Low Voltage and High Voltage Cable Testing
June 7, 2011 68 Comments
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
JIGNESH PARMAR
thank you very much it is realy importent programs thank you so much
Dear sir,
could you provide
me your Email Address please
Thank you…..for info thank UUUuuuuuuuu Mr Jignesh.Parmar
My Salute to your knowledge & your wish to share
Sir, actually during meggaring of cables the polarisation index is considered…can u give me the tabulation of values of polarisation index
thank u sir for sharing elect. notes,can u give installation of UPS,Lighting and power layout,equipment layout and LV wiring details.
God bless you my dear really are you doing very good job keep continue Thanks,
Thank You sir..
thanks for sharing knowledge on tht net it is very beneficial for all god bless u
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.
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.
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.
Is there any test for cable outer sheath
Very helpful and important information. Thank you!
tks for your good information…. i m sumesh working as an electrical engineer in a construction company
Dear Sir,
Hatss off to u. Hope person like can make our world more beautyfull.
All the very best
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
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
thank you sir.
its very useful
Thanks For Important Information Keep Sharing
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
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?
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..
Thanks a lot sir for share your great experience and knowledge with us really its very fruitful for me and other fresh engineers thanks
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 )
how to found the resistance fault of 33KV HT cable having 300 mtr length and how to rectfy.
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.
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
sir.whn we test 11kv dead cable.From one it shows ok but from other end it shows faulty what is the reason
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?
I really appreciate your post and it was superb .Thanks for sharing information.
Regards
Very useful information thanks sir.
thanks………
thanks sir its very useful to every electrical engg
Thanks for it all,it,s good way for help to people.👍✌😊
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?
hi jigneshbhai
can u give brif detail for 11kv 630A indoor panel how to do high voltage test ?
I need to test 11KV 3core 120 Sqmm cable how to test using Megger
sir how do hi-pot test for enamoured 132kV single core cable
how to do hi-pot test for 138kv outer sheeth for xple cable
Tank you
respect sir
i have one question ?? give me cable formula according to load
ex ,,maximum demand is 500 amps incoming supply if i use copper cable how can i measure if i use aluminium cable how can i measure exact formula i want can send me please sir ( cable 3phase )
Thanks
Sir. If i use high voltage cable for low voltage load, what would be the implication?
Good method for clear the topic.
Sir,
I am an Electrical Engineer working in Kerala. I need an advice on the Testing of wires used for Low voltage and High voltage. As per Indian standards what all tests are to be done to assess the quality of wires/cables. Thanks for the consideration
33kV – no TF’s connected 5kV Megger 1 minute 1000 meg-ohms.
33kV – with TF’s connected 5kV Megger 1 minute 15 meg-ohms.
What is “TF” ?
thanks.
How to test cable Armour healthy
continuity in loading condition ?
sir
can you tell me how to measure the insulation test of 240 sq mm single core cable .please provide one example
thank you
i have doubt on insulation resistance test. why the values are in mega ,giga ohms.
What is the minimum and maximum pressure testing voltage on a new installation of xlpe cable at 11kv using a AC pressure tester . ?
Thank you so much sar for the information
Dear Sir,
There is any calculation formula for to calculate acceptable leakage current while applying high voltage test for medium voltage lead sheathed cables.
what is the criteria?
Chandrakant Ghan
Dear Sir, You have not coverd FRLS (Fire resistant Low smoke cables) in your main types of cabales. Rest of course as per your normal practice EXCELLENT!
Dear Mr. Parmar,
I was looking for HV cables sheath integrity test criteria and range etc, and found your article.
Thanks for your write -up covering the details of cables test.
In one of our new project installation we found Outer Sheath Insulation Resistance is less than .5 M Ohm for 11kV Cables and pretending some cable sheath damage could be.
While referring the IEC cl. associated with Sheath test, noted that the requirement is after applying 4kv/mm or maxi, 10kv for 1 minute and to check “” No breakdown of the over sheath shall occur during the test”.
Is there any definition of Breakdown ?? What does it mean ? orcan you please advise on the reference in your write up under cl no. “Outer Sheath Insulation Resistance (Screen wire test)” for 11kV/33 kV XLPE cables ..
Many thanks in advance.
how to check HT/MV cable is dead or live we dont have drawing
Dear Sir,
Kindly provide me the applicable IEC standards for the above mentioned tests.
Thank You in advance
We are glad to know about your company and what you are able to do. We are are urgently looking for Voltage Detectors for immediate purchase.
We are a Zambian Company based in Ndola dealing in electrical supplies. We wish to urgently purchase 20 Live line testers which can work for 11, 33 and 66 kV, switchable, for our client. Kindly give us your best quote.
Description:
TYPE
1. With Housing made of polycarbonate with high mechanical properties and dielectric strength.
2. Supplied with insulated switch stick, extendable to 12.5m height or more
3. With universal tool end for test on Overhead Power Lines
VOLTAGE: 11/33/66 kV switchable
INDICATION:
Double signaling; extremely bright red light signfying presence of voltage and powerful rated audible signal >60 dB/2meters or Equivalent
SELF TESTING
Test Button when pressed a timed green LED lights to indicate detector is operational and the self-testing of the unit continues or equivalent.
POWER SUPPLY
1. Rechargeable Battery
2. With Portable Battery charger
END FITTING: Universal Stick type
TEMPERATURE: -1 to 40 degrees Celcius
ALTITUDE ABOVE SEA LEVEL: 1400m
SUPPLIED WITH: Carrying case, Batteries and two contact electrodes
APPLICABLE STANDARD: IEC/BS/SANS/EN
PREVIOUS TYPE TEST REPORT: YES
ROUTINE TESTS CERTIFICATE: YES
I shall be very glad if before the end of tomorrow, Monday we can reach a conclusion whereby we can have the quote and Pictures plus copies of Test Certificates, sent to us.
The quote may be CIF Ndola, Zambia.
Waiting to hear from you.
Thanks To Jinesh Excellent Result shown by Jinish I am satisfied from Your Answer ,I am Instrumentation Projects Engineer Construction Al Jubail KSA,We are use this type cable in Oil & Gas Company in KSA
Really a great support for the megger testing work. This like blogs will really touch with the importance.
why do we use 5KV for testing 11kv cable ?
shouldn’t be use more then how much it carry ?
for example we use 500v for IR test of 400v LV cable
Sir wich insulation tester better for meggering. Name plz i want 2 go buy now.
Sir If the result gives you a greater than 1 mega Ohm between all phases and neutral + earth(Megger results), so if you put back the cable on the main source and check the voltages which is 240V but if you go and check the other end which is 50V. SO please what happen to that fault?
Good work ht power work
Hi Sir!
Sir, it is still safe for me to energize my project despite a poor result of megger testing? The reading is just 0.50 mega ohms.
Sir!
I have a direct buried 34.5 kV armored cable. Which testing would be the best to assure that the cable is accepted: VLF or Hipot?
For the same type type of cable, which testing is the best to identify the location where could be a failure?