How to decide the voltage level for DC (48V, 110V, 220V) in a power plant?

it's all about the device features 

for example if i chose a contactor with a simple circuit for control D.O.L i'll chose the 220V contactor 

if i use a special circuit for swimming pool and the place is have not any protection and i care about the maintenance man i'll chose 48V to be insure the effect will be not affevt him.