Isolated ground

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tim89s

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I ran my ISO ground back to the service and tied it in with the neutral and ground. I then wanted to test the ISO ground so I lifted is from the neutral bar. At the ISO receptacles I had no continuity between the ISO ground and the dirty (equipment) ground. That was what I wanted but when I checked the voltage between the hot and ISO ground I got 140 volts at one and 80 volts at another. When I connected the ISO back at the service I got 120 volt between ground and hot. By the way the two receptacles were each dedicated circuits. My question is why did I get a voltage between the the?hot and ISO ground when the ISO ground was lifted?
 
Re: Isolated ground

Sounds as though you were taking readings with a digital meter, try the same thing with an analogue meter.

Roger
 
Re: Isolated ground

The extremely high input impedance of the digital meter can cause all sorts of weird readings. Most anlog meters have at most a 10,000 ohm/volt impedance so that they load the circuit enough to give reliable readings.
 
Re: Isolated ground

When you have the isolated ground unterminated, and you connect your meter to it, you are basically using it as an antenna. And with it running very close to a hot wire with 120V on it, there is no telling what you will get.

Steve
 
Re: Isolated ground

Hello-

One thing about your problem that caught my attention is the voltage values you were reading.

In one case you read 140V, but the circuit is only 120V, which seems to indicate that somewhere along the run is a higher voltage source that may be inducing this higher level onto your circuit.

I don't doubt that these are phantom voltages, but I would be concerned about noise levels that may be present on the IG conductor from this outside source.

You mentioned that these IGs are "dedicated circuits"- this term is not defined by the NEC. Could you please describe what you mean?- it may have some bearing on where these phantom voltages are originating from.

Thanks,

Mark
 
Re: Isolated ground

This should not present any problems as the induced voltage will have no effect as there is no current to back it up, as it was said a analog meter with a input resistance of 10,000 or even 20,000 ohm would short out this phantom voltage.
Digital meters have an input resistance between 2 and 10 meg ohms so this is why you can see this voltage on one. Even your body will short this voltage out.

If the circuit that was induceing the voltage had about 300 or more amps on it and the run was about 500' or more then maybe I'd worry.
 
Re: Isolated ground

Hello-

You mentioned that "The IG was ran in a raceway with opposite phase circuits." This is not permitted because it will create noise on the IG conductor.

If you have access to a book published by EC&M magazine titled "Practical Guide to Quality Power for Sensitive Electronic Equipment" , on page 93 they list about 15 pitfalls for the installation of IG circuits.

The two that apply in this case:

"Running three insulated conductors but sharing the conduit with other circuits."

and

"Running the IG conductor to the grounding electrode through a raceway carrying a circuit feeding another load."

Also, take a look at the following document published by the Information Technology Industry Council:

http://www.itic.org/technical/ite_grnd.pdf

On page 6, paragraph #3:

"IT equipment should be powered from dedicated branch circuits wherever possible. A dedicated branch circuit not only has its own breaker, but also its own grounding conductor (ground) and grounded conductor (neutral) if used; neither of which is to be shared with other circuits."

If you have access to any IEEE Standards, take a look at IEEE 1100, Recommended Practice for Powering and Grounding Electronic Equipment. Chapter 9 gives details in the installation of IG and dedicated circuits. In section 9.3 they point out "Sharing common line, neutral, or grounding conductor paths by multiple electrical loads may produce unwanted interactions."

The National Electrical Manufacturers Association has an excellent publication in IG circuits for free download. Go to www.nema.org and search for the "Application Guide for Isolated Ground Wiring Devices"- this is a 21 page .pdf file and discusses in greater detail what I have mentioned here.

I agree that you are measuring phantom voltages, but again my concern is that you will also have unwanted noise on this IG circuit due to induction from the other phase conductor(s).

Best wishes,

Mark
 
Re: Isolated ground

Originally posted by pqtest:
Hello-

You mentioned that "The IG was ran in a raceway with opposite phase circuits." This is not permitted because it will create noise on the IG conductor.
Click to expand...
Mark this is required by the NEC, the EGC, IG type or not, must run with the circuit conductors.
 
Re: Isolated ground

Originally posted by pqtest:

I agree that you are measuring phantom voltages, but again my concern is that you will also have unwanted noise on this IG circuit due to induction from the other phase conductor(s).

Best wishes,

Mark
Click to expand...
I would think the best thing to do here would be do away with the IG altogether and we can eliminate this concern. :D

Roger
 
Re: Isolated ground

Originally posted by pqtest:
You mentioned that "The IG was ran in a raceway with opposite phase circuits." This is not permitted because it will create noise on the IG conductor.
Click to expand...
It is required to be ran with circuit conductors, 250.146(D)
 
Re: Isolated ground

Originally posted by pqtest:
Hello-

You mentioned that "The IG was ran in a raceway with opposite phase circuits." This is not permitted because it will create noise on the IG conductor.


Mark
Click to expand...
Mark if you mean that an IG circuit with say A phase, neutral, IG ground and equipment ground (conduit or conductor) AND another circuit with B phase (the opposite phase), neutral, IG ground and equip. ground all in the same conduit; then that would be a design issue but not a code violation.
I think what you are saying is that IG circuits should be run in seperate conduits.

I think iwire and dereck were thinking that you meant that the IG should be run outside of the raceway containing the circuit (hot and neutral) and that would be a violation.
 
Re: Isolated ground

More or less that was what I was thinking but how does that IG eventually get back to the bonding point?

At some point the IG will have to run with the panel feeders to the source.

Bob
 
Re: Isolated ground

Originally posted by iwire:
At some point the IG will have to run with the panel feeders to the source.
Click to expand...
True if branch-panels are involved (aka sub-panels). But there is a practical limitation. How many dedicated IG circuits can be run in individual conduits before you run out of knockouts and still serve the other non-critical loads like lights, GP, and HVAC.

IMHO if you have a real need for dedicated IG circuits, you are better off with a isolation transformer using SG circuits. You will get much better common mode noise atenuation and rejection ratios.
 
Re: Isolated ground

Hello-

Sandsnow, your statement is correct regarding my last post- I should have proof read it more carefully.

Thanks all!

Mark
 
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