Chemical Ground

[hwilke]

My question is about Chemical Grounds.
I could not find any information on chemical grounds in the NEC. We have currently finished a research facility at UTD Dallas that has a chemical ground system. There will be an electron microscope connected to this system. The University is concerned that there will be noise on this ground because it is bonded to the main ground bus for the building. the university is wanting to remove the bonding jumper from the main ground bus. I'm telling them this would be a code violation. Can you have a compleatly seperate grounding system for this microscope? Is there any information out there that we could use to help with this problem? Your hthoughts on this matter would be greatly appreciated.
Thanks
Hank Wilke


Edit, removed contact info per forum rules, use PMs to exhange that info.

 

[iwire]

the university is wanting to remove the bonding jumper from the main ground bus. I'm telling them this would be a code violation. Can you have a compleatly seperate grounding system for this microscope?

No, they can add all the grounding electrodes they want but they all have to be tied together.

Take a look at 250.54 .

 

[dereckbc]

Bob actualy you can have a seperate ground if you use a seperately drived system dedicated for a specific system.
However there is no reason to do so as long as as a dedicated feeder/branch is provided to the specific equipment.

Lyncole XIT ground rods are the most popular chem rods on the market.
http://www.lyncole.com/

 

[iwire]

Bob actualy you can have a seperate ground if you use a seperately drived system dedicated for a specific system.

I don't see how that could be done under the NEC.

250.30 and 250.58

 

[jack horner]

I don't see how that could be done under the NEC.

250.30 and 250.58

Bob is correct.

 

[LarryFine]

Bob is correct.

"Uh, Mr. Trotter?" ~ Judge Haller in My Coucin Vinny

 

[dereckbc]

I don't see how that could be done under the NEC.

250.30 and 250.58

Bob I use to think the same thing until John Caloggero straightened me out. Do you or any other members remember his user's name on this forum?

He addressed this very subject not long ago and should be easy to find if I can spot his usr's name to perform a search.

 

[iwire]

Do you or any other members remember his user's name on this forum?

john m. caloggero

 

[dereckbc]

Thanks Bob found the post quickly. After reading his response to this thread:
http://forums.mikeholt.com/showthread.php?t=53312
I PM'ed John and ask him specifically if the ground rods had to be bonded to the GES to comply.
He simply replied NO assuming that a properly sized EGC ran with the primary conductors satisfied the requirement.

Don and I have hit this very subject a few times of the last 4-years, that since an EGC is ran with the primary, there is no need to bond the ground electrode of an SDS to the facility electrode.

So let the friendly debate begin. I side with John and the NEC.

Edited to correct spelling.

 

[iwire]

He simply replied NO assuming that a properly sized EGC ran with the primary conductors satisfied the requirement.

I agree with that.

Now tell me how that keeps the two grounding electrodes or GES's separate or independent as the OP was suggesting.:grin:

The added ground rod must be tied into the EGC, there is no NEC way for that EGC to not also be connected back to the buildings GES.

 

[dereckbc]

I agree with that.

Now tell me how that keeps the two grounding electrodes or GES's separate or independent as the OP was suggesting.:grin:

The added ground rod must be tied into the EGC, there is no NEC way for that EGC to not also be connected back to the buildings GES.

I am assuming anyone installing scientific equipment is going to power it from a SDS. It would be foolish not too if noise was a concern

 

[winnie]

The added ground rod must be tied into the EGC, there is no NEC way for that EGC to not also be connected back to the buildings GES.

This, however, might be a solution to the original poster's dilemma.

If this grounding electrode is tied to the building GES by some jumper that goes off to the nearest other grounding electrode, then we have a potential for a ground loop and noise pickup.

However if this grounding electrode is tied to the building GES by the EGC in the feeder to this piece of equipment, then any noise induced in the EGC will likely be 'common mode' with noise induced in the feeder conductors. Since everything (including the chassis of the electron microscope and the _soil_ where this grounding electrode sits will be at the same 'noise voltage', then the noise won't get noticed by the electronics.

