Oh, now I understand what you mean. Yes, I think you are right, I can't rely on the transient to be fast enough to be detected by the transformer
Are there other ways to detect an earth leakage of DC current? Maybe I can move the current transformer before the diode bridge and use a power transformer with a central tap
As an alternative I can put a small resistor (e.g 0.05Ohm) between the circuit ground and the earth and detect the voltage drop across it, although lifting up the ground doesn't sound very safe to me (although the resistor is very small), not sure if this is allowed