Ground Faults on Ungrounded Systems

Аннотация

Ground Faults on Ungrounded Systems. В данной работе рассмотрены процессы протекающие при замыкании на земли в сети с изолированной нейтралью.

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.

The current transformer will always read zero current in a healthy system even under a full load condition. In accordance to Kirchhoffs laws, Incoming and Outgoing currents will cancel each other out. Assume a 10A load connected to a 480/277VAC system. 10A will be fed from the source into the load, therefore 10A will have to return from the load back to the source. The CT will measure both simultaneously since it is placed around all conductors. The values were randomly chosen. Below is what the CT would see at a specific moment in time:

In accordance with the schematic below: 10A - 5A -5A= 0A for a healthy system

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. <