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Why -48V is used as power for telecom?

mahesh khandagde - 4th January 2018  (06:28 GMT)

To make returning power subside we r using -48 and 48 is just stander set by IEEE

KJB - 20th June 2010  (10:18 GMT)

Agreed with Manooj:)

manoj - 20th June 2010  (06:14 GMT)

Why -48V is preferred?

(i) Positive voltages cause comparatively more corrosion in metal than negative voltages.
(ii) Negative voltages are safer for human body while doing Telecom activities.
(iii) Lightning may cause positive voltages in the equipment circuitry. In that case, negative voltages (lack of electrons) neutralize positive charges and prevent excessive heat.
(iv) Negative voltage is safer for long telephone line for transmitting power trough it.
The reson for electing -48V in reference to ground is to avoid the nasty galvanic effects in telephone cabling. +48V would have nastier effect on wet cabling.

naglaa - 8th April 2006  (19:44 GMT)

can i put telephony cables with twinaxial cables in the same conduit?

Harjeet - 8th May 2004  (20:08 GMT)

HI every body,
I'm replying very late
Sammour has rightluy pointed about chemical effect.
Actually in olden days there was over head bare cables used for telephone systems & in order to reduce the chemical effect from halogens (which are the most one responsible for corrosion)which are -ve charged and are repelled by -ve voltage. Hence -V. The figure of 48 was taken becuse it was a compromise between safety (of electrocution) & power loss due to R of cables HENCE -48V

Pankaj - 6th April 2004  (17:54 GMT)

You should also keep this in mind that all of USA uses 24 (+Ve) system not 48 (-Ve) system. Hence all the arguments do not really hold much grounds. these are two different systems, that's all. the voltages had to be in the multiples of basic 2 V hence -48 V or 24 V, actually it turns out to be -54 V and +27V respectively. However there is one fact though, positive ground is more efficient then negative ground.

Vijay - 6th April 2004  (10:58 GMT)

Ahmad, dont go mad on it, :) actually there was no technical reason behind it. Bell first used 48 v bcoz they used 4 batteries of 12 volts , that time only 12 volts batteries were common so they used it. Now every body following them thats all.


Ahmad Anas - 5th April 2004  (00:32 GMT)

As for as I've heard, -ve volt is used because to avoid any interfernce due to noises as noise voltages used to be in positive side. Why 48V is used?

waleed - 4th April 2004  (07:31 GMT)

sorry dear ahmad i miss understand your question, I thought you are asking about the voltage (why 48 instaead 24)not why - 48and not + 48,in case of -48 the +ve polarity is connected to the ground(equipment chassise)and the dc current flows from +ve to -ve so in this way the we will avoide this current stream carring distortion and interference to log in the electronic circuits gaining high reliability and pure DC supply.

Ahmad - 1st April 2004  (00:25 GMT)

Can some experts answer for this question..

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