What is Fault Current?

You might notice that breaker (or MCCB) always denote with the “KA” value. But what is KA actually?

KA is the amperage value (in Kilo Amp) for the fault current.

Fault current, also called "short-circuit current" (I_SC) (and sometimes called symmetrical fault current) describes current flow during a short.

It passes through all components in the affected circuit. Fault current is generally very large and, therefore, hazardous which due to "fault condition" caused by the low-impedance, phase-to-phase or phase-to-ground connection.

The electrical distribution system should be designed to minimize the effect of a fault. When we experience a fault on an electrical system, the protection equipment will detect the fault and trip out the faulty circuit. It cannot however trip instantaneously, and a delay, (of up to a few seconds) will occur.

In a simple word, it prevents a short at an outlet from shutting down power to the entire building!

The formula for the short circuit current (I_SC) withstand capability of an electric cable is:

Where:
Isc = Short circuit rating of cable (kA)
A = Area of conductor (mm²)
t = Time to trip (in seconds)
K = 96 for PVC, Copper conductor
    = 62 for PVC, Aluminium conductor
    = 116 for PAPER, Copper conductor
    = 78 for PAPER, Aluminium conductor
    = 143 for XLPE, Copper conductor
    = 98 XLPE, Aluminium conductor

Example: For a 70mm², PVC insulated Copper conductor, the 1 second short circuit rating is:

Isc= (96 X 70) / sqrt 1 = 6.72KA

If the protection is fast, (say 0.2 seconds) then the conductor will have a 0.2 second short circuit rating of:

Isc= (96 X 70) / sqrt 0.2 = 15KA