Difference between Unearthed Cable & Earthed Cables
January 2, 2013 32 Comments
Introduction:
- In HT electrical distribution, the system can be earthed or unearthed. The selection of earthed/unearthed cable will depend on system. If distribution system is earthed then we have to use cable which is manufactured for earthed system. (Which the manufacturer specifies). 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. Examples: 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. Examples : 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 (some times 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.
JIGNESH PARMAR
Plz mention where unearthed cables are used in detail.(with applications)
Yes Sir,
Thank a lat for your Electrical Notes.
hi sir
above note is very useful one
Thank you.It is very useful to me
Lot of useful information u have provided..Thanx to u sir..
Please describe me that can I used 11 kv xlpe HT cable in place of 23 KV HT cable ?
thank you for your explanation
Really useful one.
Thanks a lot for providing these much valuable information. But I am unable to download your Electrical Tools. Pl. brief me how to download the same.
Thanks in advance
Satyam
if your lose “a” phase HV earth on your 11 kv transformer what will happen.
is 11kV(UE) equal to 22kV(E)?
We have power transformer 132 kv /11kv , primary star connected and solid earthed & secondary 11 kv delta connected & two out put ckt .The impedence voltage of the transformer 20%.My question is 11kv earth panel is suitable for this application or going for 11 kv unearth panel for motor & transformer feeders.
THANKS FOR YOUR INPUTS
The explanation of ungrounded and grounded cable is very useful.
But considering the IEC 60502-2, the cables need not to be ungrounded cable if the fault is cleared in the MV system within one hour explained as category A,B and C.
Generally, most of the MV system (3.3, 6.6 and 11 kV) are earthed through LRG. In IEEE 141, it is mentioned that the ungrounded system whose fault cannot be cleared within 1 hour need 173% insulation.
So, I am really confused, do we need unearthed cable for a MV system with LRG clearing the fault within 1 min or 1 hour.
Sir,
Can you give any comments if semiconducting screens are required/ provided to make earthed 3.3kV grade cables suitable for 3.3kV/ 3.3kV unearthed system, which is normally not provided with 3.3 kV system?
Dear sir,
In a distribution transformer(11/440 DYn11) primary is not grounded.So,which kind of cable is preferred out of E/UE.
Kindly send your response pls…….
Which cable is better for 33kv underground work ?
XLPE Insulated, Armored, PVC Jacketted size, voltage, material of conductor
When using wye-broken delta earthing transformers, should we rate cables as like unearthed systems or earthed systems?
Nice and useful…
Thanks for above IMP information bcs I was selected a UE cable for 11kv distribution transformer which have delta primary thank’s allot. …
Dear Sir,
I have 33 KV/ 11KV 2.5 MVA Delta /Star solidly grounded Step Down Transformer. The 33 KV Input is through over head Conductor and output is 11 KV cable to VCB. Now We are going to install one more 11KV/415 volts Delta Star Transformer at a distance of 390 Meter distance through overhead 11 KV 3 core cable. Please sugest which cable we have to select Earthed or Unearthed. Also exp;lain why we selected for the same.
This is only applicable for IS, in IEC no specific un-earthed cable.
Dear sir can please mention which specific standard shall be followed for 33KV Unearthed cable. As you know that IS 7098 part-II and IEC 60502 (up to 30 KV)don’t cover it
Thanx sir. Above explanation was very useful.
sir,if the cable tray is not earthed any problem for the mv cable running on the cable tray
In unearthed system,Neutral is earthed through a resistance to reduce the fault current,obviously,it can be called as restricted neutral system rather than unearthed system.
Can we use 22 KV earth HT Cable in place of 11 KV unearth cable
In which system[Earthed/Unearthed] ELR to be used?
Thanx a lot for information.
can anyone pls. advice the cable voltage grade for 22kV resistance earthed system with IEC standard reference.