How to check whether the line is short-circuited or grounded with a multimeter?
As the most common and practical instrument for electricians, the multimeter is simple to use, but if it is used well, it can be said that it is powerful. Here, I will tell you a little about the introduction usage of multimeter and how to check whether the line is short-circuited or grounded, hoping to give some inspiration to beginners of electrician.
The purpose of multimeter
1. Use a multimeter to detect the breakpoint in the wire.
Because the outer layer of the wire is wrapped with insulation, the internal breakpoints are hard to see. It is very troublesome to detect with the traditional multimeter, which needs to be blocked by electricity. It is time-consuming and easy to damage the wire when detecting one by one. Now the digital one is much simpler. Connect one end of the wire with the phase line, and the other end is suspended. Holding the nib of the black stylus in one hand and holding the red stylus in the other hand, let the nib slowly move backwards from one end of the phase connection wire along the insulation layer of the wire. When the voltage reading of the multimeter suddenly becomes smaller (equivalent to one tenth of the original reading), 15CM back from here is the breakpoint of the wire.
Is it convenient to solve the problem by destroying only one point of the wire in this way? This method can also detect the breakpoint of electric blanket.
2. Use a multimeter to detect the short circuit fault in the circuit.
When there is a short circuit between the live wire and the neutral wire due to broken wires or aging of the circuit in the home, it is generally difficult to determine where this short circuit point is. At this time, it can be easily found by using the resistance method of multimeter. After the line is short-circuited, turn off the main switch and unplug all electrical appliances. Put the multimeter in the resistance range, and connect the two probes to the live wire and the zero wire respectively. If the resistance value is zero or very small, it can be concluded that there is a short circuit. It is necessary to measure the resistance value between the live wire and the zero wire section by section, and if necessary, cut off a section of wire to determine the short circuit point.
How to check whether the line is short-circuited or grounded with multimeter?
It can be seen from the above that it is very convenient to measure the short circuit with a multimeter, but it is not so accurate to measure the grounding with a multimeter. In fact, it is most reasonable to use a shaking table. Let's introduce how to check the short circuit or grounding with a multimeter.
Let me tell you about short circuit first: in fact, this problem itself is problematic. We know that the short circuit of the line refers to the connection between phases and between phases and between phases and the ground outside the normal operation of the power system; Therefore, the grounding of the phase line is also a kind of short circuit. If the neutral line is grounded, it will produce leakage current and the leakage protector will trip, so the subject's question is not very rigorous. Personally, I understand that he wants to ask how to detect the short circuit and leakage of the line.
1. How to detect the short circuit of the line with a multimeter?
1) Power off first, turn the multimeter function switch to the buzzer position, and put the two probes of the multimeter on the two terminals to be tested. If there is a short circuit, there will be a buzzer and a small on-voltage value will be displayed. At this time, there will be a short circuit between the two tested points.
2) Using a multimeter to measure the line insulation can tell whether the line is short-circuited. For example, the measurement sheet is relatively insulated. If the insulation value is zero (metallic grounding) or very low (non-metallic grounding), it can be judged that the line in this phase is grounded. If it is not grounded, the insulation value is high. Measure the phase-to-phase insulation again. If the phase-to-phase insulation is zero, it means that there is a short circuit between the two-phase lines.
3) When there is no electricity in the line, use a resistance gear (RX10 gear is put in the pointer meter, and the on-off gear is called for a while in the digital meter), and touch the two meters (or two wires) to be measured. If the pointer meter does not move, it is an open circuit, and it is a short circuit. The number of digital meter open circuit has not changed, and there is no sound. Short circuit will call, or the number is zero.
4) Separate the wire cores at the two ends of the wire, and don't touch each other. Then, hit the multimeter to the above position, and put the probes on the wire ends of two different colors. If the measured value is above 0.5 megohm, or it shows infinity, there is no problem with the insulation of the line, that is, there is no leakage in the line; If the measured value is less than 0.5 megohm, then the line insulation is unqualified and there is leakage. Find out all the connectors and junction boxes in the line after this idle opening, whether the insulation of the connectors is not done well, and then use a multimeter to check each connector and junction box by resistance measurement. The reason is that the short circuit will generate a large current at the moment of short circuit, and the line will not burn out much. Generally, the short circuit position can be determined by resistance measurement in the joint or junction box.
2. How to detect short circuit or grounding?
But I still answer the question according to how to detect whether the distribution line of the knife switch is short-circuited or grounded without the protection of circuit breakers and leakage circuit breakers. (In fact, if there is a short circuit on the line, the power distribution of the knife switch; The consequence is either to burn out the conductor or to burn out the switch of the knife, however; I still answer questions according to the testing principle.
(1) Turn off the power switch at the beginning of the distribution line, disconnect all the load switches on the line, including the plug-in load plugged into the socket, and measure the resistance of the two outlets at the outlet end of the power switch with a multimeter resistance× 100. If the resistance of the multimeter is measured to be very small (that is, the pointer swings to the right almost to the end), it proves that there is a short circuit between the phase line and the zero line, otherwise there is no short circuit. It is also the same detection method to measure whether the phase line is short-circuited to the protective grounding (zero) line and the zero line is short-circuited.
(2) If there is no short circuit between the phase line and the zero line, between the phase line and the protective grounding (zero) line, and between the zero line and the protective grounding (zero) line, it can be detected whether there is grounding between the phase line and the zero line.
If you have a clamp meter at hand, it is best to use it to detect the grounding current. The detection method is as follows: firstly, disconnect the power switch, remove the distribution neutral line from the outlet end of the power switch (and mark it), then close the power switch, and use the clamp meter to measure whether the phase line has grounding current (if the clamp meter is set to the 100A gear first, if it can't measure current, then slowly set it to the smaller current gear), if it still can't measure grounding current; You can rule out the grounding of the phase line. After detecting the phase line, disconnect the power switch to remove the phase line, connect the zero line to the phase line outlet contact of the power switch, and close the power switch to detect the zero line by measuring the phase line with the above clamp meter.
