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.
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.
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.
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.
Related electrical guides & articles
In furnace transformer, (DYn11), the secondary star point is not earthed.
Which one suitable cable for primary side. (ie) earthed or unearthed cable?
That will depend on the primary side voltage (whether it is grounded system or ungrounded system)
Use unearthed cable for 11KV system.
Hi I use 11kv AB cable Route Length 8km.
But many problems cables failed falut creat of up side note conductor side
Why
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.
Please explain the ground to nutral voltage is restricted in less than two voltage ..In ESD safe work table.. please explain all things.
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.
Use of earth cable
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.
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.
HOW we can identify the given cable is earthed or unearthed without checking nomenclature mentioned on the cable top.
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
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.
Hello team.,
Can we use 1.1kV/650V cable for 800Vac unearthed system rated at Inverter O/P terminal.
Thanks & Regards
Srikanth
dear peter,
can we use 11kv/11kv UE cable to 11kv/22kv Earthed system? are they same?
Yes u can use 11kv ( UE ) / 22 KV ( E ).
Hi Guys,
Can any one explain can we use 11KV cable instead of 3.3KV cable, if not what is the reasons?
Regards
NB
Nice and updated information .
A good platform of Electrical engineering.