The need for backup power for lighting and boiler operation is a problem in rural communities. A battery inverter system is an excellent short period solution but power outages can last longer than the battery capacity.
For a more substantial secondary supply an ICEPEG (Internal Combustion Engine Powered Electric Generator) is a good choice. However, an auto change over controller can be expensive. The above circuit has been designed to be a cheap alternative.
With the mains power applied RES, DIS, THR and TRIG are set to 0v. Thus the OUT tag is set to LOW. Power is applied to the load (Represented by the 18 ohm resistor) via the 2 c/o relays which in practice would be a 240v contactor with 2 n/c (mains supply L &N) & 2 n/o sets (secondary supply L &N).
Unfortunately, confusion arises because the earth symbol function is used for both 0v DC and Neutral on AC.
Upon mains power failure the (open the I/S) Rly1 will switch the common rail to HIGH (courtesy of the 12v battery) causing the TRIG tag to drop to LOW and thus the OUT tag to go HIGH. The npn transistor will then be at high impedance thus de-energising the relay on the right (Rly2).
After about 5 secs he timer will then set OUT to LOW the transistor will then activate Rly2. This delay is to ensure that the mains is off. Rly2 then applies the secondary power source (lower left) to the contactor which applies power to the load & isolates the mains.
The battery would require an overload protected trickle charger from the mains supply it could be in parallel with the load represented by the red resistor. This would mean that the battery would charge from either source!
With electric generator start the on board starter battery could also power the timer circuit etc., however, some form of voltage regulator would be required.
In addition the starter button assembly could be extended (pick-a-back) into a convenient box next to the mains Distribution Board.
If auto start is required then watch this space.
NOTE
Both Rly1 & 2 could be optoisolators.
Emergency lighting situated appropriately would be helpful. This could also taken from the common supply rail.
