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#6104 - 12/23/01 01:02 PM afraid to ask  
Cindy  Offline
Member
Joined: Jun 2001
Posts: 196
PDX, OR, US
embarassing maybe, but I don't get this. In a single phase 3-wire system for instance, I've always read that the center point of the secondary winding has had this neutral wire physically spliced/connected to the winding, and then going to a ground connection point.
But why isn't that a short circuit to ground? the same as it would be 50 feet away from one of those phase wires if they were connected to ground?
if this is too complicated, never mind


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#6105 - 12/23/01 01:22 PM Re: afraid to ask  
sparky66wv  Offline
Member
Joined: Nov 2000
Posts: 2,236
West Virginia
Good Q, Cindy...

It is my understanding that any one phase (or point between) may be grounded... But only one. The ground reference helps to define what 0 volts is, and also provides a (rather high impedance) return path.

Please correct me if I have erred, I wish to understand this a little better too.


-Virgil
Residential/Commercial Inspector
5 Star Inspections
Member IAEI

#6106 - 12/23/01 01:22 PM Re: afraid to ask  
maintenanceguy  Offline
Member
Joined: Dec 2001
Posts: 300
Southern NJ, USA
If I understand your question, you are right. The neutral wire (which comes off the center tap of the secondary transformer at the pole) is shorted to ground. Or more accurately, forced to stay at ground voltage or zero volts.

If the world was wired without grounding neutrals everything would still work fine...but the neutral in my house would be at a different voltage than it would be in yours or for the guy next door.

A 220/110V transformer provides 100V between each hot leg and neutral but that can be done by providing three wires at -110V, 0V, and 110V or by providing 4,300V, 4,510V, and 4,620V. The difference between the wires is still 110V and equipment would operate properly if you don't mind your plumbing, building framing, electrical boxes all being at 4000V above ground. Hurts to turn on a light while standing outside.

Hope this helps.


#6107 - 12/23/01 01:37 PM Re: afraid to ask  
Cindy  Offline
Member
Joined: Jun 2001
Posts: 196
PDX, OR, US
last time i asked someone this question i got the same dumb look on my face when i listened to the answer. this may be one of those questions like who came first, the dinosaur or the egg?

Mxguy, you said it is a short to ground. why don't it go boom?


#6108 - 12/23/01 01:50 PM Re: afraid to ask  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
The answer is the same as that for a corner grounded delta, where one of the phases is intentionally grounded. As long as it is the sole connection to ground, there is no short circuit, because ther is no current path. That is, until a second (unintentional) ground connection (fault) occurs on another phase.


#6109 - 12/23/01 03:22 PM Re: afraid to ask  
Cindy  Offline
Member
Joined: Jun 2001
Posts: 196
PDX, OR, US
whether single or 3-ph, like the 3 windings on a trxfmr or generator stator, they say that all phase conductors are connected together at a common point in a wye system, or looped together in a delta system, and my understanding of phase conductors is that they are bent on finding the easiest path to ground. [my next question was going to be how can these phase conductors connect without another boom? but i won't go there now]
I'm reading that the common conductor attached to that midpoint or corner is "grounded to earth" so why isn't that a "current path" as Redsy says? its line to ground, right? why does it need to be a different phase to ground before its a short? why does calling it "intentional" make this midpoint or corner grounding less of a current path than if grounding those phase conductors 2 feet away or 10 feet, etc.
if you look at an impedance grounded neutral system i think they put a resistor between the trxfmr and the GE, so if they are putting a resistor there, then what are they resisting? i know, dumb questions probably, but i'm missing something fundamental obviously


#6110 - 12/23/01 03:45 PM Re: afraid to ask  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,311
Scott had a number of X-former pix in the Tech ref area @ one time, I found this,,
https://www.electrical-contractor.net/ubb/Forum15/HTML/000044.html

The theory of a 'center tap' is viewable here. Myself, i think it helps to think of electricity simplistically as a circle
( all hardcore theory aside) [Linked Image]

It's just that one point in that circle should be at 0 volts....


#6111 - 12/23/01 03:52 PM Re: afraid to ask  
sparky66wv  Offline
Member
Joined: Nov 2000
Posts: 2,236
West Virginia
Cindy, you apparently understand it better than I do... You point out a lot of interesting things...

I have a similar Q concerning what would happen if a non-grounded delta conductor were to "fault" to a grounded guy wire of a wye system... like on a shared pole, 19.9KV Delta on top and 4160/7200 Wye on the bottom...

[Linked Image]

In this photo, there are fiberglass insulators to prevent the scenario I've created here for discussion...

Go here for the other thread about this photo where the insulators were "brought to light"...
https://www.electrical-contractor.net/ubb/Forum5/HTML/000095.html

[This message has been edited by sparky66wv (edited 12-23-2001).]


-Virgil
Residential/Commercial Inspector
5 Star Inspections
Member IAEI

#6112 - 12/23/01 03:57 PM Re: afraid to ask  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,311
Hoo Boy!,now were gettin' technical! It's funny how one 'afraid to ask' Q can lead to us all partaking in the Tarte de corneillefor X-mas din-din.... [Linked Image]

& by the way, Merry Xmas to all !!!

Steve
aka sparky


#6113 - 12/23/01 04:36 PM Re: afraid to ask  
maintenanceguy  Offline
Member
Joined: Dec 2001
Posts: 300
Southern NJ, USA
Here's an experiment that might make sense. Of course just do it in your head.

Take two AA batteries and put one stacked against the other just like they would be in a flashlight. Pretend this is your tiny little secondary winding on the pole. How the electric gets into these batteries doesn't really matter and the fact that it's 1.5VDC instead of 110VAC really doesn't either.

So take the point where these two batteries touch together and ground it...for no reason at all. With your meter, you measure 1.5V between on end of this battery stack and ground and the same between the other end and your ground, right?

Now from one end of your stack to the other end is 3V, right? Now you've got two hot legs and a neutral.

What if instead of grounding the center point, you left it ungrounded, or even attached it to one end of some other battery somewhere? You still get 1.5V on each leg to ground and 3V across both batteries. But...the voltage of the center point is not necessarily at the same voltage as ground anymore. That's okay, there's nothing wrong with that. you still get the voltage you had before.

The problem is that in our houses, we want our neutrals to all be at the same voltage. To do this they all have to be tied to some reference point, and we chose to use the ground.

But even the earth's ground voltage changes all the time. When there's a lightening storm, it vary's all over the place, light hitting the earth causes a negative charge to form as do all sorts of radiation. But we don't notice because we all change our ground voltage along with the earth. As long as the whole neighborhood is on the same ground, no body gets hurt.

When you walk across a carpet and get a shock touching the doorknob that happens because you and the doorknob had a different ground voltage. Had you both been attached to a driven rod, that wouldn't have happened.

If your house and garage aren't attached to the same ground, the results of moving from one to the other would be worse than that static shock. So everything gets grounded for no reason other than to keep us all working at the same ground voltage...whatever that is today.

I know this is long but I can't come up with a short way to say it. Hopefully somebody more elequent than me will come along.


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