How to measure the resistance of insulation resistance?

Resistance (R) hinders the current in the conductor. Resistance is measured in ohms. The Greek letter Ω is used to represent ohms. According to the magnitude of the resistance value, you can add a prefix before Ω to form a composite unit to express, such as kiloohms (kΩ), megaohms (MΩ) or milliohms (mΩ), microohms (μΩ). According to Ohm's law, resistance limits the current in a circuit, and the current is measured in amperes (A).
●Conductors and insulators
All materials have a certain resistance to current.
※conductor
Conductor refers to a material that has a small resistance and allows electrons to pass through easily. Most metals are good conductors, copper (Cu) is a common conductor; silver (Ag) is a better conductor than copper, but silver is too expensive for most applications; the conductivity of aluminum (Al) Not as good as copper, but lower cost, and lighter for high-voltage applications like overhead power lines. It is for this reason that aluminum is often used as a conductor. Conductors can be wires, power cords, or cables, and can be bare, insulated, or covered.
The factors that affect wire resistance include the cross-sectional area, length, material, and temperature of the wire. The larger the cross-sectional area, the smaller the resistance; the longer the wire length, the greater the resistance; the higher the temperature, the greater the resistance.
※Insulator
Insulators refer to materials with very high electrical resistance. Common insulating materials include rubber, plastic, air, glass, and paper. When the conductor insulation material is degraded by moisture and/or damaged by overheating, its resistance will drop.
All conductors must be protected from possible contact with other conductors, metal parts and personnel. The insulating layer of the wire can protect the wire from damage and isolate the electrical energy in the wire, but not all energized parts of the circuit are protected by an insulator.
When the energized part of the circuit is exposed, such as when the wire is connected to a fuse or a circuit breaker panel, the distance or air gap acts as an insulator. The greater the distance between the energized wire and the part, the higher the resistance; the higher the voltage, the larger the air gap required to establish resistance to prevent undesired electron flow (such as a deadly arc).
●Insulation resistance measurement
When measuring the integrity of an insulator, a meter specially designed for insulation resistance measurement must be used. Before starting the insulation resistance test, first perform basic voltage, current and resistance measurements. The type of device under test and the purpose of the insulation resistance test determine the required measurement items. The two basic insulation resistance measurements for insulating materials are insulation resistance measurement and leakage current measurement, and an insulation resistance value is finally generated.
The true condition of the insulator is determined by measuring the insulation resistance with an insulation resistance tester. The measurement is made between the wire that carries current when it is energized and other parts of the system where no current flows during normal operation. Regardless of the size of the circuit, system, or electrical load, wires are used to provide appropriate voltage, current, and power; the insulating layer on the wires is used to prevent current from flowing outside the design path. No insulator can completely prevent current from flowing through the insulator to the ground or other conductors. All insulators will pass a small amount of leakage current.
Generally speaking, the leakage current is very small and will not cause any faults and can be ignored; unless the leakage current reaches a certain level, it starts to cause electric shock, temperature rise or equipment damage. The higher the resistance of the insulator, the smaller the leakage current flowing through the insulator. When it is first put into use, the resistance of the insulator is correct.
Basically all insulators will degrade over time, resulting in a decrease in resistance. Factors such as humidity, extreme temperature, dust, dirt, oil, vibration, pollution, and other mechanical stress or damage can cause the degradation of the insulator. The total value of the insulation resistance depends on the size of the conductive leakage current and capacitive leakage current in the system.
※ Conductive leakage current

Conductive leakage current refers to a small amount of normal current flowing through the insulation layer of a wire. Conductive leakage current flows from the wire to the wire, or from the live wire to the ground. The conductive leakage current can be determined by using a formula according to Ohm's law, or a megohmmeter can be used to measure the leakage current. An increase in conductive leakage current will cause further degradation of the insulation layer, and the resistance of the wire insulation layer will decrease.
Keeping the insulating layer clean and dry can ensure leakage current.
※Capacitive leakage current
Capacitive leakage current refers to the current that passes through the insulating layer of the wire due to the capacitive effect. This effect occurs when two or more wires are arranged in the same tube.

Dielectric materials refer to materials with relatively low electrical conductivity, and insulators are often referred to as dielectric materials. Two wires that are very close together form a small capacitor, the insulating layer between the wires is the dielectric, and the conductor is the metal sheet.
In the DC circuit, since the capacitive leakage current only lasts for a few seconds and then stops, the wire carrying the DC voltage generally produces a small capacitive leakage current; the AC voltage will produce a continuous capacitive leakage current, but it can be arranged in the whole arrangement. Separate the wires in the pipeline to lower it.
※Surface leakage current
Surface leakage current refers to the current flowing from the area where the insulation layer is stripped off the wire for electrical connection. In the circuit, the wires are connected at different points using nut insulating heads, connectors, flat lugs, terminals and other connection fixing devices. The connection point where the insulating layer is stripped provides a low-resistance path for the surface leakage current, and dust and moisture will generate a larger surface leakage current.

The surface leakage current will increase the heat at the connection point, and the increase in heat will promote the degradation of the insulation layer, thereby making the wire fragile. Keeping all connections clean and tight will cause surface leakage currents. In 600V and below systems, the surface leakage current is yes. In medium voltage (1kV to 35kV) applications, the surface leakage current becomes an important factor.