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).]