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This question already has an answer here:

Why does telecommunications equipment in the field (e.g. a BTS site) run on -48V? What is the advantage of running it at a voltage lower than ground rather than using a positive voltage?

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marked as duplicate by PeterJ, Andy aka, Brian Drummond, Daniel Grillo, helloworld922 Feb 17 '16 at 11:24

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • \$\begingroup\$ I was about to post about the corrosive aspect but you found a dup \$\endgroup\$ – JonRB Feb 17 '16 at 10:23
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To reduce electrochemical reaction on grounded metal poles or parts which form a part of the circuit. Ionic reaction tend to move particles from negative (cathode) to positive ( anode).

Thus, by having the negative part of the circuit grounded, the metals associated with that grounding points will stay more healthy.

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  • \$\begingroup\$ If it's a -48V supply, then isn't the positive side of the circuit grounded? \$\endgroup\$ – Simon B Feb 17 '16 at 17:29
  • \$\begingroup\$ Simon B, you are right... I miss spoke here. Wrong fact...!! I'm sorry \$\endgroup\$ – soosai steven Feb 17 '16 at 22:37