Are RCDs any different?

Well, the mechanism notwithstanding, the current will flow until the next
zero crossing point. I didn't mean to imply that there was any magic or
electronics in this, just that it is what AC switches do.

However, RCDs are often much more complicated than MCBs and do often have
internal analogue electronics, rather than being simple mechanical/inductive
devices. This means that they could be designed to have the ability to sense
an impending zero crossing point before attempting to move the contacts. I
don't know if they actually do that, though, but I'd guess not.

The effect on the system as a "black box" though is that the current will
cease at a zero crossing point whether the RCD attempts to nurse its
contacts or not.

Christian.