Insulation is a safety measure that uses non-conductive materials to isolate or enclose live conductors to protect against electric shock. Good insulation is the most basic and reliable means of ensuring the safe operation of electrical equipment and lines and preventing electric shock accidents.
Insulation can generally be classified into three categories: gas insulation, liquid insulation, and solid insulation. In practical applications, solid insulation remains the most widely used and most reliable type of insulation material.
Under the influence of strong electric current, insulating materials may break down and lose their insulating properties. Among the three types of insulating materials mentioned above, gaseous insulating materials, once broken down, can recover their inherent electrical insulation properties after the external factor (strong electric field) is removed; however, solid insulating materials, once broken down, irreversibly and completely lose their electrical insulation properties. Therefore, the selection of insulation for electrical circuits and equipment must be matched with the voltage level and adapted to the operating environment and conditions to ensure the safe functioning of the insulation.
Furthermore, corrosive gases, vapors, moisture, conductive dust, and mechanical operations can all reduce or even destroy the insulation properties of insulating materials. Moreover, the long-term effects of environmental factors such as sunlight, wind, and rain can also cause insulating materials to age and gradually lose their insulating properties.
In summary, the main indicators affecting the performance of insulating materials are:
1. Insulation resistance and resistivity: Resistance is the reciprocal of conductance, and resistivity is the resistance per unit volume. The lower the conductivity of a material, the higher its resistance; the two are inversely related. For insulating materials, it is always desirable to have the highest possible resistivity.
2. Relative permittivity and dielectric loss tangent: Insulating materials have two main applications: mutual insulation between components of an electrical network and dielectric (energy storage) in capacitors. The former requires a low relative permittivity, while the latter requires a high relative permittivity. Both require a low dielectric loss tangent, especially for insulating materials used in high-frequency and high-voltage applications. To minimize dielectric loss, insulating materials with a low dielectric loss tangent are required.
3. Breakdown Voltage and Dielectric Strength: When an insulating material breaks down under a strong electric field, losing its insulating properties and becoming conductive, this is called breakdown. The voltage at which breakdown occurs is called the breakdown voltage (dielectric strength). Dielectric strength is the quotient of the voltage at which breakdown occurs under specified conditions and the distance between the two electrodes subjected to the applied voltage; in other words, it is the breakdown voltage per unit thickness. For insulating materials, generally, the higher the breakdown voltage and dielectric strength values, the better.
4. Tensile strength: This is the maximum tensile stress that a specimen can withstand during a tensile test. It is the most widely used and representative test for the mechanical properties of insulating materials.
5. Flame Resistance: This refers to the ability of insulating materials to resist combustion when in contact with a flame or to prevent further combustion when removed from a flame. With the increasing application of insulating materials, their flame resistance requirements are becoming more important. Various methods are used to improve and enhance the flame resistance of insulating materials. Higher flame resistance generally translates to better safety.
6. Arc Resistance: Under specified test conditions, the ability of an insulating material to withstand the action of an electric arc along its surface. The test uses a small current at high AC voltage. The arc generated between the two electrodes by the high voltage is used to determine the arc resistance of the insulating material. The longer the time value, the better the arc resistance.
7. Sealing: It provides good sealing and isolation against oil and water.
The four fundamental constants that affect dielectrics are:
Dielectric constant: refers to the electrical properties that transmit, store, or record electricity in an electrically polarized manner.
Electrical conductivity refers to the leakage current that exists in a dielectric under the influence of an electric field.
Dielectric loss: The loss of electrical energy in a dielectric material under the influence of an electric field.
Dielectric strength: refers to the potential for dielectric breakdown under a strong electric field.
Good dielectrics require easy polarization, high dielectric constant and dielectric strength, and low conductivity and dielectric loss. When the electric field strength exceeds a certain critical value, the dielectric changes from a dielectric state to a conductive state; this phenomenon is called dielectric breakdown. The critical electric field strength at which the dielectric breaks down is called dielectric strength, or breakdown electric field strength. Therefore, dielectric strength is a measure of the electrical withstand capability of a material as an insulator. It is defined as the maximum voltage that a unit thickness can withstand when the sample is broken down, expressed in volts per unit thickness; the higher the dielectric strength of a material, the better its quality as an insulator. The table below lists the reference values for the dielectric strength of commonly used cable insulation materials, where rubber is referenced using the virgin rubber.
Dielectric strength of commonly used cable insulation materials (for reference only)
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
Gas-insulating materials possess high ionization and breakdown field strengths, rapidly recovering their insulation properties after breakdown. They exhibit good chemical stability, being non-flammable, non-explosive, non-aging, non-corrosive, and resistant to decomposition by electrical discharge. Furthermore, they have high specific heat capacity and good thermal conductivity and fluidity. Air is the most widely used gas-insulating material. For example, air is used for insulation between overhead conductors in AC and DC transmission lines, and between overhead conductors and ground. High-voltage standard capacitors also use gas-insulating media; early versions used high-pressure nitrogen or carbon dioxide, but now sulfur hexafluoride (SF6) is more commonly used. SF6 is also used in the manufacture of high-voltage circuit breakers, metal-enclosed switchgear, gas-insulated transmission cables, and gas-insulated transformers.
When the insulating material we are discussing contains air bubbles, the relative permittivity ε of the bubbles is very small, resulting in a high electric field on the bubbles after applying voltage. Furthermore, the dielectric strength of the bubbles themselves is much lower than that of the solid dielectric (Eb≈33kV/cm for air), so the bubbles break down first, causing further discharges (ionization), generating a large amount of heat, which can easily lead to the breakdown of the entire dielectric. Because of the high internal stress generated during heat production, the material also easily loses its mechanical strength and is destroyed.
The relative permittivity of air is close to 1, while that of all solid insulating materials is greater than 1. The strength of an electric field is distributed according to the permittivity of various insulating materials, inversely proportional to the permittivity. Therefore, ideally, the permittivity of all insulating materials should be equal to that of air (1), which is impossible. Thus, we must choose materials with the lowest possible permittivity (while also considering other factors such as temperature resistance, insulation performance, insulation resistance, and cost). However, if the permittivity of two materials differs significantly, the electric field strength between them will be extremely uneven, resulting in a higher electric field strength for the material with the lower permittivity (such as air), leading to premature breakdown. Therefore, the relative permittivity has no direct and necessary relationship with the electrical properties of a dielectric.
