Hello everyone
I am new to SDMB, I sent a question to UC that has been bugging me and Dex sent me here, I have refined the question a bit so here goes (Note Ground is electrical Earth is dirt at least in this post)
In electrical theory a three phase system sums to 0V due to phase cancellation at the load end, but in the real world loads are not balanced and lots of voltage goes to Ground, So what happens to it? I have been told that the Earth is an electrical sink and that it returns to the generator via the Earth following transmission lines, but there have to be limits on this, Earth is an insulator and insulators generally are not good conductors so there would be resistance(heat) as to the electrical sink is there a capacitance value for the Earth?( yes I am aware it would be huge) but wouldn’t the eventual discharge be devastating? I know that I am leaving out a bunch of variables, just trying to make a complicated question simple. Any takers? Thank you
Lots of voltage doesn’t go to ground.
There is an inbalance in the current between the phases, which the power company doesn’t like.
I worked at a plant where no thought was given to which leg a single phase load was connected to .So many circuits were connected to the same leg that the power company took notice.
The Earth is a good conductor. It may not be a good conductor at the micro level, but a bad conductor that is miles wide has low resistance overall.
I think you’re mixing up voltage and current here. On a three-phase line driven with the same voltage across all three phases and the same load across all three phases, current will ideally sum to 0.
In practice everything won’t be balanced, and you can have a neutral wire to carry the unbalanced current, or send it through ground. If you don’t have the neutral wire and don’t allow current to flow to ground, the currents will be forced to sum to zero, so with an unbalanced load on the three phases, the voltages will have to be unbalanced as well, to satisfy Ohm’s law.
I believe most three-phase power transmission is delta rather than wye, with no neutral or ground. Any imbalance in the load results in higher current flow in the phase conductors themselves.
Normally, the only time earth ground is carrying current is in case of a fault. With the exception of certain rural power systems, (The curious can Google “SWER.”) earth ground is a safety rather than a current-carrying conductor.
A well run 3 phase operation has a set of phase ground lights, and somebody at least on call to come find and fix the problem if one of the light goes out due to a ground.
I am not sure about the amount of electrons the dirt can yield or accept, but with AC, you switch back and forth 60 times a second. Even in Edison’s DC days, I never read about it being a problem. I think the standard ground rod driven into the ground is 8’ long. Usually that is long enough to get down to moist soil. Many electricians like water pipe grounds. Code calls for all the conductive material in a house to be connected to the electrical ground unless the gas company forbids connecting their pipes. Of course, my gas pipe connects to my furnace which in turn connects to my electrical ground.
First of all, you seem to have a fundamental confusion between voltage and current.
A generator generates voltage, but if there’s no load connected to it no current flows and nothing exciting happens. When you have a load, then the voltage causes current to flow.
I hate to use the water pipe analogy because it leads to a lot of misconceptions, but voltage is kinda like how much water you have (more water = more pressure) and current is kinda like how fast the water is moving through the pipe.
Anyway, if you take your handy dandy multimeter and stick the probes in the ground, you’ll find that dirt has a pretty high resistance. This will make you think that the earth is a pretty poor electrical conductor. If you use a higher voltage though, and make a better connection to the dirt (like say you pound a 5 foot copper rod into the ground) then you will find that the earth is actually a much better conductor than you thought it was.
Let’s take a very simple example. Forget about all of that three phase stuff for now. Just take a simple generator and a simple load, and two wires to connect them. Electricity goes out through one wire, though the load, and back through the other wire to the generator, completing the circuit. Simple. Now, remote the second wire. Take the second connection from the load and connect it to a copper rod that has been pounded into the ground, and do the same at the generator side. Most of the time, you will find that this actually works just fine. It’s like the earth can act as a great big wire and can return the current for you.
Note however that I said “most” of the time and not “all” of the time. In some areas, the earth doesn’t make all that great of a connection. But, there actually were electrical systems set up this way. The benefit of it was that you only had one wire instead of two. If you need to run a few hundred miles of wire, that’s a good benefit. The down side was that it didn’t work well in all areas, and in some areas it worked better or worse depending on when the last time was it rained and that sort of thing (moisture can affect the ground conductivity). So, this sort of thing isn’t done very often, only because it’s not as reliable as we would like our electrical systems to be.
So now let’s go back to our simple generator, and put both wires back in place. We could have a DC generator or an AC generator. If you want to transmit electrical power any great distance though, you need to boost the voltage. This is really easy to do with AC. All you need is two coils of wire wound around an iron core. With DC it is much more difficult. This is why we all use AC systems these days (mostly). Now with our AC generator, the current goes out one wire and comes back the other, then it switches direction and goes out the second wire and back on the first, over and over, 60 times a second (or 50, depending on what part of the world you live in). There is no real difference between one wire and the other at this point.
