Usefulness of Isolated Grounds

[big john]

First things first, I've never found a good answer for this:
A) What introduces noise into a ground line in the first place? Is a lot of it contributed by the fact that devices with a lot of solid-state elctronics [computers, copyers, etc.] use the ground as a voltage reference and this contributes to the ambiant noise? Are larger transients caused by the EMI that accompanies the operation of motors, compressors, etc.? If so, would this not also carry down the line conductors to affect any ground not in the proximity of the motor/compressor but run parallel with the line conductors?

B) Assuming at least some of the above is correct wouldn't it make the most sense to run a dedicated circuit with an isolated ground to help eliminate noise introduced by the line conductors? Is it not contrary to the whole idea to run a "clean" ground in along side "dirty" power?

C) Why, in a wood-frame house wired in Romex, is it said that IGs are a waste? If any of the above is true, then certainly devices in the average home from dishwashers to blenders have the potential to introduce noise into the ground line.

D) What makes a steel frame building so different than a wood frame house? Is it because the framework can almost act as an antenna of sorts and will transmit any EMI it picks up via the framing members into the metal boxes to the EGC? If that is the case, wouldn't it do just as much good to simply put recepticals in plastic boxes when mounted to metal studs? Or would you sacrifice some shielding provided by the grounded box?

E) What power quality problems would arise from having two clean interconnected devices on the same circuit but connected to two different IGRs? E.g.: Two computers only connected to each other, no printers, scanners, etc. Would, in this case, the problem not be power quality but the concept that the datacom cable connecting the combuters could conduct fault current if any difference in potential between the grounds developed?

F) It is often said that IGs are mostly installed to do away with common-mode voltage. Try as I might, I can't find a simple explanation of what this is or is even caused by. Any links or ideas?

G) On top of all this, from reading Awwt's recent question and one of Derek's replies CMV is best eliminated not by an IG but by an isolation transformer.
If this is the case under what circumstance would a IG even be necessary? You can not use them on clean equipment interconnected with dirty equipment, which right away eliminates almost all computers. Is it only truely effective to install an IG to a piece of equipment that is completely stand-alone, introduces minimal noise into the line, and needs a clean ground? If this is the case, shouldn't IG installations be limited to dedicated laboratory equipment or similar?

Anything else you'd like to add, feel free.
Many thanks.
-John

 

[dereckbc]

Re: Usefulness of Isolated Grounds

Big John, you have a lot of questions and I cannot answer all of them at this time, but lets try a few.

A. How is noise introduced into ground?

There are several sources. Lets start with cable capacitance. There is capacitance between the phase conductors and ground. The longer the circuit the more capacitance. A capacitor will conduct AC. The 60 hz primary frequency, harmonics and any ac noise will couple into the ground circuit and return through the N-G bond at the transformer. Its inherent to the unbalanced power architecture we use.

Secondly most of the electronic equipment we use will have line filters installed from L-G and N-G for FCC requirements. These filter will also inject noise currents into the ground circuit. This could be avoided by connecting filters L-L and L-N but not often practiced.

There are other mechanisms, which you mentioned such as EMI, EMF, and RFI

B. Assuming at least some of the above is correct wouldn't it make the most sense to run a dedicated circuit with an isolated ground to help eliminate noise introduced by the line conductors? Is it not contrary to the whole idea to run a "clean" ground in along side "dirty" power?.

It would if you had clean power to start with. The problem is where you originate the IGR may already have common mode noise. If that is the case IGR will do nothing but make the problem worse from the high impedance on the IG.

C. Why, in a wood-frame house wired in Romex, is it said that IGs are a waste?

Because by the very nature of the wood frame you EGC is isolated and from incidental contact from ground provided it is run dedicated. Think about it.

That’s all for now.

 

[dereckbc]

Re: Usefulness of Isolated Grounds

D What makes a steel frame building so different than a wood frame house?

Commercial and industrial buildings are commonly constructed of concrete and/or steel. The installations use metal conduit. Conduit is installed on concrete and metal frame members. Concrete and metal are conductive and usually have some kind of bond to earth either planned or incidental. The EGC installed in the conduit have to bond to enclosures and all junction boxes, which are installed on metal framing or concrete. Where the conduit and junction boxes are installed, they are at slightly different potentials from the contact with concrete/steel, which are in contact with earth. These potential are equalized by the conduit and EGC, hence common mode noise. Refer to questions A, B, & C.

