“Negative” 48 Volt Power: What, Why and How
Back in the day, when Telephony equipment was being developed, 48 was the chosen system voltage because it''s considered safe “low voltage”, and reduced amperage requirement of
Back in the day, when Telephony equipment was being developed, 48 was the chosen system voltage because it's considered safe “low voltage”, and reduced amperage requirement of equipment powered at this voltage.
Products basically use -48V power supply system, and the actual measured voltage is generally –53.5V. This is because for reliability reasons, communication equipment is equipped with a backup battery (-48V). In order to ensure reliable charging of the battery, the supply voltage needs to be slightly higher than the battery voltage.
For -48V system equipment, the required operating voltage range is -38.4V ~ 57.6V, but in fact we generally require the operating range -36V ~ -72V. The main consideration is that -48V system equipment must be compatible with –60V power supply system, which requires –48~-72V.
In fact, -48VDC allows telecom operators to use 12-volt lead-acid batteries wired in series to act as a backup power source in the event of a power failure. Negative 48VDC (-48V), or positive grounded, was selected for use by Bell when it was found to be superior to positive voltage.
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