# Differences Between Earthed and Unearthed Cables

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## 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.

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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. Membership No:M-1473586.He has more than 13 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 Deputy 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.

1. 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?

2. 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

3. 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

4. snehalkumar
Nov 11, 2015

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

• Vijayakumar Jayapal
Mar 06, 2017

why?

5. SUBRATA
Aug 05, 2015

Thank’s for those type of information !!!

6. 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

7. Manuel Bolotinha
Mar 15, 2015

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.

9. vikas j trivedi
Nov 02, 2014

How to calculate feeder htvr through metlab or etap

Jul 25, 2014

It is good and interesting web. Informative topics are covered

11. noureddine
Jan 26, 2014

Mr. Peter
may be the setting is wrong.
thanks.

12. 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…)

13. 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.

14. 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