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I've been advocating against isolated grounds in data centers for years. The fact is, unless you use very special mounting hardware on everything and take an unrealistic level of care with the installation of each piece of equipment, you will corrupt the "IG" with the first device you mount.
Why? Because it has a metal chassis with a built-in safety ground (that's code) and that chassis is screwed into a metal cabinet that had better also be grounded. You now have two ground paths: one to the standard power ground, and one to your so-called "IG." Each piece of installed equipment creates another dual-ground path, so the whole "IG" system is no longer "isolated."
"Isolated grounds" were developed for early, sensitive computers. Those computers were installed in an office environment where all sorts of other equipment were also connected, which put electrical noise on the line.
Today's boxes are much more stable, as evidenced by the fact that nearly every home has one, and power corruption problems are rarely seen. The much more sophisticated servers and storage we install in data centers does need good grounding, but that does not mean a true "isolated ground."
ABOUT THE AUTHOR: Robert McFarlane is a pioneer in the field of building cabling design. He has been asked to speak at countless seminars on building infrastructure for electronic communications, evolving technologies and the requirements of trading floor and data center design. Mr. McFarlane served for twelve years as President of Interport Financial, Inc., a firm specializing in designs for financial trading floors and critical data centers.