Why -48volt in telecommunication?

Answer 1

Let's take the voltage first. The exchange 'battery' (or CO if you are in the USA) is set between -48 to -55v depending on where you are in the world, and to some extent, the type of exchange you are connected to. The figure of -48v is not set in stone. No need to worry if it is above this.

This battery voltage drops the minute a telephone is taken off of hook. The exchange 'equipment' senses the current flowing and drops the voltage to around 9 to 11 volts. The On-hook voltage stems from a day of long lines, electro-mechanical switching and phones.

They next point is that in an idle state a telephone line has a NEGATIVE potential with the EARTH being positive. Take out a voltmeter and set it so you can measure 50v DC. Place the POSITIVE probe to a known good earth and the NEGATIVE probe to the A and then the B legs respectively. (Ring & Tip in the USA). You will measure 48-55 volts on one leg, and around 4v on the other in relation to earth. The exchange battery has it's positive terminal put to Earth. Why? Simple. Corrosion. Positive earth systems don't suffer from corrosion (caused in a similar way to electro plating) in the same fashion as negative earth systems. Something the telephone companies worked out a very long time ago.

Answer 2

48 VDC "Ring and tip" is used for for fixed home telephones.

Answer48 VDC "Ring and tip" is used for for fixed home telephones. Answer48 VDC "Ring and tip" is used for fixed home phones by US telephone companies today.

Yes they do. It is still called the central battery voltage.

(and the abbreviation is Vdc, upper case V for volts, lower case dc for direct current)

Answer 3

Why is any voltage needed ? Power is needed on an analogue wired telephone to power the telephone from the telephone exchange or central office, especially to amplify the low energy levels in human speech. Its also used for signalling things like you want to answer a phone call and powering your tone dialler

Some DC current flowing on the line also has the advantage of "wetting" the line. Joints and switching equipment are less likely to add a crackle to your phone conversation with "wetting"

AC or DC ?

The power must be DC rather than AC so that the power supply does not interfere with the voice transmission.

Why 48v

The voltage needs to be high enough to power the telephone over reasonably small diameter cable, over a pair of cables which may run for more than 5km.

The voltage needs to be low enough not to be dangerous to linesmen or not so high that it breaks down the insulation between the conductors or arcs across gaps.

48v meets the above criteria, its also the sort of voltage you can get from a reasonable number of lead acid cells in series ( or even 4 x 12v car batteries)

Exceptions

• Many PABXs which don't have phones on such long lines only use -24v
• Bell's first telephone didn't need any DC voltage on the line because his microphone was not amplified, you just had to shout very loudly.
• Pair gain systems use higher DC voltages as they have to power more than one phone over the same phone line e.g. the BT 0+2 DACS uses 140v
• In the early part of the twentieth century many telephones where operated with local batteries, rather than the "common battery system we use now. Your local telephone man would come round to replace the "dry cells" every once in a while.

Why minus ?

The - in -48v DC refers to the voltage when compared to earth on one of the conductors.

Telecom lines sometimes come in contact with the ground, if there was a positive potential compared to earth on them in damp ground electrolysis would eat the copper of the phone line away.

but with a negative potential compared to earth electrolysis tends to deposit material on the cable, which means the fault can be repaired with cleaning rather than replacement, which is cheaper end easier. Search for

"Sacrificial Anode" to find out more.