(Earth) grounding transformers

[ryan_618]

Why do we (earth) ground transformers?

 

[peter d]

Re: (Earth) grounding transformers

For lightning induced surges?

 

[charlie b]

Re: (Earth) grounding transformers

Also because we bond them. If the center point of a WYE secondary is connected (bonded) to the case, and if that same point is not ?earthed,? then there would be a potential difference between the case you touch with your hands and the dirt you touch with your shoes. This would create a ?human GEC.?

 

[charlie]

Re: (Earth) grounding transformers

Why do we (earth) ground anything?

250.4(A)(1) Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.

That is the end of the story. Everything else is bonding, not grounding. Is that where you are going?

 

[ryan_618]

Re: (Earth) grounding transformers

I'm just thinking lightning. How would lightning differentiate between the transformers earth ground and the original systems earth ground? I can understand bonding the structural metal and the water pipe, so that you have an effective ground fault current path. I just am having a hard time seeing how lightning or unintentional contact with a high voltage line come into play. I mean, if you don't have structural metal or water pipe, you would drive a ground rod, and I just can't figure out why. I admit, my knowledge of the finer points of electrical theory are lacking to say the least, perhaps that is why the confusion.

 

[sandsnow]

Re: (Earth) grounding transformers

I have heard that it stabilizes the voltage to ground on the secondary side. I think this was the combination of bonding and grounding the neutral.

Someone please debunk or support that.

 

[charlie]

Re: (Earth) grounding transformers

Ryan, don't think of the earth as non-conductor of electricity. It is of no value for fault return for low voltage (600 volts or less) but for medium and high voltage, it will work as a return for fault current. Plug in the 8.66 kV (grounded 15 kV) class to ground and look at its return current. Assuming 25 ohms, you will have something over 300 amperes of fault current depending on the ? to ground voltage of the 15 kV class system.

Lightning and higher voltage to ground are returning to the source. The more paths that are available, the better.

 

[charlie]

Re: (Earth) grounding transformers

It does Larry. A static charge can build up on an ungrounded system so the AC actually rides on the static charge. Everything actually works well but the static could really hurt someone.

 

[sandsnow]

Re: (Earth) grounding transformers

Originally posted by charlie b:
Also because we bond them. If the center point of a WYE secondary is connected (bonded) to the case, and if that same point is not ?earthed,? then there would be a potential difference between the case you touch with your hands and the dirt you touch with your shoes. This would create a ?human GEC.?

Charlie
Where would the current be trying to flow to? What I mean is if we have a properly bonded and grounded system and out at the load end instead of an equipment grounding conductor we drive a ground rod for the non-current carrying metal parts, if the case is energized then the current would flow through a person to earth to try and get back to the source. So how does it play out if we don't ground the neutral and someone touches the XFMR case?

 

[ryan_618]

Re: (Earth) grounding transformers

Charlie, I understand what you are saying regarding high voltage, but I am thinking in terms of a typical 480 Delta 208Y/120 transformer.

Thanks for your help, even though I'm still not perfectly clear

 

[sandsnow]

Re: (Earth) grounding transformers

Originally posted by charlie:
It does Larry. A static charge can build up on an ungrounded system so the AC actually rides on the static charge. Everything actually works well but the static could really hurt someone.

I was thinking more along (with what I remember) that the voltage to ground would be unequal. Say 100 on one leg and then 150 and 90 on the others. Or something like that.

I don't recall about the static charge. Is that for HV primaries? or for 480volt as well?

[ December 21, 2004, 06:57 PM: Message edited by: sandsnow ]

 

[iwire]

Re: (Earth) grounding transformers

Originally posted by ryan_618:
Charlie, I understand what you are saying regarding high voltage, but I am thinking in terms of a typical 480 Delta 208Y/120 transformer.

Thanks for your help, even though I'm still not perfectly clear

I am in the same boat as Ryan, I am not seeing the link to the transformers I deal with.

Bob

 

[charlie]

Re: (Earth) grounding transformers

If a transformer coil fails (one of mine) and the primary voltage goes to the secondary side, that is a higher voltage line coming in contact with a lower voltage line and the path is the return to the substation.

 

[pierre]

Re: (Earth) grounding transformers

To add to this confusion

I will try to look at the type that Ryan is asking about, such as 480v/208v type transformers located INSIDE if a building.

As far as lightning protection of this situation, it is a far stretch to actually see lightning effect this type of installation.

So grounding will be effective to stabilize the voltage, establish a grounded (neutral) conductor, and helps to establish an "equipotental plane" in the area of the transformer to keep the potential as close to zero as possible.

