Following on from this thread to save dragging it Off Topic:
http://www.vintage-radio.net/forum/s...273#post966273
There's a Wikipedia article here:
https://en.wikipedia.org/wiki/Ringer_equivalence_number which says that it is a thing that's used internationally, though with differing definitions.
I know that the GPO/BT maximum of 4 is conservative and, unless you are on a very long line, it'll really be more. When I was young and daft (guess which one of those still applies
) I connected a load of 700s to a BT line and they all rang. Assuming the ones with 500Ω coils and no resistors have a REN of 4 I must have been pulling about twenty.
The problem I've had with trying to work it out is not knowing the inductance, and therefore the Inductive Reactance of bell coils.
The only problem I've had was quite recently. I converted an ATM T3903 to a 232 configuration. I remarked at the time that it's bell coils were 1,000Ω and didn't expect this to matter. With a 2.2kΩ resistor that bell made a pathetic tinkle but was fine without. I then found that for all that that one worked it was pinching all the current so changed those coils for 2,000Ω ones. I did notice that the originals seemed very skinny so am inclined to think that it was their Inductive Reactance, or lack thereof, that was the problem. I also wonder if this is why things like 250s and 704s have two 250Ω coils which are short and fat so, maybe, more inductive than the usual type.
I've never fully understood the need for resistors as if I put several bells in parallel the total Inductive Reactance will increase and reduce the current drawn anyway. If this is the case though I can't see why the GPO would have bothered to change over to 2,000Ω.