Short circuits

A short circuit is an unintended low-resistance path, often across a supply. Current can become very large, but real sources have internal resistance, wiring resistance and sometimes current limiting; the actual current is finite.

Picture a river that suddenly finds a wide-open shortcut around the waterwheel. All the water rushes through the easy path and the wheel stops turning, while the shortcut channel floods. In a circuit, all the current floods the short and your real components get nothing.

Fuses and circuit breakers interrupt excessive current according to their time–current characteristics. They do not necessarily act instantly at one threshold; choose protection appropriate to the circuit and its wiring.