Why -48 Volts used in telecom equipments?

Answer 1

(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.

Answer 2

Telecom carriers use -48vDC because it is easy to store in deep cycle batteries and can backup equipment during a power outages for much more time than a regular UPS. Depending on the load of the equipment you can easily get 8+ hours of backup before having to switch to a Generator.

Answer 3

During older days, telephone cables used to be laid underground. We know that when two metals are at different voltages then electro - depositing process starts and ions of metal at high potential start depositing on metal at low potential. This was the reason telephone circuitry used to be fed -ve supply in earlier days to maintain cables at low potential in comparison to the Earth to stop the gradual decay of telephone cables. All the communication related circuits developed afterwards supported the same practice of maintaining -ve supply to communication system. This is the reason most of the communication systems available today use -ve supply as a power source. Vineet Kumat vineet.kumar@in.abb.com

Answer 4

It is actually 55.2 volts, because it originally started out as the voltage of a serial stack of 24 lead acid cells on charge.

It all goes back to the very early days of telephony where signals had to be transmitted over long distances, before the invention of the triode (audion) tube, so amplifiers were not possible. Consequently a high voltage was needed, but not so high as to cause arcing or other breakdown effects, and not so high as to be dangerous. Thus the 55 volt standard was chosen as a reasonable compromise.

I suspect that the negative polarity would have been chosen because it reduces corrosion. There was a brief period in the 60's when there was a fashion for cars to have negative 12 volt electrical systems, but it caused so many problems that it soon went away.

Answer 5

For better Noise clipping we use -48v as all the noise are on +ve side