Isolated Ground - Computer Power (8-3-99)
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
With the panelboard only 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?
Response No. 1
1999 NEC Article 250-146(d) - If the receptacle is in the same building then a grounding conductor
is allowed to be installed from the receptacle to the panel board grounding terminal. A second grounding
means shall be run to ground metal boxes, conduits, etc. The ground on the secondary side of the transformer
is supposed to be connected to the grounding electrode. The isolated ground is allowed to pass through
panels without connecting to the grounding bus or metal casing.
Response No. 2
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"
Response No. 3
First of all I will be using the 1999 National Electrical Code.
In your question you stated that your use of Flexible Metal Conduit was in lengths over the length that would allow it to be used as an equipment grounding conductor. I would then say the Flexible Metal Conduit is in lengths exceeding 6 feet [250-118 (6) (c)]. If the Flexible Metal Conduit does not comply with the requirements in 250-118 (6), it is not an equipment grounding conductor. When you install a conductor inside the Flexible Metal Conduit, which is described in 250-118 (1), then that becomes the only equipment grounding conductor. You have not installed an isolated equipment grounding conductor. Then the point of termination of the equipment grounding conductors, at the source of power, does not matter but it is very important that it be properly installed as an equipment conductor on the load side of the separately derived AC system. One point to be concerned with is that this conductor installed inside the Flexible Metal Conduit to an isolated receptacle does not fall under 250-146(d).
Note: When you install a separately derived AC system you should play close attention to 250-30 (a) so not to produce parallel paths and objectionable currents over grounding conductors [250-6 (a)].
Fred W. Brown
Response No. 4
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.
Response No. 5
The isolated grounding wire connects to the isolated ground bar in the panelboard. The isolated grounding
conductor will be fastened to only the isolated grounding terminal of the isolated ground receptacle.
It shall not make electrical contact to any conduit, "j"-box or any other item in contact to the common
ground. I have a great schematic, if you send me a Fax # I would be happy to send it to you.
Response No. 6
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.
Response No. 7
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.
Response No. 8
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. But 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.
Response No. 9
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.
Response No. 10
You must have a totally separate source of ground to have a true isolated ground system. For some reason
people think if they have two ground wires they have an isolated system and that's not true. To give
you an example, we wired a law office in Jax, FL they insisted on having an isolated ground system.
So we wired it as drawn by the electrical engineer. The only problem was the office was on the 38th
Maybe Mike can tell us both how to get an IG system on the 38th floor of an existing building.
Response No. 10
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. It sounds like you can terminate the isolated ground in the panel since it is just down stream of the transformer and it is bonded. With out seeing a drawing of the system I'd be afraid to guess. I welcome any further enlightenment on this subject as grounding and bonding in my opinion is the hardest part of the code to master. I don't know how many wires you need to pull but you might consider jacketed MC with the isolated ground already in it.
Richard D. Currin Jr.
Response No. 11
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. So, 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.
Also check out the article attached on how isolated grounds work with surge protectors.
Response No. 12
Many manufacturers will sell you as an accessory another insulated neutral bar that you can mount in the breaker panel, either by bolts, sheet metal screws or by taking the place of the existing ground buss. (See Westinghouse Power-line) In the latter case the ground bus needs to be moved to another location and attached and bonded to the tub of the panel accordingly to deal with faults in the panel itself. This "ground" bar needs to be labeled prominently as the "Equipment Grounding Conductor" and attached to the same ground that the 480vac supply is grounded to.
The new and empty neutral bus will need to be labeled "Isolated Ground" and connected to the PDU ground
with appropriately sized wire for the service. Then you connect the phase conductors to the breakers,
the white to the neutral buss bar, the ground from the conduit and boxes to the "Equipment Grounding
Conductor", and the last, isolated ground to the "Isolated Ground". This wire does need to be a green
wire, but I suggest that you pull green with a yellow stripe. This practice is allowed by many inspectors
and allows you to easily keep the isolated ground separate from "common ground" or "Equipment Grounding
Conductor", and lets whoever may come along in the future that something is slightly different in
the system. And don't forget the isolated ground receptacle with the little orange triangle printed
on the face of it.
Response No. 13
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.
Mike Holt's Response
All of the above comments are correct for different reasons. However for the application of the questions, Response No. 13 is the most correct answer.