Questions about Electricity

Factual Questions
1

No one (even an electrician) has been able to explain a few questions on electricity to me. Perhaps I’m not understanding it enough to ask the correct questions, but here goes:

We have in our homes, 110v electricity running in 60hz cycles. That means that the electricity “flows” in and out of our home flip flopping back and forth in direction at 110v, right? Shouldn’t we need a “Positive” and a “Negative” line? Technically, both would be positive and negative depending on the point in time one was to measure the current. Instead we have a “Hot” line, and a neutral. Why is this second line called Neutral? It isn’t really neutral as I understand it, It carries just as much juice as the hot line. But if you touch the neutral line, nothing happens. If you touch the hot line, you get a good shock. This doesn’t seem possible to me as I’m under the understanding (obviously wrong) that both lines are basically the same. Where does this neutral line go? Is my home in a complete circuit back to the electric company? Next, there is a ‘ground’ line as well. This line is named appropriately as it actually goes into the ground, but what does it do? I believe it is for discharging DC charges that build up due to static electricity, but how does this ground wire connect into my homes electrical system? Is it a direct connection for both hot and neutral lines into the ground, or just one? Lastly, since the hot and neutral wires are the ame in the way that they cycle the 110v on and off, why is there polarity on a wall outlet???

2

This is an incorrect undertanding of AC. It doesn’t flow into and out of our homes. It flows over a circuit that reverses direction 120 times a second for 60 cycle power. A wave has to go in both directions to make one cycle. That’s pretty basic but AC can be hard to understand. It can do some tricky things such as flow across a gap where there is no physical connection with no arcing, something DC can’t do.

You actually have three wires coming into your house. One is a common ground and the other two have AC power in opposing phases. At a moment when one wire is positive in a DC sense the other is negative. There reason for this is versatility. When you measure AC voltage from either wire to common you’ll see 110v. If you measure the two opposing phase AC wires against each other you’ll see 220v. This way your waffle iron and TV can run from 110v and your clothes dryer and arc welder can run from 220v.

Note that modern 110v wall sockets have three holes. One is AC, one is common and the third is a safety ground that may be connected to cold water pipes or a ground spike near your house. The safety ground is only used if there is a failure in an applicance so that current flows to ground and not through you.

3

Oh hell, where are my manners. Welcome to SDMB CanTak3. Let me be the first to extend this laurel and hearty handshake. Good first post.

I should have added that the earth is part of the circuit.

4

From reading how your question is worded, I think you aren’t clear on the difference between voltage and current. The current flows back and forth between the hot and neutral lines, but that doesn’t mean they have the same voltage as each other.

Think of it like water in pipes. Current is equivalent to the flow of water through the pipes, and voltage is equivalent to pressure. Water flows back and forth between the neutral and hot “spigots,” 60 times per second. But if you disconnect them and just measure the pressure at those points, you’ll find that the neutral spigot has very low pressure, but the pressure on the “hot” spigot alternates between pushing and sucking. If you put your mouth over the neutral, nothing will happen, but if you put your mouth over the hot side, you’ll get a surprise.

This analogy has its limitations. There’s not a good model of a transformer, which decreases or increases the voltage on one side relative to the other, while increasing/decreasing the current. Power comes from the electric company at a relatively high voltage on a pair of wires which are symmetrical - they both push and pull at opposite times. Then it goes to a transformer which brings that voltage down to 240V across a symmetrical pair of wires. The transformer also has a center tap which is tied to ground. So if you measure the voltage from one side of the transformer to the center tap, you’ll see 120V, with one side basically ground. That’s what gets run to the standard outlets in your house. Some will use one side of the transformer’s output, and the others will use the opposite side.

5

First lets start off with a two wire DC system.

We measure voltages just like we measure distance - between two points.

When distances are measured, one can say that the total distance is taken in a positive direction from point A to point B, and you could also say that by measuring in reverse, from point B to point A, then you have a negative distance, but the actual value is still a real existing number, just that it has a minus sign in front of it.

