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Defects in the insulation

The measurement of insulation resistance is a common routine test performed on all types of electrical wires and cables. As a production test, this test is often used as a customer acceptance test, with minimum insulation resistance per unit length often specified by the customer.

Megger MIT1020 10-kV insulation resistance testers are all designed specifically to assist the user with the testing and maintenance of high voltage equipment
Megger MIT1020 10-kV insulation resistance testers are all designed specifically to assist the user with the testing and maintenance of high voltage equipment

The results obtained from IR Test are not intended to be useful in finding localized defects in the insulation as in a true HIPOT test, but rather give information on the quality of the bulk material used as the insulation.

Even when not required by the end customer, many wire and cable manufacturers use the insulation resistance test to track their insulation manufacturing processes, and spot developing problems before process variables drift outside of allowed limits.

Selection of IR Testers (Megger):

Insulation testers with test voltage of 500, 1000, 2500 and 5000 V are available. The recommended ratings of the insulation testers are given below:

Voltage LevelIR Tester
650V500V DC
3.3KV2.5KV DC
66Kv and Above5KV DC

 Test Voltage for Meggering:

When AC Voltage is used, The Rule of Thumb is:
Test Voltage (A.C) = (2X Name Plate Voltage) +1000.

When DC Voltage is used (Most used in All Megger)
Test Voltage (D.C) = (2X Name Plate Voltage).

Equipment / Cable RatingDC Test Voltage
24V To 50V50V To 100V
50V To 100V100V To 250V
100V To 240V250V To 500V
440V To 550V500V To 1000V
2400V1000V To 2500V
4100V1000V To 5000V

Measurement Range of Megger:

Test voltageMeasurement Range
250V DC0MΩ to 250GΩ
500V DC0MΩ to 500GΩ
1KV DC0MΩ to 1TΩ
2.5KV DC0MΩ to 2.5TΩ
5KV DC0MΩ to 5TΩ

Precaution while Meggering

Before Meggering:

Make sure that all connections in the test circuit are tight. Test the megger before use, whether it gives INFINITY value when not connected, and ZERO when the two terminals are connected together and the handle is rotated.

During Meggering:

Make sure when testing for earth, that the far end of the conductor is not touching, otherwise the test will show faulty insulation when such is not actually the case.

Make sure that the earth used when testing for earth and open circuits is a good one otherwise the test will give wrong information. Spare conductors should not be meggered when other working conductors of the same cable are connected to the respective circuits.

After completion of cable Meggering:

  • Ensure that all conductors have been reconnected properly.
  • Test the functions of Points, Tracks & Signals connected through the cable for their correct response.
  • In case of signals, aspect should be verified personally.
  • In case of points, verify positions at site. Check whether any polarity of any feed taken through the cable has got earthed inadvertently.

Safety Requirements for Meggering:

  • All equipment under test MUST be disconnected and isolated.
  • Equipment should be discharged (shunted or shorted out) for at least as long as the test voltage was applied in order to be absolutely safe for the person conducting the test.
  • Never use Megger in an explosive atmosphere.
  • Make sure all switches are blocked out and cable ends marked properly for safety.
  • Cable ends to be isolated shall be disconnected from the supply and protected from contact to supply, or ground, or accidental contact.
  • Erection of safety barriers with warning signs, and an open communication channel between testing personnel.
  • Do not megger when humidity is more than 70 %.
  • Good Insulation: Megger reading increases first then remain constant.
  • Bad Insulation: Megger reading increases first and then decreases.
  • Expected IR value gets on Temp. 20 to 30 decree centigrade.
  • If above temperature reduces by 10 degree centigrade, IR values will increased by two times.
  • If above temperature increased by 70 degree centigrade IR values decreases by 700 times.

How to use Megger

Meggers is equipped with three connection Line Terminal (L), Earth Terminal (E) and Guard Terminal (G).

Megger connections
Megger connections

Resistance is measured between the Line and Earth terminals, where current will travel through coil 1. The “Guard” terminal is provided for special testing situations where one resistance must be isolated from another. Let’s us check one situation where the insulation resistance is to be tested in a two-wire cable.

