- This topic has 10 replies, 1 voice, and was last updated 7 years, 2 months ago by Mr Raju kumar.
16th March 2008 at 16:50 #51569ShazzadGuest
Why we use -48v for GSM/CDMA equipment.16th March 2008 at 17:10 #51570zoyGuest
In Telecommunication generally we have 3 standards power supply to the BTS
1. 220/230 V AC
2. -48 V DC
3. +24 V DC (for outdoor BTS)
for -48 V DC generally we use negative polarity due to reduce corrotion loss.17th March 2008 at 03:46 #51571MKTGuest
First Part: Why -(negative)
Answer: To cancel the effect of noise, prevent ionic reactions etc etc.The line voltage was selected to be negative to make the electrochemical reactions on the wet telephone wiring to be less harmful. When the wires are at negative potential compared to the ground the metal ions go form the ground to the wire instead of the situation where positive voltage would cause metal from the wire to leave which causes quick corrosion.
Second Part:Why 48V?
Answer:The -48V voltage was selected because it was enough to get through kilometers of thin telephone wire and still low enough to be safe (electrical safety regulations in many countries consider DC voltages lower than 50V to be safe low voltage circuits). 48V voltage is also easy to generate from normal lead acid batteries (4 x 12V car battery in series).
Also, Telecommunications started with wireline first. Since GSM/CDMA came after it , they were designed to be compatible with the existing power arrangements available with the PSTN operators.
Regards19th March 2008 at 11:05 #51572Nyihemba A.Guest
we use -48v for GSM/CDMA equipment to reduce ionic reactions, electrical safty of many countries consider DC voltage lower than 50v to be safe.-48v can travel through thin wire faster.3rd April 2008 at 16:36 #51573mabsGuest
Q: “To cancel the effect of noise, prevent ionic reactions etc etc”
But isn’t it that the telephone wires are well isolated from ground. How would ions flow then? Also not sure with your chemistry explaination. Is there some refernce available to support your point?21st July 2009 at 14:18 #51574saeedGuest
why telecommunication uses negative voltage?16th March 2012 at 11:52 #51575StarkadhrGuest
Grounding the negative side of the telecom power plant was done to combat electrolysis. There was significant cathodic problems between the lead sheathed telecom cables and trolly car rails.
The nominal 48vdc was selected because it afforded the necessary potnetial to reach the furthest subscribers but still be safe to handle21st March 2012 at 11:49 #51576TKTGuest
its just simple
negative because – it reduces the chances of Noise ,, as noise tends to add more towards +ve signal
48 because – its safe ( you will not get a shock ) as in higher DC voltages , one may stick to the conductor ( In higher AC however you may be thrown off )…. and also 48 as 12 Volts was once an standard battery – 12*4=4822nd November 2012 at 13:08 #51577MikersGuest
Sorry, but this:
“negative because – it reduces the chances of Noise ,, as noise tends to add more towards +ve signal ”
is garbage. Electrical noise picked up on a conductor is not affected by the potential of the conductor.
This too is drivel: “as in higher DC voltages , one may stick to the conductor ( In higher AC however you may be thrown off )”
It’s an old wives tale, nothing more. When subjected to a shock, your muscles contract. In an Ac shock, they do not contract and expand at the line frequency (50/60Hz). Generally if you get a belt to the hand, your arm folds at the elbow and your fingers close. That may throw you off, or it may close your grip… ac or dc makes no difference.30th June 2014 at 07:02 #51578AykutGuest
-48V DC 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 through several kilometres of telephone copper cable, low enough not to cause major electrical hazards and suitable for battery backup system.
why -48V DC instead of +48V DC. The reason for electing -48V in reference to ground are avoiding the nasty galvanic effects in telephone cabling11th August 2014 at 12:11 #51579Mr Raju kumarGuest
V – 48 canvter