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Posted by Tomi Engdahl (195.238.204.193) on July 18, 2001 at 04:09:48:
In Reply to: why -48Vdc posted by jack on July 18, 2001 at 02:43:29:
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: Does anyone who can tell me why the most of telecom equipments use the -48Vdc for their input voltage rather than the 110/220 Vac?
-48Vdc has been a traditional voltage for telephone
applications. The reason for selecting that voltage
has need that the voltage was enough to supply enough
power to telephone equipments though several kilometers
of telehone copper cable, low enough not to cause
major electrical hazards and suitable for battery backup
system. Large battery backups are used to guaranee
that telecom equipments can get constant power
no matter if mains power fails. Telephone switches
started using -48V DC in the beginning, and more
equipments needed to be placed on telecom company
premises adopted this practice because this was the
system telephone companies had already the infrastructure
for this.
The text above described why 48V and DC, but you might
wonder one other thing. You might ask why -48V DC
insted if +48V DC. THe reson for electing -48V in
reference to ground are avoiding the nasty
galvanic effects in telephone cabling
(+48V would have nastier effect on wet cabling I have read).
When comparing -48VDC system to 110/220V AC system
in telecom environment it can be seen te following
points (plus and minus facts):
For -48VDC system:
+ directly suitable for powering normal telephone lines
+ widely installed in telecom company premises
+ easy to do bagttery backup
+ less dangerous than 110/220V AC system (electrical shock sense)
- larger losses in wiring when power is transferred (larger currents needed)
- equipment power supplied need to be lower volume special designs
compared to normal mains power supply (not more complicated though)
+- different installing and safety practices than normal mains power
For 110/220V AC systems:
+ widely used electrical distribution
+ most power supplies are designed for those voltage
+ voltage can be easily converted using transformers
+ high voltage transfers high power with pretty low current
- dangerous voltage and dagerous frequency
- potential source for AC frequency noise
- hard to do battery backup (UPS devices are complicated)