27.18: Household Wiring And Electrical Safety
Companies that supply power to most modern households use three conductors, typically called a three-wire line. While one is neutral, the other two are both at 120 V but with opposite polarity, giving a voltage of 240 V between them. With a three-wire line, high-power appliances that require 240 V, such as electric stoves and clothes dryers, are linked between the two hot lines. 120 V appliances can be connected between the neutral and either of the hot lines. The neutral side, which is always connected to the ground, is one of the sides of the line. The ground is a real electrode driven into the Earth's surface in houses.
The power supply is constantly connected in parallel with the numerous bulbs, motors, and other devices that need to be powered. If appliances were wired in series, turning off one would turn off the others.
The resistance of the wires limits the maximum current available from an individual circuit. Fuses and circuit breakers protect against overheating and overloading of circuits. A fuse has a link made of a lead-tin alloy with an extremely low melting temperature; when the current exceeds the rated current, the link melts and breaks the circuit. An electromechanical device known as a circuit breaker serves the same purpose by tripping the breaker and cutting off the circuit when the current exceeds a specific limit. The advantage of circuit breakers over blown fuses is that they can be reset after being triggered.
A short circuit results from contact between the line's hot and neutral sides. Such a situation, which can be brought on by faulty insulation or mechanical malfunctions, offers a very low-resistance current path, allowing a huge current. If this current is not cut off by a fuse or circuit breaker, it would quickly melt the wires and ignite their insulation. A damaged wire obstructing the current route and causing an open circuit is similarly risky. The possibility of sparking at the site of sporadic contact makes this dangerous. According to accepted wiring practices, only the hot side of the line should have a fuse or breaker, as hot/live wires supply the current to operate the appliances.