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Home / Technical Articles / Differences Between Earthed and Unearthed Cables

Introduction

In HT electrical distribution, the system can be earthed or unearthed. The selection of unearthed or earthed cable depends on distribution system. If such system is earthed, then we have to use cable which is manufactured for earthed system. (which the specifies the manufacturer). If the system is unearthed then we need to use cable which is manufactured for unearthed system.

Differences Between Earthed and Unearthed Cables
Differences Between Earthed and Unearthed Cables

The unearthed system requires high insulation level compared to earthed system.

For earthed and unearthed XLPE cables, the IS 7098 part2 1985 does not give any difference in specification. The insulation level for cable for unearthed system has to be more.


Earthed System

Earlier the generators and transformers were of small capacities and hence the fault current was less. The star point was solidly grounded. This is called earthed system.

In three phases earthed system, phase to earth voltage is 1.732 times less than phase to phase voltage. Therefore voltage stress on cable to armor is 1.732 times less than voltage stress between conductors to conductor.

Where in unearthed system, (if system neutral is not grounded) phase to ground voltage can be equal to phase to phase voltage. In such case the insulation level of conductor to armor should be equal to insulation level of conductor to conductor.

In an earthed cable, the three phase of cable are earthed to a ground. Each of the phases of system is grounded to earth.

Example: 1.9/3.3 KV, 3.8/6.6 KV system


Unearthed System

Today generators of 500MVA capacities are used and therefore the fault level has increased. In case of an earth fault, heavy current flows into the fault and this lead to damage of generators and transformers. To reduce the fault current, the star point is connected to earth through a resistance. If an earth fault occurs on one phase, the voltage of the faulty phase with respect to earth appears across the resistance.

Therefore, the voltage of the other two healthy phases with respect to earth rises by 1.7 times.

If the insulation of these phases is not designed for these increased voltages, they may develop earth fault. This is called unearthed system.

In an unearth system, the phases are not grounded to earth .As a result of which there are chances of getting shock by personnel who are operating it.

Example: 6.6/6.6 KV, 3.3/3.3 KV system.

Unearthed cable has more insulation strength as compared to earthed cable. When fault occur phase to ground voltage is √3 time the normal phase to ground voltage. So if we used earthed cable in unearthed System, It may be chances of insulation puncture.

So unearthed cable are used. Such type of cable is used in 6.6 KV systems where resistance type earthing is used.

Nomenclature

In simple logic the 11 KV earthed cable is suitable for use in 6.6 KV unearthed system. The process of manufacture of cable is same.

The size of cable will depend on current rating and voltage level.

  • Voltage Grade (Uo/U) where Uo is Phase to Earth Voltage & U is Phase to Phase Voltage.
  • Earthed system has insulation grade of KV / 1.75 x KV.
  • For Earthed System (Uo/U): 1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33 kV.
  • Unearthed system has insulation grade of KV / KV.
  • For Unearthed System (Uo/U): 3.3/3.3 kV and 11/11 kV.
  • 3 phase 3 wire system has normally Unearthed grade cables and 3 phase 4 wire systems can be used earthed grade cables, insulation used is less, and cost is less.

Thumb Rule

As a thumb rule we can say that 6.6KV unearthed cable is equal to 11k earthed cable i.e 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.

Because each core of cable have the insulation level to withstand 6.6kv so between core to core insulation level will be 6.6kV+6.6kV = 11kV

For transmission of HT, earthed cable will be more economical due to low cost where as unearthed cables are not economical but insulation will be good.

Generally 6.6 kV and 11kV systems are earthed through a neutral grounding resistor and the shield and armor are also earthed, especially in industrial power distribution applications.  Such a case is similar to an unearthed application but with earthed shield (sometimes called solid bonding).

In such cases, unearthed cables may be used so that the core insulation will have enough strength but current rating is de-rated to the value of earthed cables.

But it is always better to mention the type of system earthing in the cable specification when ordering the cables so that the cable manufacturer will take care of insulation strength and de rating.

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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. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

41 Comments


  1. sudip mukherjee
    Mar 14, 2024

    Can anybody tell is it righ as I heard the bounding metal (Cu.) tape for unearthed HV cable is thicker than its earthed counterpart


  2. BALAMURALI S B
    Jul 14, 2022

    In furnace transformer, (DYn11), the secondary star point is not earthed.
    Which one suitable cable for primary side. (ie) earthed or unearthed cable?


