ECN Forum Forums General Topics of the Trade General Discussion Area isloated ground receps in plastic boxes

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