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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 13 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

34 Comments


  1. 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.


  2. 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.


  3. 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.


  4. S. MADHUSUDHAN
    Feb 17, 2020

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


  5. 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


  6. 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


  7. tej
    Dec 03, 2018

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


  8. 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


  9. O SHRINU
    Apr 06, 2018

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

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