Rich, Sorry to contradict you but...
1) Agreed that doorbells operate at ELV and are typically double insulated >from mains circuits. Some standards would even call this SELV <grin>. 2) The ringing voltage for a domestic telephone (POTS) is above the ELV limits. When such a Telecommunications Network is connected to an item of ICT equipment then IEC 60950-1 would require you to apply the requirements for a TNV-3 Circuit to internally connected parts (IEC 60950-1 and its regional derivatives (being equipment standards) do not apply to the wiring in the installation). FYI, it is common for one pole of the telecommunications network to be 'earthy' - i.e. connected to earth at the central office but having an impedance to ground and a current-carrying capacity that would certainly not be adequate for safety purposes. Otherwise, broadly agreed. Richard Hughes In a message dated 09/12/2003 00:18:46 GMT Standard Time, [email protected] writes: In the USA, power for doorbells is floating SELV and is installed in the walls and ceiling voids. Same for telephone conductors. There are no requirements for the insulation of these wires as voltage is SELV and the power is considered less than that necessary for fire. Such wiring cannot be in contact with the mains wiring in those walls and ceilings. Furthermore, the mains wiring must be protected against insulation or conductor damage due to alterations, etc., by installing various protective devices and by routing limitations. Best regards, Rich
