Voltage monitor for emergency supply - has monitoring relay switched, during AC failure from main- and sub-batteries to main

摘要

The voltage monitor, for emergency power supply systems, has its voltage-monitoring relay switched from the main and sub-batteries to the main battery when the a.c. supply fails and switched back again to the main and sub-batteries when the a.c. supply returns. All the batteries are connected over DC/AC converters to the a.c. supply. This manner of switching the relay keeps the maximum change in the load voltage during switch over as low as possible, i.e. keeps its change smaller than the voltage swing between the batteries' lowest discharge voltage and the constant charging voltage. In a further embodiment a protective resistance is provide for each opposed contact of the switching relay to limit the current in the event of a short circuit.