Overcurrent Protection, Part 2

June 9, 2014
Circuits experience overcurrent due to overload, short circuits, or fault current. What are these, and how do they differ?

Circuits experience overcurrent due to overload, short circuits, or fault current. What are these, and how do they differ?

An overload is a relatively small amount of excess current resulting from too much load. It might help to explain it with an analogy. If you normally carry a 50lb backpack and someone sticks an 8lb rock in it, you’ve got an overload.

A fault condition, by contrast, produces large amounts of excess current. Instead of a heavier backpack, someone hits you with a truck.

A fault is an abnormal current path. It adds to the normal current path, and this can easily be enough to melt the circuit conductors. That’s why circuits need fault protection.

Faults come in various forms. A fault may occur between phases. In the case of a ground fault, current flows between one phase (or more) and ground. Fault current isn’t not necessarily all of the available current.

A short circuit is a special case of a fault current. In this scenario, the fault is of such low impedance that the fault current path bypasses the normal current path, creating demand for all available current. Conductors subjected to abnormal stress (e.g., transient voltage) and/or excess moisture can short phase to ground, causing this kind of fault.

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