Here's a new method to calculate short-circuit currents, one we like to call the “Easy Way kVA Method.” You can use in it in place of the abstract “per-unit” method of short-circuit calculations from the past. But for many, fault current calculations have always been difficult to get a handle on, until now. Otherwise, the equipment will explode as it attempts to interrupt the fault. With today's high fault currents, it's more important than ever to protect electrical equipment from extremely high current levels. The second largest source is from motors within a facility. Obviously, the largest source is the electric utility, although the high- and medium-voltage lines leading to the facility do have finite impedances, as does the utility service transformer.
There are three basic sources of short-circuit current: the electric utility, motors, and on-site generators. Using basic Ohm's Law (E = I × Z or I = E ÷ Z) as a guide, it's obvious that if the voltage remains constant and the impedance suddenly decreases, approaching zero, then the current must simultaneously increase, approaching infinity, to satisfy Ohm's Law. However, a short-circuit path bypasses the normal current-limiting load impedance, resulting in excessively high current values that are restricted only by limitations of the power system itself, and by the impedances of the conductive elements that still remain in the path between the power source and the short-circuit point ( Fig. 1, the impedance of the electrical load limits the current flow to relatively small values. Short-circuit current is the flow of electrical energy that results when the insulation barrier fails and allows current to flow in a shorter path than the intended circuit. Major changes would be considered a change in feed by the electric utility, a change in the primary or secondary system configuration within the facility, a change in transformer size or impedance, a change in conductor lengths or sizes, or a change in the motors that are energized by the system.Įvery electrical system confines electrical current flow to selected paths by surrounding the conductors with insulators of various types. Baseline short-circuit studies should be performed when the facility electrical system is first designed, and then updated when a major modification or renovation takes place - but no less frequently than every five years. Short-circuit currents represent a tremendous amount of destructive energy, which can be released through electrical systems under fault conditions.