Re: And then there was silence
"For something that wasn't his fault."
Only it was his fault - his actions directly caused the power outage and I am assuming it was made very clear to him that powering down the building was bad.
The correct course of action would have been to clearly identify what he was working on and if he was unable to do that, advise who he was working for that there was an issue and come up with a way that the risks involved could be reduced (i.e. carry out the work out of hours or determine how to investigate further or even discover the incorrect cabling and plan to replace it all). i,e, perform the task in a professional manner rather than bodge it.
Instead, the electrician committed the same mistake as his predecessors who decided not to do things properly. The difference is they got away with it. And I suspect the investment bank likely had a history of working to JFDI with the unwritten rule being don't get caught.
If you substitute the electrician for an IT contractor, does your view of the actions/consequences change?