I think you've got the question turned around just a bit.
Anything with a metal case ought to have a three wire cord come with it. The wires in that cord are sized to the load.
Your metal cased equipment is attached to the rack with metal fasteners; it's pretty hard to do that without bonding the rack to the case in the process. So, having multiple devices mounted to the rack, each with it's own ground wire, is as sure a ground as you can get.
Using a power strip? Again, the cord to the strip ought to have a ground wire on it that is already plenty large for the power delivered.
Off the top ... if there is a 'problem' with static shocks ... that is proof of a good ground. Why? Because you are bringing electricity to the rack, and the rack is letting the static escape. Your solution is to stop carrying electricity with you.
For example, I once was called to 'fix' a desk that was 'shocking' the executive that used it. This gent had just bought a stylish desk chair, which had a heavy nylon mesh, rather than the usual cushioning. Well, nylon rubbing against his wool suit .... the man was a static factory.
Where there are static problems, the first step ought to be sure that there is some humidity - maybe 35% RH. Next, look for things like the carpet that might be making static electricity.
#167117 - 08/06/0709:59 AMRe: Data rack ground bar wire size?
Oh, boy does this topic bring up memories. We had a customer that had one salesman who continuously had fried phones and computer keyboards at an upscale office furniture company. See, he wore expensive leather-soled Italian shoes and tended to "kick" himself around in his office chair while he spoke on the phone. With the phone handset that had some eventual reference to ground, along with the computer keyboard, this was an ongoing problem. Something was broken every week with no explaination.
It turned out that the static that he managed to build up "skooting" back and forth on the swivel chair from the carpet, all that it took was for him to touch the keyboard or phone and create yet another $500.00 repair job.
Fabric softener and water (about a 50/50 mix) in a spray bottle applied to the carpet around his office area was all that it took to solve his expensive Italian leather shoe problem.
As for racks, we always bond them with #6 copper; just a simple Ilsco lug connected to a rack screw space near the bottom (or top) and a few inches of copper between them. We tie them back to the nearest grounding facility to make it look good, but in reality, like Reno said, it's the equipment's grounding conductor that is all that is needed.
Architects mandate ridiculous bond bar setups in closets, some connected by silly 2/0 copper connections to the GEC, but in reality, they are serious overkill. Why is it that 3-prong receptacles provide suitable ground connections for the equipment connected through them, yet a 2/0 is required to protect equipment connected through the very same means?
You can't keep people from causing problems. You just have to take steps to minimize the impact of the problems that they cause.
Tel/data/video for an student apartment building is being provided by the local telco. They are installing several racks in a tel/data distribution room. Each with a ground bar on the rack. I think they want a ground bar in the room to bond each of their bars to. Any suggestions on the size of the bar? Then a #8 or #6 back to the service ground?
Last edited by cgw; 08/07/0707:29 AM.
#167156 - 08/07/0709:18 AMRe: Data rack ground bar wire size?
Bars can be anything from a mandated .25"x2"x12" or greater on insulated spacers or something as simple as a standard grounding bus bar like a Square D #PKxxGTA style on (or not) an insulator kit (#PKGTAB). The size and such are dependant on the building, the equipment in the closet, the closet, and the specs. Same with the size of the conductor to the main service which depends on length also.
For very small buildings I would not size it smaller than the grounding electrode conductor and typically limit my contracts to #4 as a minimum size. This is just a general shot in the dark.
I do require the bonding of the racks as mentioned above, but also bonding is acomplished as needed through the cord sets. There can be ground loop problems with radio systems with this arrangement, but I have not experienced them yet.
When we were installing "drain" wires (be careful calling them "grounds" or you might get into 250 trouble) our enginerers said a 16ga was plenty. It all depends on exactly what you think you are bleeding off. If it is really high frequency "noise" you want a very fine stranded or braided wire. You probably get more surface area in a 16ga fine strand wire than in a regular 2ga "stranded" THHN. When this actually fixed something (notably a noise problem on long printer cables) 16ga stranded THHN did the trick.
I do not know how the telco will be using it. They just asked for a ground bar. An 1/4" isolated bar is what came to mind but it seems an over-kill. The biggest room has a 100A panelbd which serves everything in the room including air conditioning. This is where I came up with #8 ground. The other rooms only have a few circuits serving them.
#167212 - 08/08/0708:00 AMRe: Data rack ground bar wire size?
The sizing is also dependant on how knowledgable the person requesting it is.
I've had experienced phone guys ask for #6, but the hotrod "I'm the HEAD of this company's IT Department" just told me yesterday that he thought that the #2/0 that he requested during the initial design wouldn't be big enough and is whining that it should be changed out to be larger. The less these folks know, the more that they're likely to blame their problems on 'improper grounding'. Heck, these are the same folks who invented cutting off the ground plug on the equipment to 'solve' the noise problems on their little networks.
What's located in this room? A single 19" rack, a third filled with patch panels and 1 router. Where's it located in their existing building? In a storage room plugged into an extension cord and working just fine.
BTW, I get the same grief about isolated grounds for computer equipment when it's just a standard PC. I always ask if they have a PC at home; do they have an IG receptacle at home; and why do they think that what works at home won't work in the office.