The protection for an electrical system should not only be
safe under all service conditions but, to insure continuity of service, it
should be selectively coordinated as well.
A coordinated system is one where only the faulted circuit is
isolated without disturbing any other part of the system.
Overcurrent protection devices should also provide short
circuit as well as overload protection for system components, such as bus,
wire, motor controllers, etc.
Procedures and Methods
To determine the fault current at any point in the system,
first draw a one-line diagram showing all of the sources of short-circuit
current feeding into the fault, as well as the impedances of the circuit
components.
To begin the study, the system components, including those
of the utility system, are represented as impedances in the diagram.
Below are the links to methodologies on short circuit
calculations.
OHMIC METHOD
Most circuit component impedances are given in ohms except
utility and transformer impedances. (Note that the transformer and utility ohms
are referred to the secondary KV by squaring the secondary voltage.)
PER UNIT METHOD
The per-unit method is generally used for calculating
short-circuit currents when the electrical system is more complex.
POINT TO POINT METHOD
The application of the point-to-point method permits the
determination of available short-circuit currents with a reasonable degree of
accuracy at various points for either 3ph or 1ph electrical distribution
systems. This method can assume unlimited primary short-circuit current
(infinite bus).
A comparison of the three method including comments can be found on this very informative paper made by Cooper Bussmann, which can be found here.
Other highly informative links about Fault Current Calculations below:
Short-Circuit Calculation Methods
All electrical systems are susceptible to short circuits and the abnormal current levels they create. These currents can produce considerable thermal and mechanical stresses in electrical distribution equipment. Therefore, it's important to protect personnel and equipment by calculating short-circuit currents during system upgrade and design. Because these calculations are life-safety related, they're mandated by 110.9 of the NEC, which states: Read more...
Fault Level Calculations (A USAID Presentation)
Fault level at any given point of the electric power supply network is the maximum current that would flow in case of a short circuit fault at that point. Read more...
Power System Circuit Breakers
To explain the difference between typical and series amp-interrupting capacity (AIC) ratings, let’s review the typical method. Fault current calculations are needed in an electrical system to ensure that the system can safely handle and protect the specified equipment during a fault. The interrupting capacity of a device is the “the maximum short-circuit current that a protective device can safely clear.” Read more...
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