Has anyone used IG receps in plastic boxes. 12-3 NM-B for a wiring method. My question is, how do you ground the yoke of the recep with the EGC? there is only 1 screw on the recep? Do they make an IG with 2 screw?

Good question, the only thing that I would suggest is to install a metal box. Unless it's already to late.

If you are using a non metallic box there is no reason to use an IG receptacle. What is this IG receptacle going to be used for?

Curt

What would the rules be on that? I mean if the box has no ground and the iso ground runs back to the isolated buss in the panel with isolated cable using the insolated ground conductor would it pass? Also since an IG receptacle has the ground pin isolated from the rest of the receptacles some metal parts such as the tang would not be grounded as the screw grounds the the tang to the service grounded metal box. You can't use a bare ground in either BX or NM because it would compromise the system if it touches the panel box or metal sheath. The panel should have two grounding buss bars and one being isolated if anywhere in the wiring the grounds touch it's not isolated. If its L.I.M system however they can be joined and IG receptacles are not used. I once asked two different inspectors the same type of question and got two different answers. Best to call inspector who will be involved.Never seen an IG with two grounds but who knows.

While the NEC has rules on the correct installation of an isolated ground, it never requires one. The NEC also never really says haw far back the isolation has to go; to the panel?; to the separately derived system?; to the service? These are questions that are left to the engineer to specify.
In the case of a non-metallic enclosure for a isolated ground receptacle, there must be a means of bonding the yoke provided. If the receptacle cannot be found that has two ground screws, then the box must provide the bond. There are plastic boxes manufactured that do provide a means of bonding the yoke via metallic screw inserts bonded internally to a grounding screw inside the box.
Like everything else, we have to install the right box for the application.

In this case a standard receptacle would achieve the same results as an isolated ground receptacle, and at the same time bond the yoke with no adverse affects.

As Curt said, there is no reason to install an isolated ground receptacle in a non metallic wiring method or enclosure.

As far as using plastic boxes in a "Line Isolation Monitored" (in a hospital) system, this would be a violation of 517.13, although isolated ground receptacles are allowed in 517.16.

Roger

[This message has been edited by Roger (edited 07-18-2004).]

the receps are for computer equipment in a rewire of an existing bank. Specs call for IG receps. Building is all wood construction, using 12-3 NM for the IG. Another question I have, can you put more than 1 recep on 1 circuit? wouldn't this cause "noise" on that circuit if two computers where on the same circiut? Thanks for the answers so far.

The use of IGs at all is controversial at best.

To do it the 'best way' no you may not have more than one outlet per IG conductor and that conductor has to go all the way back to the point that the electrical system is bonded.

Most times I do not see it done this way, at best there will be separate IGs back to the first panel where an IG terminal bar will be installed and from that point a single conductor is run back to the bonding point.

Do it however your job specifications allow, however I agree with Roger and Curt an IG outlet in a plastic box in a wood frame building does nothing that a standard outlet will do.

To satisfy the job requirements go ahead and put the IGs in but you will have to use a metal box or the special plastic bonding boxes as has already been said.

Strictly speaking you can not use 12/3 for this, remarking the red for use as the IG is a violation.

Bob

[This message has been edited by iwire (edited 07-18-2004).]

Let me just say that isolated ground receptacles are the biggest scam on the market. They will do absolutely nothing about protecting computers from surges or other problems. A UPS power supply is the way to go. Having said all that a lot of specs call for them. Generally we put 4 IG receptacles on a isolated ground circuit. If possible I would also recommend a metal box instead of plastic.

I didn't think the 12/3 was an issue, but I see now the code section involved,250-119(b). It just doesn't make sense to me, I mean I can put purple tape on a white wire to reidentify it for ungrounded use. I can put white tape on a black wire to reidentify it for nuetral use, but I can't put green tape on a red wire to identify it for grounding use?? Does anyone know why this is??

You can only remark a white to another color in a cable (never in a raceway) for specific applications.

I can not tell you we have never used 12/3 for IGs but you better be sure the inspector is OK with this, changing it after is tough.

The cheapest code compliant option that I know is to use 12/2 AC hospital grade.

The metal armor can be the 'normal' ground and the insulated green conductor can be the IG.

Another option is 12/2 IG MC which has two insulated green conductors one having a yellow stripe for the IG.

I think the 12/2 IG MC is more $ than the 12/2 AC hospital grade.

I don't beleive you are required to ground (bond) the yoke. If you look at 406.2 It seems to imply that the yoke will not be bonded and therefore a metal faceplate is not permitted because it cannot be bonded by the yoke.

