Answer Posted / ganeesh jain

There are earthed and unearthed cables for earthed and unearthed systems respectively.
Here, 1.9/3.3 kV = Uo/U = Phase to Earth Volt / Phase to Phase
1.9 X 1.732 = 3.3
Previously our systems were of small ratings and hence the short circuit faults were less and therefore lesser insulations were required for conductor. Whereas in advancement of time in modular system, huge fault currents are present and hence the insulation level on cables.
In earthed systems, Phase to earth voltage is 1.732 times the phase to phase voltage, where as in unearthed systems, phase to earth voltage can be equal to phase to phase voltage.
Therefore in earthed system ,stress on conductor to armour is 1.732 times lesser than the stress on conductor to conductor and hence lesser insulation is required between conductor and armour while in unearthed systems, since Ph-Earth voltage can be same as that of Ph-Ph voltage therefore the stress between conductor and armour will be more and hence the insulation.
Therefore manufacturers have designed 3 insulation classes for cables as per system design.
Insulation Class:
1. 100% : For solidly earthed systems, or on any system where the protective device will clear earth faults within 1 minute.
2. 133% : For systems where the protective device is expected to clear earth faults within 1 hour, and is typically specified for high impedance earthed or unearthed systems.
3. 173% : For systems where the time to clear an earth fault is indefinite. This is typically recommended for unearthed or resonant earthed systems.
 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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