Residential wiring connections are useful to be learnt if we try to analyze the electricity problems around us.
In the United States, most household lighting and appliances operate on 120-V, 60-Hz, single-phase alternating current. (The electricity may also be supplied at 110, 115, or 117 V, depending on the location).
Make sure to read what is ac circuit first.
The local power company supplies the house with a three-wire ac system. Typically, as in Figure.(1), the line voltage of, say, 12,000 V is stepped down to 120/240 V with a transformer.
|Figure 1. A 120/240 household power system. (Source: A. Marcus and C. M. Thomson, Electricity for Technicians, 2nd ed. [Englewood Cliffs, NJ: Prentice Hall, 1975, p.324.)|
The three wires coming from the transformer are typically coloured red (hot), black (hot), and white (neutral). As shown in Figure.(2), the two 120-V voltages are opposite in phase and hence add up to zero.
|Figure 2. Single-phase three-wire residential wiring|
That is, VW = 0∠0 ◦, VB = 120∠0 ◦, VR = 120∠180 ◦ = −VB.
Since most appliances are designed to operate with 120 V, the lighting and appliances are connected to the 120-V lines, as illustrated in Figure.(3) for a room.
|Figure 3. A typical wiring diagram of a room. (Source: A. Marcus and C. M. Thomson, Electricity for Technicians, 2nd ed. [Englewood Cliffs, NJ: Prentice Hall, 1975], p. 325).|
Notice in Figure.(1) that all appliances are connected in parallel. Heavy appliances that consume large currents, such as air conditioners, dishwashers, ovens, and laundry machines, are connected to the 240-V power line.
Because of the dangers of electricity, house wiring is carefully regulated by a code drawn by local ordinances and by the National Electrical Code (NEC).
To avoid trouble, insulation, grounding, fuses, and circuit breakers are used. Modern wiring codes require a third wire for a separate ground.
The ground wire does not carry power like the neutral wire but enables appliances to have a separate ground connection.
Figure.(4) shows the connection of the receptacle to a 120-V rms line and to the ground.
As shown in the figure, the neutral line is connected to the ground (the earth) at many critical locations.
|Figure 4. Connection of a receptacle to the hot line and to the ground.|
Although the ground line seems redundant, grounding is important for many reasons.
First, it is required by NEC.
Second, grounding provides a convenient path to ground for lightning that strikes the power line.
Third, grounds minimize the risk of electric shock.
What causes shock is the passage of current from one part of the body to another. The human body is like a big resistor R.
If V is the potential difference between the body and the ground, the current through the body is determined by Ohm’s law as
The value of R varies from person to person and depends on whether the body is wet or dry.
How great or how deadly the shock depends on the amount of current, the pathway of the current through the body, and the length of time the body is exposed to the current.
Currents less than 1 mA may not be harmful to the body, but currents greater than 10 mA can cause severe shock.
A modern safety device is the ground-fault circuit interrupter (GFCI), used in outdoor circuits and in bathrooms, where the risk of electric shock is greatest.
It is essentially a circuit breaker that opens when the sum of the currents iR, iW, and iB through the red, white, and the black lines is not equal to zero, or iR + iW + iB ≠ 0.
The best way to avoid electric shock is to follow safety guidelines concerning electrical systems and appliances. Here are some of them:
- Never assume that an electrical circuit is dead. Always check to be sure.
- Use safety devices when necessary, and wear suitable clothing (insulated shoes, gloves, etc.).
- Never use two hands when testing high-voltage circuits, since the current through one hand to the other hand has a direct path through your chest and heart.
- Do not touch an electrical appliance when you are wet. Remember that water conducts electricity.
- Be extremely careful when working with electronic appliances such as radio and TV because these appliances have large capacitors in them. The capacitors take time to discharge after the power is disconnected.
- Always have another person present when working on a wiring system, just in case of an accident.
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