Hello zia,
I think your fresher to telecom field, all the telecom equipment using -48 v standard , the volatage range in 48 v dc human body withstanding voltage more than this voltage you feel shock, below this volage system takes more currunt so we need to uncrease the conductor size so we are using 48 v dc ( why exactly 48 , the dc system normal standard is 6v, 12v, 24v, 48v. this is the systematic dc .