Isolated Ground

Hello Philip, I used to work with a lot of equipment that required isolating grounds for a number of reasons. It is very common to isolate grounds for equipment. What you need is an isolation transfomer that is capable of handling the power requirments of all the equipment on the circuit. So if there are 5 computers on the line and each draws 500W then you would need at least a 2500W isolation transformer. I would also allow for and extra 20% over that to allow for other things being added that you may not know about. Make sure that the ground line (the green wire) is not conducting all the way through the transformer. I have run into equipment that isolates the Hot and Neutral lines and the Ground line runs straight thru the transformer. This definitley defeats the purpose of isolation so be sure to check this with an ohm meter. The science behind this is to allow for all the equipment to 'float' with the circuit the equipment is attached to. In the case of high voltage testing isolation is needed to avoid aliasing the results. The same is probably true for your case. Your equipment needs to float to avoid screwing up any data that the system wants to capture. On some test and sampling equipment they isolate the inputs to the input channels to avoid aliasing the data captured.

The client doesn't want the expense of the isolation transformer, just the isolated gnd. I have the panel with the computer outlet gnds passing through the sub panel and connecting to the building safety gnd some 200 ft away. Can I run one isolated gnd from my sub panel to 5 quad outlets? I would also like to read up on this wiring configuration to better understand the true science. There are often urban legend type designs that are not what we think. Thanks for your comment. P2

Few people still try to use an isolated ground system because networking (etc) essentially undermines it. With or without an isolated ground, the 120VAC ground is tied to the DC ground of the computer is tied to the ground conductor of network cable. If a netwok hub is connected to a computer (on the isolated ground network) and connected to a commercial laser copier/scanner/copier (not on isolated ground network as a pulsing load), then the two grounding networks are connected and nothing is isolated. I don't know of any isolated ground networks that aren't inadvertently undermined by scenarios like this.

A great citation from: http://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/Grounding-versus-Bonding-Part-9-of-12~20050602.php

Computer circuits are prime candidates for the "install IG receptacles first, ask questions later" approach. But, the results are more cosmetic than substantial and tend to mask underlying problems.

In most cases, an IG is a waste of money. IEEE 1100, Powering and Grounding Sensitive Electronic Equipment (Emerald Book) states, "The results from the use of the IG method range from no observable effects, the desired effects, or worse noise conditions than when standard equipment bonding configurations are used to serve electronic load equipment [8.5.3.2]."

Isolated earth ground is dangerous and can cause injury or death. Using one wire for a panel depends on the fault (short) current worst case it is best to use a green/bare wire for each socket. There is some reasons to use an isolated ground due to ground loops but one socket needs to be a solid earth ground.