Low Voltage and High Voltage Cable Testing

Low Voltage and High Voltage Cable Testing

 Low Voltage XLPE Distribution Cables:

Insulation Resistance:

  • Cables shall be tested for insulation resistance with an insulation tester (i.e. Megger) at 1000 Volts for 1 minute.
  • The minimum insulation resistance to earth or between phases shall be 100 meg-ohms.
  • The instrument used for this measurement shall have a minimum resolution of 10 meg-ohms on the 0 to 500 meg-ohm range.
  • At the conclusion of LV insulation resistance testing, the neutrals must be connected to the earth stakes.

Phasing Test:

  • The correct phasing of all LV circuits shall be checked at all positions where the LV cables are terminated into fuse bases and where any LV cable is run from point to point.
  • This test shall be performed with an instrument designed for the purpose. Mains frequency voltage of 240 Volts is not acceptable for this test.
  • The neutral conductor shall be connected to the earth stake for this test.

Continuity Test (resistance of bolted connections):

  • For loop LV systems, a continuity test shall be carried out on each LV circuit to ensure that all bolted connections are complete and adequate. The test shall be carried out as follows:
  • (1)     At the transformer firmly bond all 4 conductors together
  •  (2)   Undertake a continuity test at every point where there is a service provision or open point. In a fused service pillar the bottom row of fuses bases must be the point at which the test is undertaken as that is the furthest extent of the network.
  • The difference between the readings of each phase conductor and the neutral for each individual test shall not be greater than 10% of each other. Any difference greater than this may indicate a loose or dirty connection and will require further investigation.
  • The instrument used for this measurement should have a resolution to the second decimal point in the 0 to 5 ohm range.
  • A typical instrument would be the earth “Megger” type and taking into account the resistance values of the test leads.

Earth Resistance Test:

  • In any overhead or underground network the earth resistance at any point along the length of a LV feeder is to have a maximum resistance of 10 ohms prior to connection to the existing network.
  • In any overhead or underground network the overall resistance to earth Shall be less than 1 ohm prior to connection to the existing network.

 11 KV AND 33 KV XLPE Cables:

Phasing Test

  • The correct phasing of all HV circuits shall be checked at all positions where the HV cables have been terminated.
  • This test shall be performed with an instrument designed for the purpose. 240 Volt mains frequency is not acceptable for the performance of this test. The test may be conducted on either the wire screens or the aluminum conductors.
  • Where the test is performed on the wire screens, they shall be disconnected from earth.

 Outer Sheath Insulation Resistance (Screen wire test)

  • The purpose of the test is to determine soundness of the outer polyethylene sheath against water ingress, mechanical damage and termite attack.
  • Values below 0.5 meg-ohms (500 kΩ) can indicate sheath damage. Values between 1.0 and 10 meg-ohms may not indicate damage in a single location. Fault finding can often be very difficult. In new cables, values of greater than 100 mega ohms are required.
  • The integrity of the outer sheath shall be checked after cables have been buried by an insulation tester (Megger) at 1000 Volts.
  • The test shall be conducted for 1 minute between each wire screen and earth after the cable has been jointed and terminations installed.
  • For cables after repairs, the resistance must not be less than 10 meg-ohms.
  • Where HV cable circuits are cut and joined to new circuits, sheath testing must be carried out on the existing old circuit prior to joining to the new cable.

 HV test on XLPE cables already in service or previously energized

         Except for New Cables, Testing at Voltage greater than 5.0KV is not permitted

  • Studies carried out on DC high voltage testing of XLPE cables now conclude that;
  • DC testing above 5kV of field aged XLPE cables generally increases water tree growth and reduces service life.
  • 5kV is not considered a “High Voltage DC Test”. The test voltages for tests on XLPE cables is now limited to 5kV after in service repairs and 10kV for new installations.
  • A 5kV Megger is suitable for a 5kV test on cables after repairs.
  • The changes to this section will also make it possible for a repaired cable to be tested by repair crews and made available for immediate return to service.

Application

Test Voltage

Criteria

After repairs – Sheath

1kV Megger 1 minute

10 meg-ohms min.

After repairs – Insulation

5kV Megger 1 minute

1000 meg-ohms min.

After repairs – Insulation

5kV DC 1 minute

5.0 μA (micro-amps) max.

