Type A R.C.D. 6mA tolerant.

I'm not sure about all D-locks, but I have played with one of that  type that injected a few hundred mA between L and N, very well smoothed, and ramping up quite slowly - took over a second . 

The Zs test pulse from L-E was then in comparison huge (something like 20A or so from memory) but, and this is key, only present for the half cycle of mains that was in the same polarity  as the locking current already applied. In terms of the BH curve, the ‘lock’ current took it up onto one of the flat wings of the curve where more primary amps (H) does not mean any more secondary volts (B'), and then the test pulse took it further from the centre in the same direction. 

Had the test pulse been reversed so it was un-doing the lock, then a test current comparable to the lock current would have returned the magnetic core back to the high slope region near zero magnetisation, where it does work like a proper transformer, and it would have tripped. Even so the locking current was not enough to fully saturate  all designs of RCD, and even with that tester they still tripped.

Mike.

PS

volts on secondary is volts per turn = B' = dB/dt = rate of change  of magnetisation over time in teslas/second, so you keep the volts right down by changing the current slowly, or for things like ignition coils you get a large back EMF on an inductor by changing current quickly.

I'm not sure about all D-locks, but I have played with one of that  type that injected a few hundred mA between L and N, very well smoothed, and ramping up quite slowly - took over a second . 

The Zs test pulse from L-E was then in comparison huge (something like 20A or so from memory) but, and this is key, only present for the half cycle of mains that was in the same polarity  as the locking current already applied. In terms of the BH curve, the ‘lock’ current took it up onto one of the flat wings of the curve where more primary amps (H) does not mean any more secondary volts (B'), and then the test pulse took it further from the centre in the same direction. 

Had the test pulse been reversed so it was un-doing the lock, then a test current comparable to the lock current would have returned the magnetic core back to the high slope region near zero magnetisation, where it does work like a proper transformer, and it would have tripped. Even so the locking current was not enough to fully saturate  all designs of RCD, and even with that tester they still tripped.

Mike.

PS

volts on secondary is volts per turn = B' = dB/dt = rate of change  of magnetisation over time in teslas/second, so you keep the volts right down by changing the current slowly, or for things like ignition coils you get a large back EMF on an inductor by changing current quickly.