What does the multimeter show if the line is grounded or shorted?
When the power is out, use the 10k range of the multimeter to disconnect the load and measure the line. In the case of three-phase four-wire, the resistance multimeter of the four lines should show infinity, and the single-phase line should also show infinity when the resistance is normal. If there is a resistance value, it can be determined that there is a short circuit in the line. To determine the grounding, use one test lead of a multimeter to touch one end of a line, and use another test lead to measure whether there is a change in resistance. If it is infinite, it is normal. Otherwise, there is a grounding phenomenon. Use this method to measure all the lines one by one to find the problem. Also note that if it is a steel tape cable, place the other end of the test lead on the steel tape for measurement. In order to ensure the accuracy of the measurement, it is best to first check whether there is an open circuit in the line, and then use a multimeter to check for short circuit and ground fault. In fact, to troubleshoot these two faults, you can also use megohmmeters, detectors and other tools to make maintenance faster and more accurate.
Take home appliance lighting as an example: Home appliance lighting has a live wire, a neutral wire, and a ground wire. If the live wire and the neutral wire are short-circuited, the circuit breaker will trip. At this time, turn off all electrical appliances and remove the two output terminals from the circuit breaker (the live wire and neutral terminal of the user line). Use a multimeter to measure the resistance of the two wires. is infinite. If the resistance is zero or only a few ohms, 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 250-speed AC: one test lead is connected to the air conditioner lead, and the other test lead is connected to the user lead. If there is a voltage of about 200 volts, it is proved Line ground.
In my country's low-voltage power system, it is stipulated that the neutral point must be grounded, the neutral wire cannot be used as a local line, and a special protective ground wire must be equipped. The live wire and neutral wire after entering the user's electrical box must be kept insulated from the earth in order to work properly. When the resistance of the live wire 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, you can use a multimeter to detect whether the line and load behind the circuit breaker are short-circuited or grounded. Methods as below:
Turn off the main power switch and disconnect the neutral wire from the power supply. (The neutral wire of some circuit breakers is not disconnected in the open position, and the neutral wire must be removed.) Then, use the maximum resistance setting of the multimeter (the range must be greater than 1M ohm) to measure the live wire and the neutral wire to the ground respectively. Resistor, if the resistance value is lower than 0.5M ohm, it will be judged as unqualified.
If the resistance of the live wire and neutral wire to the ground wire is qualified, it means it is not a ground fault. Then you can only use a multimeter to measure the resistance 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 wiring, it will be more troublesome. The problem lies with the electrical appliances. Check them one by one!
Regardless of whether it is a short circuit or a leakage, the circuit breaker will definitely trip. If the circuit breaker fails to trip due to a short circuit, the consequences will be disastrous, otherwise the little nest may be destroyed by a fire. When encountering such a situation at home, you must not close the switch again, but first check the fault. First, throw off all the corresponding loads of the tripped circuit breaker, and then disconnect all the neutral and live wires at the outlet end of the circuit breaker. Use a multimeter to measure whether there is a path between them. If the multimeter buzzes, the short circuit will make a sound. If you use a resistor to short-circuit, the resistance value will show zero ohms. If there is a short-circuit, look for the circuit.
Find all the joints and junction boxes in the line after this circuit breaker to see if the insulation of the joints is not good, and then check with a multimeter at each joint and junction box using the resistance measurement method. The reason is that a large current is generated at the moment of short circuit and the circuit breaker automatically trips. The circuit will not be burned much. Generally, the position of the short circuit can be determined by resistance measurement at the joint or junction box.
