Household Circuits and Electrical safety

Friday 7-12-96

The relevant section in the textbook is 17.5

+++ This entire section is just for your own interest +++

Main concepts:

• Household circuits use alternating current, which in North America is at a frequency of 60 Hz. This means the current continually reverses direction, and reverses direction 120 times per second. The voltage is 120 V, and the currents in household appliances can often be 10 A or more. This combination of voltage and current is sufficiently dangerous that a number of safety features are built into a standard household electricity supply, as well as into standard household appliances.

• Fuses and circuit breakers provide a certain level of protection by limiting the current that flows through household circuits. These are useful, for example, for preventing dangerously-large currents from flowing for long in the case of a short circuit, which occurs when a conducting path of low resistance is somehow set up in a circuit (for example, by two wires coming in contact with each other that should not be touching). With the voltage set to 120 V, by V = IR when the resistance drops to a small value the current goes up to large values. This large current flowing through a fuse or circuit breaker will produce enough heat to melt a wire (in the case of a fuse) or bend a contact (in the case of one type of circuit breaker), interrupting the conducting path and stopping the current. Circuit breakers may be re-set after the problem is removed, while blown fuses must be replaced.

• Fuses and circuit breakers in household circuits are usually rated to handle relatively large currents, 15 A, 20 A, or 30 A. Many electrical appliances don't need nearly this much current, so they may have their own built-in fuses rated for smaller current. To provide the highest degree of safety, the fuse should be connected to the "hot" (high voltage) side of the circuit.

• Two ways in which individual appliances (meaning everything from electric lights to televisions to air conditioners) are made safer are by dedicated grounding and by the use of polarized plugs. A polarized plug has two prongs, one wider than the other, so it can only be plugged in one way. The thinner prong is connected to the hot side of the circuit. Because the manufacturer knows ahead of time which prong will be connected to the hot side, this allows them to build in safety features such as fuses on the hot side of the circuit, where they'll do the most good.

• A plug using dedicated grounding is a three-pronged plug. The middle prong is connected directly to ground, and the circuit is wired in such a way that if a short circuit develops, the current should flow directly to the ground through this middle prong, rather than to the ground through you.