Loss of Mains

What ever the generator manufacturer recommends.

I'm so glad to see that the OP recognizes the limitations of their expertise. The generator manufacturer should be able to recommend a suitable switch but safe usage of this switch will require the selection and implementation of many more components than just this one switch. As I've often repeated, consult your local codes. Better yet, hire a licensed electrical engineer or electrician.

"Is there a recommendation for a specific device?"

Yes! An electrical service technician will work well for this application.

If DG sets are running in parallel with mains and mains is lost is not dangerous for DG Sets. There is a chance of Over loading of DG Sets and their tripping in cascade in case part of load of plant is shared with mains. In this case automatic load shedding can save total loss of power - designing dynamic load shed scheme is a complicated procedure which is achieved through PLC based Automatic Load Shed Control System (my firm deals in this).

If the grid feeder to your plant is a radial feeder dedicated to only your plant, then most probably you will not know that this has tripped.

But if the feeder is branched off to other consumers as well, once the grid fails, your plant will become the source of power supply to other consumers. This can be sensed by df/dt relay which can immediately isolate your plant from the Grid by opening grid incomer.

What is dangerous for DG Sets is that if main fails and suddenly it returns before you can know of its failure. In this case Synchronization between DG Sets and Main can occur out of Step - resulting in damage to DG Sets.

To ward off this situation - it is mandatory that remote end substation from where mains feeder is coming to your factory has a Check Synchronising Relays - so that Operator at remote end can not energise the feeder without out of synchronism with your DG Sets (which are back feeding the mains line up to that substation). similar facility shall be provided at your end on Grid CB.

In large Power Plants interconnected via 66kV lines and above, we have Carrier Communication and if interconnecting line between two source of power generation trips at one end it is immediately tripped by carrier tripping at remote end as well. Reconnection is via Central Load Dispatch Officer and through check syn system. Yours being a small station, carrier tripping is not feasible.

Hope your query is answered.

Best regards,

Ramesh Kapur