Thanks, read that but I still feel I'm missing something... in that description it says..
but that doesn't apply if inverters are outputting to grid...
Yes it does. It periodically pushes a phase wiggle against grid to test that grid is still present.
But if there are 2 or more inverters, won't they keep each other in sync - or at least for quite a while?
When hybrid grid interactive inverter is connected to AC input source it is slave to AC input freq/phase/voltage. Once pass-through relay releases the inverter goes back to being its own master.
If you have inverters running in parallel, or series connected 240/120vac for split phase, there is usually an interconnect cable to keep units in sync when there is no AC input.
In addition to sync'g the two hybrid units, there is usually other interconnect control lines, like if one unit shuts down the other unit also shuts down, or if one is charging battery, the other is also charging battery and they both switch charging states (bulk, absorb, float) at the same time based on first unit jumping states. Again, if AC is present, cross sync'g control is overridden and they obey the phase of their respective AC input as slaves to AC input.
Inverter-generators that can run in parallel require AC output be connected first before one generator is started first and it becomes master for phasing. Second inverter-generator, seeing AC voltage present on its input when started, becomes slave to first generator started.
But for UK ENA Type-tested inverters, disconnection has to take place within 500ms, and I'm guessing the same for other jurisdictions?
Only way to detect an open circuit disconnect on AC input is by inverter pushing against phase lock to AC input. Pass-through relay allows inverter output to be present on open circuit AC input so voltage detection is of no use. There will be a current deviation push back when a legitimate grid is present. Grid is an 'immovable' object when inverter tests for grid presence by attempting to wiggle it phase lock to AC input.
Most cheaper, non-UL1741 compliant inverter just bias their phase locking circuitry so if AC input goes open circuit there is no phase locking reference and the inverter frequency will wander higher in frequency until it hits maximum allowed frequency spread limit of inverter before it releases pass-through relay. Some count how many zero crossings occur in AC cycle without any phase correction generated. If too long a period without phase corrections then it assumes AC input has been lost and releases pass-through relay.