Sub-panel replacement then dead appliances
Location: San Francisco, CA
We had to have a sub-panel replaced to get our system up to code for a larger remodel. The electrician hired by our GC turned off the power to the unit (3 BR condo), replaced the panel, then re-energized the system. My partner noted hearing several "pops" at some point during the repair (unsure if prior to re-energizing or not) We later discovered that several appliances in the house were dead, including dishwasher, microwave, stereo receiver, and others. A repair technician for the dishwasher confirmed that the controller board was fried and the entire appliance needed to be replaced.
The electrician did say at one point that they found a "back feed issue, but it was on a different circuit. Not the ones the appliances were on". When we pressed them on their responsibility for frying the electronics, they said that the back feed was probably due to some unpermitted electrical work in a previous renovation, and since the panel was a like-for-like upgrade, they could not be held responsible. We did not perform any unpermitted work, so if this is the case, it was done by previous owners.
The GC sent the electrician back out after we expressed our concerns, and he plugged a voltage detector into several outlets to show that it was reading 120V. His claim was that was sufficient evidence, and that there was nothing in their work that could have caused the fried electronics.
I have a million questions, but I'll try to keep them tight.
Assuming that they are telling the truth about a back feed issue, was there a way that they could have and should have detected it before re-energizing the panel?
Do they have a responsibility to detect these kinds of issues and disclose them?
If there was improper wiring from previous work from before we owned the home (3+ years), why would it only manifest itself on a panel upgrade?
Is there a risk to us or the home since the issue was not corrected?
What should our next steps be?
We are currently out thousands of dollars in appliance replacement cost. We're getting stonewalled. We had a third party electrician come out to examine the work. He said that the workmanship looked good, and the evidence points to a "lost neutral", but couldn't pinpoint a definitive cause. We only have the effects, no causes.