    • Giri
      Nov 14, 2022

      That will depend on the primary side voltage (whether it is grounded system or ungrounded system)


  3. Vivek Kumar Singh
    Oct 17, 2021

    Use unearthed cable for 11KV system.


  4. Ajay
    Dec 31, 2020

    Hi I use 11kv AB cable Route Length 8km.
    But many problems cables failed falut creat of up side note conductor side
    Why


    • Krishna Chandavar
      Jan 29, 2021

      You may have used 11 kV (E) cable for an 11 kV (UE) system. The insulation level for 11 kV (E) cable is lower. You are advised to check the upstream system if it is an UE system which means that the upstream transformer neutral is grounded through a resistance. You should use a 11 kV (UE) cable.


      • Hareesh
        Sep 29, 2021

        Please explain the ground to nutral voltage is restricted in less than two voltage ..In ESD safe work table.. please explain all things.


  5. Ramesh bavarava
    Sep 16, 2020

    sir, let me know which type of cable we can use for 11 kv supply from discom (PGVCL sub station). 11 KV earth cable or 11 KV unearth cable. please inform me.


    • Ajay
      Dec 31, 2020

      Use of earth cable


    • Krishna Chandavar
      Jan 29, 2021

      You should use 11 kV (UE) cable. All the Discom receiving stations have their transformers grounded through a resistance. They are not solidly grounded like LV systems. Therefore you must use a 11 kV (UE) cable.


  6. Amit Mishra
    Feb 18, 2020

    Dear Sir,

    I need to know that 11KV Cable can be used for 6.6KV System for Operation.
    I also want to know the advantages & disadvantages of usages of 11KV cable for 6.6KV Voltage operation.


  7. S. MADHUSUDHAN
    Feb 17, 2020

    HOW we can identify the given cable is earthed or unearthed without checking nomenclature mentioned on the cable top.


  8. Kiran R G
    Oct 14, 2019

    Hi team,

    If NGR using 11kV DG’s normal we use UE cables. Suppose we are not used UE cable from DG 11kV panel to its feeding transformer (2.5mva ,11kV/433V ) . One major L-G fault happened during the 11kV DG time what would the effect of fault in DG etc


    • Prashant
      Jan 11, 2022

      The 11 kv Cables can sustain for a higher phase to ground voltage. which is evaluated during the load cycle of the cable accessories and the test voltage is 2.5 Uo and time is 60 cycles of 8 hours. Here the point of discussion is unearth cable will have a ph to earth voltage 1.73 Uo during a fault situation. So I think the 11 KV cable will sustain for this application.


  9. srikanth
    Jan 09, 2019

    Hello team.,

    Can we use 1.1kV/650V cable for 800Vac unearthed system rated at Inverter O/P terminal.

    Thanks & Regards
    Srikanth


  10. tej
    Dec 03, 2018

    dear peter,
    can we use 11kv/11kv UE cable to 11kv/22kv Earthed system? are they same?


    • Lal parth
      Jan 27, 2023

      Yes u can use 11kv ( UE ) / 22 KV ( E ).


  11. NB
    Sep 29, 2018

    Hi Guys,
    Can any one explain can we use 11KV cable instead of 3.3KV cable, if not what is the reasons?
    Regards
    NB


  12. O SHRINU
    Apr 06, 2018

    Nice and updated information .
    A good platform of Electrical engineering.


  13. ravi kumar
    Mar 06, 2018

    In 33 kv unearthed system 3CX240SQ.MM Unearthed cable we can use the Earthed straight through kits and earthed end kits .if we use what will be the result.


  14. rajesh Thapa
    May 11, 2017

    Can any one tell me the logic/ reasons, why do we make use of the cable 1.1kV grade /insulation for low voltage system ( house wiring wires 220 440 V AC) or reasons for using 1.1 kV insulation cable in 11kV transmission lines and as well as in internal wiring system?


  15. Alok
    Mar 09, 2017

    What if the system is effectively earthed grounded through NGR to Limit the earth fault current upto 50A (33/3.3KV, Delta star) Should I use 1.9/3.3KV or 3.3/3.3KV Grade Power cable


    • Bharat BHosale
      Mar 29, 2018

      3.3/3.3KV Grade Power cable(means UE cable) shall be used (at secondary i.e. 3.3 kv side i.e star side) as the system is with NGR (means neutral is grounded with resistor).


