Why did my mains trip when I used a multimeter on a disabled socket?
That can happen very easily by testing, or shorting, the neutral to earth (ground).
To the elcb for the supply that presents as an excessive trip current so it shuts off the suppy.
Best way to avoid that is to isolate the neutral for that circuit at the consumer unit as well, this avoids the neutral currents from other working devices being an issue.