An electrical circuit should send as much current back to the source, the plug for our needs, as it sends to the equipment being powered. If leakage occurs then that could be electricity going through somebody or something and to ground rather than back through the plug.
So a difference of, from memory, 30mA will cause the RCD to function for safety. It should happen faster than you will take to die from electrocution.
That is very reassuring to know Neil, I only intend on dying once so if I can put it off for a while then that's a good thing?! LOL
My RCD's kept tripping on the first job just by pushing the "test" button on (machine was switched off), wouldn't let me even turn them both on?!
I managed to find out that the circuit that my auto pump out was on had become damaged during cleaning it out at the weekend (okay, I dropped the waste tank on its side, my fault!!) & I thought nothing of it.1 new little part later & all's good, I'm just thankful for RCD's as without them I don't know what damage I could have done?
I know the whole point of an RCD is to prevent the use of an iffy supply. However Mark, I can tell you that one day you will be wishing you hadn't replaced your 13 amp plug with the RCD. It cuts down your options when you have a particularly sensitive property.
Also, they don't last forever. They need replacing. One day they work, the next not. Then you have to fiddle about fitting a new one on site.