Question

overhead cables are never kept taut why should they be kept slack

Answer 1

Over head cable lines are usually made of aluminium alloys which is a good conductor of electricity as well as heat. They are kept sagging so that when they expand due to too much heat in summer and contract in winter they don't break or become dis shaped or affect the flow of current

Answer 2

because they expand in summer and contract in winter, so that they dont break they are kept loose.