|
| Ek's Home | Forum | Chat | Electrical Links | |
| Subject - How electricity works | |
| John A. Peters |
This is the text of an e-mail that I'm preparing to send to Carol Fey, a territory manager for Honeywell, who has worked in the controls industry for more than 20 years. She writes an article in P.M. magazine titled Hydronic Controls. She's not an electrician and I think she needs a little help from us with her understanding of grounding. Please read over the below and let me know what you think should be changed, before I send it to her. -------------------- In the March issue of P.M. magazine you added a correction in which you state that your friend says "No, the neutral leg goes to ground" and that he assures you that there's no problem except for farmers whose cows get hoof-to-mouth transmission and don't like it. I hope to clear up some confusion of, or at least the risk of confusion. I think you were correct when you wrote that "the neutral electricity goes back to the power plant." Perhaps a better way to state this is that "the electricity goes back to the powerplant on the neutral wire." The way that cows get shocked is when the electricity in the barn, does not have a good solid path back to the generator through the white neutral wire. Cows get shocked when some of the electricity is traveling back to the generator via the earth. If a cow is standing on a wet concrete floor which makes it a good ground and is being milked by a milking machine and eating food from a metal trough which may be grounded in a slightly different way at another location there's a potential to have a difference in voltage between the two places. When there's a voltage differential between two places current can flow and a cow will feel a shock. This can be a big problem when a cow is being milked. It may refuse to come in to the barn. To correct this problem the farmer has to use a technique called bonding. By running a good sized copper wire between the cement floor, the milking machine and the feed trough you'll eliminate voltage differences between the equipment and the floor, thereby protecting the cows from shock. This usually requires a grid of copper wires or steel rebar to be included in the cement floor of the barn. I believe is correct to say that the neutral leg is grounded. According to the code this connection is allowed at only a few restricted places. One place where the system is grounded is at the meter, and another one is at the transformer on the power pole and a third is generator at the power company. Grounding may also be installed at an indoor transformer in a large commercial building. I think it might be realistic to imagine the electricity as flowing through the wire as not to different form from water flowing through the cold water line. The water in the water pipe is under pressure the same thing is voltage and when you release it into the sink (unloading it) rather than it returning to the ground it returns to the sewer line and eventually via the outfall into the ocean, not the ground. It only leaks into the ground when there's a problem. From there you can imagine the sun evaporating the ocean water into the sky as being like the power generator. Eventually the water falls as rain and runs into the reservoir or lake. (Please ignore the fact that it is essentially grounded) From the reservoir it continues on the circular path or circuit back to your district water company where they put it under pressure. Circuits have that name for a reason. It is a circle. Maybe a better explanation is the electricity comes in under pressure (voltage) through the black wire and flows (GPM or volume = Amps) to the generator on the white wire. The only exception to this, is during a problem (broken or lost neutral, person touching the black wire while touching a grounded surface, etc) in which case some of the electricity may be flow through the ground and through the earth back to the ground rod at the generator. So normally there's a circuit or circular path from the generator out through the black wire to the load. It then should return back on the white wire. The only exception is when there's a problem. The reason we call the white wire the neutral wire is because in the early days of electricity, both wires were black and neither wire was grounded. Therefore a person could get a shock from either or both wires. Someone had a good idea. They realized that if they grounded one of the two wires at the generator and/or transformers, that one wire would be neutralized! That way you could not get a shock from that wire. To make things more clear they marked the neutral wire white. They actually used white paint on the neutral wire to indicate which wire was the neutral. To this day in the neutral wire is one with some kind of an indication. If you take a close look at an extension cord or zip cord, as we call it you'll notice that one of the two wires has a raised rib. This indicates the neutral wire. Sometimes it's easier to feel this raised rib and is to see it. Thank you, John A. Peters, Owner, Brookline Electric Co. 415 239-5393 Office 415 509-3599 Cell http://home.pacbell.net/japeters/ is the web page I am building. |
| Romex Racer | I like that explanation. I always use plumbing analogies to explain electricity. I never stopped to consider the fact that the power generating station has the neutral grounded. Maybe they have a driven ground rod. Probably a series of buried conductors, it would be interesting to find out. |
| kbsparky | A couple of minor suggestions: First, the description of a cow in your sentence "...If a cow is standing on a wet concrete floor which makes it a good ground and is being milked by a milking machine and eating food from a metal trough which may be grounded in a slightly different way he had another location there's a potential to have a difference in voltage between the two places." A cow should be referred to as she or it as you did in other parts of your response. That sentence itself may need some additional grammatical revision as well. Another thing I would suggest is use the word source instead of generator as that might be confused with an on-site standby unit, instead of the utility's unit many miles away. I would refrain from using your water/drain/evaporation analogy since that may only confuse the average reader. |
| jc808 | Good 'un!! my lecturers use the 'plumbing' analogy as well :) |
| lucky1122 | I believe the process of bonding which you are refering to is known as "equipotential plane". It is the process of putting all metalic paths at the same potential through effective bonding . It is especially used where farm animals are housed or kept since they are very susceptible to the slightest voltage gradients . It is also used in swimming pool locations to ensure that swimmers are protected from the same.
