What are the differences between and reasons for -48 VDC and +24 VDC communications equipment?

Question

And, more generally, could you help me with an easy (ie. non-engineering) explanation of the differences between and reasons for positive and negative voltage based systems?
ie. Why would a service provider choose a +24 VDC base transmission station vs. a -48 VDC base transmission station?

Answer 1

From what I was tought in the Military, the US Army based its radios and equipment on the +24VDC standard to be powered by vehicles and DC generators already used by the Military. The +24VDC is used because the +12VDC used in most other vehicles made in the US at the time could not provide the power ( amperage) needed to supply radios and charge batteries at the same time. By using +24VDC vs +12VDC, existing wiring could be used by the civilian contractors that make vehicles for the Military; and the power capacity of existing systems can essentially be doubled. Otherwise all the wiring and components would have to be upgraded; increasing weight, manufacturing time, and cost. For example if you ever used a cheap set of jumper cables to jump your car you noticed that the cables got hot. If you jumped the same car with a set of jumper cables that were double the thickness of the cheapo set, the cable would allow you to jump the car, not get as hot and probably sent much more current directly to your battery.

Now the -48VDC was a Naval standard, also used by the telephone industry because the positive ground produces less corrosion on the contacts of the equipment than negative ground. Also inportant to the Navy was that with the large amount of wiring in a ship, and the longer distances spanned they needed to multiply the voltages again to make up for current losses without increasing weight. Military aircraft went to 48VDC for this same reason.

For the most part the invention of the AC alternator has eliminated most of these concerns, but the standards remain in place.

Answer 2

-48vVDC is for telecom equipment because it's based on batteries that powered the phones. Telcom equipment uses 21" racks unlike 19" regular network equipment uses.