Posted by Ashley on April 10, 2007, 10:36 pm, in reply to "-48 volts" Similarly in the days of vacuumtube carrier systems there were two other voltages used, these being -24V for the filament supply which was positive grounded as for the switching equipment, and +130V for the plate supply of the vacuumtubes which was negative grounded. Note also that -48V is used to distinguish from the +48V supply used in Strowger type exchanges that is used as a meter battery source. This supply is also negative grounded. Additionally, there is also the +48V supply used in doublecurrent telegraph working where a Mark is -48V and a Space is +48V. This source is also negative grounded. And furthermore there is also the +48V supply used on very long two and fourwire physical trunks so that the effective operating voltage of the circuit becomes 96V instead of 48V; -48V is applied to one wire while +48V is applied to the other wire. --Previous Message--
The term -48 refers to the fact that the operating voltage for the exchange equipment is negative 48V with respect to the ground - the positve pole of the 48V supply is grounded.
: An instructor at a class posed the question,
: Why do phone companies use the term -48?
: Were did it originate? Were did it start? He
: says he is not actually referring to the -48
: power used in the central office but
: something else. I don't know? Can some one
: help?
:
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