This article was posted before 01/01/2011 and is most likely outdated.

Isolated Ground Reference One

Grounding & Bonding Articles Menu  

Isolated Ground Circuits

I do a great deal of work in information technology rooms (IT), specifically installing branch circuits in liquidtight flexible metal conduit. My question is regarding isolated ground circuits and their proper installation. All branch circuits are feed from a power distribution unit (PDU), a step down transformer (480 to 120/208) and panelboards in one enclosure. An IG circuit has two grounds, one terminates in the outlet box since the flexible conduit is always over the length that would allow it to be used as this ground, I would not use it anyway, and the isolated ground conductor that connects to receptacle.

With the panelboard having one ground bar, where should the safety ground terminate in the panelboard and where should the isolated ground conductor terminate? If both grounds are terminated on the same ground bar, do you still get the advantage on the isolated grounded conductor on the circuit?

Steve Spooner,

Response No. 1

It is best that the separate isolated ground conductor be taken back to the first panel after the transformer, and connected to the common ground bus bar. There should be a separate neutral bus bar and a separate equipment ground bus bar in each panel after the first panel after the transformer. The separate equipment ground bus bar in the panel just after the transformer, has on it the isolated equipment ground conductor from the receptacles and all the other equipment ground conductors. See IEEE Std 1100, "Recommended Practice for Powering and Grounding of Sensitive Electronic Equipment"

Don Zipse <

Response No. 2

How do you like those PDU's? If they are the same as I've been working with, good luck. I found them very hard to deal with for a number of reasons. I hope you are using bolt-in breakers. Anyway, I believe you have to ground the two grounding conductors to the same ground bar since this is the source of the derived circuit. You can keep them separate only until the source [1999 NEC 250-146(d)]. I think that manufacturers of CNC machines have been trying to get around this for quite sometime to no success. I tell my students that grounds are grounds are grounds and at some point, they all have to be connected together without regard to the usage.

Jeffrey Neumann,

Response No. 3

The main advantage of the isolated ground is to isolate the equipment being served from electrical noise (typically high frequency) from the mass of raceway, which acts as an antenna in a building. The isolation ground should be terminated on the same ground bar as the equipment ground bar in your situation since this is the source of your separately derived system.

Keith Barth,

Response No. 4

See NEC 250-146-(d) FPN, regarding the need to ground the safety ground to the ground bar at the panel. This action should not defeat the purpose of the isolated ground, which separates the receptacle and any equipment from noise on the ground line from other sources - between the receptacle and the N-G bond point.

Ken Riches,

Response No. 5

There will not be much advantage in the IG circuits since they originated so close to the equipment. Please consider the following:

(1) The sealtite serves as a raceway AND a metal enclosure for the power circuit. The metal enclosure isolates the circuit from electromagnetic interference that might affect the power circuit. Less of a problem when the circuit is short and not around any power equipment, motors, HID fixtures, etc. However- with the use of the sealtite (excuse me; liquidtight flexible metallic conduit) the equipment ground is mandatory to ground the far end and box. This may offer some protection from 'noise' in your application.

(2) With the IG circuit the receptacle and load equipment is isolated from the noise or static that may be imposed on the ground circuit by loose conduit fittings, contact and movement against large areas of building steel or other conductive surfaces.

(3) The IG panel allows ALL the equipment connected to the network to be grounded to the same point. If a computer on the next floor is connected to the network with a coax or shielded cable and that area is fed by another remote panel or even separate transformer, the shield or braid on the data cable could be called on to serve as a parallel ground path to carry current between the two systems. This is read as data and/or errors in data and may fry delicate electronics trying to deal with power system currents. In that case, the IG would pass through each remote panelboard on an isolated buss and finally connect other equipment. Not perfect- but perhaps needed.

(4) Do you have equipotential plane and raised floor bonding around the data room? If so; and there is no equipment out of the area connected to the PDU; the IG system may be completely un-necessary. However, most computer manufactures have bought into the idea that the most dangerous thing on the system is the equipment ground; they would like to remove it. They will remove it if they get a chance so you may be compelled to use the properly installed IG set-up to satisfy warranty requirements and keep them from looking on you and the electrical system as scapegoats.

Lynn Adams,

Response No. 6

All the grounding conductors should terminate on the same bus. The purpose of the isolated ground is to protect the load from a ground fault that might occur on the grounding system.

Mike Cuellar,

Response No. 7

Consult the NEC art 517-19(g). This is on health care facilities, not IT equipment, but I think that is how the isolated ground was first used and the needs are the same. Basically you need a single path back to ground that is not shared with any other branch circuit The ground needs to be at the same potential as the ground the other branch circuits use.

Richard D. Currin Jr. <

Response No. 8

The reasoning behind an isolated ground is not to have a separate ground reference from all other equipment, but to have a 'clean' ground path back to the panel. If nothing else is connected to this isolated ground wire, there should be less interference caused by other equipment that may introduce noise on the ground wire. Therefore, as long as the conduit has no other connections to ground (building steel, etc.) before it reaches the panel, your isolated ground is fully functional.

Steve Newell,

Response No. 9

The isolated grounding terminal and the equipment ground would terminate on the same ground bus in your panel. The NEC requires the isolated ground to terminate at the "applicable derived system or service" 250-146(d). Since the panel is fed from a step down transformer, it is a separately derived service, and the grounded conductor (neutral), grounding electrode conductor and equipment grounds are all terminated together, in accordance with Section 250-30.

Yes, you'll still get the same advantage to the isolated ground even though they are at the same point. Probably the best reduction in noise is from having the nearby step down transformer, in the area.

Tom Baker <

Mike Holt's Response

All of the above comments are correct for different reasons. However, for the application of the question, Response No. 9 addresses the question more completely.

For more information, see Isolated Ground Reference Two