A short circuit occurs when a low-resistance path bypasses the normal load. This causes current to surge dramatically. The (often denoted as I_sc ) is the maximum current that flows during this fault condition.
$$I_sc = \fracVR_sc$$
I once saw a 22kA-rated breaker installed just 10 feet from a 150kVA transformer. The available short circuit current was 38,000A. Two weeks later, a fault welded the breaker contacts shut. The fire department had to cut the main feed. short circuit current formula