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Ground Faults on Ungrounded Systems

Floating systems are derived from a power source where there is virtually no connection to ground. 480VAC delta configured transformers are a typical supply for a floating system. Some deltas in the mining industry can be found in hoists. 480VAC deltas are also in wide spread use to supply 1000Amp - 2000Amp main feeder circuits in general industrial applications. Floating systems are often used in areas where a sudden shut down must not occur. Examples are intensive care units (ICUs) in hospitals, signal circuits, and emergency backup systems.

The magnitude of ground fault current in an ungrounded system on a first ground fault is very small. It depends on the system voltage, the resistance of the ground fault causing part and the system capacitances.

Example: If a grounded object with low resistance touches a live conductor, the resulting current flow will be negligible. The ground fault loop will be incomplete because the return path to the source is missing. Grounding may only occur through system leakage capacitance to ground. The Possible resulting current is also known as charging current.

Figure 1: The ungrounded system with single ground fault path

On a single ground fault, ungrounded systems will not produce the amount of fault current needed to trip a common GFR. The IMD is the device of choice for the protection of floating systems. BENDER IR series products are ideal for detecting ground faults on both AC and DC ungrounded systems.