Size of grounding grid

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electricaldoc

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At my work we have a 480V/277V 3PH Y system with a total of about 15KA. We started out a lot smaller and pulled less than 40% of this at beginning. Now we are noticing a lot of grounding problems. Such as potential difference of 40V-50V between grounds. We are currently in a NEC class for 2005, and one of the electricians asked instructor about this. Electrician asked about our grounding grid being inadequate now that we are pulling so much more amps. The instructor agreed. This left me confused because I was under the impression that once you achieve a low impedance ground it was sufficient for any size system. I know the bonding jumpers and conductor sizes will change, but this is done as system is upgraded. I mean they will add proper sized bonding jumper every time switch gear is added, which should all add up to correct size for entire system. I feel the problem would most likely be with our bonding system that we install on equipment and circuits, and not the grounding grid its self. I know the grounding grid can derogate and get higher impedance, but this place is only seventeen years old. The code only requires 25OHMS or less to ground and this is suppose to be sufficient no matter how large of system you have. Now I realize this is only a minimum, and probably not good enough for quality power for a place our size so much lower value needed. The original grounding grid should have reached this lower level, and adding more power should not make this grounding grid inadequate. I still feel as if it is our own bonding that would be problem, and not the grounding grid. I am trying to learn most of this grounding and bonding on my own with help from soares and you guys. So please let me know your opinion, about if I am wrong or correct, and why.

Thanks
Dan

:confused:
 
Re: Size of grounding grid

OK so you got these two ground conductors.
Now you said ground conductor (green) not GROUNDED conductor (neutral or white)

You got a 40 volt difference at their ends.
First off where are "you getting the voltage"
AND why is there a 40 volt difference between them?
when they are supposed to be at the same potential difference and at ground potential?

They both go to the same grid.
Ideally they both go the the same "bus" that is tied to the grid.

I would check the volt differance at the point where they tie to the grid. If zero (and should be) start taking readings downstream on each ground conductor and find out where your bad connections are.
 
Re: Size of grounding grid

Not sure I understand the system you are describing, but here is what I know. If you have 40 volt difference between two systems, they are not bonded period. You said 17 years old correct? If the soil PH is acidic a GES can disolve in as little as 6 months.

If you have a true grid in tact there will be no potential difference, and you should not be able to measure any appreciable impedance @ 60 Hz between any two points on the grid. Such a small system of 15KVA is not large enough to produce such large potentail differences unless something is drastically wrong. Heck most homes have bigger transformers and pull more than that. Sure you got your numbers straight? Besides why would you need a grid for a single transformer? Sorry for the questions, but something is not right.

My guess is either your grid is completely deteriorated, your equipment is improperly configured, or both. Without details and analysis, any guess is as good as any.

[ October 15, 2005, 11:09 PM: Message edited by: dereckbc ]
 
Re: Size of grounding grid

Dan,
Did you say 15,000A @ 480/277V?
Have any grounding tests been performed to determine if the grounding electrode system is still is in tact?
 
Re: Size of grounding grid

Dan
I, like Derek, do not think your problem is because of the resistance to ground value. It is a bonding issue, not a grounding issue. You, or someone in the facility needs to start checking the bonding to see what is actually happening. In a situation like this, your best tool is your "2-eyes".
 
Re: Size of grounding grid

Originally posted by electricaldoc:
So please let me know your opinion, about if I am wrong or correct, and why.
Click to expand...
Your terminology is correct. I wonder if they were speaking the same "language."

Such as potential difference of 40V-50V between grounds.
Click to expand...
Between what? Grounding electrodes? Bonding conductor and Neutral at some point? I just want to be sure that we're all on the same page.

I know the bonding jumpers and conductor sizes will change, but this is done as system is upgraded.
Click to expand...
You don't know anything until you visually inspect the system. Assume nothing.

The code only requires 25OHMS or less to ground and this is suppose to be sufficient no matter how large of system you have.
Click to expand...
The code requires no such thing.

The code requires that if any single ground rod is installed, and that single rod's resistance to ground is greater than 25 ohms, it shall be supplemented with one more rod. That doesn't necessarily mean that the second rod will bring this value down to 25 ohms. The resistance of the GES could be higher, there's no requirement for the overall performance of the GES.

I say this with the mutual disclaimer that we're talking about code minimums, not actual desired performance. Just being clear. :)

Back on topic, it is necessary for us to know what two points read 40V potential between them to discuss this accurately.
 
Re: Size of grounding grid

"I still feel as if it is our own bonding that would be problem, and not the grounding grid"

That is exactly what I am trying to say is grounding grid is fine but our bonding procedures in our plant are in dire need of help.

As far as the potential deference it is between the chassis ground on one piece of equipment and the metal frame of building.

yes if I ad all switch gears together it is about 15,000A.

This difference is exactly why I suspect the bonding in our plant, but others think grounding grid is now to small. Our instructor then agreed with them. I do not want to debate this with him , but I feel they are both incorrect.

Dan :confused:
 
Re: Size of grounding grid

Dan, first please accept my apology. I miss-read your original post and thought you had a 15 KVA system rather than a 15 KA. OOPS :eek:

First thing I would do is make sure the grid is intact by using a ground loop clamp-on tester. The size of the grid, the wire size, and impedance is irrelevant, only it's integrity is important. A 17-year old grid is old and susspect IMO.

Your statement:

"As far as the potential deference it is between the chassis ground on one piece of equipment and the metal frame of building".

Tells me right away your bonding integrity is incorrect or missing. Sounds like the building steel is not bonded properly, or not bonded at all. It is impossible to get that much voltage difference if it is bonded properly.

