Electrical Safety

The batteries we are using in our activities are low voltage, and are quite safe -- assuming, of course, that people can resist the temptation to swallow them, or throw them at each other! However, household electricity can be dangerous, and so it should be emphasized that these battery experiments are not to be repeated at home using the wall-socket kind; and a discussion of electrical safety should accompany the study of electrical circuits.

Electricity poses two sorts of hazards: fire hazard and health hazard.

Fire hazard
Electrical energy can start a fire if it gets out of control. This is equally true of gas, oil, gasoline, and even firewood; electricity is actually safer than the alternate sources of energy, because it can be turned off quickly and easily, and because it has to stay in wires (or at least in conductors).

The electrical fire hazard can be greatly decreased by always using equipment that has been designed for wall-socket electricity (that is, 115 Volts), and not using equipment that has a bad plug or a wire with broken insulation. Circuit breakers and fuses help prevent fires of electrical origin, by turning off a circuit when too much power is going into it.

Health hazard
The nervous system is based on electrical signals. An electrical current in the body will cause a very painful shock (all out of proportion to the amount of energy involved) because it confuses the human signaling system. A current passing through the chest is capable of causing the heart to stop beating, because the nervous system itself uses electrical signals to control the heart.

It only takes a current of 1/100 Ampere to cause a shock -- far less than the current required to operate a light bulb (about 1 Ampere). Fortunately, people are not good conductors, so that voltages up to 10 Volts can be handled with complete safety. Beyond this level, electricity should be handled respectfully. As we all know, wall-socket voltage (115 Volts) can cause serious and even fatal shocks. The dividing line between safety and danger depends on factors such as how dry your hands are, because wetness makes skin a better conductor. It is just the size of the current that is dangerous, and not whether it is ac or dc, from batteries or from the power plant. If you were to continue piling washers and damp paper until you had 200 washers of each kind, you would have a 90 V battery which could give you a very nasty shock. You should consider any voltage greater than 20 volts to be hazardous.

It is the current through the chest, and not the voltage, that is dangerous. Birds can perch on power lines because they are not part of a circuit: their feet are in contact with very high voltage, but there is nowhere for a current to go, through the bird. The first rule of electrical safety is to avoid being part of a circuit. Using wall-socket equipment in the bathtub or in a flooded basement or while standing on a metal ladder on damp ground are all bad ideas, because they make it more likely that you will become part of a circuit.

We use several strategies to make electrical power safer.
The first is to

The lamp and the user are grounded, as is one of the power wires

have all the conducting objects that you are likely to touch be at the same voltage. We say the objects are grounded because the traditional way to establish this condition is to connect everything to a pipe driven into the ground. Having the lamp base and the stove frame and the water pipes all at the same voltage means that you will not get a shock if you happen to touch two of them at once.

Whenever you plug something in, one of the slots in the wall socket is supposed to be connected to ground.

The lamp is grounded by the third wire

Fuses and circuit breakers lower the chance of a fire originating in an electrical device. They are detecting the amount of current that is flowing. When the current becomes large it means a lot of electrical power is going somewhere. If the current becomes too large, the circuit breaker decides there must be a short circuit, and disconnects the line. Unfortunately, the current needed to give a lethal shock is too small for a circuit breaker to notice.

The most hazardous place (electrically speaking) used to be the bathroom, because there were good conductors (pipes and water) and electrical devices (shavers, hairdriers, heaters) in close combination. This has changed somewhat by the requirement that all bathroom circuitry be protected by a "ground fault interrupter (GFI)." This is a device that compares the amount of current going into a device to the amount that returns from it. If these differ by as little as 0.005 Amperes, the circuit is broken or interrupted -- in 1/1000 second! Of course you should still be careful when using appliances that operate on household power, especially in situations where there is water and pipes around.

Outside power lines (overhead and buried) are at even higher voltages than in the home. The distribution lines out in the street are already at 7200 V. Things that we usually consider to be insulators (such as wood) are sufficiently good conductors to deliver a lethal shock, when high voltages are involved. Let's stay away from these power lines when trimming trees or working with ladders -- even wooden ladders.
Check the box when you are done:  
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