What happens if we touch one wire and the earth? Remember, the earth is electrically conductive. If all we have is our generator, two wires, and our load, then nothing happens. You can quite easily touch either wire (but not both at the same time) and the earth and not get shocked. It’s pretty safe, right? Well, what happens if a tree branch touches one of the wires? Then if you touch the other wire and the earth, you get shocked, because the current will go through you, through the earth, and up through the tree, to the other wire. That’s not so safe, because then you never know which wire is safe to touch and which isn’t.
So, what we do instead is we intentionally connect one of the wires to the earth. Since this wire is now electrically connected to the earth, it is safe to touch this wire and the earth at the same time. So we call that the “neutral” wire. The other wire is not safe to touch, since if you are touching it and the earth at the same time you will get shocked. We call that the “hot” wire.
There are “isolated” systems where the electrical system isn’t intentionally grounded. You will find them in hospitals, especially in operating rooms and other “wet” locations where tiny flowing ground currents can flow through the patient’s chest in open heart surgery and kill them, for example. You also find them on ships sometimes. It is very difficult to keep an isolated system isolated, though. Hospitals for example have to test their isolation transformers every year to make sure the isolated parts stay isolated. If you tried to keep a big residential type electrical system isolated you’d find that it’s just too hard to do. Mother Nature is just a bit too good at throwing random ground connections into your system.
The important point here is that grounding in a modern electrical system is done for system stability and safety. Although we could use the earth as a source and sink for electrical charge, we usually don’t.
You will notice that there is a third connection in your electrical outlet. That is the safety ground. Electrically, it is connected to the neutral (and both are connected to earth ground) fairly close to where the electricity comes into your home. It may seem kinda silly to have a second connection that goes to the same place, but the safety ground serves two purposes. First, it carries no current, so its voltage is always exactly the same as earth ground. Second, if something has a metal case (like a stove or a fridge) that case is grounded for safety. If you use the neutral as your safety ground and the neutral wire breaks, then the case of your appliance will be “hot” if you turn it on, and you could be shocked if you touch the case and earth ground at the same time. If you have a separate ground, if the neutral breaks, the case stays safe, and if the safety ground breaks, the case still stays safe. With a separate safety ground system, it is much harder to create a fault that will leave the case hot.
Now, getting back to 3 phase. If everything is perfectly balanced, then the currents will sum to zero. If they aren’t perfectly balanced, you could use the earth as a fourth wire to get the current back to the generator, but this isn’t done very often. Most of the time, there is either a fourth wire which is much smaller than the other three (it doesn’t have to be big since it doesn’t carry much current) or there is no connection, and they just let the system run a bit unbalanced. It’s usually close enough to balanced that it’s not off by a noticeable amount.
Allright, someone who knows, case in point I work in the Concert and Event Business
One of our worst case scenarios a 3 phase dimmer, potential 120V@ 200 amps per leg, leg A load 100%, leg B 35%, leg C 50% = 200% of line on neutral(most 3 phase dimmer racks carry a double neutral conductor for this very reason, they distort the wave form) at some point Ground and Neutral are bonded and the Neutral is generated in the transformer, so to ground it goes. My question what happens to this? Sorry to not be specific in the first question, this is beyond the scope of the layman. I know I must be mis understanding something but this has to balance out somewhere, thanks please prove me to be an idiot
Thank you,
Such a simple answer, I know most of what you told me, I work in concert production, but it never occurred to me the the Earth would be a good conductor at a little depth, Doh I have hammered in so many Ground rods over the years. I have been trying to better understand Electrical theory ever since I almost killed myself on a panel last year(100a @ 121.3 Vac Right in right arm out knees, hurt like…) thank you again
Kirk:smack:
Why didn’t you ask Scotty, is he on shore leave? 
Look, someone was going to say something about it soon or later. I choose me & now. :p;)
Ah yes, theatrical lighting is a bit of a specialty. If you’re with IATSE,your local probably has someone who’d be able to help out a curious deck electrician.
Failing that, some manufacturers of equipment offer training - either formal classroom programs or collections of articles on their websites.
At the transformer normally wired in a y configuration, the neutral and ground should be bonded. 5 wires should run out to the dimmer packs.
The dimmer packs will have three hot busbars. A triact willl be connected to one buss bar and the neutral bus. The current comming back from your example’s B bus will cancel out some of the current from the A and C legs. and current comming back from the C leg will also cancel some of the current comming back from the A leg. The neutral does not carry a sum of the legs. The wave distorsion can be interesting, but to me that is Black Box stuff.