 HV test on new XLPE cable:

  •  Prior to the performance of this test, the screen wires must be connected to the permanent earth position.
  • The cable shall be tested at the test voltage and the pass criteria shall be in accordance with the following table:

Application

Test Voltage

Criteria

New cables – Sheath

1kV Megger 1 minute

100 meg-ohms min.

New cables – Insulation

10kV DC 15 minute

1.0 μA (micro-amps) max

New cables – Insulation

10kV DC 15 minute

1000 meg-ohms min.
  • If further repair works are undertaken, and they require additional joints to be installed, the complete HV testing procedure shall be repeated.

Alternative HV Test Requirement on Insulation for 11kV Cables

  • Where it is not practical to conduct a high voltage test, the test requirements for insulation (core to screen wire) may be limited to testing for the condition of “safe to energize”. The following list of circumstances and conditions must be met as a minimum requirement:
  • The cable circuit voltage shall be 11kV,
  • The circuit outage duration shall be not more than 48 hours,
  • The work shall involve extending or repairs to existing circuits,
  • The insulation test shall be applied for 1 minute between each
  • phase core and screen with a 1000 Volt minimum insulation tester (Megger),
  • Typically the test result should be in the order of 1000 meg-ohms.

Paper Insulated Cables:

Tests on LV Cables

  • An insulation resistance test shall be conducted with a 1000 Volt megger. Test results as low as 10 meg-ohms on old cable circuits are common and therefore considered safe to energies.

Test on 11kV and 33kV Cables between Cores and Earth

  • For three core belted cables, the test on any core shall be conducted between the core and lead sheath with the remaining two cores connected to earth.
  • The test voltages and pass criteria shall be in accordance with the table below.

Application

Test Voltage

Criteria

11kV new cables

5kV Megger 1 minute

100 meg-ohms.

11kV after repairs

5kV Megger 1 minute

100 meg-ohms.

33kV – no TF’s connected

5kV Megger 1 minute

1000 meg-ohms.

33kV – with TF’s connected

5kV Megger 1 minute

15 meg-ohms.

66kV XLPE CABLES

Core to Sheath Test after Repairs:

  • After repairs have been carried out, the 66kV XLPE cable shall be energized at power frequency for 24 hours without load. DC testing is not permitted.
  • The cable sheath link box/cross bonding system shall be put into its normal condition.

Outer Sheath Integrity Test:

  • An insulation resistance test between the metallic sheath and earth shall be conducted. The anti-termite barrier must be connected to the metallic sheath and the insulation test performed to earth.
  • The test voltage applied for 1 minute shall be 5kV DC applied with either a high voltage test set or insulation resistance tester (Megger).

References:

  • ETSA TECHNICAL STANDARD
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About Jignesh.Parmar
Jignesh Parmar has completed his B.E(Electrical) from Gujarat University. He has more than 11 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Assistant Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. 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.

37 Responses to Low Voltage and High Voltage Cable Testing

  1. farouq faisal says:

    thank you very much it is realy importent programs thank you so much

  2. Irwan Chargeman says:

    Thank you…..for info thank UUUuuuuuuuu Mr Jignesh.Parmar

  3. Murali says:

    My Salute to your knowledge & your wish to share

  4. Ginju Thomas says:

    Sir, actually during meggaring of cables the polarisation index is considered…can u give me the tabulation of values of polarisation index

  5. thank u sir for sharing elect. notes,can u give installation of UPS,Lighting and power layout,equipment layout and LV wiring details.

  6. Mohammad Arif Hussain says:

    God bless you my dear really are you doing very good job keep continue Thanks,

  7. rao says:

    Thank You sir..

  8. moahmmed akber ahmed says:

    thanks for sharing knowledge on tht net it is very beneficial for all god bless u

  9. Trajanco says:

    Dear Sir,

    I have one question:

    Quote: Cables shall be tested for insulation resistance with an insulation tester (i.e. Megger) at 1000 Volts for 1 minute.

    Can you please provide schematic/wiring diagram for this type of test, and also the length of the cable to measure.