E. What power quality problems would arise from having two clean interconnected devices on the same circuit but connected to two different IGRs?

If you are using a cable to interconnect the two pieces of equipment that contains a grounded circuit conductor of ground conductor, you are connecting the two IG together thereby creating a loop and defeating the IGR. RS-232, and coaxes are common offenders. You have to use optical isolation modems or balanced signal transmission between the two units with out any type of shields or grounds….

F. It is often said that IGs are mostly installed to do away with common-mode voltage. Try as I might, I can't find a simple explanation of what this is or is even caused by. Any links or ideas?

See all above questions. Or try a search on Google. I get over 10,000 hits.

G. G) On top of all this, from reading Awwt's recent question and one of Derek's replies CMV is best eliminated not by an IG but by an isolation transformer.
If this is the case under what circumstance would a IG even be necessary? You can not use them on clean equipment interconnected with dirty equipment, which right away eliminates almost all computers. Is it only truely effective to install an IG to a piece of equipment that is completely stand-alone, introduces minimal noise into the line, and needs a clean ground? If this is the case, shouldn't IG installations be limited to dedicated laboratory equipment or similar?

IG is useful for A/V equipment if applied correctly. For example in an office you can run a dedicated 20-amp IGR to a meeting room to supply power for PA’s, projector, DVD, what ever. A good idea is to install a quad receptacle and power all the A/V equipment from the quad. Then you can use all the grounded signal cables you want as long as they all share the quad single point ground. In a home you can use the same approach, just no IGR. Hospitals have there own reasons for using IGR, but that is not my field, ROGER where are you???

 

[Guest]

Re: Usefulness of Isolated Grounds

Thank you for starting this thread. I am learning a lot about IGR's!

G) On top of all this, from reading Awwt's recent question and one of Derek's replies CMV is best eliminated not by an IG but by an isolation transformer.

Is an "isolation transformer" the same as an Auto-Transformer? Doesn't an auto-transformer introduce it's own set of risks? My knowledge is dusty, rusty, and confused still so go easy on me. I remember looking at Auto-Transformers years ago for noise reduction and the stats were incredible-- especially if you "stack" them up end-to-end. The noise reduction goes geometric as I recall when you put more than one auto-transformer in a series.

C) Why, in a wood-frame house wired in Romex, is it said that IGs are a waste?

I would think that the Romex would have to be x/3 Romex w/IG (i.e.- 12/3 B/W/Green/Bare) for a Romex IGR to work. You need one box ground (EGC), one conductor, one neutral, and one insulated ground (green) to pull off a Romex IGR circuit. Correct me if I'm wrong. How else to you ground the IGR box (oh, are you assuming plastic boxes)? The Romex would have to make a home run, or at least the ground would have to pass through the subpanel if any. If a pass-through the physics would dictate an insulated ground. Mike Holt talks about passing through the insulated ground in one of the links in the second IGR thread I posted.

I have a million dollar invention in mind that will make all this IGR and isolated transformer stuff for computers a moot point. I hope to get busy with it soon. Anybody know somebody that can draw a simple electronic circuit? If so, send them my way. I'm ready to get going with a whole new way of powering computers. I already have a patent agent lined up.

../Wayne

[ September 05, 2003, 12:53 AM: Message edited by: awwt ]

 

[hurk27]

Re: Usefulness of Isolated Grounds

AWWT

Is an "isolation transformer" the same as an Auto-Transformer?

No as a auto-transformer has a primary/secondary connection and does not isolate.
This is why spikes and noise from the primary side of the power companys lines is passed onto the coustomers service drop. as there is a primary/secondary bond on the transformer at the pole.

 

[ty]

Re: Usefulness of Isolated Grounds

Dereck,
Where did you get your training and how long have you been doing your line of work??
I've never seen you state any of this before.
Good job on your replies. Your posts on this type of subject are always well explained.
Are you back to work yet?
TY

 

[iwire]

Re: Usefulness of Isolated Grounds

Originally posted by awwt:
Why, in a wood-frame house wired in Romex, is it said that IGs are a waste?

