John Coombs did an excellent job of describing the C4A ringer (sorry about forgetting the 4 in my original).
What must be borne in mind is that the REN is there as an On-Hook DC Resistance measurement. The intent is to prevent premature ring-trip. Ring-trip occurs when the DC component of the current drawn from the Central Office exceeds a certain value. If you have too many ringers in parallel than the DC resistance will become so low that the current through the Central Office increases to the point where the Central Ofcie thinks the telephone(s) at the far end are off-hook and the Central Office will cease supplying ringing current and switch from the alerting mode to the connected mode. Ergo, if the combined DC resistance of the customer premises equipment, in the on-hook state is too low, the phone will never ring because the CO ring detector will think that the customer is busy. (Telephone companies don't like permanent off-hook conditions either). The Canadian regualtion CS-03 gives a formula to calculate the Evaluation Number as well as a method of how to measure it. It is a straight DC measurement and the capacity and the conductance of the circuit are not taken into consideration. Ciao, Vic
