[Dekatron]

Really nice that you found this problem but what a job to fix it!

Do you have any serial number or any thing else identifying the date on your three transformer tester, it seems strange that AVO didn't take the wire diameter into account (or perhaps not if you count all other errors that you can find in an AVO VCM).

Sometimes it is a good idea to check the contact resistance even in the mains circuit with the mains switch and of course checkking the cut out relay contacts for high contact resistance - briding them with wires like you did and comparing the result is a good way of finding these faults.