1. To have good insulation performance, so that in dry and rainy conditions, can withstand the standard stipulated pressure;

2. insulators bear not only the vertical and horizontal load of the wire, but also the additional load such as wind pressure and ice coating on the wire, so it is required that insulators must have sufficient mechanical strength;

3. The overhead line is in the field, which is greatly affected by the ambient temperature. It is required that the insulator can withstand large temperature changes without breaking;

4. Insulators are subjected to high voltage and mechanical force for a long time, which requires slow aging rate of insulation performance and long service life;

5. Corrosive gases in the air will degrade the insulation performance of insulators, so insulators should have sufficient anti-pollution and anti-erosion ability of chemical gases.

Installation of insulators

1. Before installing the insulator, the porcelain parts and flange should be checked to be complete without cracks, and the filler at the bonding place should be complete and firmly combined.

2. When pillar insulators and wall bushing are installed, their bases or flanges shall not be buried in concrete or plastering layers.

3. Cushion washers of rubber or asbestos paper with thickness not less than 1.5mm shall be used between the contact surface of low-pressure pillar insulators without base and top cap.

4. When the wall casing with rated current of 1500A or above is directly fixed on the steel plate, no closed magnetic circuit should be formed around the casing.

5. When the wall casing is installed vertically, the flange should be upward; when it is installed horizontally, the flange should be outside.

6. The aperture of the installation casing shall be more than 5mm larger than the embedded part, and the maximum thickness of the concrete installation plate shall not exceed 50mm.

Metal splints (except fasteners) at the end of wall bushing for 7.600A and above bus shall be of non-magnetic material. There should be a metal connection between it and the bus, and the contact should be stable. The thickness of the metal splint should not be less than 3mm. When the bus is two or more, the pieces of the bus should be fixed.

8. When the oil filling casing is installed horizontally, the oil storage tank and oil sampling pipeline should be free from leakage, and the oil level indication is clear. The position of oil injection and sampling valve should be installed on the mobile monitoring side, and the oil injected into the casing must be qualified.

9. Casing grounding terminals and unused voltage extraction terminals should be grounded reliably.

Damage to insulators

Insulator damage is caused by:

1. Artificial damage, such as wounding, crushing, etc.

2. The installation does not meet the requirements, or the stress under exceeds the allowable value.

3. Due to sudden cold and hot climate, the internal stress of electric porcelain is generated, or it is broken by hail and other injuries.

4. Pollution flash accident due to dirt, or surface discharge phenomenon (flashover) in rain, snow or thunderstorm days and damage.

5. When operating under overvoltage, the insulation strength and mechanical strength are not enough, or the insulator itself is of poor quality.

Inspection of insulators

Cracks in insulators can be inspected either on patrol or during power outage. Methods of inspection are as follows:

1. Visual observation. The obvious cracks of insulators can be found by naked eye during line patrol.

2. Telescope observation. Closer inspection with the aid of a telescope usually reveals less obvious cracks.

3. Acoustic judgment. If the insulator has abnormal discharge sound, you can judge whether it is damaged and the extent of damage according to the sound.

4. In case of power failure, use a megohmmeter to measure the insulation resistance, or use a fixed spark gap to measure the insulator with electricity.