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# Fault Finding Solutions Guide by MEGGER

## Good Cable Insulation

When voltage is impressed across any insulationÂ system, some current leaks into, through, andÂ around the insulation. When testing with DC high voltage,Â capacitive charging current, insulationÂ absorption current, insulation leakage current, andÂ by-pass current are all present to some degree.

ForÂ the purposes of this document on cable faultÂ locating, only leakage current through the insulationÂ will be considered.Â For shielded cable, insulation is used to limit currentÂ leakage between the phase conductor andÂ ground or between two conductors of differingÂ potential.

As long as the leakage current does notÂ exceed a specific design limit, the cable is judgedÂ good and is able to deliver electrical energy to aÂ load efficiently.

Cable insulation may be considered good whenÂ leakage current is negligible but since there is noÂ perfect insulator even good insulation allows someÂ small amount of leakage current measured inÂ microamperes. See Figure 1.

The electrical equivalent circuit of a good run ofÂ cable is shown in Figure 2. If the insulation wereÂ perfect, the parallel resistance RP would not existÂ and the insulation would appear as strictly capacitance.

Since no insulation is perfect, the parallel orÂ insulation resistance exists. This is the resistance measured during a test using a MeggerÂ® InsulationÂ Tester. Current flowing through this resistance isÂ measured when performing a dc hipot test asÂ shown in Figure 1.

The combined inductance (L),Â series resistance (RS), capacitance (C) and parallelÂ resistance (RP) as shown in Figure 2 is defined asÂ the characteristic impedance (Z0) of the cable.

When the magnitude of the leakage currentÂ exceeds the design limit, the cable will no longerÂ deliver energy efficiently

1. Aiyidu Ehigiator
Jun 03, 2020

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Apr 18, 2020

i have a 16 m/m twin and earth cable laid in the ground over a distance of 500/meters load10/amps, it worked well for over a year and then all of sudden it tripped the c/b reset, then after 1/min it tripped again. No short, except when I tested with an amp/meter i noticed when i switch on the amps it started to climb rapidly, like a heating circuit , is it bad joints or installation break down, no idea please help ? Circuit controls a camera circuit, tested with megger shows dead short live neutral earth, no bang when engaging c/b stays engaged for short time?

3. GABRIL M ELFAKHRI
Nov 19, 2017

Dear sir

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4. inderpreet kaur
Feb 19, 2016

sir,

5. LAKHAN
Jun 03, 2015

Good work

6. EB1000
Jun 04, 2013

Very good article. As for the cable model, I must say that I’ve never seen this kind of cable equivalent circuit. Aren’t cables supposed to be symmetrical reciprocal systems? Input impedance should be the same from either end. Cable model is usually a Pi network consisting of half the values of both capacitance and parallel resistance on each end, plus a series resistance and inductance in the middle. The model shown here looks unsymmetrical. I think both series resistance and inductance should be added to both sides.

7. salariah
Feb 28, 2013

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Nov 06, 2012

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10. revannavar
Jul 11, 2012

Thanks for articles…

• Edvard
Jul 11, 2012

You’re welcome!