.5 megohms, it is determined to be unqualified.
If the resistance of the live wire and the neutral wire to the ground wire is qualified, it indicates that there is no grounding fault. Then, use the multimeter to measure the resistance between the live wire and the neutral wire (all plugs must be pulled out and all electrical appliances must be turned off). Similarly, the resistance must be greater than 0.5 megohms. If there is no problem with the circuit, it will be more troublesome, and the problem lies in the electrical appliances. Check them one by one!
