Surely the test button of an ordinary 4-wire RCD passes the test current from one pole of the supply side to the other pole of the load side, to create an imbalance without a connection to the CPC? Anyhow, dumping 30mA into the CPC shouldn't have any effect on the apprentice, so I'm with the inductive / capacitive load idea or the neutral opened first allowing it to jump towards line for a moment.
My thought would be that if you were in the position of having someone working on a circuit that was not DP isolated, you would want them to be hands-off at the moment you opened the DP isolation, or as an emergency measure at least trip all other the MCBs first so that there's no load on the system in case the neutral was the first to break.
And, um, lockouts?
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