The question at this point that begs answering is: what if there is no means of grounding in the vicinity of the transformer (no steel,cold water, etc...), and a grounding electrode conductor has to be installed 3 floors downstair to ground rods outside? What does this really accomplish?

Pierre

 

[don_resqcapt19]

Re: (Earth) grounding transformers

Pierre,
For the type of installation that you have described, I see no electrical reason(only a code reason) why the primary EGC cannot be used as the secondary GEC. I submitted a proposal about this but CMP5 rejected it.
Don

 

[friebel]

Re: (Earth) grounding transformers

After reading all of the threads on "Earth" Grounding Transformers, I am going to make a suggestion. If you ever get the opportunity to attend one of Mike Holt's Grounding & Bonding Seminars, do it. It will help to answer all of your questions that you may have.
I attended Mike's Grounding & Bonding Seminar in Newark, Delaware. I also invited four of my students that I have taught the NEC to go to the Seminar with me. Since then I have purchased the Grounding & Bonding Tapes, and we offer two, five-hour sessions on Grounding & Bonding at our college.The electricians in the State of Delaware need 10-hours of CEU electrical training to renew their license. I have been certified by the State of Delaware Licensing Department in Dover to teach the electricians that want to renew their license.

 

[ryan_618]

Re: (Earth) grounding transformers

Originally posted by friebel:
After reading all of the threads on "Earth" Grounding Transformers, I am going to make a suggestion. If you ever get the opportunity to attend one of Mike Holt's Grounding & Bonding Seminars, do it. It will help to answer all of your questions that you may have.

I never thought about it until I heard Mike Holt ask the same queestion I did. Why do we do this?

I agree with Don's reply, personally.

 

[charlie b]

Re: (Earth) grounding transformers

Originally posted by sandsnow: Charlie. Where would the current be trying to flow to? . . . So how does it play out if we don't ground the neutral and someone touches the XFMR case?

If the neutral is not ?earthed,? then it can ?float? at any voltage with respect to planet Earth. Think of it as a charged capacitor. In order for the capacitor to have become charged, it had to have currents flowing, however briefly, through a not-yet-complete path, from the source (i.e., the transformer) to both sides of the capacitor (i.e., the transformer case and planet Earth). When a person touches the case, the capacitor is discharged through the person.

Does this answer your question?

 

[charlie b]

Re: (Earth) grounding transformers

Originally posted by ryan_618: How would lightning differentiate between the transformers earth ground and the original systems earth ground?

It would depend on where the lightning strikes. I?ll address Pierre?s question next, so let?s talk now about an outdoor pad-mounted 480V ? 120Y/208V transformer. Recall that we are talking about two separate connections: (1) The connection (bond) between the center point of the WYE and the transformer case, and (2) The connection (ground) between that same point and planet Earth.

Lightning occurs because of the build-up of net-negative charges in the clouds, and net-positive charges on planet Earth. The build-up is possible because there is some distance between them, and a great deal of resistance in the path between them. Sound like a capacitor? Lightning is essentially the breakdown of the dielectric (not the ?discharge?!) of a capacitor. The electrons are flowing through the dielectric (i.e., atmosphere), and they are headed for the dirt. When lightning strikes an outdoor transformer, it finds a current path from the case to the neutral point, and via the GEC to the ground rod. That is the lowest resistance path that it will find. It will find other paths, of course, one of which will somehow include your TV and your stereo system. But if there were no direct path from the case to the dirt, then more of the current will flow through your house, perhaps setting it on fire. That is why we pound the ground rod.

Now let us set aside any considerations about lightning. Let?s enact a ?No Lightning Here? law, and eliminate that problem. Result: We still need to bond and ?earth? the center point of the WYE. Why? See my previous posts about floating voltages and human GECs.

Does this help?

 

[charlie b]

Re: (Earth) grounding transformers

Originally posted by pierre: The question at this point that begs answering is: what if there is no means of grounding in the vicinity of the transformer (no steel, cold water, etc...), and a grounding electrode conductor has to be installed 3 floors downstairs to ground rods outside? What does this really accomplish?

An indoor transformer is not going to be struck by lightning, so that consideration does not come into play. But the possibility of having the case of the transformer ?float? at a voltage different from, say, the floor next to it, is still a consideration. ?Earthing? the center point of the WYE accomplishes the same thing that I have described earlier. From the perspective of a possible ?Human GEC,? running a long copper GEC is not as desirable as having the transformer on an outdoor pad. But that is not possible in a high-rise, and this is the next best thing.