In uk.d-i-y, Tony wrote:

When I first commissioned my sauna and spa, the RCD was forever tripping.
The manual said this was expected for the first few hours of use as the
heating elements (and there are lots of them) were new and could have
absorbed moisture. Somehow (perhaps someone could explain?) this trips the
RCD. After about a dozen resets all settled down as the manual said it
would. If its a new iron could this also be the case?

Your sauna heating elements have a heating wire, surrounded by mineral
insulation, with an earthed metal sheath around that. If the insulation
has absorbed a bit of moisture, it (to be anal about it: various ion
species now in solution and therefore unexpectedly mobile) will conduct
a small proportion of the electrickery - which would otherwise flow only
from live to neutral - from the heating wire to the earthed metal sheath.
Because the sheath is earthed, it's not dangerous to you, since the sheath
is kept at the same potential (a) as any other bits of metal in the sauna
- that's what your Supplementary Bonding is doing for you, and (b) as the
general mass of your surroundings. But the RCD is saying "hmm, there's a
bit more current going down the live wire than wot's coming back up the
neutral; don't like that; is it still going on? yup? ah well, time to
trip then". As the mineral insulation dries out from the heating, it
stops being nearly so conductive, and your RCD goes back to a state of
wariness rather than at action stations.

Although an iron *might* leak to earth a little more when first used than
after a while, I'd'a thunk it'd settle down much sooner than the 6 months
our original poster reports (unless in their household they have a
few-times-a-year ironing binge... like us ;-) It sounded more like the OP
had pre-existing leaks-to-earth which put their RCD right on the edge of
popping already.

Stefek