Safety Precautions in the Household

Electrical safety systems and devices are designed and widely used to reduce the risks of thermal and shock hazards.

Electrical Safety and Household Appliances

Electricity has two hazards. A thermal hazard occurs in cases of electrical overheating. A shock hazard occurs when an electric current passes through a person. There are many systems and devices that prevent electrical hazards.

AC Circuit Lacking Safety Features: A schematic of a simple AC circuit with a voltage source and a single appliance represented by the resistance R. It lacks safety features.

In practice, a simple AC circuit with no safety features is not how power is distributed. Modern household and industrial wiring requires the three-wire system, which has several safety features. The first safety feature is the familiar circuit breaker (or fuse) that prevents thermal overload. Secondly, there is a protective case around the appliance, as with a toaster or refrigerator. The case prevents people from touching exposed wires and coming into electrical contact with the circuit, helping prevent shocks.

Three-Wire System: The three-wire system connects the neutral wire to the earth at the voltage source and the user location. It exists at zero volts and supplies an alternative return path for the current through the earth. The case of the appliance is also grounded to zero volts. A circuit breaker or fuse prevents thermal overload and exists in series on the active (live/hot) wire. Wire insulation colors vary by region. It is essential to check locally to determine which color codes are in use, even if they were followed in one particular installation.

There are three connections to the earth or ground (earth/ground, ). An earth/ground connection is a low-resistance path directly to the earth. The two earth/ground connections on the neutral wire force it to exist at zero volts relative to the earth, giving the wire its name. This wire is therefore safe to touch even if its insulation is missing. The neutral wire is the return path for the current to follow in order to complete the circuit.

The two earth/ground connections supply an alternative path through the earth to complete the circuit, since the earth is a good conductor. The earth/ground connection closest to the power source could be at the generating plant, while the other is situated at the user’s location. The third earth/ground connection involves the case of the appliance, through the green earth/ground wire, forcing the case to be at zero volts. The live or hot wire (live/hot) supplies the voltage and current to operate the appliance. The three-wire system is connected to an appliance through a three-prong plug.

Three-Prong Plug: The standard three-prong plug can only be inserted one way to ensure the proper function of the three-wire system.

The Three-Prong Plug

The three-wire system replaced the older two-wire system, which lacks an earth/ground wire. Under ordinary circumstances, insulation on the live/hot and neutral wires prevents the case from being situated directly within the circuit, so that the earth/ground wire may seem like double protection. Grounding the case solves more than one problem, however. The simplest problem is worn insulation on the live/hot wire that allows it to contact the case. When lacking an earth/ground connection (some people cut the third prong off the plug because they only have outdated two-hole receptacles), a severe shock is possible. This is particularly dangerous in the kitchen, where a good earth/ground connection is available through water on the floor or a water faucet.

With the earth/ground connection intact, the circuit breaker will trip, thus requiring appliance repair. Some appliances are still sold with two-prong plugs. These appliances, including power tools with impact resistant plastic cases, have nonconducting cases and are called ‘doubly insulated. ‘ Modern two-prong plugs can be inserted into the asymmetric standard outlet in only one way, ensuring the proper connection of live/hot and neutral wires.

Color-Coding

Insulating plastic is color-coded to identify live/hot, neutral, and ground wires, but these codes vary throughout the world. Live/hot wires may be brown, red, black, blue, or grey. Neutral wires may be blue, black, or white. Since the same color may be used for live/hot or neutral wires in different parts of the world, it is essential to confirm the color code for any given local region. The only exception is the earth/ground wire, which is often green but may be yellow or ‘bare wire. ‘ Striped coatings are sometimes used for the benefit of those who are colorblind.

Induction and Leakage Current

Electromagnetic induction causes a subtler problem solved by grounding the case. The alternating current in appliances can induce an EMF on the case. If grounded, the case voltage is kept near zero, but if the case is not grounded, a shock can occur. Current that is driven by the induced case EMF is called a leakage current, although current does not necessarily pass from the resistor to the case.