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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