Here's a basic description of how the electric power gets to your house. Most homes are supplied with a 220 volt service capable of about 200 amps. Some older homes might not be wired that way. They may just have a 110 volt service. The following describes a 220 volt system in the United States. The system is as basic as how American cars run, how hamburgers are made and flag poles for sale.
Back at the power generating station electricity is generated by huge turbine generators which are driven by either fossil fuel, nuclear energy or hydro power, like Niagara Falls. The power is generated at very high voltages and sent out over a network of distribution lines. As it gets closer to your house it goes to a sub station where the voltage is lowered and then sent on its way. It finally arrives in your neighborhood and is sent to a transformer where the voltage is lowered to 220 volts. This is what comes into your house and goes to the circuit breaker box.
The next section describes a bit of technical mumbo jumbo about sine waves and phase relationships. You can skip it if you aren't interested.
Alternating current (AC) is what is delivered to you and is what runs everything in your home. The alternating part means that the voltage changes constantly from zero to plus 110 volts and then back to zero and then to minus 110 volts. This cycle repeats itself 60 times a second thus the description of your service is 110VAC 60Hz. The Hz is the abbreviation for Hertz, a famous scientist from Germany who also invented the rental car business (and as a sideline invented radio and wireless communications along with a lot of his co-workers, Avis, Enterprise and Alamo).
The cyclic variation of the voltage occurs very smoothly and is described as sinusoidal variation, named after Samuel Sinus, a famous mathematician and nose doctor. Now this is important - the 220 volts comes to you in a kinda weird way. There are three wires coming to your house. The one in the middle (so ta speak) is called the common. On each side of that wire are 110 volt wires - 110 volts is measured between that wire and the common. If you measure across the two outside wires you will get 220 volts. It all happens in the complex vector math of the two 110 volt sinusoidal waveforms, but suffice it to say that it works and you get your choice of 110 or 220 volts to run your appliances.
Remember the middle wire I called "common"? It comes all the way from the power generating station as the common wire and it also is attached to the cold water pipe that comes into your house through the ground. It's important because you should realize that the earth is part of a circuit of our power distribution system. What it really means to you is that you can get zapped if you happen to be in contact with some form of ground, be it a kitchen sink or a puddle of water on the ground outside, and come in contact with a "hot" wire. Ground is ground and if you touch a "hot" wire and are grounded elsewhere on your body, current will flow through the circuit. If one of your fingers is in a light socket and another finger on the same hand touches ground, you will feel a shock across your hand. If no other part of your body is grounded that's all that will happen. It will scare you but it won't hurt you or kill you. Well, it might hurt you if you yank your hand away fast and hit it on the corner of the kitchen cabinet or any other relatively sharp object!
Confused yet? Well here is one more thing to contemplate. There is one more wire traveling through your house. It is sometimes covered with green insulation and sometimes it is a bare wire buried inside the plastic insulation of the Romex. It is a ground wire and is connected to the grounding buss inside your electrical box. It is connected to the same place that the white or common wire is connected but is used ONLY for grounding of circuits for safety reasons. Don't think you are smart and try to use that green wire to carry current in a circuit! It is unsafe and ILLEGAL.
I once heard of a plumber who was wiring a disposal under a kitchen sink. He thought he knew what he was doing and decided to be lazy and not go down the cellar to turn the circuit off. He worked with one hand in his pocket (an electrician's safety trick) so as to not risk grabbing a hot wire with one hand and ground with the other. Then he made one big mistake. As he reached for the hot wire to connect it to the disposal, his forehead, now soaked with sweat, touched the bottom of the steel sink. Needless to say, he is no longer installing disposals.
I have to mention something here about grounding an appliance. In some cases it is recommended to attach a grounding strap from the appliance to a water pipe since water pipes all go into the earth and are considered the absolute ultimate in grounding a device - well, that is not always true. Consider two devices which are 99.9999% of all homes. Well at least the first is, it is the water heater. One side of the water heater is connected to the cold water supply. The other is connected to the hot water pipes in your house. It is absolutely possible that the electrical path to ground is interrupted in the water heater. This may be due to plastic components inside the water heater or to the the teflon thread tape used to seal the pipe joints. In either case the the ground path can be broken by the water heater. The cure for this is to install a copper grounding strap across the water heater pipes, one end to the outlet pipe (hot water side) and the other to the inlet side (cold water). Everything is now grounded throughout your house, right?? Well, not exactly.
Did I mention water softeners?? No, but I will. Water softeners. There. Oh, the problem with them? Same thing. Plastic parts inside the water softeners offer the same electrical isolation as the water heater does. So, when you are out buying copper grounding straps and clamps, get two - no, make that three. The water meter is the last problem in interrupting the electrical ground system in the copper pipes in your house. So put one across the water softener and one across the water meter - same way - one end connected to each of the inlet and outlet pipes.
Why is this so important, you may ask? Well, if the installer of an appliance uses the hot water pipe as a "safety ground" and no ground straps are installed, and there is a failure in the appliance, say for example a disposal, there is a very good chance that the frame of the device will be connected to the "hot" side of the guts of the device. Therefore all the hot water pipe everywhere in the house will then be wired to 110 volts! Next person to grab the hot and cold faucets at the same time will not ever do that again, dirty hands or not! They won't even be around to get their hands dirty, they will be dead!! That's why it's important.
The main message here is one of safety. There are a lot of sources of ground in or near where you will be working on an electrical circuit. Don't be lazy - take that extra few minutes to find the right circuit breaker and turn it off. Check with a circuit tester to make certain that the "hot" wire is dead and proceed with your work.
One final note. Do you think that you could ever be electrocuted by touching the white (common) wire of a perfectly wired house where everything is done to code? You betcha you can! Consider this - you are at the circuit breaker box and decide to remove the white wire from one of the house circuits for whatever reason - there it hangs in the air, the white wire. Now everyone knows that white is common so what does it matter if you are standing in a puddle of water and grab the ground wire?? Don't try it!! You may very well find yourself shaking hands with the disposal installer. If the circuit which the white wire is a part of has any device plugged in and turned on, even a 60 watt light bulb, that white wire is connected through the appliance to the black wire! Get it?? The white wire is looking for an earth ground to make the light bulb light! So if you supply that path to earth ground through your sneakers the bulb may not light up, but you surely will!! Helllooooooo Saint Pete!!
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