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Measuring cable insulation resistance

time:2020/12/31   source:华天电力  reading:580 time

1. Cable insulation resistance and cable insulation resistance value

Insulation resistance refers to the DC resistance of insulation under specified conditions, and is the most basic insulation index for electrical equipment and electrical circuits.

2. Standard calculation formula for cable insulation resistance

The insulation resistance of the cable is related to the temperature and length of the cable. Use 20°C as the temperature measurement unit, generally expressed in resistance per kilometer.

Namely: R20=Rt×Kt×L

In the formula, R20-MΩ/KM per kilometer of cable at 20℃; Rt-Insulation resistance of cable with length L at t℃, MΩ; L-Cable length, Km; Kt-, the coefficient at 20℃ is 1.0.

Good (qualified) cables are usually very high in insulation resistance, and their minimum insulation resistance can be specified as follows: new cables, the insulation resistance of each cable core and outer sheath (resistance value per kilometer at 20°C).

Insulation Resistance Tester (3).png

Three, power cable insulation resistance measurement regulations

The insulation resistance should be measured before the power cable is put into operation, and the insulation resistance value should be the value after 1 minute through the megger and insulation resistance tester.

Use 1kV (1000V megohmmeter) below 1kV: its value should not be less than 10MΩ

Use 2.5kV megohmmeter above 1kV (2500V or 5000V insulation resistance tester)

The insulation resistance of cables of 3kV and below is not less than 200MΩ.

The insulation resistance value of 6~10kV cable is not less than 400MΩ,

The insulation resistance of the 20~35kV cable is not less than 600MΩ,

The insulation resistance value of 220kV cable is not less than 4500MΩ/km,

Four, cable insulation resistance tester

The cable insulation resistance tester is suitable for measuring the resistance value of various insulating materials and the insulation resistance value of transformers, motors, cables and electrical equipment. The YMG2500 insulation resistance tester has 5 levels (100V/250V/500V/1000V/2500V), YMG5000 The insulation resistance tester has 5 levels (250V\500V\1000V\2500V\5000V). When the user is measuring the insulation resistance of the cable. Please select the appropriate model according to your needs. The YMG2500 and YMG5000 cable insulation resistance testers produced by Wuhan Zhuoya Electric Power Automation Co., Ltd. output short-circuit current: ≥2mA, insulation resistance measurement range: 0.01MΩ~200GΩ, and leakage current test with polarization index (PI) and absorption ratio ( DAR) function. It is widely used in China.

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