Ohm's & Kirchhoff's Laws and Isolated Ground Recep

Guest · Sep 3, 2003

I started this conversation under a different thread called "Best Way to Get 'Clean' Power to a Computer Outlet". I want to start a new thread. I got my clean power answer, but now want to move on to the physics of the solution. It involves Ohm's law; electrical engineering; and field experience. I welcome all your input on this. I have read enough about IGR's to know that nobody understands them completely. If you know somebody who does, please have them join this discussion. I have learned a lot in the past two weeks researching this. I am not an apprentice. I have a lot of experience. It's just dated, rusty, and dusty. Let's dust this one off! I have witnessed the magic of an IGR in the field and now I want to understand it's pros and cons.

Here is some reading you might want to do before you offer up any off-the-hip suggestions:

See NEC 250.146(D) and 406.2(D) for the requirements for isolated grounding-type receptacles.
The Basics of Isolated Grounding Receptacles by Mike Holt, Mike Holt Enterprises, Inc., Electrical Construction & Maintenance, Oct 1, 2001

Preventing Isolated Ground Abuse by Bryan Lundgren, P.E., Haworth, Inc., Power Quality, Sep 1, 2002

Code Basics: Article 645, Information Technology Equipment by Mike Holt for EC&M Magazine

Isolated Ground Reference One by Mike Holt
Should I use Isolated Ground for my Sensitive Electronic Equipment?

Responses to "Grounding Sensitive Electronic Equipment"

If you want to do more reading you can do a Search for "Isolated Ground" on the Mike Holt Newsletter Search page:
http://www.mikeholt.com/searchsite.php#

Leviton ProGrade Isolated Ground Receptacles 15A & 20A

HERE IS THE ISSUE I AM CONCERNED WITH:
The computer will be plugged into the IGR. Other devices will be plugged into the IGR, or they will be plugged into another IGR, or they will be plugged into a "standard" receptacle (SR). There will be cables running from the computer to various devices (printer, monitor, mouse, keyboard, speakers, Ethernet, etc.).

QUESTION #1: In an IGR environment what stops an electrical potential from developing in the various devices? The law of physics says the stray current will take any path of least resistance. Kirchhoff's Law & Ohm's Law will prevail.

QUESTION #2: What prevents the cabling between the computer and the devices (i.e.- the printer cable, or the speaker cable) from becoming a current carrying conductor? The laws of physics says the stray current will take any path of least resistance. Is the path of least resistance going to be through the printer or speaker cable? Is this a hazard? Will this defeat the IGR?

I welcome your constructive comments during this time of new beginnings for me. Let me know if you have any other web links to read up on IGR's.

[Note: This post was spellchecked and edited after initial posting. Thanks to charlieb for pointing out a typo: Kirchhoff. Spell Checked in a jiffy with ieSpell ]

[ September 04, 2003, 12:37 PM: Message edited by: awwt ]

dereckbc · Sep 3, 2003

Re: Ohm's & Kirchhoff's Laws and Isolated Ground Recep

First let me say the IG form of grounding is only intended to be used as a possible method of obtaining common-mode noise reduction on the circuit in which it is used. It has no other purpose and its effects are variable from none, desired, or worse.

I can also tell you the three largest telephone companies do not use them in ITE application on their own equipment. They use a much more effective means, a UPS, isolation transformers, or both in combination. A common dry type isolation transformer will provide 40 db of common mode rejection ratio (CMR). A shielded isolation transformer or ferroresonant isolation transformer CMR goes up to 120 db. For those not familiar with the unit of db 40 db CMR equal a ration of 10,000 to 1, and 120 db equal a ratio of 1,000,000,000,000 to 1. The whole point of using an isolation transformer is to establish a new N-G bond point close to where it is utilized. You cannot get any better CMR than a isolation transformer or a combination of a UPS with PDU’s….. From the isolation transformer or PDU we then use dedicated SG outlets with another secret called a SRG that references all the transformers/PDU’s

Now to answer your two questions.

QUESTION #1: In an IGR environment what stops an electrical potential from developing in the various devices?

Answer: No current, no voltage

QUESTION #2: What prevents the cabling between the computer and the devices (i.e.- the printer cable, or the speaker cable) from becoming a current carrying conductor? Is the path of least resistance going to be through the printer or speaker cable? Is this a hazard? Will this defeat the IGR?

Answer: Nothing. Lets say you install a dedicated IGR for one computer system, and another for a different system. You then interconnect the two systems with a RS-232 cable. The RS-232 cable contains chassis ground and signal ground. Thereby creating a loop that defeats the purpose of the IGR. If you had a fault in one system the fault current would split via RS-232 and create havoc in both circuits. If you are going to use IGR you would be well advised to use optical isolation modems or balanced signal transmission that uses no grounded circuit conductor or ground conductors like shielded cable.

karl riley · Sep 3, 2003

Re: Ohm's & Kirchhoff's Laws and Isolated Ground Recep

Awwt, probably Dereck can give an experienced answer to this, though your question is not quite clear to me.

In any case, my one comment is that it is not true that electric current takes the path of least resistance. It takes all paths available, and flows in each according to the relative impedance.

IGs have to remain insulated until they get to the service neutral bus, so it would make no sense to connect equipment together that was grounded both by IGs and EGCs. If that is what you meant.

Karl

dereckbc · Sep 3, 2003

Re: Ohm's & Kirchhoff's Laws and Isolated Ground Recep

AWWT, here is another reason we in the telecom buisness will not use IGR. In an internet hub somesimes called a co-locate hotel, the various vendors install thier routers, servers etc in an equipment rack. The equipment racks are bonded to an SRG under the raised floors. We do have some vendors demand IGR which we will install if they pay for them.

Here is the problem. They install thier equipment in the rack without isolating the chassis ground from the equipment rack. This shorts out the IGR to SRG defeating the purpose. To my knowledge there is no equipment that uses four wires (L, N, G, & IG) nor do I know of any plugs. Then they further complicate thier life by using grounded signal cables like RS-232 or coax.

We get a pretty good laugh when they complain that there IGR is no longer IGR. We then show them what they did to themselves. You should see the looks on thier faces.

Guest · Sep 4, 2003