As per what Barren said, secondary side of the transformer won't show up as a loss in a residual current device.
RCD only reads current in and out of the primary winding.
Also, bear in mind you can still get belt from active to neutral even with a RCD installed.
You can also get a belt of less than 30ma with a RCD installed, you will still feel it but it isn't enough to trip the RCD.
