Final answer:
Blown fuses and open breakers are typically caused by a short circuit, where electricity is directed to ground with no intermediate component, leading to an excessive current that triggers these protective devices.
Step-by-step explanation:
Failed circuit protectors, such as blown fuses and open breakers, are almost always caused by battery power going directly to ground with no component in between to stabilize the circuit. Fuses and breakers act as safety devices that automatically open or break the circuit when sustained current exceeds safe limits.
This usually occurs due to a condition known as a 'short circuit' where the electricity follows an unintended low-resistance path, often directly to the ground, bypassing the normal load of the circuit. A short circuit can lead to excessive current flow, which in turn can cause the protective devices to activate and interrupt the flow of electricity to prevent damage and hazards.
Circuit breakers are also designed to handle situations with high voltage, using insulating gases and jets of gas to help extinguish sparks that may occur as they interrupt the current. Particularly for AC currents, where the current goes through zero 120 times per second, giving opportunities to more easily extinguish any arcs that form due to high voltage currents.