Differences Between Earthed and Unearthed Cables

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.

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.


author-pic

jiguparmar - Jignesh Parmar has completed his B.E(Electrical) from Gujarat University. He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 12 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 Assistant 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.



7 Comments


  1. noureddine
    Jan 26, 2014

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


  2. guest
    Jan 03, 2014

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


  3. 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.


  4. 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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