Good Cable Insulation
When voltage is impressed across any insulation system, some current leaks into, through, and around the insulation. When testing with DC high voltage, capacitive charging current, insulation absorption current, insulation leakage current, and by-pass current are all present to some degree.
For the purposes of this document on cable fault locating, only leakage current through the insulation will be considered. For shielded cable, insulation is used to limit current leakage between the phase conductor and ground or between two conductors of differing potential.
Cable insulation may be considered good when leakage current is negligible but since there is no perfect insulator even good insulation allows some small amount of leakage current measured in microamperes. See Figure 1.
The electrical equivalent circuit of a good run of cable is shown in Figure 2. If the insulation were perfect, the parallel resistance RP would not exist and the insulation would appear as strictly capacitance.
Since no insulation is perfect, the parallel or insulation resistance exists. This is the resistance measured during a test using a Megger® Insulation Tester. Current flowing through this resistance is measured when performing a dc hipot test as shown in Figure 1.
The combined inductance (L), series resistance (RS), capacitance (C) and parallel resistance (RP) as shown in Figure 2 is defined as the characteristic impedance (Z0) of the cable.
When the magnitude of the leakage current exceeds the design limit, the cable will no longer deliver energy efficiently
|Title:||Fault Finding Solutions – MEGGER|
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