Ground Fault Detection by Differential Monitoring of the Float Current

The safety requirements for battery ground fault detection were recognised by UPS industry from its infancy. Detection of leak to ground as low as 4 mA is a standard feature in UPS battery installations. In Telco applications one of the battery busses is usually grounded, therefore ground fault detection appears to be redundant. Placement of the battery in closed cabinet initially removed the requirements of ground detection. However, the issue of serious failure of the system in the case of ground failure initiated the search not so much for ground detection as for leakage of current to ground. In some instances it is possible to use the same technique as for the "floating" battery. In this paper the specifics of the leak detection when the battery is intentionally grounded or connected directly to the AC will be discussed