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This is due to the fact that the sky-lighting is negatively charged and because the negative-negative polarity repels each other. Hence, in this way you minimize the chances on the ground to get hit by sky-lightng
It is to prevent corrosion. When -48 is supplied to equipment the cabinet will be +v ground and this will make the cabinet more positive that ground. This will prevent the cabinet from corrosion and hence longer equipment life. Something called "cathode protection is the concept.