At the transformer the seconday coils are connected in a Y manner with the neutral at the center tap. The current comming back the neutral splits and corrects the unballance at the transformer. Remember each leg 120 degrees different than the other legs.
The ground should not be carring any current. But the neutral and the ground are donded at the transformer so they will be at the same referance point. If the neutral is not bonded then the voltages can float.
Before hooking up a light board to any power source the ground and neutral shold be checked for any voltage differance.
Sorry for the hijacking but this is related.
The other day I noticed that some pressure readings were not reading on my screen. So I went to check this equipment out. I saw that it was plugged in but the lights were not ON. The unit doesnt have a off/on switch. Plugged in is ON. So the unit itself is portable and was setting on the concrete floor which has a layer of floor epoxy. Near the unit is a metal floor covering a pit and the metal floor is grounded.
Anyways I leaned down with my right hand supporting my weight on the metal floor and when I touched the unit I got shocked with 120v AC on my hand for a moment. I yelled very manly-like and jerked my hand away from the unit. My safety engineer congratulated me on not dying. Was it possible I could have kicked the bucket? Why didn’t I?
How was it that all the lights on the unit were not lit, but I still got shocked? The extension cord was a twist lock and was made in the electricians shop that day. I was told that the “ground may have lifted” and also that somehow the “hot” also energized the chassis of the unit. I think that is BS and what really happened was someone made a cord that had the Hot wire connected to the ground pin. What say you?
A few points to clear things up:
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If the loads are isolated from ground (i.e. there’s no ground current, which is usually the case), the currents in all the wires will *always *sum to zero, even if the loads are not balanced. In other words, if you add the three currents in a three-phase delta system (hot hot hot), they will always equal zero regardless of the loads, and if you add the four currents in a three-phase Y system (hot hot hot neutral), they will always equal zero regardless of the loads. Yes, the currents may not be the same on each wire, but all of them *will *add to zero. Note also that, when you add the currents, you must include their phases and not just magnitude, obviously.
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In a three-phase delta system, the magnitude of the current in each wire (hot hot hot) will be the same if each load is identical.
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In a three-phase Y system, the magnitude of the current in each of the three hot wires (hot hot hot) will be the same if each load is identical. Furthermore, there will be no current in the neutral wire if each load is identical.
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When we say “the three wires have the same current,” we are saying the magnitude and frequency of the current in each wire is the same. The phases will not be the same. When you include the phase of each current, they algebraically add to zero when the loads are identical.
congrats on not dying.
i’ve seen miswiring of devices the same way where a chasis was hot. only good safety practices, during troubleshooting, might prevent death or injury with grounded objects inches away.
now a days checking can be easier with a noncontact voltage detector. they are a life saver.
it is a deep philosophical question for me what an appropriate response to the person(s) who would cause that to happen would be.
So with the lights OFF on the unit but me getting shocked…does that mean the cord was wired wrong? I assume if the ground had lifted as stated by a lead electrician and the hot wire had somehow frayed and touched the chassis…wouldn’t the circuit breaker for that have tripped instantly? However, the breaker for that unit was checked and it wasn’t tripped. Not only that I took my fluke and measured from ground to the spot I touched and it read 120v AFTER I was zapped.
Again, sorry about the hijacking but I am still mad about this.
Was this at work? Was it reported to someone? What steps are being taken to ensure this doesn’t happen again, with something worse than a manly yell the result?
I believe nothing was done. Usually we have near miss reports to warn others but nothing was released after a couple months now. The electrician who supplied the cord is in the union. I think it was covered up because it was such a huge Fuck up. Like lose your job kind of Fuck up.
The story told was that the cord was damaged due to a technician not knowing it was a twist lock (lie because I wrote the request for the cord and specified it was a twist lock) and that by yanking in it the technician had damaged it. Total BS…to protect the electrician.
only a person who is competent looking at your device can say why it happened. a newly made cord causing a device to fail and be a fatal hazard was not done by someone competent, someone other should be checking it.
if the chassis was isolated from from an electrical ground (painted floor might do it, rubber pads on feet also would) it would not have to trip a breaker.
the light being off only means that the light is off. it could malfunction or be blown for many reasons including too high voltage on it. good safety practice is to check your measurement devices (to indicate safety) before each days use and trust their measurement and not any indicator on the device you are trying to fix.
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If the chassis of the unit had been properly grounded, then you should not have received a shock. There is a discontinuity in the safety ground somewhere in the circuit. It could be in the unit itself, the unit’s power cord, the receptacle on the wall, in the breaker box, etc. Use a meter to figure it out.
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For most devices, you should not receive a shock even if the chassis is not connected to earth ground. The fact that you got shocked means 120 VAC somewhere inside the unit may be leaking to the chassis via some insulation that is breaking down.
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GFCIs are good things.