How to use a multimeter to check short circuit, short circuit and leakage
Today I will bring you the essential skills of an electrician, using a multimeter to check for short circuits, short circuits and leakage. Commonly used instruments for electricians include multimeters, clamp ammeters, megohmmeters, ground resistance meters, electric bridges, etc. Among them, multimeters are the most widely used in daily maintenance of electricians. Multimeters have many functions, such as measuring current, resistance, voltage, diodes, transistors, capacitance, inductance, temperature, etc.
Check short circuit with multimeter
A "short circuit" means that the two ends of the power source are directly connected by a wire. Using a multimeter to check for a short circuit actually means measuring the resistance to determine whether the circuit is connected. Since the resistance range of the multimeter cannot be measured while the power is on, the resistance measurement must be carried out when the power is turned off.
Judgment method: First disconnect the power supply to ensure that there is no electricity, and then use the resistance range of the multimeter to measure both ends of the object being measured. Under normal circumstances, there is a certain resistance value. If the measured resistance is zero, it means that the object being measured is short-circuited. You can also use the buzzer mode or diode mode of the multimeter to measure both ends of the object under test when the power is off. If a sharp beep sounds, it means that the object under test has a short circuit.
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 measuring. For example, if you press the multimeter pen a little tighter, the measured result will become smaller; if you relax the test pen a little, the measured result will become larger. This is because the test leads and the wire under test are not connected properly, and the contact resistance of the connection point is also measured. In order to reduce measurement errors as much as possible and restore the authenticity of the results. Therefore, the test leads and the wires of the object under test must be tightly connected during measurement; if the wire under test is severely corroded or oxidized, the oxide layer on the surface of the conductor must be scraped off. (Under normal circumstances, copper is bright golden yellow, and after being oxidized, it turns dull black; the same is true for aluminum wires)
Check circuit break with multimeter
After you know how to use a multimeter to test for short circuit, testing for open circuit is very simple.
Judgment method: First disconnect the power supply to ensure that there is no power, and then use the resistance range 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 there is no connection at both ends of the line, that is, an open circuit has occurred. You can also use the buzzer or diode setting of a multimeter to test both ends of the line when the power is off. If there is a buzzing sound, it means that the line is connected; on the contrary, if there is no sound, it means that the line under test may be short-circuited.
Note: The resistance at both ends of the wire is zero (or close to zero) under normal circumstances; the electrical equipment has a certain resistance under normal circumstances, but the specific resistance depends on itself. Therefore, to determine whether the circuit or electrical equipment is open, 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, which cannot be measured with a small range.
Multimeter to measure leakage
First of all, we have to think about a question: What is the difference between equipment leakage and normal conditions? Under normal circumstances, the neutral line of electrical equipment is insulated and has no connection with the equipment shell. Once the insulation of electrical equipment is damaged, for example, the neutral wire or live wire is connected to the shell, then current will flow to the shell, making the shell electrified. Therefore, you can determine whether the electrical equipment is leaking by measuring whether the neutral wire or live wire is connected to the shell.
Judgment method: First disconnect the power supply to ensure that there is no electricity, set the gear to the maximum range of the resistance gear, and then use a multimeter to measure the resistance between the neutral 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 there is leakage.
Note: A multimeter for measuring leakage is only suitable for situations where the leakage is very serious, such as when the neutral wire or live wire touches the metal casing. Leakage caused by insufficient insulation strength cannot be detected, such as when the wire skin is aging but still has a certain insulation capability. Since the wire sheath 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 a more professional equipment---a megger (megger).
