Earthed and Unearthed Power Cables

Power Cables can be classified into Earthed and Unearthed cables. The terms "earthed" and "unearthed" refer to the application for the which the cables are to be used.

By Earthed system, we mean a three phase system whose star point is grounded directly. In this case, in the event of the earth fault, the voltage in the faulty phase will become zero. The voltage between the healthy phases and the ground will be the same, 11kV/1.732 or 6.6/1.732.

However, in case of an earth fault in an ungrounded system, the voltage between the healthy phases and the ground will be equal to the phase to phase voltage. i.e. 11kV or 6.6 kV.

Hence, the insulation level of these cables needs to be higher.

Thus a cable suitable for a 11kV earthed system will be suitable for a 6.6kV unearthd system.

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