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#51277 04/27/05 10:55 AM
Joined: Apr 2005
Posts: 3
A
Junior Member
At our college, a researcher in his electrophysiology lab wants an isolated dedicated ground, separate from the building ground system. He wants "to connect his equipment to this isolated ground system, thereby preventing ground loops and minimizing the electrical/magnetic fields that result from ground loops". Is his resoning correct, and is this allowed per the NEC?
Can we not run a separate isolated ground back to the main grounding point for the building and acheive the same and meet code? Thanks in advance. Andrew

#51278 04/27/05 11:02 AM
Joined: Jan 2003
Posts: 4,391
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The isolated dedicated ground can never be entirely isolated from the buildings grounding system.

You may run a separate insulated grounding conductor in the same raceway or cable as the other circuit conductors back to the point of bonding.

That may be the main bonding jumper at a service disconnect or the bonding jumper of a separately derived system.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#51279 04/27/05 12:59 PM
Joined: Feb 2002
Posts: 375
G
Member
You can have an engineer design the work and the AHJ should approve it. It should be installed and inspected to the design plans.

#51280 04/27/05 06:59 PM
Joined: Oct 2000
Posts: 2,722
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Quote

researcher in electrophysiology lab wants an isolated dedicated ground, separate from the building ground system. He wants "to connect his equipment to this isolated ground system, thereby preventing ground loops and minimizing the electrical/magnetic fields that result from ground loops"

If this Grounding System is not physically bonded to the Power System, of which the Equipment is driven from, there WILL be extreme levels of noise (and other fun issues), because this is:
"A Great Method To Create Ground Loops!" [Linked Image] [Linked Image] [Linked Image] [Linked Image] [Linked Image]

If you want to drive a rod or a few rods (or create another type of Electrode System) locally to this Lab, that's OK - just include them in the complete Scheme.

The "Complete Scheme" would be a sufficiently sized Eq. Grounding Conductor, ran from the Lab, to the Main Service Equipment (or "convenient" "effectively Ground Bonded" item - as pointed out in Article 250).
If terminated at the Service Equipment, land to the Grounded Conductor bus.
If terminated elsewhere, well - land it via an approved and mechanically solid termination means.
At the Lab, Bond the new Ground Rods (or other acceptable Electrode System) to this Equipment Grounding Conductor.
(by "Bonding" I mean to mechanically and electrically connect the GES to the newly ran EGC).

Without a solid bonding between the two Grounding Electrode Systems, the only connection is via a realitively high Impedance circuit - the Earth (Dirt, Soil, Actual "Ground", etc.).
Place a High Z (Impedance) load across any Power Supply, and the result will be current flowing through the circuit / load!

Try explaining this to the guy in the Lab, so there is no question as to how the grounding scheme should be done.

Exception to this whole thing:

If the Lab Equipment is driven from a Local Isolation (1:1) Transformer - or some other SDS means, then the Primary side feeders should (hopefully) include an EGC for bonding the Transformer's enclosure.

Along with that, there "Should" be a Local GES for the Transformer(s)' Secondary side output.
The GES would be Bonded to the Secondary side (typically at "X0") for a Grounded AC Power System, or to the enclosures only (not to the Electrical System) if the output is an Ungrounded System.

Let us know what the outcome is!

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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