Introduction to Judging the Insulation Resistance of Wires or Electrical Equipment with a Multimeter
The so-called wire leakage actually means that due to various reasons, the insulation resistance of the wire or electrical equipment fails to meet the normal working conditions, resulting in a very small resistance value between the insulated wire and the ground. In the case where the insulation resistance value is equal to zero for some wires, it is not simply a leakage problem, but is equivalent to a short circuit.
Here, I would like to popularize the concept of insulation resistance. The so-called insulation refers to using relatively non-conductive substances to isolate or wrap the charged body itself or charged bodies with different voltage levels from each other. The function of insulation is to ensure the safe operation of electrical equipment and power supply lines and prevent the occurrence of personal electric shock accidents.
The definition of insulation resistance is the resistance value presented by the insulation layer of electrical equipment under the action of a DC voltage. There is no absolutely "insulating" substance in the world. When a DC voltage is applied across an insulating substance, there will always be a current passing through the medium, just depending on the level of the voltage. The current in the medium can be further divided into leakage current (leakage current, polarization capacitive current).
In simple terms, the leakage grounding current of the wire is related to the moisture and aging of the insulating substance, etc. The magnitude of the leakage current is related to the applied voltage. The higher the voltage, the greater the leakage current. The volt-ampere curve between them is not a linear relationship.
Both digital multimeters and analog multimeters are commonly used auxiliary measuring tools for electricians. Strictly speaking, multimeters cannot measure the leakage value of wires. In fact, using a multimeter can only preliminarily determine whether the casing of household electrical equipment or motors is leaking electricity when they are powered on. At this time, the multimeter is equivalent to an electric test pen.
To determine whether there is leakage in wire circuits or electrical equipment, a megohmmeter should be used for measurement. Commonly used megohmmeters include those with ratings of 250V, 500V, and 1000V. As long as the megohmmeter is shaken at a speed of 120 revolutions per minute and the insulation resistance is greater than 0.5MΩ, the insulation resistance is considered basically qualified, and the electrical appliance can be used with confidence.
Don't think that a multimeter is all-purpose. Generally, the batteries used inside a multimeter are 1.5V and 9V stack batteries. It is simply impossible to measure the insulation resistance of the circuits or electrical equipment of single-phase 220V or three-phase 380V alternating current.
When the conditions do not allow, the high resistance range of the multimeter can be used to preliminarily judge the insulation resistance of wires or electrical equipment, but this method is not recommended. Using a multimeter to measure the grounding of a circuit or the grounding resistance is also just a preliminary judgment. To truly measure the grounding resistance, a single-arm or double-arm bridge should be used.
