System Design: If the positive terminal of the power supply is grounded (i., set as a 0V reference point), then the entire casing, cabinet, and wiring of the communication equipment will transmit a negative voltage (-48V). Physical Principle: In humid environments, metal conductors carrying a positive voltage (positive pole) are more likely to attract negative ions from the air, leading to electrochemical corrosion and causing cables and terminals to gradually rust and break. Additionally, operating at a safe voltage reduces the need for costly, thick wiring. The -48V is also known as positive ground. If a certain degree of search on this topic, there will always be a "convention" or "there is a large number of negative charges in the air, according to the knowledge. For 48-volt sites, these typically operate with a positive-ground configuration, or occassionally with a negative-ground configuration. Remember that voltage is defined only as a difference between two points; neither of these has a defined voltage relative to earth.
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