Lighting circuit trips RCD

[Shinner]

Hi
Have recently had a house extension fitted and when the builders connected up their electrics they connected their neutral to the wrong side of my 17th edition consumer unit.
Both RCDs were then tripping.
I spotted the mistake and corrected it, however the RCD which is not on the new circuit side would not reset.
I removed the neutrals from that side and got it down to one lighting circuit.
That circuit contains transformers, so I assumed that they had blown them, however I took them out of the circuit along with any dimmer switches and bulbs, but the RCD will still not make.
Nothing has been changed to this circuit, so I can only assume something has blown that I don't know about.

Any thoughts or ideas as I have no hair left?

 

[spadxiii1918]

When in someones elses property I drop the test voltage to 250 for IR test just to make sure I don't blow their elecronics.
If in doubt I normally connect phase to neutral and test to earth as per guidance note 3 for a 500V IR test.
But i agree with spark1 check out where they have been poking around, might be a trapped cable.
Time consumng I know, good luck with it.

 

[scotsparky]

spadxiii if you drop the test voltage to 250 you arnt doing a full test. If i am unsure its the GM3 method 2 way at 500v