Electrician's Skills

How to using a multimeter to check short circuit, short circuit and leakage

Common electrical instruments include multimeter, clamp ammeter, megohm meter, ground resistance meter, bridge, etc., among which multimeter is the most widely used in the daily maintenance of electricians. Multimeter has many functions, such as measuring current, resistance, voltage, diode, transistor, capacitance, inductance, temperature and etc,.

• Short-circuit checking by a multimeter ?

"Short circuit" means that the two ends of the power supply are directly connected by wires. Use a multimeter to check short circuit, that is measuring the value of resistance to check whether the wiring is breakover. Since the resistance test function of a multimeter cannot be worked with power on, the resistance must be measured without power.

Method: Disconnect the power supply to ensure that it is out of power, and then use the resistance test function of the multimeter to measure both ends of the V- and V+ ?to be measured. Usually ,there will be appear the value of resistance on multimeter, if the multimeter shows zero , or when using a multimeter buzzer or diode testing function to test two end of power supply, and there is a sharp beep, it means the parts of power supply have short circuit situation.?Note: If the measured resistance has a certain value but is very small, it is likely that a short circuit has occurred, which is caused by contact resistance. We sometimes encounter this situation when we measure. For example, if you press the multimeter pen a little tighter, the measured result will become smaller; if you loosen the test pen a little, the measured result will become larger. This is because the test lead and the wire under test are not connected properly, and the contact resistance of the connection point is also measured. In order to reduce the measurement error as much as possible and restore the authenticity of the results. Therefore, when measuring, the test lead and the wire of the measured object must be connected tightly; if the measured wire is corroded or oxidized seriously, the oxide layer on the surface of the conductor should be scraped off. (Copper is bright golden yellow under normal conditions, and becomes dull black when oxidized; the same is true for aluminum wires)

• Circuit breaker checking by a multimeter ?

Once you know how to use a multimeter to test for short circuits, it's easy to test for circuit breaker.

Method: first disconnect the power supply to ensure that there is no electricity, and then use the resistance file of the multimeter to measure both ends of the line. Under normal circumstances, the resistance is zero or has a certain resistance value. If the measured resistance is infinite, it means that the two ends of the line are blocked, that is, an open circuit has occurred. You can also use the buzzer or diode file of the multimeter to measure both ends of the line when the power is off. If there is a beep, it means that the circuit is connected; on the contrary, if there is no sound, it means that the circuit under test may be short-circuited. Note: Under normal circumstances, the resistance at both ends of the wire is zero (or close to zero); under normal circumstances, the electrical equipment has a certain resistance value, but the specific resistance value is related to itself. Therefore, to judge whether the circuit or electrical equipment is open circuit, it is best to use a resistor with a large range. Because the resistance of some electrical equipment is very large, such as energy-saving lamps, it cannot be measured with a small range.

• measure leakage checking by a multimeter ?

First of all, we have to think about a question, what is the difference between the equipment leakage and the normal state? Under normal circumstances, the zero live wire of the electrical equipment is insulated, and it is not connected to the equipment shell. Once the insulation of the electrical equipment is damaged, such as the neutral wire or the live wire is connected to the shell, then there will be current flowing to the shell, thus making the shell electrified. Therefore, to judge whether the electrical equipment is leaking, you can judge whether there is leakage by measuring whether the zero line or the live wire is connected to the shell.

Method: first disconnect the power supply to ensure that there is no electricity, set the gear position to the maximum range of the resistance range, and then use a multimeter to measure the resistance between the zero wire (or live wire) and the shell (or the ground wire connected to the shell). Under normal circumstances, the resistance value should be infinite. If a certain resistance value is measured, it means that there is leakage.Note: Leakage measurement with a multimeter is only suitable for very serious leakage situations, such as neutral or live wires connected to metal casings, etc. The leakage caused by insufficient insulation strength cannot be detected, such as the case where the wire skin is aging but still has a certain insulation capacity. Since the wire skin has a certain insulation strength, it is non-conductive at low voltage, but leakage will occur when the voltage is high. If you want to accurately measure whether the circuit or equipment is leaking, you need to use more professional equipment---megohmmeter (shake meter).

Summarize

From the above point of view, the multimeter to measure short circuit, open circuit, and leakage is actually a simple application of measuring resistance. As a beginner electrician, it is very good to know how to use a multimeter to measure voltage and resistance and be able to use it flexibly. As for the probability of using other functions of the multimeter is relatively small, if you can also master it, it will be the icing on the cake.