[jacquesm]

That you might not be there when you need to go to island mode or that you might not even be aware that the grid is down because you and a bunch of neighbours are capable of sourcing enough power that your 'off grid' detection mechanism fails.

There are solutions for this, it costs a little bit of money and depending on local regulations it may or may not be allowed.

So yes, you can override this and if you mechanically disconnect from the grid then you are typically allowed to operate your installation in island mode. But if you get caught backfeeding the grid when the grid is down then you will more than likely lose your hookup.

Most cheap/light inverters need an external power source to sync to and will automatically shut down if the grid is not available for synchronization purposes, which makes them compliant with the demands of the utilities. If you want to go to island mode you'll need a system that is considerably more expensive (batteries, load based inverter rather than generation capacity based) than one that can't.

[balabaster]

There is no "detection" system. You are the detection system. There is no way of enabling your off-grid connection without shutting off the on-grid system. The interlock switch physically prevents you from switching on the trip for your off-grid system while your on-grid system trip switch is on https://www.youtube.com/watch?v=GbtRxcb-cmA

When the grid goes down, your power goes down. You then have to go trip the switch in your box to shut off the power between the grid and your house. Only then can you switch on the trip for off-grid power bringing the power back up in your house.

I guess you could have some kind of a relay where on-grid power would flow right through, but if on-grid power goes off, the relay redirects the circuit to your off-grid power - I think this is the basic principle behind the automatic transfer switch. Being on vacation or "not detecting a power failure" isn't really an option because as long as there's power, the power would flow into your electrical box from the grid. As soon as the power goes out, the relay that connects the circuit from the grid to your power box would flip to your off-grid circuit. The two circuits aren't connected but your household power can run from either one or the other.

I guess if you're wanting to sell power back to the grid, or supplement your power needs, that's different, but if it's a grid vs. off-grid situation, there isn't a problem.