A friend in Bangalore lives in a apartment complex that has power backup for all the flats. While the backup power is on, the residents can use only low power appliances like lights and fans. The moment they switch on a geyser or microwave oven, the power to the flat trips and is restored after a brief while. When it returns, if that appliance is still ON, it trips once more and this happens ad infinatum. He has no intention of bypassing the restriction but just wants to know at any given time if he is on the city power or backup power. What exactly is the mechanism that trips the connection for higher loads when the power is supplied by the backup source?