To measure insulation resistance from a conductor to the outside of the cable, we need to connect the “Line” lead of the megger to one of the conductors and connect the “Earth” lead of the megger to a wire wrapped around the sheath of the cable.


Megger configuration
Megger configuration

In this configuration the Megger should read the resistance between one conductor and the outside sheath.

We want to measure Resistance between Conductor- 2 to sheaths but actually megger measure resistance in parallel with the series combination of conductor-to-conductor resistance (Rc1-c2) and the first conductor to the sheath (Rc1-s).

If we don’t care about this fact, we can proceed with the test as configured. If we desire to measure only the resistance between the second conductor and the sheath (Rc2-s), then we need to use the megger’s “Guard” terminal.

Megger - Connecting guard terminal
Megger – Connecting guard terminal

Connecting the “Guard” terminal to the first conductor places the two conductors at almost equal potential.

With little or no voltage between them, the insulation resistance is nearly infinite, and thus there will be no current between the two conductors. Consequently, the Megger’s resistance indication will be based exclusively on the current through the second conductor’s insulation, through the cable sheath, and to the wire wrapped around, not the current leaking through the first conductor’s insulation.

The guard terminal (if fitted) acts as a shunt to remove the connected element from the measurement. In other words, it allows you to be selective in evaluating certain specific components in a large piece of electrical equipment. For example consider a two core cable with a sheath.

As the diagram below shows there are three resistances to be considered.

Meggering wiring
Meggering wiring

If we measure between core B and sheath without a connection to the guard terminal some current will pass from B to A and from A to the sheath. Our measurement would be low. By connecting the guard terminal to A the two cable cores will be at very nearly the same potential and thus the shunting effect is eliminated.

Continued here – Measurement of insulation resistance (IR) – Part 2

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Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control) ,B.E(Electrical). He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 13 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.


  1. sagar galani
    Sep 18, 2018

    what about for 3 core cable? how we can use guard?

  2. Wint Zar Zar Theint
    Jun 13, 2018

    I want to know ” how much require resistance for 500 V.

  3. charles
    Feb 04, 2018

    Solar panel 0- 24v

    Please tell me requied testing range of voltage level and test results

  4. james
    Oct 07, 2016

    hi Jignesh, what about the other end of the cable? We need to make sure cores A and B are insulated from each other and not earthed right?

  5. Khalid
    Aug 25, 2016

    I have 6.6KV transformer what the is IR DC voltage test should I use

  6. Youssef Adel
    Feb 15, 2016

    Really appreciate your Effort.

  7. FELIX JR.
    Jan 21, 2016


  8. Toni
    Jan 10, 2016

    Why is IR Value decreasing after AC high voltage test?
    What is the dc voltage to applied for 3.3 kV switchgear for IR Test?

  9. soumya ranjan behera
    Dec 03, 2015

    great sir.
    my qstn is how a electricity synchonise with the frequency from grid to our cpp.
    what are the symptoms.

    Nov 28, 2015


  11. Md. Rezaul Haque
    Sep 30, 2015

    Please i want simple diagram of how to test insulation resistance between two single core pvc insulated cable. Thank you.

  12. jonchhen biren
    Aug 30, 2015

    I’m going to measure the insulation resistance in 132kv line, what should be the required value of phase R- y, R-B, n Y-B & Earth resistance of R- Earth, Y- Earth n B- earth ?

    Aug 29, 2015

    Please i want simple diagram of how to test insulation resistance between conductors. Thank you.

  14. Abbas Mohamed
    Aug 09, 2015

    Sir I want know what is the voltage drop for the following 11KV cable.
    Considering 3 core 70Cq mm XLPE cable;
    Cable resistance and Resistance per KM = 0.164 + j 0.082
    Length of cable, = 2.5 Km.
    Current carried —– ?
    Full Load Transformer = 500KVA
    Usage Current Amperes 200 Amperes

  15. Gopinath
    Jul 29, 2014

    1.what is the safe value ,if i am test LT cable / equipments .
    2.Any counting restriction (No. of times meggered)

  16. Paras mani Sharma
    Jan 29, 2014

    Why is it that the IR value of 66 KV line is differnt when measured with 2.5 KV, 5 KV, 10 KV meggers

  17. prodaja
    Apr 29, 2012

    Looking forward to read next article!

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