To the original point, you can get a number of high-end SPDs that have alarm contacts in them, and those alarm contacts could certainly be wired to a Shunt Trip on the Main CB so that if the SPD ever did fail, the contacts would trip the Main CB. But the big problem I see with this is that on all the ones I have seen, the alarm contact ALSO changes state when the power fails. Now technically, if the power fails, you also lose the power necessary for the Shunt Trip coil to function. But because of the way that works, I'm not sure you could ever energize the breaker in the first place. That's because the Main CB feeds the panel, which then feeds the SPD. So the alarm contact on the SPD will remain in the tripped state until it says "Ready", which means power is applied to the SPD. But that means the Main CB must close first, and any delay in the SPD alarm changing state means the Shut Trip will shut the Main CB off again immediately. You could get around this with a push button that opens the ST coil circuit I suppose, but then EVERY time the user wants to re-energize after a power failure, someone has to stand there and hold the button down to bypass the SPD shunt trip circuit, then energize the Main CB, then let go of the button AFTER the green "OK" light on the SPD comes on. That won't happen right on a dark and stormy night at 3AM...
I suppose that could also be done via a timer that delays every time power is energized, but the whole thing is getting a little ridiculous.