|
| John A. Peters | I corrected the typos - thanks. I already sent the letter in and she said "To bad she did not know about us before she published her book on electricity." I wrote back that if she were to send me a copy I could help with the second edition. No response to that one. |
| John A. Peters | quote: Could you send us a copy of how you use the analogy, please. |
| electricpete | ElectricPete here, I would like to share my take on my old boss's explanation of grounding/bonding for Ms. Carol Fey. I get my info from my collection of antique electrical books/magazines. The lightning rod and conductor and ground rod came first.(think Ben Franklin-circa.1760)). Then Edison 2 wire D.C. c.1880. Then Tesla and A.C.c.1890. A.C. power won the battle of the currents and by the mid 1890's, Edison, (My 1915 electric textbook says)came up with the idea of using 3 wire, 220V w/neutral not because of any safety factor of grounding the white wire, but to save money on the copper wire installed. Now J. Peters in the next to last paragraph of his post says that in a 2 wire system, you can get a shock from either or both wires, and by neutralizing one leg, it's safer. Not so. In a non-grounded 2 wire system, you can ONLY get a shock by touching BOTH at the SAME time. One hand on either wire and other hand on water pipe will NOT shock you. Get it? When you Ground a conductor, you make you whole house's metal pipes/radiators/sinks the other half of a circuit. If you're still with me, I promise to come full circle :-) SO NOW WHAT ABOUT GROUNDING THE NEUTRAL? The NEC says (in Part) "conductor(s) are grounded to "limit voltages due to lightning, and facilitate overcurrent device operation"(and some more stuff, but look it up if you want). PETE"S CONCLUSIONS: 220V 3wire(w/neutral) was invented to save wire; connecting the neutral to the ground(earth) was done mostly for lightning strikes. I understand it's alot to think about, and I may have left out some details, but all in all it's in the right direction. Let me know what you guys think of my theory. |
| jc808 | quote: Not sure what you mean. :S |
| John A. Peters | In your lectures, how do you word the analogy or what you say about plumbing as an analogy for electricity? |
| jc808 | Im not a lecturer. When I say 'My lecturers' I mean the lecturers who teach me (I am a trainee electrical engineer) Youre way off the mark, but I am flattered ;)... |
| bigbillnky | I would like to make a few comments about this and maybe help to better explain how important this area is. 1. The ONLY way to receive an electrical shock is by placing yourself between a source and it's return path. Lightning is generated by the earth's magnetic field and the movement of the atmosphere in this field. When there is a conductive path between the earth and the atmosphere, a circuit is completed and current in the form of lightning flows back to the earth. This is the reason ground rods are installed in electrical systems. If a system did not have ground rods, the power from lightning strikes will flow through an electrical system it has struck until the power has dissipated or it finds a path back to earth(the source). Since the generator powering the electrical system that the lightning struck did not produce the power of the lightning, it will not return to that generator. 2. A ground rod does little in helping to open protective devices. The neutral conductor is the conductor which provides the fault path on line to neutral loads. This is why older, ungrounded 3 wire systems still could blow fuses. This would have been impossible if the grounding conductor serves this purpose. This is why BONDING is so critically important and why livestock receive shocks when all points are not at 0 potential difference. This also places an emphasis on the MAIN BONDING JUMPER. Fault current passes thru bonded metal parts, to the grounding electrode conductor, thru the main bonding jumper, and back to the source. If this path has a low impedance as required, a high current will flow and cause the PROPERLY SIZED overcurrent/ short circuit protective device to open. If the fault path has a high enough impedance, the protective device will simply see a load and will not open. I believe these are the most important issues we all should thoroughly understand and be able to apply to any circumstance we find in the field. Good luck to all |