Input Power Supply for Telecom Equipment: -48VDC

[juanpico]

Dear Experts

Until now I don't have the real reason why do all telecom equipment power supply is in -48 VDC

Any bright answer for everyone's info

Best Regards

juanpico

[zeuscane]

The value 48 volt derives from the telephony, in fact at the end of 1800, the 48 volts they became the standard of the batteries for this technology.
The negative value is given by the fact that, in the first fittings, this tension was reported to the earth and the copper of the electrodes it consumed him in smaller way.

zeuscane

[rfworld]

Dear Experts

Until now I don't have the real reason why do all telecom equipment power supply is in -48 VDC

Any bright answer for everyone's info

Best Regards

juanpico

In telecom systems D.C. Power supply is used due to the following reasons

1. Harmonics of A.C may affect the speech signals.
2. Relays used in telecom systems are more sensitive to D.C than A.C
3. Transistors and I.Cs etc. being unidirectional devices, the use of D.C had become necessary.
4. Earlier arranging standby source to A.C was difficult compare to D.C for which secondary cells(i.e.battery) can be used as S/B source.

Reasons for earthing of one pole of D.C are as follows:
1.Switching can be single pole.
2.Cross talk and other disturbances can be avoided.
3.Earth return signalling can be used.

Reasons for earthing positive pole of D.C
1. In electrolysis positive electrode will be normally corroded. If we keep our lines and equipment at negative potential, we can minimise troubles from the corrosive effects.
2. Partial Earth faults can be definitely identified if the conductor is negative. Otherwise fault is liable to seal up owing to oxidation.

and -48v is used as zeuscane said above.