  10. hussain says:

    Dear Sir,
    thank you for these valuable information
    I have the following points :
    You mentioned that ” Except for New Cables, Testing at Voltage greater than 5.0KV is not permitted” how about VLF test.
    During fault location some time you need to surge the fault to reduce its resistance , please explain.
    Now HV Ac Test is using for new cables ( i.e 2Uo).
    Do you have any idea for the testing of in service oil filled cable

    • Agree with You.
      In used cable more than 5KV may be damage insulation resistance of Cable. Even For new cable more than one time HIPOT test at site is not advisable by manufacturer also (Manufacturer also do HIPOT at time of manufacture).
      We use High voltage spike to detect Fault in HT cable by Thumber but it may be affect property of Cable.

      • azeem says:

        Dear sir,
        thanks a lot, for that type of informations.
        I m Azeem Electrical Officer in Karachi Electric Supply Company.
        sir i have a question?
        that why manufacturing fault is produce as a pin hole fault in H.T 11kv 3 core armur cables.

  11. jewan seepersad says:

    Very helpful and important information. Thank you!

  12. sumesh says:

    tks for your good information…. i m sumesh working as an electrical engineer in a construction company

  13. vinod says:

    Dear Sir,

    Hatss off to u. Hope person like can make our world more beautyfull.

    All the very best

  14. GS says:

    Dear Sir,

    Thank you for the excellent information, is it possible to share how frequent we should do the sheath test and insulation test? (yearly? half yearly?) Thanks

  15. Tom Burnett says:

    When performing hi-pot or better yet, megger testing, what is the closest the tester can be to the conductors that are being tested? I understand there is an approach boundary for unqualified persons, but what about the tester?
    Please advise or direct me to where I can find the information. The tester manufacturers do not specify a safe distance in their manuals.
    Thanks

  16. srinivas says:

    thank you sir.
    its very useful

  17. Thanks For Important Information Keep Sharing

    • D.Thirunavukkarasu says:

      33kv cable single core after sheath fault repair what is the minimum sheath insultion resistance requried to declare fit for service.Can you please update with relavant standard

  18. kuda says:

    How can you test that there is no breakage on sheath for PVC(non metallic sheathed acbles) with Galvanised steel armour wire?I need a test method that is not passing the cable through a spark tester,like testing Insulation resistances?

  19. prajod k says:

    sir
    plse give me the wiring details of meggring of 11kv armoured cble for insulation resistance test…

    • Wiring Detail!!!. Connect Megger L and E Terminal between Phase to phase and Phase to armored.. No need for wiring diagram…for more detail read the post of “IR Value ” of this Blog..

  20. Shehzad Rana says:

    Thanks a lot sir for share your great experience and knowledge with us really its very fruitful for me and other fresh engineers thanks

  21. Rahul says:

    Is there any test to find out a damage on outer PVC sheath of an 11kV cable?

    ( In case during installation / pulling on tray a minor cut has occurred on cable which was not identified during physical investigation )

  22. kameswar Behera says:

    how to found the resistance fault of 33KV HT cable having 300 mtr length and how to rectfy.

  23. Juan 1120 says:

    Dear Sr. I’m in the case of receiving a new MV (72,5 kV) cable at site for installation. The client is asking me to do a Megger test between core and screen/earth to verify the insulation resistance. I assume the test shall be done at 10 kV, 10 min; but I would like to know the acceptace criteria for this kind of test. The cable is completely new.

  24. Shiekh Tariq says:

    Dear sir I have a question cable shell be tested for insulation resistance with an insulation tester 1st test we do with 5 kV magger then the value is 5000 mo this time the cable length is 500mts then 2nd test we do then cable length is 1000mts then value is 1500 to 2000 mo why

  25. tanvirsarfaraz says:

    sir.whn we test 11kv dead cable.From one it shows ok but from other end it shows faulty what is the reason

  26. Desderius Luhaga says:

    Dear Sir,I have a question,I have insulation resistance test of 500v,what would be the results,would you help me to know the governing equestion for insulation resistance test?

  27. High voltage test set says:

    I really appreciate your post and it was superb .Thanks for sharing information.
    Regards

  28. VAISHALI says:

    Very useful information thanks sir.

  29. kumar says:

    thanks………

  30. karthi says:

    thanks sir its very useful to every electrical engg

  31. MOHAMMED AZEEM says:

    Thanks for it all,it,s good way for help to people.👍✌😊

  32. Hayder I Khairi says:

    Thanks for this information,
    I am very interesting to know the standard that we based on when test insulation of cable, applied voltage and time of test. Would you please tell me when can I find it?

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