Hmmm... Interesting problem. I'm trying to sketch out the arrangement and figure what might have caused it.
I'm assuming that the RCD(s) are located after the transfer switch (i.e. that you don't have separate RCDs on each supply before it).
How was the gen-set frame grounded before? Was it just bonded to the neutral/PME earth? If so, then now that the temporary rod has been added, is that frame-to-neutral bond still in place?
