If the grid supply goes down for some reason (natural disaster, car hits transformer, significant sustained overload blows pole fuse etc) then everything connec

If the grid supply goes down for some reason (natural disaster, car hits transformer, significant sustained overload blows pole fuse etc) then everything connected to the grid in that area will see some or all of the following effects: * sudden change in supply voltage * sudden change in frequency (and rate-of-change of frequency) * sudden change in phase angle You can't mask these signals and they are particularly pronounced on the section of grid that was affected (i.e. the part that is now blacked out). These (and more) are the specific effects that inverters monitor for, and they will disconnect if they notice anything unusual. Inverters aren't checking for the presence of a grid supply, they're monitoring it's health. Your local lines company dictates the trip limits based on your address and size/type of installation. As for safety, compare some scenarios that a lines worker could actually find himself in: 1) he has turned up to a fault and found that a HV line is on the ground and the auto-recloser is still trying to re-liven it at periodic intervals. The voltages are so high that walking anywhere near the fault could result in him being electrocuted due to a step voltage. The power levels involved are high enough to cause a major explosion. 2) he has to open or close a circuit breaker in a switch yard. He has no way to know whether the breaker is going to explode when he racks it in. At least he probably won't know about it if it does blow up. 3) he turns up to a fault on a 230V line, tests it and finds that there's still voltage present despite the earlier fault. He visits the houses connected to that part of the circuit who have a grid tied inverter and manually isolates them before investigating further. That last one is hypothetical, since I've never heard of an inverter failing to turn off and even if one did stay on initially it would very quickly trip on overload after trying to supply your house and your neighbors all by itself. Testing the safety of a circuit before you work on it is an everyday part of the job for electricians. Finding that a circuit is still live is very common, and having one re-liven unexpectedly happens too from time to time. You just take precautions, wear protective gear, and do your best to minimize the danger.