I delta M

This is a RCD so breaks only one kind of fault. Are they suggesting 1000A if no death and glory 100A fuse but 10000A with one? I have searched a lot of literature but cannot find a definitive answer.

I think that is probably what it means.

This link may help - see page 11.

As I understand it, I_m is the rated breaking capacity and I_Δm is the rated residual breaking capacity.

Confusingly, in BS7671, I_m is something completely different.

yes - because the fuse goes first and by cutting the arc short limits the energy available that would presumably otherwise irreversible burn up or weld the  RCB internals - presumably to an energy equivalent to about 1 % of the time that the RCD part alone takes if damage goes as I squared....

Energy limiting  fuses really do save the day. It is not really a cleanly switched rectangle of current of ten times the magntude for 100th of the time of course -that's just the mathematical equivalent energy.
 
The actual effect is much weirder; as the fuse starts to open it inserts a time dependent resistance, as the energy from the voltage dropped accross it is dissipated to create first the molten metal then vaporising it and then  throwing it  about, hopefully all the while staying inside that ceramic cylinder.

I really can't see why they need to put Im -= I delta m as you say is an RCD - so a plain  Im would do.

The many alphabet soups that different makers use to describe the same thing does not always make things clearer.

Mike.

I really can't see why they need to put Im

Just thinking aloud... I guess the RCCB could be asked to break L-N currents that are larger than the residual current in a few circumstances - e.g. a bolted L-N-PE fault on a TT system. Hopefully in that situation the fault protection would open first, but maybe the thinking is that by specifying Im, the effects of L-N faults on the RCD have to be considered as well.

   - Andy.

I really can't see why they need to put Im

Just thinking aloud... I guess the RCCB could be asked to break L-N currents that are larger than the residual current in a few circumstances - e.g. a bolted L-N-PE fault on a TT system. Hopefully in that situation the fault protection would open first, but maybe the thinking is that by specifying Im, the effects of L-N faults on the RCD have to be considered as well.

   - Andy.