Wayne you answered this partly yourself (plastic box) however a metal box nailed to a wood stud would also be fine.

As far as passing through a sub panel, yes you are correct you would want to get the IG back to the the bonding point of the system the circuit originates from. As we are taking houses that would be the main panel.

This would not change if the branch circuit was in 12/2 NM or 12/3 NM.

Although remarking the red of a 12/3 NM green in a dwelling unit would be a violation. (see 250.119 Identification of Equipment Grounding Conductors.)

I would also like to thank Dereck, your info is greatly appreciated, I would enjoy being an electrician working on your projects, your common sense approach is refreshing.

[ September 05, 2003, 04:49 AM: Message edited by: iwire ]

 

[big john]

Re: Usefulness of Isolated Grounds

Originally posted by dereckbc:
E. What power quality problems would arise from having two clean interconnected devices on the same circuit but connected to two different IGRs?

If you are using a cable to interconnect the two pieces of equipment that contains a grounded circuit conductor of ground conductor, you are connecting the two IG together thereby creating a loop and defeating the IGR.

Derek,
I understand that it does defeat the isolation aspect of the IG, but it seems to me that the whole point of the IG is to eliminate noise. So my question is, would this setup still work to eliminate noise on the EGC even though you no longer have two isolated grounds?
Theoretically, could you interconnect a bunch of relatively clean devices that were all fed from IGRs and still have a much cleaner ground than you would if you were utilizing SGRs?
Thanks. -John

 

[dereckbc]

Re: Usefulness of Isolated Grounds

Big John, an IGR cannot eliminate any noise. It does not have any active, passive, or reactive components. The only purpose of an IGR is to possible prevent common mode noise, that's it. However, that assumes that the origination point of the IGR is clean to begin with. If you put garbage in you get garbage out.

"So my question is, would this setup still work to eliminate noise on the EGC even though you no longer have two isolated grounds? "

Not really. Every circuit will add a small minute amount of current from capacitive coupling and line filters. When you interconnect the circuits the currents are additive and subtactive. You just do not know exactly what you will end up with. It probable will work ok.

Computer and data circuits are not as sensitive to common mode noise as analog circuits are, so a lot of IGR debate is mostly hype created by equipment manufactures own design defiencies. They create the problem by installing filters from L-G rather than L-L and L-N, and use unbalanced signal transmission lines that reference the signals to ground rather than use balanced transmission techniques. It is changing, but it is not there yet.

 

[dereckbc]

Re: Usefulness of Isolated Grounds

Ty I graduated from Oklahoma State University in 1979 with a BSEE. I started working for a electric utility in OK as a relay protection engineer, didn't like electrical and transfered to the communication department as a RF engineer/technician and did that for 10 years. I left the utility and went to work for Northern Telecom as a digital switch engineer for three years.

Then in 1994 I went to work for a company called WorldCom (now MCI) as a digital switch engineer. In 1995 WorldCom growth exploded and they had no one with any electrical experience and they moved me into the electrical engineering department. There I mostly designed DC power plants, UPS, protective grounding, generators, AC distribution, TVSS, lightning protection, and wrote the companies engineering practices, etc. I eventially managed the department until I was laid off last Feburary. I was hired back as a contractor in July, but was laid-off again last Friday.

Guess I am going to have to leave home sweet home Tulsa Oklahoma, the economy here is terible and companies are leaving in droves or going belly up.

 

[don_resqcapt19]

Re: Usefulness of Isolated Grounds

Dereck,
Sorry to hear that you are out of work again. Based on the major power outage last month and the questions about our power distribution grid, it appears that there should be a large demand for relay protection engineers for the next few years.
Don

 

[ryan_618]

Re: Usefulness of Isolated Grounds

Dereck, like Don I too am sorry to hear of your terrible luck. The industry needs more people with the wealth of knowledge that you possess, not less.

 

[hurk27]

Re: Usefulness of Isolated Grounds

Dereck

Have you tried to call Comcast they were looking for some EE's in our area and since they took over the cable TV and Internet here they need somone that knos what there doing.