Good Catch Mr. Jackson.

On the last job I did that involved a lot of computers the engineer had us install an over sized neutral. I asked him about IG receptacles, and he replied that he saw no useful purpose in installing them. I don't understand all I know about IG systems, but it seems to me that unless you have a complete separately derived isolated system they would be of little use in reducing line noise.

Thanks Mr. Badger

Its nice to be the one that finds the "abstract code section of the day" once in a while

Do I understand this correctly:

Wood frame construction + NM cable + Plastic box = IGR. Why would you want to put IG receptacle in place, when the nature of the installation already makes it IG. Either someone does not know what they are doing or scamming.

The IG receptacle has a grounding connection separate from and redundant with the EGC that will bond the enclosure (if metal) and the yoke of the receptacle. The theory is that the sensitive electronic device plugged into the receptacle will be compromised by the stray voltages and currents present in the normal EGC. So, some engineers decided to invent this redundant IG receptacle. The IG is run all the way back to the service ground, and kept insulated all the way.
Some of us feel that this is dumb.
I recall years ago when this issue came up, the engineers wanted to disconnect the ground altogether, and for us to drive a ground rod in the middle of the building (and sometimes bond to the building steel), and use this for our grounding conductor. They would spec that the last few feet/inches of the EMT run would be changed to PVC, and then the sensitive device would be bonded to the ground rod. They called it a static charge drain.
It worked, but the NEC folks determined that this ground would not facilitate the operation of the OC device, and the installation was unsafe. The IG is the compromise they worked out.

Early, Just to let you know dereckbc designs tel co plants and things of that nature.

He can tell us more about grounding than we would every want to know.

I would also bet my paycheck that he would never tell someone to run the IGs to an isolated ground rod.


Bob

Bob, I don’t know that I am worthy of your comment but thanks for your vote of confidence. I do not know, I might tell someone to use an isolated rod if it were your paycheck and not mine.

There is a lot of churn in regards to IGR circuits usefulness in today’s communications systems. They were originally used in the early days of data processing when signal transfer media between processing units used RS-232 cables. RS-232 cable uses 4-wires (in reality only 3) for signal transfer. TX, RX, Signal Ground, and Shield Ground. The signal ground carried the return for both TX & RX. Obviously the processing units used different branch circuits using a multi-grounded EGC system ran with phase and neutral conductors. Inside the processing units the AC line side will use filters between L-G and N-G for FCC EMI/RFI compliance. Also an EGC ran with multiple branch circuits will couple some energy into the EGC via induction. These filters and induction would couple noise and some 60-Hz line energy into the EGC circuits creating what is known as “common-mode noise”. This common-mode noise can create some voltage differences along the various EGC paths as it is multi-grounded creating multiple loops, or paths for current to enter and leave.

So some engineer came up with the concept of IGR, a fourth wire, ground, that is only bonded once at the source, usually the N-G bond point. This single point ground eliminates any possibility of current entering, thus eliminating any voltage potential difference, or a zero reference point. However the circuit must be dedicated. As I mentioned before the AC line side of data processing equipment has EMI/RFI filters, if you allow multiple circuits on a IGR this noise is accumulative and completes loops to allow noise to get in the IGR. Here is a kicker. Now let’s say we connect two processing units on their own dedicated circuits via RS-232 cables. You have a loop and compromised the IGR. Probable not a problem as long as it is limited to two or three interconnected units over a short distance.

Here is another problem I see in data centers I have designed where you have multiple customers. I call them Co-Locate Hotels (Co-Lo or CLEC). A customer demands an IGR circuit for their equipment rack that we supply. The equipment rack is bolted to the one next to it, and bonded the Signal Reference Grid (SRG) under the raised floor. Then the customer mounts their router, server, black box, whatever in the equipment rack without isolating it from the equipment uprights, plug the equipment into the IGR strip, and guess what? They lost their IGR they pay extra for each month. When and if they discover the corruption, they naturally blame us, until we point out their ignorance. Then if they use RS-232 cables to interconnect elsewhere they get another corruption and blame us again, until we point it out again. Usually the customer gives up and asks how do they obtain an isolated single point signal ground. We inform them there is a Signal Ground (SG) cable located on “J” hooks attached to overhead cable racks just for that purpose. Then contact their equipment manufacture on how to isolate the chassis ground from signal ground (usually a horseshoe jumper located on the back on inside the equipment). DUH read your contract technical specs.

