Why Bother with a Short Circuit Analysis?
What a Short Circuit Analysis Does...
The Short Circuit Analysis, standing alone, reveals important information concerning the protective devices in the electrical system. Protective devices, such as breakers, fuses, etc. must be properly rated in for the potential current during a fault - in other words, during a short circuit. Protective devices that are improperly rated will not function correctly during such an event, possibly resulting in damage to equipment in the facility's electrical system. The Short Circuit Analysis provides a critical component of any Arc Flash Study. Without it, an incomplete picture of an electrical system's safety condition is presented.
How a Short Circuit Analysis Works...
A short circuit analysis evaluates the short circuit current at locations in the electrical system based on the utility source voltage and the impedance between the source and a point in the electrical system. All short circuit contributors are taken into account during a short circuit study. Sources include: utility power source, generators, transformer impedance, reactors, cable impedance and motor contribution.
Short circuit currents introduce large amounts of destructive energy in the forms of heat and magnetic force into a power system. Calculations are performed to ensure equipment short circuit ratings are adequate to handle the available fault currents. The magnitude of the short circuit current is independent of the load and is directly related to the size or capacity of the power system. When determining the magnitude of short circuit currents, it is extremely important that all sources of short circuit current be considered and that the impedance characteristics of these sources are known.
Generally, the largest contributor of fault energy is the power source. It is important to have the electric utility provide its short circuit contribution at the point of service, along with the X/R ratio: the reactive component per resistive component. Other sources such as generators and motors must be accounted for, as they also produce fault current.
It is important to calculate the duty imposed on the electrical equipment when an electrical fault occurs and to determine the necessary rating of the equipment. Short circuit momentary duties and interrupting duties are calculated according to the maximum available fault current at each switchgear bus, switchboard, motor control center, distribution panelboard, and pertinent branch circuit panelboard, as well as at other significant locations throughout the system.
In addition, the settings and selection of protective devices, such as protective relays, fuses, and other direct-acting trip devices, should be made based on the short circuit current study results.