raghu wrote
To me it appears a very odd way of wiring up an UPS or inverter - normally the line source is given to the input of the UPS and THAT decides whether it feeds the load or the incoming line feeds the load

dear raghu.
you are correct BUT what when a wireman unknowingly
connects main phase to any output load . i.e in larger
KVA systems the ups or inverter may be instaled on ground floor and the load maybe on first and second floors. during a fault in inverter or low battery the output will be lost. so in one case out of 100 on the
customer complain normaly the local wireman may think
that phase is absent and may short the output to any nearby phase on first floor to restart the lighting or computer that has stoped working due to absense of supply.
THIS WILL RESULT in shorting of two phases of INVERTER and MAINS and when the battery charges or inverter is repaired( when inverter output returns) . HENCE this has to be DETECTED and added to inveter protection tripping
ankas