A new terminology for electrical currents now include all faults those between live conductors as well as earth or grounding faults.
If we not design our circuit protection properly followed standard guide,it can cause serious injury or death for human and equipment damage.
Fault current be determined at every relevant point. For small and medium installations with LV utility supplies, this is satisfied by determination of the fault currents at the incoming power supply.
When we sizing the breaking capacity for circuit breaker,we need to measure the incoming fault currents or loop impedance.So when get the value,it can used these value to selected the right kA.
Protection for short circuit
In actual application,the major cause for electrical shock and equipment damage or fired came from short circuit.So when we design for electrical protection,overcurrent protection or circuit breaker must be able to
1) Withstand to short circuit current
Because it generated high value of ampere typically in KILO ampere.If you notice manufactured state the value of device breaking capacity at their circuit breaker in kA unit.Example 4.5 kA or 6 kA .
2) Tripping capability
For the circuit breaker,the time for disconnect the supply with sufficiently quickly very important to prevent damage to the cables or equipment.In other mean is to minimize the defected cause by high ampere of current.
The fault currents to be considered include faults between line conductors and earth, line conductor and neutral, and line-to-line conductors.
For most domestic installations the prospective fault current,the value can exceed until 6 kA, up to which value the Icn and Ics values are the same.
The highest fault currents will arise with three-phase line conductors shorting together and to earth.That why protection device must proper selected and meet the standard requirement.