What is the available short-circuit current?

The available short circuit current is the measured or calculated current that can be delivered to a circuit by a power source when a short circuit is present. This information is extremely important as an overcurrent protective device has only a finite ability to safely open a circuit when a fault condition occurs. Therefore the amount of fault current available is a critical piece of information in order to select the proper protective device.


The available short circuit calculation can be very complex and generally should be left
to qualified engineers to calculate. These calculations are generally based on the
following factors:


• How much short-circuit current is available from the utility?
• What is the wiring resistance from the utility to the equipment
where the fuse is installed?
• What is the internal resistance of the piece of equipment where the fuse is
installed


Once all of these factors are known, then the engineer can calculate the available short
circuit current to the fuse.


The fuse must be selected as to have a greater short circuit rating than what is
available in the circuit (otherwise the fuse can explode and cause great harm to
people and equipment!)

How to Calculate Fuse Rating for Electrical Appliances?