Grounding Electrode for Seperately Derived System

[fishin' electrician]

What is the purpose of a grounding electrode system on the secondary side of a transformer installed within a building? Isn't it already connected to the buildings GES through the required supply side grounding? Is it merely because the supply EGC is likely to be smaller than a properly sized GEC when stepping down? I was taught that the GES is to limit the voltage potential in case of accident contact with utility high voltage lines and for lightning protection, I just don't get how that would effect anything here. Thanks in advance, I've learned an awful lot on this forum.

 

[Ed MacLaren]

Re: Grounding Electrode for Seperately Derived System

Are you referring to grounding the separately derived system, or the transformer enclosure?

The EGC run with the primary feeder only grounds the transformer enclosure. (sketch below)

If you are referring to grounding the secondary system, I agree that there isn't much lightning or hi-voltage lines inside the building.

However, one of the reasons given in most codes for grounding, that does apply to separately derived systems, is the one that reads - "to limit the maximum potential to ground under normal conditions".

In the example shown the voltage to ground is limited to 120 volts, which puts less stress on conductor insulation, whereas if it were an ungrounded system, and a ground fault existed on one live wire, the voltage would be 208 from the other two to ground.

Ed

[ June 21, 2003, 05:32 PM: Message edited by: Ed MacLaren ]

 

[fishin' electrician]

Re: Grounding Electrode for Seperately Derived System

Thanks Ed.

I was referring to grounding the seperately derived system. We have it connected exactly as your diagram indicates with a GEC to the building frame steel, however the inspector is telling us that we also need to run a GEC back to the water main unless we can prove that the building steel is effectively grounded. Of course I can't prove that the water main is anymore effectively grounded than the building steel without an engineers stamp so I don't know why he assumes it is. What gets me is that all we did was move the transformer from the floor to above the ceiling and it was originally bonded only to the units interior water piping.

 

[bennie]

Re: Grounding Electrode for Seperately Derived System

Fishin: You have a good concern. Not only do you not know if the steel is effectively grounded you will not know if it is bonded to the other available electrodes.

Should the steel be effectively grounded and not bonded, there will be two points of earth connection. This is two individual ground electrode systems.

 

[fishin' electrician]

Re: Grounding Electrode for Seperately Derived System

The main carrying beam we ran to is bonded to the building's 480 service. The state electrical inspector overided the town inspector and OK's us as is. Thanks!

 

[bennie]

Re: Grounding Electrode for Seperately Derived System

Here again, no one will agree with me; To be effectively grounded, and bonded, would require a ground ring around the perimeter, with a tail cad-welded to each steel support column.

Depending on building steel to have and retain low impedance is only wishful thinking.

To validate my position...Take an aluminum ladder, connect a neutral conductor to one side at one end, connect a wire from the other side and end to the neutral bus. Apply a load that will be about 1200 watts. Measure the current in each rung. There is a lot of difference. This will be what happens with building steel .

 

[fishin' electrician]

Re: Grounding Electrode for Seperately Derived System

bennie,

Given my grounding/bonding/effectively grounded/etc education, I am really in no position to agree with anyone . Being a mostly silent member here that uses this forum as a learning tool, I often do find myself in agreement with you as your real-world experiences and education far outweigh mine. In a perfect world, your idea of the ground ring setup would surely constitute the most ideal scenario, however; one can only imagine the cost of such a system for instance, in a large or high rise building.

Oh, and sorry, I might try that experiment, but I do not own an aluminum ladder !!!

 

[bennie]

Re: Grounding Electrode for Seperately Derived System

fishin: I am illustrating an extreme perfect world situation. The procedure you are using is OK with the majority situations.

Your knowledge is better than you think. As stated by another individual, "a smart person is one who is aware of what they don't know"

I am very smart, there's a lot I don't know

 

[bennie]

Re: Grounding Electrode for Seperately Derived System

Another ladder demonstration is to connect the same side member, in a circuit from one end to the other. Establish a 10 amp flow. Measure the current in each rung. The current will be the same at each end but will progressively drop towards the middle rung. This is what takes place in the power distribution system using the multi-ground principal.

The voltage drop decreases towards the middle. This is the cause of the phenomenon of stray voltage, cows gettin mad, and pool tingle shocks.

 

[tom baker]

Re: Grounding Electrode for Seperately Derived System

"Here again, no one will agree with me; To be effectively grounded, and bonded, would require a ground ring around the perimeter, with a tail cad-welded to each steel support column."

Bennie I agree with you. This type of connection is shown in the Erico grounding guide, and I have seen IEEE papers referencing this.

 

[bennie]

Re: Grounding Electrode for Seperately Derived System

Tom: An endorsement from you means a lot. I have not read either one of the papers you mention.

I wrote the standard for a ground plane in remote areas, that were impossible to make the earth an equi-potential medium. One project was in the silica sand of the Sahara Desert of Libya. The other was the coral reefs of the Marshall Islands, and the ice at Siple Station in the Antarctic.

We used the building and floor supports as the ground, just like an aircraft.

In the Antarctic I was asked by one of the scientists "How are you going to get a ground on this 2 mile thick ice cap" I said " In the dirt we call it grounding, here I call it icing" I'm not sure how that went over. Some scientists have a weird sense of humor

I have hit post number 1200, this makes a total in excess of five thousand. What the heck have I wrote?

[ June 21, 2003, 09:56 PM: Message edited by: bennie ]

 

[tom baker]

Re: Grounding Electrode for Seperately Derived System

Congrats on the posts.
On of my students worked on an fishing island in Alaska. They had a problem getting a ground for the village generator there, so they put an old D-8 cat in the river and grounded to it. the permafrost is tricky to deal with. It will pop ground rods out of the soil.

 

[bennie]

Re: Grounding Electrode for Seperately Derived System

Tom: Of course you informed your student that a D-8 Cat is not an approved grounding electrode