Answer / chetram sharma 9928737032

As per my opinion the neutral point in case of ac supply system is earthed at every point viz generation,transmission and distribution for having balanced three phase supply so the Earth always provides us with the neutral and we can tap earthing anywhere from Earth and make use of supply system
But in case of a dc system the supply system network is not big as in case of ac system and Earth is not a part of it. It is always a small and local supply system which has to have its positive and negative lines live through wires. However we also provide earthings to equipments working on dc supply but to safeguard the equipment as well as signals from outer disturbances like thunders etc.

Answer / guest

BECAUSE DC IS NOT AC AS THEIR GRAPH IS LINEAR.

why transformer is rated in kva instead of kw?

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