In the cabinet where your circuit boards have been turning to toast, do you have a receptacle fed from the same panel that supplies the electronics in that cabinet? If so, this allows you to check the panel that supplies the cabinet without having to remove the panel cover (or even find the panel).
Take these two voltage measurements: line-to-neutral (L-N) and neutral-to-ground (N-G).
A low L-N voltage could be due to:
Assuming your system is correctly wired and bonded, you'll see the other 2V on L-G. Why? Because current doesn't (normally) flow in the ground conductor, it has no voltage drop. This means it acts like an extended test lead from your DMM to the NG bond.
In Part 8, we'll see why this is useful.