Premium Membership ♕

Save 50% on all EEP Academy courses with Enterprise Membership Plan and study specialized LV/MV/HV technical articles & guides.

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.

Megger - Fault Finding Solutions
Megger – Fault Finding Solutions

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.

As long as the leakage current does not exceed a specific design limit, the cable is judged good and is able to deliver electrical energy to a load efficiently.

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.

Good cable insulation
Figure 1 – Good cable insulation

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.

Equivalent circuit of good cable
Figure 2 – Equivalent circuit of good 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
Size:2.4 Mb
Download:Right here | Video Courses | Membership | Download Updates
Megger - Fault Finding Solutions
Megger – Fault Finding Solutions

Premium Membership

Get access to premium HV/MV/LV technical articles, advanced electrical engineering guides, papers, and much more! It will help you to shape up your technical skills in your everyday life as an electrical engineer.
50% Discount 💥 - Save 50% on all 90+ video courses with Enterprise Membership plan.

More Information


  1. Aiyidu Ehigiator
    Jun 03, 2020

    This one of the greatest exposition site into the field of Electrical Engineering that has ever existed

  2. Paddy
    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?

    Nov 19, 2017

    Dear sir


  4. inderpreet kaur
    Feb 19, 2016

    I am not been able to download article about powet transformer maintenance and condition monitoring

    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

    great work, very informative

  8. vhparmar
    Feb 06, 2013

    Dear All,

    This site is very usefull for all electrical engineer in true manner.

    I complete my day at office after reading article send by your site.

    I want to join in your site with articles and small Excel software developed by me.

    I look forward to upgrade my knowledge.

  9. mark2012
    Nov 06, 2012

    Hello Sir,

    I am new here in your site and I find it very informative and very helpful to me as a fresh electrical engineer. I just want to request for a topic about generators. The methods, principles, preventive maintenance and etc. will be very significant to me sir.

    Many thanks,


  10. revannavar
    Jul 11, 2012

    Thanks for articles…

Leave a Reply to LAKHAN

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

  +  13  =  sixteen

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge