sorry dear ahmad i miss understand your question, I thought you are asking about the voltage (why 48 instaead 24)not why – 48and not + 48,in case of -48 the +ve polarity is connected to the ground(equipment chassise)and the dc current flows from +ve to -ve so in this way the we will avoide this current stream carring distortion and interference to log in the electronic circuits gaining high reliability and pure DC supply.