I know that most telco switch rooms run their equipment from a common DC supply (at least in the US).
I understand the advantages of a common supply. But what is the advantage of using a negative rather than a positive supply voltage?
A Hamlet*:
Galvanic corrosion takes place on the anodic surface. With a positive supply voltage, this means that the any part of the wiring with leakage to the ground plane will corrode. With a negative supply voltage, the ground plane will become pitted and corroded, in the presence of a leakage path and an electrolye. This is often less of a problem as there is usually far more of it to corrode..
*Wild stab in the dark...Bet you'll find more 24v than 48v now.
Most larger switches (phone systems like sbc or att and ect use) are negative 48 volts....
*All* larger switching systems are...
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