Cable insulation resistance testing instruments and on-site testing methods

Cable insulation resistance test

The test of cable insulation resistance refers to the measurement of insulation resistance between cable cores and between cable cores and skin.

It can preliminarily determine the defects of cable insulation aging due to moisture, and can also determine whether there are any defects in the insulation of the cable during the withstand voltage test. The DC resistance of the cable conductor can be measured using the bridge method or the voltage drop method. When measuring the DC resistance of the conductor of a phase cable, other phase cables can be used as voltage and current leads at the other end.

The simple and basic method is the megohmmeter method

Measuring cable insulation resistance with a megohmmeter

The measurement of insulation resistance includes direct measurement method, comparison method, charging method, and self discharge method;

The direct method is a commonly used measurement method, which applies a known stable DC voltage to the tested workpiece. Determine the insulation resistance by the steady-state current of the tested object.

Test methods for insulation resistance testers

The L terminal is connected to the cable core, using a connection wire with high insulation resistance and being careful not to place it on the ground or come into contact with other objects. The E terminal of the megohmmeter is connected to the cable sheath and ground. To eliminate the impact of surface leakage current, the G terminal should be connected to the shielding ring of the insulation at the end of the cable core.

For the sake of comparison, the insulation resistance values measured at different temperatures are generally converted into values at a temperature of 20 ℃ and a length of 1Km.

Precautions for measuring cable insulation resistance

Before the test, the conductor core and cable metal sheath must be grounded to fully discharge, and an appropriate megohmmeter must be selected based on the rated voltage of the tested cable

At the same time, wipe the surface of the cable terminal clean and perform surface shielding.

When measuring, the megohmmeter should be placed in a stable and free from significant vibration place, and the megohmmeter should be checked for normal operation according to relevant requirements;

After each insulation resistance measurement, the cable should be discharged. The longer the grounding cable line, the longer the grounding time, usually no less than 1 minute.

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