Insulation is a safety measure that uses a non-conductive substance to isolate or enclose a charged body to protect against electric shock. Good insulation is the most basic and reliable means for ensuring the safe operation of electrical equipment and circuits and preventing personal electric shocks.

Insulation can usually be divided into three categories: gas insulation, liquid insulation and solid insulation. In practical applications, solid insulation is still the most widely used and most reliable insulating material.

Under strong electric current, the insulating material may be broken down and lose its insulating properties. In the above three kinds of insulating materials, after the gas insulating material is broken down, once the external factor (strong electric field) is removed, the inherent electrical insulating property can be restored by itself; and after the solid insulating material is broken down, it is irreversibly completely lost. Its electrical insulation properties. Therefore, the insulation selection of electrical circuits and equipment must be compatible with the voltage level, and must be compatible with the use environment and operating conditions to ensure the safety of insulation.

In addition, due to corrosive gases, vapors, moisture, conductive dust, and mechanical operations, the insulation properties of insulating materials can be reduced or even destroyed. Moreover, the long-term effects of environmental factors such as sunlight and wind and rain can also cause the insulating material to age and gradually lose its insulating properties.

In summary, the main indicators that affect the performance of insulating materials are:

1. Insulation resistance and resistivity: The resistance is the reciprocal of the conductance, and the resistivity is the resistance per unit volume. The smaller the material conducts, the greater its resistance, and the reciprocal relationship between the two. For insulating materials, it is always desirable to have the resistivity as high as possible.

2. Relative dielectric constant and dielectric loss tangent: There are two uses for insulating materials: mutual insulation of various components of the electrical network and dielectric of the capacitor (storage energy). The former requires a relatively small dielectric constant, the latter requires a relatively large dielectric constant, and both require a small dielectric loss tangent, especially for insulating materials applied at high frequencies and high voltages. Insulation material with a small dielectric loss tangent.

3, breakdown voltage, electrical strength: In a strong electric field, the insulation material is destroyed, and the loss of insulation performance becomes conductive state, called breakdown. The voltage at the time of breakdown is called the breakdown voltage (dielectric strength). The electrical strength is the quotient of the voltage between the two electrodes when the breakdown occurs under the specified conditions and the voltage applied to the applied voltage, that is, the breakdown voltage per unit thickness. For an insulating material, generally, the higher the breakdown voltage and electrical strength, the better.

4. Tensile strength: The maximum tensile stress that the specimen is subjected to during the tensile test. It is the most widely used and representative test for the mechanical properties of insulating materials.

5. Combustibility: refers to the ability of an insulating material to resist burning when it contacts the flame or when it leaves the flame. As the application of insulating materials is expanding, the requirements for its resistance to combustion are more important, and various methods are used to improve and improve the flame resistance of insulating materials. The higher the resistance to combustion, the better its safety.

6. Arc resistance: The ability of an insulating material to withstand arcing along its surface under specified test conditions. During the test, an alternating current high-voltage small current is used to determine the arc resistance of the insulating material by the action of an arc generated by the high voltage between the two electrodes to form a conductive layer on the surface of the insulating material. The greater the time value, the better the arc resistance.

7. Sealing degree: It is better to seal the oil quality and water quality.

The four basic constants that affect the dielectric are:

Dielectric constant: refers to the role of transferring, storing, or recording electricity in an electrodeized manner.

Conductance: refers to the leakage current of a dielectric under the action of an electric field.

Dielectric loss: It is the loss of electrical energy in the dielectric under the action of an electric field.

Dielectric strength: refers to the destruction of dielectric under strong electric field.

Good dielectrics require easier polarization, higher dielectric constant and dielectric strength, lower conductance and dielectric loss. When the electric field strength exceeds a certain critical value, the medium changes from a dielectric state to a conductive state. This phenomenon is called destruction of dielectric strength, or breakdown of a medium. When dielectric breakdown occurs, the corresponding critical electric field strength is called the dielectric strength, or the breakdown electric field strength. Therefore, dielectric strength is a material used as insulation

A measure of the electrical strength of the body. It is defined as the maximum voltage per unit thickness that a sample is subjected to when it is broken down, expressed as volts per unit thickness; the greater the dielectric strength of a substance, the better its quality as an insulator. The following table lists the dielectric strength reference values ​​for common insulation materials for cables. The rubber is based on the original rubber.

Dielectric strength of commonly used insulation materials for cables (for reference only)

Material Dielectric strength (kV/cm) Material Dielectric strength (kV/cm)

Air 33 XLPE 250

Mica tape 180 low smoke halogen free 200

Polyester tape 1300 EPDM 300

PVC 200 CPE 220

PE 250 silicone rubber 300

The gas insulating material has high ionization field strength and breakdown field strength, and can quickly restore insulation performance after breakdown. It has good chemical stability, non-combustible, non-explosive, non-aging, non-corrosive, not easily decomposed by discharge, and has a larger specific heat capacity. , thermal conductivity, fluidity are good. Air is the most widely used gas insulating material. For example, between overhead conductors of AC and DC transmission lines, overhead conductors are insulated by air from the ground. High-voltage standard capacitors also use gas-insulated media. Early use of high-pressure nitrogen or carbon dioxide, and now sulphur hexafluoride (SF6). SF6 is also used in the manufacture of high voltage circuit breakers, metal-enclosed combination appliances, gas-insulated transmission pipeline cables and gas-insulated transformers.

When the insulating material we are talking about contains bubbles, the relative dielectric constant ε of the bubbles is small, so the electric field on the bubbles is higher after the voltage is applied. The anti-electrical strength of the bubble itself is much lower than that of the solid medium (Eb≈33kv/cm in general air), so first the bubble breaks down, causing other discharges (ionization), generating a large amount of heat, which easily causes the entire medium to break down; Due to the heat generation

, a relatively high internal stress is formed, and the material is also easily damaged by mechanical strength.


The relative dielectric constant of air is close to 1, and all solid insulating materials are greater than one. The intensity in an electric field is distributed according to the dielectric constant of various insulating materials and inversely proportional to the dielectric constant. Therefore, we hope that the dielectric constant of various insulating materials is equal to the dielectric constant of air 1, which is most desirable. Of course this is impossible. So only choose materials with the lowest dielectric constant (of course, consider other factors such as temperature resistance, insulation properties, insulation resistance and cost). However, if the dielectric constants of the two materials differ greatly, the field strength between them is extremely uneven, which will cause the field strength of materials with low dielectric constant (such as air) to be very large, and the adverse consequences of first breakdown. Therefore, the relative dielectric constant has no direct and inevitable relationship with the electrical properties of the medium.