[asterisk-users] Why Aastra uses 48V whereas other IP Phones use much less, i.e. 5-12V

[Shaun Kruger]

> Power is nothing to do with voltage (well it is, but not alone), you
> need the current too i.e. V * A.
>
> Pylon electricity lines run at very high voltage (several hundred
> thousand volts) or the current going down the lines would heat the
> cables and you'd lose a lot of power.
>
> 48V is just a telco standard, and most telco equipment (that runs in
> racks) is 48V. Probably because 110 (or 220/240 here in EU) is enough to
> electrocute an engineer, and 5V/12V would require too many Amps so
> wiring would have to be huge to carry the current.

The 48V standard came from what was the station battery.  This dates
back to very early telephone standards (think operators at desks with
patch chords).  The station battery is hooked up to power the
equipment with the positive terminal at ground.  On hook voltages
(between tip and ring) were derived from this battery.  Once a phone
goes off hook with 600 Ohms of resistance the voltage across tip and
ring drops to roughly 6 V.  The power coming over the line is expected
to be sufficient to power the phone.  Station batteries were intended
to be stable permanent power sources much an UPS except without the
conversion back to AC.

As a matter of further information regarding voltages:
Ring voltage is double the 48 volts alternating at 20Hz.  I believe
that the number of amps that can be driven determines the maximum
ringer equivalence rating for a circuit.  Ringer equivalence is a non
issue for most modern phones.  Especially phones with external power
sources.

This is reproduced from memory.  If I recalled incorrectly corrections
are welcome.

Shaun

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