To isolate the circuit. I lighting circuit has a finite resistance with ground, so if we do not use isolating transformer then the whole circuit will show practically zero meggar(IR) value. Then it will be very tough to identify a cable fault or anything like that.
2. why 380V..?? instead 415V.
In India and many country, the normal bulbs/tube lights/fan and other electrical appliances come with a rating of 220V,ac.
if we use 415V/415V lighting transformer, then P-N volt will be 415/(sqrt of 3) =240V ac. which will cause more power consumption as well as less equipment life. So, use a 415/380V LT, which will give 220vac output. saving or power & equipment life.
