What does the multimeter show when the line is grounded or shorted?
In the case of power failure, use the 10k gear of the multimeter to disconnect the load and measure the line. In the case of three-phase four-wire, the resistance value multimeter of the four lines should show infinity, and the single-phase line should also show infinity when it is normal. When there is resistance, it can be determined that there is a short circuit in the line. To determine the grounding, touch one end of a line with one test lead of the multimeter, and measure whether there is any resistance change to the ground with another test lead. If it is infinity, it is normal. Also note that if it is a steel strip cable, the other end of the test lead is placed on the steel strip for measurement. In order to ensure the accuracy of the measurement, it is best to check whether there is an open circuit in the line first, and then use a multimeter to check for short circuits and ground faults. In fact, to troubleshoot these two kinds of faults, you can also use megohmmeters, detectors and other tools to make the repairs 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, remove the two output wires from the circuit breaker, (the live wire and the neutral wire 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, it means that the live wire and the 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 level of 250: one test lead is connected to the live line of the air switch, and the other test lead is connected to the user line. If there is a voltage of about 200 volts, it can be proved Line ground.
In my country's low-voltage power system, it is stipulated that the neutral point must be grounded, and the neutral line cannot be used as a local line, and a special protective ground line must be equipped. After entering the user's electrical box, the live wire and neutral wire must be kept insulated from the earth to work normally. When the resistance of the live wire or the neutral wire to the ground wire is less than 0.5M ohms, the leakage protection action trips and cuts off the power supply.
After a trip occurs, a multimeter can be used to detect whether the lines and loads behind the circuit breaker are shorted or grounded. Methods as below:
Turn off the main switch of the power supply, and disconnect the neutral line from the power supply. (The neutral wire of some circuit breakers is not disconnected at the opening position, and the neutral wire must be removed). Then, use the maximum resistance range of the multimeter (the range must be greater than 1M ohms) to measure the ground wires of the live wire and the neutral wire respectively. Resistance, if the resistance value is lower than 0.5M ohms, it will be judged as unqualified.
If the resistance of the live wire and the neutral wire to the ground wire is qualified, it means that it is not a ground fault. Then you can only use a multimeter to measure the resistance value of the live wire and the neutral wire (all plugs must be unplugged and all electrical appliances must be turned off), and it must also be greater than 0.5M ohms. If there is no problem with the line, it will be more troublesome. The problem lies in the electrical appliances. Check them out one by one!
Regardless of whether it is a short circuit or a leakage circuit breaker, it will definitely trip. If the short circuit does not trip, the consequences will be unimaginable. Otherwise, the small nest may be solved by a fire. If you encounter such a situation in the family, you must not close the switch again, and first check the fault. First remove all the corresponding loads of the tripped circuit breaker, and then disconnect all the zero and live wires at the outgoing terminal of the circuit breaker. Use a multimeter to measure whether there is a path between them. Use a multimeter to buzz and the short circuit will make a sound. Use the resistance file to short circuit, and the resistance value will display zero ohms. If there is a short circuit, find the line.
Find all the joints and junction boxes in the line after the open circuit, whether the insulation of the joints is not done well, and then check with a multimeter at each joint and junction box by resistance measurement. The reason is that the circuit breaker automatically trips when a large current is generated at the moment of a short circuit, and the line will not be burned out. Generally, the location of the short circuit can be determined by measuring the resistance of the joint or junction box.
Then there is the inspection of line leakage by resistance method. Use a multimeter to connect one test lead to live wire or neutral wire and one test lead to ground. Normally, the resistance is infinite. If there is resistance or the resistance is very small, then the measured line exists. grounding phenomenon. The premise is to ensure that there is no short circuit in the line, otherwise it cannot be judged.
