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Explaining Isolated Grounds

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iwire

iwire

Moderator
Staff member
Location
Massachusetts
I am looking for some help here.

I have a coworker that is insisting on driving a ground rod to connect the terminals of IG receptacles to.

This is without any connection to the buildings EGC or GEC.

I have tried to explain how this will not operate a OCPD if the item plugged into the IG outlet faults to ground.

He insists any connection other than earth takes away away from it being an IG.

HELP! This is being done in areas that consumers have regular access to. :roll:

Bob
 
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flightline

Senior Member
Re: Explaining Isolated Grounds

This would seem to be a perfect example of the addage, " a little knowledge can be dangerous". It may be the biggest problem that you may have is getting this person to accept that they may be wrong. I can't find my Soares right now, but there may be something in there. Bob, I've come to respect you and what you know. You seem to be able to convince your "listeners" or readers with what you have to say. Perhaps your biggest difficulty in this is to have this person accept that they might be wrong. The forum, however, will probably provide you with enough information and example to, at least, provide substantiation for your position.

As a thought, perhaps HE can provide written substantiation for his staement: "He insists any connection other than earth takes away away from it being an IG."

Good Luck,
Gregg

[ February 15, 2004, 09:47 AM: Message edited by: flightline ]
 
russ

russ

Senior Member
Location
Burbank IL
Re: Explaining Isolated Grounds

A few months ago M. Holt had a picture of a ground rod at a light pole, with comment of how it would not protect people from shock.

It's not the same situation, but similar.

May be somebody still has on their computer.
 
L

luke warmwater

Senior Member
Re: Explaining Isolated Grounds

Bob,
it sounds to me that this co-worker does not understand the basis of what an isolated ground is or why it is used.

An isolated ground originates at the same gound bar in the MAIN PANEL as the equipment ground. So any 'sub panel' would have an isolated ground bar that would have an isolated ground wire going back to the MAIN SERVICE GROUND.

Our standard equipment ground picks up alot of noise caused by transient currents which can effect sensitive electronic equipment.

By connecting to a single point ground at the MAIN SERVICE, the Isolated Ground has fewer chances of picking up noise.

This is how it was explained to me when I first started this trade. I was never given the chance to have the misconseption of 'drive another rod'.
Todd
 
dereckbc

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Explaining Isolated Grounds

Bob, to start quote him 250.4(5). His method does not comply.

Then explain too him the IG form of grounding is only intended to be used as a "possible means" of obtaining common-mode electrical-noise reduction on the circuit in which it is used. It has no other purpose and its effects are varible. Results from on average in order are no effect, worse effect, to desired effect.

Go to ECN and look at Scott Thompson drawings http://electrical-contractor.net/ubb/Forum15/HTML/000049.html

Next download Fips 94 and look around pages 39 and 40 http://electrical-contractor.net/ubb/Forum15/HTML/000049.html

If you can get a copy of IEEE Emerald book look at figures 9.3

Hope that helps.

[ February 15, 2004, 10:42 PM: Message edited by: dereckbc ]
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Re: Explaining Isolated Grounds

Originally posted by dereckbc:
Then explain too him the IG form of grounding is only intended to be used as a "possible means" of obtaining common-mode electrical-noise reduction on the circuit in which it is used. It has no other purpose and its effects are variable. Results from on average in order are no effect, worse effect, to desired effect.
Click to expand...
:)

Trying to get through to someone who thinks they already know it all is a much tougher mission. :)

Thanks again to everyone, some real great links. :) :cool:

Bob
 
L

luke warmwater

Senior Member
Re: Explaining Isolated Grounds

Dereck, I couldn't get the first link to work.

But the second link worked fine.

I have reviewed Scott's diagrams, input, and my prior post.
While I think that it would be the best possible design choice, I actually can't find a CODE that would REQUIRE the IG to go back to the MAIN.
It would appear that you could start your IG at any sub-panel if you were limited in design choice. But you would be subject to any 'ground noise' on the system at that point.

Dereck, how 'sold' are you on IG grounds??
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Re: Explaining Isolated Grounds

Originally posted by luke warmwater:
While I think that it would be the best possible design choice, I actually can't find a CODE that would REQUIRE the IG to go back to the MAIN.
It would appear that you could start your IG at any sub-panel if you were limited in design choice.
Click to expand...
I agree with you 100% IGs are not really an NEC issue as long as the get back to the service ground at some point.

I am just an installer but my experience tells me that these IGs get corrupted quickly and the equipment they are intended for continue working just fine.

IMO it is a sham most of the time.

But if it shows it on the print I will try my best to provided it as drawn.
 
H

hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
Re: Explaining Isolated Grounds

I-wire
Simple math should do it.

