What voltage required to operate most of telecommunication related machines or boards as per my knowledge it is -48V if it is so tthen why and if its not then what voltage required?
on hook: -48vdc off hook: -10vdc answered call: +10vdc ringing voltage: 90vac at 20 Hz When the call is answered, the battery reverses, this tells the CO that the call has been answered. The CO is a negative source, a long time ago, someone discovered that there is less corrosion on the negative terminal, so the CO is a negative supply.
on hook -48VDC off hook ~9VDC ringing 90VAC
