The insulation strength of wires and cables refers to the ability of insulation structure and insulation materials to withstand electric field without breakdown and damage. In order to check the quality of wire and cable products and ensure the safe operation of products, insulation strength test shall be conducted for all insulated types of wires and cables. The insulation strength test can be divided into voltage test and breakdown test.

The voltage resistance test is to apply a certain voltage to the test object under certain conditions. After a certain period of time, whether the breakdown occurs is regarded as the standard to judge whether the test sample is qualified. The voltage of time is generally higher than the rated working voltage of the test object. The specific voltage value and withstand time are specified in the product standard. Through the voltage withstand test, the reliability of the product under the working voltage and the serious defects in the insulation can be found, and some defects of the production process can be found, such as: the insulation has serious external damage, There are serious defects on the conductor that cause the electric field to be distorted sharply; Insulation has penetrating defects or large conductive impurities in production.

Breakdown test is to measure the breakdown field strength or breakdown voltage under certain test conditions until the breakdown occurs. The safety margin between the ability of cable to withstand voltage and the working voltage can be evaluated by breakdown test. Breakdown field strength is one of the important parameters in cable design.

The cable usually bears AC voltage in operation, but it also bears DC voltage in DC transmission system and some special occasions. For high voltage cable, it may also be attacked by atmospheric voltage (lightning) and operating overvoltage. Therefore, according to the different waveform of experimental voltage, it can be divided into three types: 1. AC (power frequency) voltage, 2. DC voltage and 3 impulse voltage.

Partial discharge measurement

For oil filled cable, there is no local power generation; Even if there is partial discharge of oil paper cable, it is usually very weak as several PCs, so the partial discharge can not be measured in the factory test. For extruded cables, not only the possibility of partial discharge is great, but also the damage of partial discharge to plastics and rubber is serious. With the improvement of voltage level and the improvement of working field strength, this problem is more serious. Therefore, partial discharge measurement is required for high voltage extruded cables in factory test.

There are many methods to measure partial discharge, which can measure discharge pulse according to the instantaneous charge exchange generated by discharge; The voltage can also be measured according to the ultrasonic generated during discharge (acoustic measurement method); The intensity of light can also be measured according to the light produced by discharge (optical measurement). For cables, the electric measurement method is basically adopted.

Aging and stability test

Aging test is a stability test to keep the performance stable under the action of stress (mechanical, electrical and thermal).

Thermal aging test

Simple thermal aging test is the aging characteristic of test sample under the action of heat. Put the test sample in an environment higher than the rated working temperature and temperature. After a specified time, measure the changes of some sensitive properties before and after aging to evaluate the aging characteristics. The aging period can also be accelerated by increasing the temperature, coupled with the stress of damp, vibration, electric field, etc. to form an aging period. After each aging period, some selected sensitive performance parameters can be determined. Until the performance drops to the value of the apparent life. In this way, a shorter life l (heating time of the sample) is obtained at a higher temperature T.

Thermal stability test

The thermal stability test is that the cable is heated by current and also bears a certain voltage. After a certain period of heating, some sensitive performance parameters are measured to evaluate the insulation stability.
Insulation stability test can be divided into long-term stability test or short-term accelerated aging test.