Anyway, an IGR has one purpose and one purpose only. To possible prevent common mode noise from entering the IG. The effects range from in order to no effect, worse conditions, or desired effect. It cannot remove noise. The best way to employ IGR is to use a isolation/shielded transformer with dedicated branch circuits ran in a metallic raceway. I specify plain old dedicated solid ground circuits, no IGR. Most communications system (PC’s Servers, Routers, Hubs, etc.) now use either a balanced transmission medium like 10-base-T or optical cable, eliminating any need for special grounding techniques.

One viable use today for IGR is A/V equipment that requires multiple AC line connections and still uses grounded signal cables rather than balanced lines. Places like recording studios. But recording studios have given up IGR and switched to balance power systems, that use no grounded circuit conductor (neutral).

Sorry to be long winded but thought it might help anyone interested to read this far.

[This message has been edited by dereckbc (edited 07-21-2004).]

Thanks for the great info.

Bob

Common 12-3 MC is easily used for IG circuits. The green is the IG, the white is N, the B (or R) is stripped back all of the way creating a code compliant bare dirty ground.
In the not uncommon situation of single phase clean power -- the hot can stay on color.
This avoids ordering trick IG 12-2 MC.

I explain IG as a DC return: It is wired up just like a neutral. The typical systems demanding IG in modern specs have internal DC power supplies producing regulated DC. The a sweet DC path is part of their voltage control logic.
As new chips are moving to ever lower voltages and 3,000,000,000+ cps any spikes -- on the DC could/ should cause errant logic. The power supply can't begin to adjust in the time frame of the microprocessor.

Bad grounds ( ground impedence ) are such a problem for modern systems that it is routine to design in waves of parallel grounds throughout cables and motherboards.

The outward manifestation of such spurious voltages may be computer lock-up. In older, east coast headquarters there have been tales of headaches so bad -- while running just PCs and the like -- that the building was retrofitted with clean power and IGs. Problem gone. ( From a manufacturer's ad IIRC )

Once burned, twice shy...It is easier to over spec than chase down the ghosts. In the above story -- they spent a fortune before scoping the problem. That's is what is so costly about dirty DC returns.

Tesla: Your method is only permitted where conditions of maintanance and supervision ensure that only qualified persons service the installation. See 250.119(B).

As already stated, in a nonmetalic box with 12/2 NM, isolation has already occured, and if we are talking about a dedicated circuit, we can't get much more isolated than that, even with standard receptacles.

Tesla do you have any references to back all that up.

What can IGs do that a equipment ground can not do.

That building you describe probably had some bad wiring, and the fortune spent installing IGs could probably have been better spent correcting the existing wiring.

In my opinion IGs are useless in almost all cases, even if we say they can do some good they become connected to the equipment grounding conductor in no time at all.

Telsa, could you please explain how the IG is used as a DC return? Seeing how all DC supplies in electronic equipment are SDS in nature, I am having trouble seing how any IG could be used as a load or return conductor, not to mention forbided by NEC

Dereck Campbell, PE
C.O. Power Engineer.

Since you've written...
Ryan_J: That is code truth.... In my work, commercial, the AHJ has no problem with it. I, myself, cannot imagine how even a tyro is going to foul up such an install....
Roger: I agree entirely. IG receptacles don't get you anything in a residential application.
iwire: I run jobs for 'catfish' feeding at the bottom end of the commercial market. The IG, and associated contract specs, stops my bosses from screwing the owner on the ground path. ( And yes they would.--- I had to withstand a screamfest from two separate owner/ bosses on wasting green wire.) The requirement for dedicated ground clear to the panel provides the owner the pretext for testing it by lifting it off the isolated grounding bar.(And, yes...they do it. They've learned by experience.) Without this spec the owner would receive a system sending the ground path regular way: through the EMT -- linked to every hunk of metal on the way....
I have built raceways
under major high voltage lines ( inducing 270V on the skin of parked cars and a hefty induction to the T-bar over the sales floor. -- Lighting crews had to pre-ground the grid every time they worked on it from a lift.)
in the path of intense microwave radiation ( with all of the side lobes/ refections triggering voltage induction beyond 40V in the EMT. This had the fire alarm tech choking.