In electrical terms this means, take a voltmeter, and connect across the two wires and a voltage differance will be detected.Reverse the two wires on your voltmeter and a minus voltage would be detected.
Add also that if you reversed the power supply terminals instead, then you would also get a minus voltage.

With a DC circuit, it would not matter how long you kept the meter connected, the voltage would always remain the same, the current flows from one part of the supply straight through the intervening electrical circuit to the other.

Keeping with a two wire circuit, now an AC system.
Connect your voltmeter as before, and we will also add a condition that the AC voltage changes very slowly.(slow enough for you to see what is going on)
You connect up and you get a reading that rises from zero to some maximum level, then falls, and then your meter(if it is capable) will then start to read a voltage that gets more and more negative, until it reaches some minimum value whereupon it will rise back up to zero volts.

At this time, all you really know is that the voltage differance between the wires is varying, you do not actually know if the variation because one or the other wire is rising and falling in value and the other is remaining fixed.

The reason you do not know this, is that you also need something as a referance to the whole system.

Back to the measurement of distance analogy - knowing a measurement of distance is fine, but it would be helpful perhaps to put that distance into a wider context, like where on the face of the planet those two points are, and this would then become our referance.

We do something very similar with electricity.

If one connects one end of the circuit to ground, or earth, we have a fixed referance point, since we always treat earth as zero volts.

Since the earth is our largest available referance point, its value can be taken as fixed and no changes to our circuit will alter this.

Since one wire is now fixed to zero volts, then any differance of voltage between our two parts of the circuit must be due to variation in voltage from the other point.

That point can rise to some maximum value, fall down to zero again, and now it can also fall below zero - to a minus value.

The earthed end still has not changed - it is still zero volts, but current is now flowing from it, and into the other point.

Now here comes the good bit,

This wire that is fixed to the earth, it carries electrical current, therefore it is not a true earth, because earths do not carry current, earths are used as referance points.
It is also clearly not live, because that is the property of the other point which supplies and recieves current in turn.

This zero volts line attatched to earth must then be called something else - neutral.
There are many ways that earths are used, but in general an earth is a referance zero point for other parts of an electronic/electrical circuit.

To add further explanation might actually detract from what I am saying here as you can easily get confused.

6

Just a few comments from an old 2-wire electrician regarding getting shocked by the “neutral” wire.

In electrical trade school, one is taught to always break the hot wire first and connect it last, making splices for the grounding conductor and common conductor first and second. You’re also taught never to switch the neutral without also switching the hot conductor (in other words, if you are going to mechanically open the neutral by means of a disconnecting device, you need to open the hot as well).

While it’s normally true that you can touch the neutral bar in a panel and get no shock, an ‘open neutral’ is a different cat completely and can be as deadly as any energized conductor. I learned this the hard way after being knocked off a ladder when I took the wire nut off the neutral splice first and brushed against the bare free end.

Now, I’ll probably get beaten up by the electrical theorists in the crowd, but this has been my experience.

7

There was another thread about grounding about a week ago that you might want to search for. It covers some of the same info.

You are correct that the two wires in an AC circuit are essentially the same. There are two types of AC circuits, grounded and isolated (not grounded). Isolated systems are actually safer. You can touch either wire and ground and not get shocked. If you are ever in a hospital, look for the red outlets. Those are isolated.

The reason we don’t run large distribution systems (like what goes out to your house) isoalted is because mother nature likes to randomly insert ground connections into the system by doing things like dropping tree branches across lines and such. Hospitals go to great pains to keep their isolated systems isolated. It would just be too difficult for the power company to do the same.

So, what we use instead is an intentionally grounded system. This way you know what is the safe conductor to touch and what isn’t, instead of having mother nature randomly decide for you.

