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Name: Jose
Status: educator
Grade: 12+
Country: Portuga
Date: Spring 2012



Question/Answers:
Q. I have the following electric Power grid questions:

Before getting to your questions, let's take a quick look at of some of the concepts we have to deal with.

Transmission of electricity from a generator to a location near your home is usually accomplished

using potentials of hundreds of thousands of volts. For a variety of reasons the use of high voltage is more economical, but it means that the voltage must be reduced before it's connected to your house. That's done with a transformer.

A very versatile means of connecting electric power to a house involves the connection of two power phases, each of which are 120Vac. Since the power to your house has to come through a transformer anyway, a good way to get the other needed connection, neutral, is to use a transformer with an

output of 220Vac, but add a center tap (meaning a wire connected to the transformers secondary, or output winding at it's center. The result of this is that measured between the center tap and either end of the secondary coil, (each phase), is 110Vac.

(For more about transformers and to see a picture illustrating these connections, please see http://hyperphysics.phy-astr.gsu.edu/hbase/electric/hsehld.html.) br>
What is most important to us about the transformer connections is that the center tap becomes our neutral line. It is connected to earth ground at the transformer before it gets to your house. Your house will have another wire connected to earth ground that is distributed throughout, so we

effectively have two ground wires. Why? Safety is one reason. If there was a break in a ground wire we wouldn't want the electric current to find that the easiest path to another ground was through your body, for example. But we also need a ground point that will be at the same potential as a

ground point, say, at the power generator. Otherwise what's 110V at the generator might be some other voltage--higher or lower--at your house.

That's because any time you measure a voltage, you are always measuring it with respect to a reference point which may or may not be at ground potential, and voltmeters have two inputs for exactly that reason. So when we measure voltage, we are checking the difference in potential

between two points. For example, let's say you had a terminal coming out of a grounded black box and checked it with a meter. You place the positive meter probe in contact with the terminal on the box, and the negative meter probe is placed in contact with earth ground. You measure 100V. Now you

find that you also have a similar grounded green box with a terminal on it. You measure that one and find it's at 105V.

What would you expect the reading to be if you placed your positive meter probe on the black box

terminal (100V) and the negative meter probe in contact with the terminal from the green box? If you said 5V you were correct. But if the ground on one or both of the boxes was missing, you would be unable to make a valid measurement. Without the ground/reference connection the voltage is undefined.

Now suppose you made the same measurement on two other grounded boxes, placing the positive probe on a box terminal and the negative probe on the box ground, and found that one box measured -10V and the other -25V. (I'm going to call the boxes -10V and -25V in the next paragraph.)

If you place these boxes side by side, and connect the meter positive probe on the -15V terminal and the negative probe on the -25V, what would be the meter reading? The answer is 10V. You measured between a reference that was more negative (-25V) than the -15V terminal, or to put it

another way, you measured a rise in potential between the negative and the positive meter probe.

Still confused? Maybe an analogy will help. Some buildings have many stories below ground level as well as above. You might refer to the underground stories as negative numbers and those above

ground as positive numbers. As you descend in an elevator to below ground level, the floors run -1, -2, -3, etc. If you were on floor -10 and got on the elevator and rode it to floor -5, you went up five floors, didn't you? Both floor numbers were negative, but when you rode the elevator it rose.

Whether you go from the -10th floor to the -5th floor, or the 3rd floor to the 8th floor, you have still gone up five floors; moved five floors in the positive direction.

If there is a rise in potential when going from the negative to the positive meter probes, your

result will be a positive voltage. If you have a reduction in voltage when moving from the positive to the negative terminals the result will be a negative voltage. The value displayed will always be (as long as you are measuring DC voltage) the simple difference in potential between the two

probes. The sign of the voltage (+/-) is determined by the direction of current flow.

Q. I have the following electric Power grid questions: Utility poles have a grounded wire. Is this just for lightening?


A. Utility poles may have lightning rods, in which case there would definitely need to be a ground wire which would go directly to earth at the bottom of the pole. If you're seeing a cable from a transformer connected directly to ground, then it's almost certainly the neutral, center-tap line

we discussed previously.

Q: Is not connected the neither one of the 3 phase wires, is it?


A: A ground cable would not be directly connected to any of the "hot" phase wires, since it would short them out.

Q: What does the multi-grounded neutral wire do, and has it always have to be present?


A: It's there as a point of reference. Again, a voltage potential only exists on a conductor with respect to a reference with a different charge. Without that no current can flow, there is no

transfer of energy and no work can be done. The neutral wire is often grounded close to it's source and again at the destination. This is to be sure that the reference point is at the same potential at both ends of the circuit.

Q: The 3 phase wires are only grounded at destination when electricity is being consumed?


A: Again, grounding a charge carrying wire, (a phase), is a dead short and may cause a fire or other damage. It makes no difference whether it is a single phase or one of a three phase group.

You would ordinarily never want to do this.

Q: If they were grounded at the destination then what is the use of transmitting the power anyway?


A: They are not grounded. There must be a path to ground/neutral/other point of charge differential

so current may flow. We extract energy to do work from the flow of current.

Q: If domestic consumers receive just one phase current, why there are 3 wires coming from the pole to the house?


A: Most houses are supplied with two live phases and a neutral.

Q: Is ground always the return path for electrons?

A: Current flows from a region of higher potential towards a region of lower potential. Strictly speaking electrons flow from negative to positive poles, but we are used to thinking of it in the

opposite sense. This is called "conventional" current flow, and is rationalized through the idea

that, when an electron departs it leaves a "hole" that will be filled by another electron. Thinking of electric current as flowing from positive to negative is sometimes referred to as "hole flow". So no, the ground or neutral line is a stable point of reference, and with AC electricity electrons move back and forth in both directions. In the US this happens 60 times per second.

Q: Is there a ground connection at every house directly or at the nearest utility pole?


A: Most houses have an earth ground established within the house itself.

Q: In power plants the wires in the generator are connected to the ground, to close the circuit?


A: You have to be careful in the way you refer to the wires leaving the generator. As you know, the phase wires exiting a generator would not be connected to ground. The neutral wire most likely will be grounded close to the generator. This is to ensure that when the neutral connection is grounded

at the users location, the difference in potential between the neutral and phase connections will be the same as it was near the generator.

Best Regards, Jerry Gardner


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