[ October 16, 2005, 03:27 PM: Message edited by: dereckbc ]
 
Re: Size of grounding grid

Originally posted by electricaldoc:
This difference is exactly why I suspect the bonding in our plant, but others think grounding grid is now to small. Our instructor then agreed with them. I do not want to debate this with him , but I feel they are both incorrect.
Click to expand...
Make a stand. You're correct. Be calm, cool and collected, but resolute. This is an important topic, and if the instructor learns something, better still. :)
 
Re: Size of grounding grid

I think that finding the source of this voltage is of even more importance than the bonding problems. Even without bonding, a correctly installed and operating electrical system should not show this potential difference. The voltage is coming from somewhere and you need to find out the source. Installing the proper bonding many locate the source because it may provide a path so that the fault current will open the OCPD.
Don
 
Re: Size of grounding grid

As far as source of voltage we have a lot of 480V heater bands, and they some time will have some connection to ground. This happens as heater band wear out, but it is small enough not to blow any fuses. Unfortunately when you add enough of these together it makes a larger amount of current flow through the equipment grounding conductor. Since work will not pay to replace them until it fails, we are stuck with this situation.

Thanks this site is great!
Dan
 
Re: Size of grounding grid

Dan,
How old are these heaters? The following code section had been in place for at least 2 code cycles.
427.23 Grounded Conductive Covering
Electric heating equipment shall be listed and have a grounded conductive covering in accordance with 427.23(A) or 427.23(B). The conductive covering shall provide an effective ground path for equipment protection.
(A) Heating Wires or Cables Heating wires or cables shall have a grounded conductive covering that surrounds the heating element and bus wires, if any, and their electrical insulation.
(B) Heating Panels Heating panels shall have a grounded conductive covering over the heating element and its electrical insulation on the side opposite the side attached to the surface to be heated.
Click to expand...
Compliance with this section should provide a path to bleed this leakage current off without causing a voltage potential between grounding and bonding conductors.
Don
 
Re: Size of grounding grid

These heater bands are insulated heater which are placed on the barrel of an injection molding machine. They have insulation built into them, and the barrel they are on "should be" an effective ground path. It will measure as one, but the heater bands may only leak .5A to ground; as they were out. Now this is not enough to blow fuse even with proper bonding. Unfortunately there may be 4 of them on each phase, of three phases, and four different zones each having their out set of fuses. So 4X.5A= 2A per phase per zone so 2A on 30A fuse not enough to trip. Take that times # zones so 4X2A= 8A total, but total OCP may be any where from 350A to 700A breaker. So it still will not trip even if bonded properly. It is kind of like insulation on wire it may be worn enough just to let small current threw, but not enough to trip. The guys as work think potential deference is coming from too small of grounding grid, But I say it is improper bonding on our part.

Dan
 
Re: Size of grounding grid

Dan,
The grounding grid has nothing to do with this problem. The leakage is not trying to get to ground, but back to its source. The problem is with your equipment grounding and bonding path. These paths should handle that amount of current without an excessive voltage drop. The only way to get the voltages you have is if the bonding and grounding path for this leakage current has an impedance high enough to cause that much voltage drop.
Don
 
Re: Size of grounding grid

Dan, I have to back Don up on this. The size of your grid has absolutely nothing to do with the problem. To get 40 or 50 volts potential difference with 8 amps leakage current would mean the path resistance of 5 to 6 ohms.

As Don stated earth has nothing to do the operation, it is only a reference point and not used for load, leakage, or fault currents unless something is terrible wrong. Earth is not used in low voltage sytems, it is only used in high voltage systems.
 
Re: Size of grounding grid

I also agree with Dereck and Don, It's not your connection to Earth that's the problem, it's the connection to the 480/277 neutral or the EGC bonds at the equipment, Or if you have a transformer after the main service you have a bad X0 bond to your EGC on the secondary of the transformer or between the neutral to EGC buss in the first panel. Also another common mistake done in industrial installations where multiple SDS transformers are used is not bonding the X0 (secondary) to the building steel or a common grounding electrode conductor ran between each SDS system to keep them all at the same potential. I have seen where they would sink a ground rod for each one but not make any connection between them. This must be done because any voltage over 30 volts could be lethal.
All neutral to EGC bonding must take place at the first means of disconnect or in the case of a SDS transformer at the transformer or first means of disconnect after it. Then each separate SDS system must be bonded together with a common bonded electrode conductor that runs to the grounding electrode system. You can use the building steel to do this if the building steel is connected through out the plant.
I'm very surprised your not having problems with any electronic systems if used. voltages like that on the grounding can cause all kinds of havoc with electronic systems that also have more than one connection to the grounding system.

[ October 22, 2005, 12:43 AM: Message edited by: hurk27 ]
 
Re: Size of grounding grid

I would like to add that it's a good idea to deenergize a panel when doing this sort of work. You never know when you're going to find an unusual situation.

Oh, wait. Yes, you do.
 
Re: Size of grounding grid

Thanks That is what I thought it's the bonding system, and not the grounding grid. This is was I was trying to say but instructor disagreed with me. Also I do de energize the system before working on "when possible", but now all you have to do is touch one piece of equipment and building frame, and you will feel the tingle

This needs corrected "now", but instructor telling other electricians I am wrong?

Dan
 
Re: Size of grounding grid

Dan
The more you tell us, the more it sounds like a very serious issue. I am not sure who this instructor is, either there is a misunderstanding, or he is not up to par with this. If your scenario is a fact, someone is going to get hurt, or a potential fire exists.

This is a problem with your "effective ground fault current path"; which seems not to be effective enough. Also as Don has said, something is creating this voltage, it does not come out of thin air - especially since you have mentioned a tingling sensation.

I guess where you stand in the pecking order for this company has alot to do with the inactivety of fixing this problem.

One other thing, is this an ungrounded system, maybe a resistance grounded system?

Are all of the switch gear located in the same area?
 
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