How to test whether the circuit is shorted 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 to be infinitely powerful. Here I will give you a little introduction to the usage of the multimeter and how to check whether the line is short-circuited or grounded, hoping to give some inspiration to electrician beginners.
1. The purpose of the multimeter
1. Use a multimeter to detect breaks in the wires.
Because the outer layer of the wire is covered with insulation, it is difficult to see the internal breakpoint. It is very troublesome to use a traditional multimeter to detect it. It needs to be blocked by electricity. It is time-consuming and easy to damage the wire. Now the digital one is much simpler, connect one end of the wire to the phase line, and leave the other end in the air. Hold the nib of the black test lead with one hand, and the red test lead with the other hand, and let the nib slowly move backwards from one end of the phase 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), it is the breakpoint of the wire that is 15CM back from here.
In this way, the problem can be solved by only destroying one point of the wire, isn't it very convenient? This method can also detect the breakpoint of the electric blanket.
2. Use a multimeter to detect short-circuit faults in the circuit.
When the wires in the home circuit are damaged or the circuit is aging and cause a short circuit between the live wire and the neutral wire, it is generally difficult to determine where the short circuit point is. At this time, it can be easily found with the resistance method of a multimeter. After the line is short-circuited, the main switch should be disconnected and all electrical appliances should be unplugged. Put the multimeter in the resistance position, and connect the two test leads to the live wire and the neutral 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 neutral wire segment by segment. If necessary, a segment of the wire can be cut off to determine the short circuit point.
2. How to use a multimeter to check whether the line is short circuited or grounded
It can be seen from the above that it is very convenient to use a multimeter to measure a short circuit, but it is not so accurate to measure a ground with a multimeter. In fact, it is most reasonable to use a shaker. The following will introduce how to use a multimeter to check whether a circuit is shorted or grounded.
Let me tell you about the 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 phases, and between phases and ground outside the normal operation of the power system; so the grounding of the phase line It can also be regarded as a kind of short circuit. If the neutral line is grounded, leakage current will be generated, and the leakage protector will trip. Therefore, the question of the subject is not very rigorous. Personally, I understand that he wants to ask how to detect short circuit and leakage of the line.
1. How to use a multimeter to detect the short circuit of the line
1) Turn off the power first, set the function switch of the multimeter to the buzzer position, and put the two pens of the multimeter on the two terminals to be tested. If there is a short circuit, there will be a buzzer and a small conduction voltage value will be displayed. At this time, there is a short circuit between the two measured points.
2) Use a multimeter to measure the line insulation to know whether the line is short-circuited. For example, when measuring single-phase insulation, if the insulation value is zero (metallic grounding) or very low (non-metallic grounding), it can be judged that the phase line is grounded. If not grounded, the insulation value is high. Then measure the interphase insulation. If the interphase insulation is zero, it means that there is a short circuit between the two phase lines.
3) To ensure that the line is not powered, use the resistance gear (the pointer meter is placed in the RX10 gear, and the digital meter has an on-off gear that will be called for a while), and touch the two meter sticks to the two points (or two lines) to be measured, and the pointer meter does not move. It is an open circuit, and it is a short circuit if it is thrown at full scale; the number of open circuit on the digital meter does not change, and there is no sound, and the short circuit will call, or the number is zero.
4) Separate the wire cores at the two ends of the wire without touching each other, then put the multimeter to the above position, and put the test leads on the wire ends of two different colors. If the measured value is 0.5M If the measured value is above 0.5 megohms, then the insulation of the line is unqualified and there is leakage. 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 short-circuit momentarily generates a large current and the circuit breaker automatically trips, and the line will not burn out much. In general, the location of the short-circuit can be determined by resistance measurement at the joint or junction box.
2. How to detect short circuit or ground
But I still answer the question according to how to detect the short circuit or grounding of the knife switch distribution line under the protection of no circuit breaker and leakage circuit breaker; (in fact, if there is a short circuit on the knife switch distribution line; the consequences Either the wire is blown or the knife switch is burned out, though; I still answer the question according to the detection principle).
(1) Turn off the power switch at the beginning of the power distribution line, disconnect all load switches on the line, including the plug-in load plugged into the socket, and use a multimeter resistance × 100 to measure the two outlets at the outlet end of the power switch. Resistance value, if the resistance value of the multimeter is measured to be very small (that is, the pointer swings almost to the right), it proves that there is a short circuit between the phase line and the neutral line, otherwise there is no short circuit. It is also the same detection method to measure whether there is a short circuit between the phase line and the protective ground (zero) line, and the neutral line to the protective ground (zero) line.
(2) If there is no short circuit between the phase line and the neutral line, the phase line to the protective ground (zero) line, and the neutral line to the protective ground (zero) line, you can detect whether there is a grounding phenomenon between the phase line and the neutral line.
Detecting Ground If you have a clamp meter on hand, it is best to use a clamp meter to detect ground current. The detection method is: first disconnect the power switch, remove the power distribution zero wire from the outlet of the power switch (and mark it), then close the power switch, and use a clamp meter to measure whether there is grounding current in the phase line (clamp Dial the meter to the 100A gear first, if the current cannot be measured, then slowly dial to the smaller current gear) If the ground current is still not detected; then the phase wire grounding can be ruled out. After detecting the phase wire, turn off the power switch to remove the phase wire, connect the neutral wire to the phase wire outlet contact of the power switch, close the power switch and use the above clamp meter to measure the phase wire to detect the neutral wire.
