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if you earth one side then you can use an RCD
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But the RCD requires you to get a shock to trip, whereas if it's floating you don't get the shock in the first place.
I just did some tests with my own bench isolating transformer. This is a 1650VA Carroll and Meynell (3kVA 30 mins) 230-230V rated. It has an earthed inter-winding screen and earthed core.
At the time of testing, the mains was 242V and the off-load secondary voltage 247V. Our mains intake at the workshop is TN-C-S and my bench is near it, so the N-E voltage at the bench is always trivial, in this case 0.15V.
Insulation test at 1kV DC. Insulation Pri-Sec, Pri-E and Sec-E >1GΩ. Using Megger MIT310.
Open-circuit AC voltage from each end of Sec to earth: L1-E 64V, L2-E 118V. Using Fluke 289.
Short-circuit AC current from each end of Sec to earth: L1-E 15μA, L2-E 29μA. Using Fluke 289.
I then attempted to measure the capacitances with the Fluke but the readings jumped around wildly. I don't know how the capacitance range works on that but it clearly didn't like the capacitance being distributed over the inductance, so I switched to a simple LCR meter on its 2nF range, which uses a measurement frequency of 1kHz. This read all three capacitances as lumped values with no change according to which end (or both) of the winding was tested.
Capacitance Pri-E 1.0nF, Sec-E 0.53nF, Pri-Sec 0.352nF.
The theoretical series value of Pri-E and Sec-E would be 0.347nF, so the very close measured value of 0.352nF, and the fact that that neither reading changes when the other winding is shorted to earth, suggests that the two windings are effectively screened from each other. Leakage current measured from Sec-E is therefore confined within the secondary, and exists only due to its stray capacitance and not to the presence of a TN supply on the primary.
I was about to calculate the equivalent source impedance of the secondary earth leakage current but I can't immediately confirm the input impedance of the 289 on AC volts which is needed as it's in the same order of magnitude as the source impedance (it's not resistive IIRC). Will return to the subject later.