Theory for Isolated Grounding

[WilliamDiosy]

I am struggling to understand the intent and purpose to installing isolated grounds. Electrically speaking, what do they do? What purpose do they serve? Why are they used?

I am currently installing dedicated power to UPS units that supply uninterupted power to telecommunication equipment. The primary contractor has specified that I use isolated grounding. Is thier need warranted? The NEC has given me limited insight into the purpose of IG.

Are there any electrical theory wizards out there that can end my quandry once and for all? Or am I doomed? SOS.SOS!

 

[billsnuff]

here is a link. http://www.p3-inc.com/whitepapers/groundcircuits.pdf

this is a white paper discussing isolated gounds.

hope this helps.

 

[WilliamDiosy]

Common-mode electrical noise

Common-mode electrical noise

here is a link. http://www.p3-inc.com/whitepapers/groundcircuits.pdf

this is a white paper discussing isolated gounds.

hope this helps.

Thank you, it helps a little. It also leaves me another question. What is ment by "Common-mode electrical noise"? Also, can you give me a common example of "stray voltages"?
I thank you and appeciate the help.

 

[K8MHZ]

Thank you, it helps a little. It also leaves me another question. What is ment by "Common-mode electrical noise"?

It is one of three possibilities:

# A normal-mode signal is any type (other than common mode) that appears between a pair of wires, or on a single wire referenced to (or returned through) the earth, chassis, or shield. Normal-mode signals are read between two wires in a balanced or unbalanced transmission path. (For a balanced 2-wire path, one wire is driven positive while the other is driven negative by an equal amount, both with respect to a static or no-signal condition in which both lines assume the same voltage level relative to circuit common.)

# A differential-mode signal appears differentially on a pair of wires in an ungrounded cable configuration.

# A common-mode signal appears equally (with respect to local circuit common) on both lines of a 2-wire cable not connected to earth, shield, or local common. Usually, but not always, this is an unwanted signal that should be rejected by the receiving circuit.

http://www.maxim-ic.com/app-notes/index.mvp/id/2045

In other words, it is an unwanted signal.

 

[WilliamDiosy]

To my understanding

To my understanding

Okay. First of all I want to say thank you to those who have given me feedback on this subject of isolated grounding. I like smart people (especially those smarter than me). I have been studying hard on this subject, and if I understand it correctly, the isolated ground (in order to be "truly" isolated) must originate @ the supplying, grounded conductor on the "XO" @ the transformer. Then end @ the device (isolated ground receptacle). Am I correct? Am I missing anything here?

I am not good on exact terminology. Those who can understand me and what I am trying to say, please respond.

 

[dkidd]

But it can't leave the building (250.146(D)).

 

[G._S._Ohm]

Also grounding for personal safety vs. EMI considerations may sometimes be at odds.

Morrison wrote extensively on this subject.

 

[iwire]

d if I understand it correctly, the isolated ground (in order to be "truly" isolated) must originate @ the supplying, grounded conductor on the "XO" @ the transformer. Then end @ the device (isolated ground receptacle). Am I correct? Am I missing anything here?

Yes or if there are no transformers it would go back to the service bond.

 

[LarryFine]

... if I understand it correctly, the isolated ground (in order to be "truly" isolated) must originate @ the supplying, grounded conductor on the "XO" @ the transformer. Then end @ the device (isolated ground receptacle).

Typically, the supply end lands where the MBJ (the building's neutral-to-EGC/GEC system bond) is made, in the main disco enclosure. The rest is wired (other than color) as you would any circuit conductor: insulated and isolated from everything except the intended terminals.

Re-grounding it at any point, such as accidentally installing a non-IC receptacle in a grounded metal box, totally defeats the intent.