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Home / Technical Articles / The Good, The Bad and The Ugly Cable Insulation

Insulation Fundamentals

The fundamental understanding of cable insulation properties forms the foundation for assessment of cable operability. These same fundamentals provide the basis for evaluating whether various electrical and physical tests and measurements are meaningful, cost-effective, and warranted.

The Good, The Bad and The Ugly Cable Insulation
The Good, The Bad and The Ugly Cable Insulation

They are a basis for evaluation of present or conventional cable test practices against the critical properties of concern for:

  • Cable operability
  • Life extension
  • Retention of the original environmental qualification, and
  • The adequacy of environmental qualification.

General Properties of Insulation

The electrical properties of concern for cable insulations are dielectric loss properties (resistivity, insulation resistance, dielectric constant and permittivity) and dielectric endurance properties (dielectric strength, breakdown strength, and ability to withstand corona attack).

Although these properties are important for higher voltage and other specialty applications, many of them lose their importance for the low-voltage cabling used in thermal/nuclear power plants. It is demonstrated that the significance of mechanical and thermal properties depend upon the application of the cable.

Insulation resistance measurements are commonly used to evaluate insulation systems.

For shielded cable, insulation resistance is directly related to the volume resistivity of the cable. For unshielded cable, the insulation resistance has a complex relationship to volume and surface resistivity because there is no shield for a return path.

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 article 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.

Cable Good insulation
Figure 1 – Cable Good insulation

The electrical equivalent circuit of a good run of cable is shown in Figure 2. If the insulation were perfect, the parallel resistance RPwould 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 Cable Insulation Is Bad?

When the magnitude of the leakage current exceeds the design limit, the cable will no longer deliver energy efficiently. See Figure 3.

Why A Cable Becomes Bad?

Damaged underground electrical cable
Damaged underground electrical cable

All insulation deteriorates naturally with age, especially when exposed to elevated temperature due to high loading and even when it is not physically damaged. In this case, there is a distributed flow of leakage current during a test or while energized.

Many substances such as water, oil and chemicals can contaminate and shorten the life of insulation and cause serious problems.

Cross-linked polyethylene (XLPE) insulation is subject to a condition termed treeing. It has been found that the presence of moisture containing contaminants, irregular surfaces or protrusions into the insulation plus electrical stress provides the proper environment for inception and growth of these trees within the polyethylene material.

Testing indicates that the AC breakdown strength of these treed cables is dramatically reduced. Damage caused by lightning, fire, or overheating may require replacement of the cable to restore service.

Equivalent circuit of good cable
Figure 2 – Equivalent circuit of good cable
Cable Bad Insulation
Figure 3 – Cable bad insulation

Cable Faults Described

When at some local point in a cable, insulation has deteriorated to a degree that a breakdown occurs allowing a surge of current to ground, the cable is referred to as a faulted cable and the position of maximum leakage may be considered a catastrophic insulation failure.

See Figure 4.

Ground or shunt fault on the cable
Figure 4 – Ground or shunt fault on the cable

At this location the insulation or parallel resistance has been drastically reduced and a spark gap has developed. See Figure 5.

Fault region simplified diagram
Figure 5 – Fault region simplified diagram

Occasionally a series fault shown in Figure 6 can develop due to a blown open phase conductor caused by high fault current, a dig-in or a failed splice.

Open or series fault on the cable
Figure 6 – Open or series fault on the cable

The Ugly Cable Insulation

In the matter of fact, there is no ugly cable insulation. It can be either good or bad. Every condition between is considered as bad.

Hipot Cable Testing Part-1 (VIDEO)


Hipot Cable Testing Part-2 (VIDEO)


Resources: Power Plant Practices to Ensure Cable Operability – Electric Power Research Institute; Fault Finding Solutions – Megger

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Melissa
    May 10, 2021

    Anyone had experience with white crystal formation on the outer PVC sheath of an XLPE cable? What is the potential cause of this and is it deteriorating the cable insulation properties?

  2. Dzulkarnain Norhalimi
    Apr 16, 2019

    Hai.. currently our cable (440Vac – XLPE) experiencing over compressed.. insulation at overcl compressed area become “bloating”.. so what is the best method of testing to see it is good or bad?.. also what standard can we reter? thanks for the help..

    fyi, our cable is new before over compressed occur during cable pulling procedure.. tq

  3. febrian
    Jun 10, 2014

    What if water penetrate and making corrosion to the cable?
    Can we detect it using megger test?
    Which one better, replace or repair the cable by scraping the rust?

    • Khawaja
      Dec 24, 2014

      What if water penetrate and making corrosion to the cable?
      Try one of the two options mentioned below;
      1-Purging with a dehydrating agent
      2-Location of the source of water and remedial actions

      Can we detect it using megger test?
      Yes but only partially.
      Try one of the three tests mentioned below;
      1- Polarization Index Test, i.e., PI = (IR value at 10 minutes/ IR value at 1 minute)
      2- Tan Delta Test (will specifically tell you about the global health of the cable insulation with respect to the amount of moisture content)
      3- TDR (Time Domain Reflectometry will indicate the changes in impedance along the length of the cable due to corrosion, etc.)

      Which one better, replace or repair the cable by scraping the rust?
      Assuming that the rust/corrosion is existing in the cable shield/ armour/ neutral only, following approach may help.
      1- It will mainly depend upon the level of required Reliabile Operation and the importance/ criticality of the connected load(s) to the cable under review for repair or replacement …
      2- Service life of the cable and the operating environment
      3- Quantitative and Qualitative analysis of the Extent of corrosion/rust and degradation of the insulation based on the results of the Three Test Methods mentioned above …
      4- Cost Benefit analysis and the ROI in case of Replacement

      In most of the cases, the above steps help in managing the water logged and corrosive cables by making appropriate rehabilitation (repair/replacement) decision.

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