A residential service is typically supplied from a transformer, which is center tapped. A transformer is just two coils of wire around a common iron core. When you do this, the voltage on one coil is the voltage on the other coil multiplied by the ratio of the number of turns in each coil. “Center tapped” simply means that the output coil going to the house has an extra conductor attached to the middle of the coil. Since it is in the middle, the voltage is halfway between the two outside conductors of the coil. The center tap is grounded, and this is the zero reference for the circuit. Therefore, you have 120 volts between the grounded center tap and either side (this is called the line to nuetral voltage) and 240 volts from one side to the other (called the line to line voltage). All of the 120 volt circuits in yur house are connected from one line or the other to nuetral, usually with half of the fuse box connecting to one line and half connecting to the other line. 240 volt appliances like your dryer and maybe your oven will connect to both lines.

The protective ground (the green wire) is connected to the nuetral near the fuse box. The reason for having a seperate protective ground is that the nuetral carries current, and since copper isn’t an ideal conductor, any time current flows through it you have a small voltage drop across is. This means that the voltage at various points on the nuetral circuit may not be exactly the same as ground. The voltage won’t be any where near as high as 120 volts, but its not zero, so to be safe we don’t use the nuetral as the protective ground. The green wire carries no current, and therefore is always at the same voltage as ground.

8

Why do hospitals use isolated systems at all, if grounded ones are easier to build?

9

Basically, to protect the patient.

Isolated power in hospitals grew out of open heart surgery. In the early days of open heart surgery, a lot of patients were dying for unknown reasons. One theory that was going around was that tiny currents from monitoring equipment and such were killing the patients, but such tiny currents would leave no trace on an autopsy, so this theory was treated with quite a bit of skepticism by some. Eventually, isolated systems did in fact become the standard for “wet” locations like hospital rooms. The national electric code now specifies that isolated systems must be used in these types of locations.

IIRC, 5 mA is the maximum amount of current that is thought to be “safe” to pass through a human heart (for obvious reasons, not a whole lot of human testing has been done to verify this). Therefore, the most current that is allowed to flow through an isolated system in the event of a ground fault is 5 mA. Somewhere near all of the red outlets, you will also find an ammeter with a test button. Periodically, hospital personell will test the system using these meters to make sure that their isolation is still intact.

There is a handbook for isolated systems, similar to the popular handbooks for the NEC. The introduction gives a much more detailed account of how isolated systems came into being, and is quite an interesting read.

10

Thanks for all the responses. I’m definately more clear on my original question, but “fighting ignorance” on electricity for me has made three new ones.

From Padeye’s post, three wires enter a home (AC, AC in reverse phase, and neutral) and difference (potential) between the AC’s and the neutral is 110v. If I had a graph of the potential difference between this neutral line and one of the AC lines, where X-axis was my time frame, would the line be a curved sine wave (as opposed to a flat peak at 110v, then a flat peak at -110v)? That’s the impression I’m getting from Casdave’s post.

It sounds like if 220v wasn’t needed, I would only need a Hot line, and a neutral line entering my house. Is this correct?

Also from Casdave’s post, the neutral wire is actually grounded making it’s reference point of 0 (or neutral). If this is the case, why does it enter my house from the power company? Couldn’t it be grounded from my home itself? What is the difference between the green grounding wire that attaches to my water pipes somewhere below my house, and this neutral wire?

11

Just a few comments from an old 2-wire electrician regarding getting shocked by the “neutral” wire.

In electrical trade school, one is taught to always break the hot wire first and connect it last, making splices for the grounding conductor and common conductor first and second. You’re also taught never to switch the neutral without also switching the hot conductor (in other words, if you are going to mechanically open the neutral by means of a disconnecting device, you need to open the hot as well).

While it’s normally true that you can touch the neutral bar in a panel and get no shock, an ‘open neutral’ is a different cat completely and can be as deadly as any energized conductor. I learned this the hard way after being knocked off a ladder when I took the wire nut off the neutral splice first and brushed against the bare free end.

Now, I’ll probably get beaten up by the electrical theorists in the crowd, but this has been my experience.

