Ok. So let me tell you the whole picture.
I have a 2.5 KW generator set. That i operate on Petrol. No ATS whatsoever is used. I am thinking to install an ATS so that it can start automatically whenever Wapda goes unavailable - which is the basic function of an ATS. But here is a problem. The load gen. is way too much (every time i start it i have to switch off two refrigerators, One 1.5 tons AC and etc etc and reduce load down to only fans and tube lights) This is the reason i can't use ATS on it.
Now to tackle this problem i made a hypothetical solution. Why not i should have another circuit that will be connected to the outlet of my UPS. Since UPS has its on dedicated wiring it only operates those appliances which are on its wiring such as fans and tube lights (UPS was the Load C i was refering in me previous example)
This circuit should check Supply A that is Wapda if it goes off it will switch to supply B that would be generator - which would have been already started by the ATS as wapda was not available.
Now an ATS is required which should have a 5 minute delay in switching circuits from wapda to generator. And in that five minutes that other circuit (which will have UPS as its laod) will switch to generator from wapda with a 10 second or so delay (less than ATS delay) and should be continuously checking wapda for its feed and ignore generator feed once wapda is restored and switch to it in a 3 minute delay time - letting the wapda stabilise. Set aside the delays for time being I will discuss the them later with more clearance. But at the moment its important to make you understand basic function and requirement i need.
This other circuit which alters the UPS supplies will make sure generator is not overloaded as UPS is a 1000 Watt capacity and generator is 2.5 KW capacity so whatever the laod on ups is generator can easily handle it. Any overloading on ups will trip the ups all the way - which is a basic function of UPS kit. Hence no overlaod on generator too.