In a way yes, it is the boiler fitters fault, if you install any new equipment you should test the supply first to see if it is OK, in the main I know we are lazy and stick in a socket tester only, but it can be debated with a FCU how much testing should be done. With a plug and socket we tend to take the attitude if they bought anything from a shop the supplier would not test socket, however we are installing a bit of fixed electrical equipment which the buyer can't simply return and say this is not good it trips my RCD.
I will admit I have a problem with RCD's as my clamp on will not measure 3.5 mA, so to measure that I would need to swap the FCU for a plug and socket and use my PAT tester, and I suppose that does raise the whole idea of using a FCU rather than plug and socket, although in an owner occupied home we may not need to inspect and test, in rented accommodation this is required.
We may call it PAT testing but correct name is "Inspecting and testing of in-service electrical equipment" so the boiler needs testing, there are two ways to test earth leakage, one is an insulation tester which uses 500 volt DC the other is a mA meter, but the latter needs inserting between the supply and appliance, so if you don't want 500 volt line to earth and neutral to earth then need to be able to unplug so a PAT tester can be used. And also of course the plug can be removed when the supply is tested.
Yes we all cut corners, but once there is a problem then that has to stop, and the appliance needs testing and the supply needs testing, if
@Sajid.Y calls an electrician who tests the supply with a RCD meter with everything but the boiler plugged in and the RCD fails to trip on 15 mA and does it again with boiler plugged in or switched on and it does trip at 15 mA extra he could justifiably say boiler at fault.
This then leaves a problem, electrician says boiler and boiler fitter says supply, far better for the boiler fitter to find the fault and if he can show the installation resistance is under the 1MΩ he can then high light the fault for electrician to fix. Not testing just leaves the boiler fitter open to claims for electricians time.
It may well be there is a fault with the supply, but it would enhance customer relations if the boiler fitter actually said to customer you have a fault on your toaster or supply it is xΩ to earth and should be over 1MΩ and it means if a capacitor is faulty on the spike filter on the boiler which is the most likely cause, then he does not get a bill for the electricians time.