So it is true, for elements as small as microprocessor transistors, induced voltages in EMT -- found so irregularly in commercial situations -- you can't trust the EMT as a clean ground path.
And without the IG/ dedicated ground wire system the owner can't spot when the EC is cheating. And the pressure on the EC to cheat is intense since the bid was a crusher.
As electricians we are instructed from earliest days that impedence is purely an AC property. That is completely untrue. DC systems also exhibit impedence...but only during the transition to steady state conditions. And, in our customary work, that is all that we deal with. So, to get the test question right -- we answer no to DC impedence.
But, in the world of microprocessors, DC impedence is a huge design concern. A next generation of processor is being planned with additional layers expressly for the purpose of removing right angle turns on the DC ground path. Each bend in the current contributes to impedence. The exact number of conductors on the 80 pin disk drive ribbon connector dedicated to ground returns is 36 IIRC. (Every signal line gets one.)
The Code does not speak to this issue because the volts and energies are dramatically below any fire or health hazard.

iwire: the NEC will accept bonding through the raceway. There is abundant evidence that microprocessors can't live with that 'fault clearing path'.
One real world example: I have an RV -- and in that RV I have a PC -- and that PC is stand alone -- not even internet -- and when my propane igniter kicked in for hot water -- the PC locked up and crashed over and over. The solution was to change locations and clean up the grounding pin. Problem gone.
dereckbc has validity but speaks mainly to signal quality. Modern systems use error correcting data transmissions and operating system firewalls. The classic signal problems have been addressed.
Falling DC microprocessor bus voltages coupled to higher speeds drives the need for cleaner grounds.
All modern regulated power supplies have feedback mechanisms working at dramatically slower speeds than the microprocessor: 3 Gig. All of the DC loads are sunk to bonded common voltage (nominal 0 Volts) and the supply voltages are biased off of it through various schemes. If -- for even a moment -- a wave of DC/AC voltage surges up into the DC sink -- the microprocessor locks up entirely. This is the malady that has people pounding their keyboards.
I call it a 'DC return' because correctly wiring an IG causes it to exactly parallel the style and form of the AC return: the neutral. It is so much easier to explain it thusly to the troops. Neutral, they understand. Otherwise, my crew is bonding green wire all over and omitting the homerun.
"SDS in nature" does not compute. Spell it
out.
I am arguing very much in parallel to derekbc except that the "noise" that he declaims seems to be coming out of signals...in my experience the trouble almost always is from major circuit loads such as igniters, elevators, motors, contactors that trigger significant wave effect distortions ( think pulsed inductions ) on proximate ground paths. And, no the energy waves do not all immediately head into the GEC. They pour out onto all adjacent conductors until the wave sinks into the earth.

[This message has been edited by iwire (edited 07-25-2004).]

So, I gather that the IG system works best when the ground is brought back to the service, is never of any use in residential applications, and could be totally eliminated by simply re-designing the electronics used, but this would be costly to the electronics manufacturer. They would much rather have the electrician eat the costs by fixing the designer's omission with elaborate IG specs.

I've often thought that electronic gadgets need to be housed in magnetically isolated metal cases (like the old anti-magnetic watch) and fed by DC batteries (i.e. the laptop). Such a plan would eliminate any stray high frequency current from being confused with valid input.

Harmonics could be eliminated as well by proper design of the harmonics producing devices. Wave chopping is cheap, but why not use the entire sine wave? The cheapest way is not the best way.

Welcome to ECN Tesla, I should have said that earlier.

by Tesla

Tesla you have to remember this forum has members from all the country (and beyond) to give a recommendation based on what your AHJ allows will not help anyone outside your area. The wording from 250.119(B) certainly leaves it open to the AHJ.

part of 250.119(B)

The opening poster describes a wood framed bank, In my opinion an AHJ could feel this building does not meet 250.119(B)

by Tesla

I also agree but Roger was not talking about a residential application.

by trekkie76

by Tesla

Unless they are lifting this IG daily and tracking down points where it has been connected to the EGC in some way it is not an IG for very long.

by Tesla

How exactly did you measure 270 V on the skin of a car or 40V in the EMT?

Between what two points and what kind of meter?

by Tesla

You run large commercial work and SDS does not ring a bell?

"Separately Derived System"


Bob



[This message has been edited by iwire (edited 07-25-2004).]

iwire...
It is true that cars parked under high voltage power lines can pick up amazing voltages...Any and all testers used by a crew of twenty DMM/ analog found the same readings on their own cars on the job site. Parking further away caused a voltage drop. Potential existed between bumpers and ground....
SDS, as in 480 delta to 208Y120, I've built many. It is not the term of art that first comes to mind in describing the internal powersupplies found in mass produced PCs. Strictly speaking I have to give it to you...their regulated DC busses are separately derived power.

Inre IG conductors: The test I described is performed - required - for each California Lotto terminal. (Watching that get fouled up is suitable for a comic thread.)

Once 'proofed' the conductor is re-bonded. It never becomes an issue again.