  16. Alok
    Mar 09, 2017

    What about 33KV/ 3.3 KV system earthed with NGR to limit the fault current upto 50A (max). Should I use 1.9KV/3.3KV or 3.3KV/3.3 KV


  17. snehalkumar
    Nov 11, 2015

    why so much stress on your face??? relax and smile!!


    • Vijayakumar Jayapal
      Mar 06, 2017

      why?


  18. SUBRATA
    Aug 05, 2015

    Thank’s for those type of information !!!


  19. DK Sharma
    Mar 24, 2015

    We use 11 KV cable to connect our Distribution transformer which has its primary or HV winding in Delta connection and Secondary or LV winding in Star connection. Since Delta connection has no earth, we always need 11 KV Unearthed (UE) cable.

    So we should always ask for 11 KV UE cable.


    • Hooshang Karimi
      Aug 07, 2015

      Dear Friend
      Please, your comment is not true. You have mixed upstream and downstream networks.
      If you are talking about earthing of 11 KV network, which have supplied your transformer, you should follow secondary winding of upstream transformer( HV to 11KV), which usually it is resistance grounded and increases required insulation level by 1.73 times.
      Bye and have a nice time.
      hooshang karimi Ahvaz – Iran


  20. Manuel Bolotinha
    Mar 15, 2015

    This article about earthed and unearthed cables is very basic and with poor technical content.


  21. AMEER AHMAD
    Mar 15, 2015

    I am not agree with “As a thumb rule we can say that 6.6KV unearthed cable is equal to 11k earthed cable i.e 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.”

    Reason: For 6.6/11kV Earthed cable, the system nominal voltage is 11kV. If we use 6.6/6.6kV Unearthed cable for 6.6/11kV Earthed system with 11kV nominal voltage, the phase to earth voltage remain 6.6kV for Earthed system(Agreed), but phase to phase voltage remain 11kV for Earthed system and this 6.6/6.6kV Unearthed cable can not meet the 11kV nominal system phase to phase voltage insulation.


  22. vikas j trivedi
    Nov 02, 2014

    How to calculate feeder htvr through metlab or etap


  23. JAYESH R MANKAD
    Jul 25, 2014

    It is good and interesting web. Informative topics are covered


  24. noureddine
    Jan 26, 2014

    Mr. Peter
    you must check set time for overload in your breaker
    may be the setting is wrong.
    thanks.


  25. guest
    Jan 03, 2014

    How’s 6.6kV+6.6kV = 11kV? As far as I know 6.6+6.6=13.2


    • Parag
      May 08, 2014

      It means it is suitable for 11KV insulation requirement (since 13.2KV>11KV). Standard cable insulation ratings are as 1.1KV, 3.3KV, 11KV, 33KV…)


  26. Balachandar
    Dec 10, 2013

    I can not agree this statement ie., 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.
    but we can use 6.6/11kv cable for 6.6/6.6kv system.


  27. Peter Abbey
    Jan 21, 2013

    I have a question and it goes thus:
    A motor Filter Fan’s RPM on motor data plate is 3545 RPM, Relay overload set is 2.9A. During solo run, motor runs at 1.6A (starting current) at 3615 RPM. The LV motor tripped at 4.9A with enclosure door closed during load run. What are the possible causes of the motor trip? What can be done to avoid such trippings? (What are your recommendations?)
    (the motor specification are: Voltage = 460 V, Power = 1.5kW (2HP), No. of Phase = 3,Frequency = 60Hz,No. of poles = 2)

    Your prompt response will highly be apprexciated.
    Kind Regards,
    Peter.


    • Ragnarok
      Mar 04, 2013

      Hi Peter, the starting current of 3 ph induction motor has nothing to do with the load. (Load affect the run up time and thermal withstand)

      The starting current for induction motors can as high as 7-8x the rated full load current. By your description of 1.5kW 460 3ph motors, I would think 4.9A sounds like a reasonable starting current. (not taking PF/eff into account, the rated full load current is at least 1.89A)


    • Teguh
      Mar 08, 2013

      Dear Mr Peter,
      if the motor trip at load run, means we have to check the load first. is it suitable for your motor or not, you can ask the mechanical engineer for that. well, for the motor itself, you can do normal check such as its impedance, megger, bearing etc.


    • AKReddy
      May 13, 2013

      I too experienced similar problem, in my case motor – fan belt coupled, we loosen the belt and it worked for us

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