I would want an engineer to very carefully evaluate how such a system would deal with a nearby lightning strike and any earth currents that would flow, or how the system would respond to 'stray voltage' between this grounding electrode and the rest of the building GES.

-Jon

 

[iwire]

Dereck SDS or not the EGC becomes the bonding jumper between the systems.

The code does not require that I specifically run a 'bonding jumper' between the grounding electrodes of the building and the SDS.

However those electrodes will be connected together through the EGCs, there is no escaping that, John even mentions that in post 6 of your link.

 

[dereckbc]

Hank, hopefully you have read the chatting between Iwire and myself.
You can do exactly what the university wants proving the microscope is fed from an isolation transformer.

An ordinary dry type transformer will provide a minimum of 40 db of common mode noise rejection. This is the noise the university is worried about. A special isolation transformer made for the purpose, can provide up to 140 db of common mode noise rejection. To put his into perspective 40 db equal a ratio of 10,000:1, and 140 db is 100,000,000,000,000:1

If I were you, I would advise your client to install a isolation transformer feeding the microscope, and use the chemical rod to establish the new ground reference.

 

[dereckbc]

Dereck SDS or not the EGC becomes the bonding jumper between the systems.

Yes I know, I am in full agreement. Like I said I am assuming the microscope is being fed by SDS transformer and they are using a chem rod as the ground electrode for the transformer, which also happens to be bonded to the facilty GES.

If the microscope is being fed by SDS, they can lift the bonding jumper from the chem rod to the facility GES.

I hope I stated my intent correctly and we are on the same page now.

Edited for grammar.

 

[iwire]

If I were you, I would advise your client to install a isolation transformer feeding the microscope, and use the chemical rod to establish the new ground reference.

And that chemical rod will still be tied to all other grounding electrodes through the EGCs

 

[iwire]

If the microscope is being fed by SDS, they can lift the bonding jumper from the chem rod to the facility GES.

I hope I stated my intent correctly and we are on the same page now.

Maybe, but I don't get it.

Lift the bonding jumper or leave it, either way it is still connected.

 

[dereckbc]

Maybe, but I don't get it.

Lift the bonding jumper or leave it, either way it is still connected.

Yes it will via the primary EGC, but there will be no circulating current to develop any external voltage on the reference point (Xo) of the transformer.

 

[winnie]

A couple of additional points. 250.54 is the explicit permission to connect _supplementary grounding electrodes_ via an EGC as we've been discussing above. An isolation transformer is not even required; this could be the EGC from a feeder.

There was an article on this sort of grounding in the October 2006 EC+M. They were discussing grounding practice for expensive CNC equipment, and describe how some manufacturers _require_ these supplementary grounding electrodes. In opposition to some manufacturer suggestions or requirements, the article suggests that having a separate grounding electrode will _increase_ the risk of damage to the machine from lighting and utility faults. I recommend showing this article to the people installing this microscope.

My _hunch_ is that having a supplementary grounding electrode of this sort is only safe for the equipment if the normal building grounding electrodes _surround_ the supplementary electrode. In which case, why have the grounding electrode at all. Establish a conductive ground plane that is insulated from the soil, except for the single point EGC bond back to the building electrode system. This gives you your electrically quiet environment, and doesn't couple your equipment to stray currents in the soil itself.

-Jon

 

[dereckbc]

Bob, thought I would take a minute to explain a little better or at least attempt to 

For years I was under the impression that a ground electrode of an SDS had to be bonded to the facility GES via a bonding jumper. I always argued that shouldn?t be necessary because the EGC of the primary should satisfy that requirement. In the democratic environment I worked in, my peers thought otherwise, so I went with the crowd. Really didn?t bother me because the code allows either way.

So as to why or why not bond the ground electrode of a SDS is a matter of design and objective. Leaving it floating achieves a couple of things. For one it allows implementation of 250.6. This leads to the second point of having a current free reference point A current free reference point in turns means no external voltages aka noise. Large systems have various current circulating around which develop voltage potential differences. By breaking the loops or paths, and establishing a new single reference point eliminates objectionable currents and voltages. Or put another way you are establishing a new Single Point Ground free of noise and external influences.