12

Crap, my first double post.

13

Yes, it would be a sine wave.

True. However, since several homes may be fed from one transformer, the power company is going to standardize what comes into your home. In fact, legally, they have to do things certain ways just to comply with local and national laws. Even if you didn’t have any 220 outlets in your house, you’d probably find that half of your 110 outlets were fed from one line and half from the other in a fairly standard arrangement. But technically, no, you wouldn’t need two 110 lines. A single (non-center tapped) 110 volt transformer would do the trick.

By the way, 110 (or 120) is just an average number. The voltage in your house will typically be higher if you are closer to a substation, and the voltage may decrease slightly the farther out along the distribution lines you are. The power company will typically guarantee something like 120 +/- 10%, which gives a range of 108 to 132 volts.

The nuetral is actually required to be grounded at your house. In older homes, you were required to make this ground connection through the cold water pipe. Sometime in the 70’s (I think) they figured out that water pipes weren’t always making the best ground connections, so the one thing you can’t use now is your cold water pipe. You have to have some other method of grounding, typically something like a copper rod pounded into the ground. However, you don’t want your water system to be ungrounded or an unsafe voltage could easily develop, so even though your electrical system doesn’t rely on the water pipe as it’s main ground, the water pipes are connected to the ground system.

If you are in an older house, all of the nuetrals will be tied together in the fuse box and the green wire will run from there to the cold water pipe. In a newer house, all of the nuetrals will be tied together in the breaker box, the same green wire will connnect to the water pipe just as in the old system, but another ground wire will be present which connects to another type of ground (most likely a copper rod).

14

Yes, normal utility is a sine wave and you can see this very clearly with an oscilliscope. Some inverter backup power supplies which convert DC from a storage battery to AC actually have a square wave output. This isn’t great but is good enough for computers which often use switching type power supplies but it isn’t safe to run all AC equipment on square wave power.

No reason it couldn’t but it may be a false economy to not provide both legs since some places do need it. Even if you don’t use 220v in your house a proper installation will split the circuits between the two legs to provide load balancing.

15

Quoth Padeye:

No, you shouldn’t have added that, because in a properly running circuit, it’s not true. The earth will only be part of the circuit if your ground wire is carrying a current, which it shouldn’t be doing. This leads into one of the questions in CanTak3’s most recent post: You can’t just make the neutral wire local, because unlike the ground, it is carrying current. Cutting the neutral somewhere short of the power company (or at least, short of the transformer) will kill the current everywhere.

16

Dear OP,

I suggest you read this: http://amasci.com/ele-edu.html

The first 3 articles on that page ought to take you to the point where you can ask maningful electrical questions and make sense of the answers.

About 1/2 way down the page noted above is another link to an article called “Why 3 prongs?”. That’ll answer the more specific questions you raise after you’ve gotten the basics clear in your head.

17

Not only load balancing, but economy and simplicity of installation. Even if you only need a couple 120v branch circuits, it’s easier to run a 220 feeder on say, 8 gage wire instead of one #2 (or so) wire. Actually, it would be three #2: hot, neutral and ground, vs four #8 wires, (two hots, neutral and ground) and correspondingly smaller conduits, junction boxes, wire nuts, lugs on breakers, etc.

18

Sorry if this has been answered, but the one thing I’m still not clear on is if the neutral wire comes into your home and is grounded there, why have it at all? Why not just use your own grounding wire that eventually attaches to the neutral wire? It’s the difference between the neutral and the ground that I’m stuck on, all other questions are answered for me.

19

And then there is the 220 volt motors that require no neutral, only a ground.

20

I’m with CanTak3…

If the neutral wire is grounded at the same plase that the ground wire is grounded (as in, the dirt beside my breaker box), then why have bother at all? Won’t the neutral wire serve the purpose of the ground wire. Or, conversely, couldn’t the ground wire carry the current instead (assuming it were insulated.)