Electronic – Max current draw from a UK Phone Line


Reading a question on SuperUser where someone received an electric shock from a modem made me curious, how much power can a phone line provide?

(I'm not looking to do anything dodgy like try to power something, this is just out of curiously/amusement; in the UK phone lines operate at ~50v (up to 100 with an incoming call) enough to overcome the resistance of human skin in the right conditions. 10mA would give a nasty shock but 100-200mA has a high chance of fatality – in theory could someone actually be electrocuted by a phone line?)

Best Answer

This source states that BT provides 40mA when the phone is off the hook.

This other source talks about the expectations of the ringer circuit:

  • less than or equal to 5mA draw at 35V/25Hz ring signal
  • less than or equal to 10.5mA draw at 75V/25Hz ring signal

PowerDsine's North America ring module generator appears to be limited to 3W, so I would expect the PSTN provider's capabilities to be similar. This signal gets shorted out when the phone goes on-hook, so the source can't be too stiff.