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>> “The FETS in the inverters are a continuing concern.” <<

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I have yet to have any solid state circuitry in an inverter fail. What usually fails is the DC input terminals. If the clamp blocks are not bright and shiny clean, they heat up if you run them for a long time, and the heat is conducted along the silver rod to the circuit board, where it melts the solder and puts you out of business. (not to mention melting the plastic insulators where the clamp blocks go through the case)

I was referring to EMP hardening.

I am happy with my old Trace inverters, but had a surprise mode of failure in the system. This was a to ground (and batteries) surge that smoked my monitors (TRI-Metric and hybrid home built) on the DC panel. There was a primary to secondary contact on our line that blew the branch transformer to our distribution leg. Everything worked and clamped as designed, sending everything to ground. Everything on the backup side was shut down during this event, except for the grounding.

NEC requires that your boxes and battery negatives be tied to ground. This is understandable for minimizing potential differences, but would be a disaster during a large EMP event.

The FETS were found to be at risk during this (defense studies). Have gone back and forth with my Electrician (good guy) on this. Separate grounds need to be tied together. Period. There is no way around a grid tie in. Transfer box is grid tied. All junction and fuse boxes are grid tied also to ground. The only solution would be a very expensive throw switch for the inverters negative cable and non-conductive shelving. This won't %100 solve the problem because again the inverters are grounded, but probably will go a long way to reduce potential damage. I hate circulating ground current.

One other thing people who are off grid may want to consider. Water well pumps have long wire runs and home wiring will be an antenna.

JMHO. YMMV.