Inre the fire alarm tech and hair pulling over EMT induction...He and I used a battery of analog and DMM to prove that such voltages were all over the fire alarm raceway. We ran shielded cables within the EMT. EMT required by AHJ for high rise office structure standing adjacent to a major fault, the eastern twin of the San Andreas fault. ( This is the fault that is splitting Berkeley Stadium in half -- ultra slow motion. )

Somehow I think that most AHJ are going to think that a bank is going to only ever use professional licensed electricians to work on their circuits. But its always going to be a case by case situation. I've never seen one of our local 'catfish' ever order/ install trick 12-2 IG MC. The price is higher, and one must wait. The technique I mentioned is quite common out here....If it is not permitted elsewhere what can I say...every EC has to make his own call.

I found this article that might help explain isolated grounding circuits.
http://www.liebert.com/support/whitepapers/documents/sl_24275.asp

Telsa, all electronic equipment with any type of solid state device uses DC power. Assuming you are using 120 or 208 VAC as the source, you have to use some type of rectifier circuit. Modern electronic equipment uses “Switch Mode Power Supplies and some still use transformer/rectifier units. Which ever type is used is irrelevant, because they both still use an isolation/step-down transformer, which is a separately derived system (SDS).

A SDS establishes a new reference point on the secondary side, whether it is an AC or DC system. In the AC systems that you are familiar with the new reference point is the N-G bond. In DC systems the reference point may be chassis, logic, or common return. It may or may not be referenced to the EGC system. It really does not matter what the reference point is, as long as it is a single point. What a SDS system is for in respect to power quality (PQ), is removing any common mode noise present on the primary side. The Common Mode Rejection Ration (CMR) on a SDS can range from 60 db to 120 db (1,000,000:1 to 1,000,000,000,000:1).

Here is the point. An IGR can only prevent common mode noise from entering the IG, it cannot remove it, impossible, only a SDS can do that economically. Since DC power supply is an SDS and forms a new reference point, any common mode noise on the input is irrelevant. The main purpose of the EGC is to provide a fault clearing path in the event of a L-G fault. In some types of systems, like PC’s, the EGC is in parallel with the Signal Ground used on RS-232, and RS-422 serial ports.

The DC logic 0 volt bus you referenced to is completely isolated from the EGC. The only disturbance that can get through the rectifiers/transformer is a normal mode event between L-N, not L-G or N-G

IEEE recommended practice for sensitive equipment is to use a either a isolation transformer or UPS using dedicated solidly grounded circuits installed in metallic raceways with a EGC ran with circuit conductors.

Dereck

CRM...
Thanks for the link...It presents my case much better than I can.... Thanks.

Tesla Did you really read the linked document?

Bob

Telsa, perhaps you need to read the section titled “Disadvantages of IG Wiring Techniques”, they far outweigh “The Benefits of IG Wiring”. To make IG beneficial, they have to be on a single dedicated circuit, installed in conduit, the only circuit in the raceway from origination to the equipment end, use a manufactured cable made for IG, and used on stand alone equipment not interconnected to anything.

I cannot recall any installation that meets all that criteria. The only situation I can think of would be a home A/V or PC system where all the components are powered from a duplex or quad receptacle. If you were to install two dedicated IG circuits and interconnect the equipment, like you would in a data processing environment, you have compromised the IG system period..

As I stated before the only purpose of an IGR is to prevent common mode noise. It cannot clean or eliminated common mode noise. Only a SDS like a transformer, balanced power, or dual conversion UPS can do that. Do not know of many home owner’s who can afford to spend as much or more for one of these systems that their PC cost them.

I design data centers, telephone office power, and protective grounding system. We do not use IGR. We use dual conversion UPS and DC battery plants on SG circuits to provide clean uninterruptible power. We are starting to use balanced power systems in conjunction with the UPS which eliminates harmonic distortion and all ground problems associated with grounded circuit conductors, be eliminating the neutral altogether for single phase loads (120 VAC).


[This message has been edited by dereckbc (edited 07-27-2004).]

My favorite "isolated ground" story:

During some electrical work at my wife's (government) place of employment, a worker lifted the ground for a block of offices. I do not know if there was a short or just plenty of stray voltage, but there was current on the ground at the time.

Electricity, being tricky stuff, found another path to ground in short order. My wife and her co workers notice the change as the network cards in their computers started smoking and catching fire. Apparently the only path to ground left was through the power cord to the chassis and then through the network cabling to the hub/switch. The worst part was that the IT staff determined the problem was corrected when the computers the were plugging in (and destroying one after the other) stopped catching on fire.

Moral:
Have at least one ground that works for every outlet.

SD