What does the multimeter show if the circuit is grounded or short-circuited?
In the event of a power outage, use a multimeter with the 10k gear to disconnect the load and measure the circuit. When there are three phases and four wires, the resistance values of the four lines should be displayed as infinite on the multimeter, and the normal value of a single-phase line should also be displayed as infinite. When there is resistance value, it can be determined that there is a short circuit in the line. To determine grounding, use a multimeter to touch one end of a wire with one probe, and measure for any resistance changes with the other probe. If the resistance value is infinite, it is normal. Otherwise, there is a grounding phenomenon. Use this method to measure all wires one by one to find the problem. Also, note that if it is a steel strip cable, place the other end of the probe on the steel strip for measurement. To ensure the accuracy of the measurement, it is best to first check whether there is an open circuit in the circuit, and then use a multimeter to check for short circuits and ground faults. In fact, to troubleshoot these two types of faults, tools such as megohmmeters and detectors can be used to make maintenance faster and more accurate.
Taking home appliance lighting as an example: Home appliance lighting is a live wire, a neutral wire, and a ground wire. If the live wire and neutral wire are short circuited, the circuit breaker will trip. At this time, all electrical appliances should be turned off, and the two output wire ends (the live wire and neutral wire ends of the user wire) should be removed from the circuit breaker. Use a multimeter to measure the resistance value of the two wires in the neutral mode, which should be infinite. If the resistance value is zero or only a few ohms, it means that the live wire and neutral wire are short circuited.
Turn on the circuit breaker, do not install the two ends of the output line, turn off all electrical appliances, and use a multimeter to measure the AC 250 level: connect one probe to the open circuit breaker end and the other probe to the user line end. If there is a voltage of about 200 volts, it proves that the circuit is grounded.
In China's low-voltage power system, it is stipulated 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 installed separately. After entering the user's electrical box, the live and neutral wires must be insulated from the ground in order to function properly. When the resistance of the live or neutral wire to the ground wire is less than 0.5M ohms, the leakage protection will trip and cut off the power supply.
After a trip occurs, a multimeter can be used to detect whether the circuit and load behind the circuit breaker are short circuited or grounded. The method is as follows:
Disconnect the main power switch and disconnect the neutral wire from the power supply. (Some circuit breakers have a neutral wire that is continuously open in the open position and must be removed), then use the maximum resistance range of a multimeter (the range must be greater than 1M ohms) to measure the resistance of the live wire and neutral wire to the ground wire separately. If the resistance value is less than 0.5M ohms, it is judged as unqualified.
If the resistance of the live and neutral wires to the ground wire is qualified, it indicates that it is not a ground fault. Then only a multimeter can be used to measure the resistance values of the live and neutral wires (all plugs must be unplugged and all electrical appliances must be turned off), which must also be greater than 0.5M ohms. If there is no problem with the circuit, it will be more troublesome, and the problem lies with the electrical appliances.
