Thanks for replies all. Since raising this question this AM I have been surfing the web for add'l input. Found a Mike Holt article back from 2/6/2006 EC&M Magazine titled "Article 645 - Code Basics" see below or (
http://68.225.22.180/mojonewsarchive/NEC-HTML/HTML/Article645CodeBasics~20030102.htm ). Am I off in my interpretation of his comments? Sounds like all ITE metal parts must be grounded. Perhaps I'm not undertsnading to what degree. Article follows:
"Grounding
This is an area where many people create conditions hazardous to equipment and people. It is typical for installation manuals to be well-intentioned, but wrong. Don?t choose between following safe grounding practices and following an incorrect manual. Contact the manufacturer and explain why you must follow Article 250. Document the discussion. If you are a contractor, work with your customer on this issue. In a negligence trial, Article 250 will carry more weight than a manufacturer?s manual. For one thing, Article 250 is backed by IEEE-142 and other industry standards, as well as basic engineering concepts like Ohm?s Law and Kirchoff?s Law. A manufacturer?s manual that conflicts with Article 250 also conflicts with industry standards and engineering fundamentals.
[645.15] You must ground metal parts of an information technology system per the requirements of Article 250. This does not mean driving a ground rod and calling the system grounded. You need to bond your ITE grounding bus to the main grounding system. Where signal reference structures are installed, you must bond them to the equipment grounding system provided for the ITE. You can?t consider power derived from listed ITE as separately derived for applying 250.20(D).
Remember, electricity is always trying to get back to the source, not the earth. If you use earth as your bonding jumper, the impedance path to actual ground may be several million or billion ohms higher than it should be. This means noise and undesirable current will seek other paths, per Kirchoff?s Law. Those paths include the backplanes of circuit cards in the ITE itself. Undesired current will circulate in equipment neutrals rather than dissipate to ground. If you fail to bond back to the source, you also create a shock hazard.
What about isolated grounding receptacles? Most people misapply these, and thus add more noise to the system while leaving a shock hazard for end-users. See 250.146(D) and 406.2(D) for the requirements for isolated grounding-type receptacles.
You can sum up Article 645 into a few key concepts. First, it pertains only to rooms that meet certain requirements, regardless of what equipment they contain. Second, you must fasten branch circuit power conductors securely in place. You don?t need to secure other cabling in place, but you must ensure it is protected from damage. Third, you must meet disconnect requirements that allow for a quick shut-off from the exit. The last concept to know is that grounding is grounding-apply Article 250."
