Earthed System :
In Transformer, 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 metallic sheath is 1.732 times less than voltage stress between conductors to conductor.
Where in unearthed system, (if system neutral is not grounded - Delta connection) phase to ground voltage can be equal to phase to phase voltage. In such case the insulation level of conductor to metallic sheath should be equal to insulation level of conductor to conductor.
Example: 1.9/3.3 KV (E) , 3.8/6.6 KV (E) cable
Unearthed System :
In transformer, 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 (UE) , 3.3/3.3 KV (UE) 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.
As a thumb rule we can say that 6.6KV unearthed cable is equal to 11kV earthed cable i.e 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.
And, in single core HT Cables, Sheath shall be earthed at one side only means single point bonding.
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.
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.
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.
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