A 120 line connected to a rod with 25 ohms resistance 120/25= 4.8 amps This will not fault a 15 amp breaker

For a 20 amp circuit the ground rod resistance would have to be below 6 ohms

for a 15 amp circuit the ground rod resistance would have to be below 8 ohms

But in eather case It is a violation of the NEC to use the earth as the sole ground
250.4(A)(5)

(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path.
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Re: Explaining Isolated Grounds

A 120 line connected to a rod with 25 ohms resistance 120/25= 4.8 amps This will not fault a 15 amp breaker
Click to expand...
I hear you Wayne and I fully understand that. :)

While many of us here can visualize this in our heads I am looking for a diagram of the current path in during a fault to the enclosure with this isolated ground rod and add to that a human coming in contact with it.

This person is set in their ways and I think a picture could make things clear. :)
 
J

jimwalker

Senior Member
Location
TAMPA FLORIDA
Re: Explaining Isolated Grounds

Maybe draw him a picture showing the rod and the person being like 2 resisters in parallel.At any rate long before the breaker trips the person will be dead.
Or better yet show him how in reality the person would act like a second ground rod

[ February 15, 2004, 01:12 PM: Message edited by: jimwalker ]
 
dereckbc

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Explaining Isolated Grounds

Originally posted by luke warmwater:
Dereck, I couldn't get the first link to work.
Dereck, how 'sold' are you on IG grounds??
Click to expand...
I could not get it to work either. It is at www.electrical-contractor.net > Technical Discussion > Technical Reference Area > IG Details #1 if you are still interested. I also have some CAD drawings, but it beats me how to post them here..

Professionally I do not recommend IG. I have designed many data centers for MCI/Worldcom, and UUNET. We do not allow them for our equipment. We use dedicated SG circuits supplied via UPS/PDU balanced power systems. We will supply them for a customer who insist on them. But we put these customers in a special area where they cannot corrupt our grounding systems. We know from experience they will corrupt the IG as soon as they install their equipment in the equipment rack (metallic shelve in a grounded equipment rack to the SRG), and again when they use RS-232 I/O cables :eek:

I do not totally condemn them as they can be useful in commercial environments where A/V equipment is used and being powered from different branch circuits. But you usually corrupt them by the interconnecting cables using a ground for signal path like coax, shielded, or RS-232 cables. You can get the same result using a dedicated SG circuit and connect all A/V equipment to that dedicated circuit. Otherwise you are going to have to use isolators to break ground loops, balanced signal tranmission media, or optical.

As far as the code requiring the IG to go back to the service N-G bond, no it is not required. But no engineer in thier right mind would do otherwise.

My personal favorite is balanced power 120/60 for technical systems. No neutral, harmonics, or common-mode-noise problems on EGC's. It is expensive, and should only be used for professional applications or home A/V enthusiast.

[ February 15, 2004, 03:04 PM: Message edited by: dereckbc ]
 
P

pierre

Senior Member
Location
Westchester County, New York
Re: Explaining Isolated Grounds

Bob
What a pain to talk to people in our industry who do not want to learn, they just think they know it all. My advice for dealing with him is forget him, ignore him and think about the snow we may get tuesday - I know you like to take your truck in it ;) .
Just for information - from two of your posts, it sound like there may be a 'bootleg' somewhere on the affected circuit. This could be caused by bonding neutrals to grounds in the panel feeding this equipment, or possibly locally where the equipment is connected to the circuit.

It is excellent to see the guys response for help on this forum - you were able to get some very informative and concise links that I say hit on the money!!!

Pierre

[ February 15, 2004, 07:36 PM: Message edited by: pierre ]
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Re: Explaining Isolated Grounds

Originally posted by pierre:
It is excellent to see the guys response for help on this forum - you were able to get some very informative and concise links that I say hit on the money!!!

Pierre
Click to expand...
You bet and I thank all of you, it was very helpful info and believe me when I say I appreciate it.

As far as ignoring him I have no problem with that it is his loss if he does not want to learn new things. :( :eek:

I agree Pierre it appears just plain old bad wiring is the culprit, as Karl Riley would say just make the installation come up to code and most problems go away.
 
hbiss

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
Re: Explaining Isolated Grounds

...is this a wood frame house? If it is, the circuit is already IG.

Dereck, I know you mention this all the time and I disagree that this is correct in all cases. Certainly with NM and a dedicated circuit it is but if the building is wired with AC instead this may not be true.

My own house for instance has runs of AC cable in chases, through soffits and stapled close together. These have to touch each other in many, many places. Some of these chases and soffits also contain plumbing lines that I'm sure contacts the AC armor also.

While I agree with you 100% about the actual usefulness of IG's, you can't assume wood framing is going to keep the grounds separated (and you automatically have an IG with a home run) if AC cable is used.
 
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