I came across a few of your articles from the early 2000s on isolated grounding. I was wondering if there have...
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been any updates since then on the usefulness of isolated grounds.
Isolated grounds (IG) are a solution that no longer has a problem. It is a waste of money to call for them unless some very special and sensitive computing system requires them. There is simply no need for isolated ground designs any longer.
Every time I start a project and tell the electrical engineers that we will not be using isolated ground circuits, they practically want to hug me because they are so grateful to be working with someone who understands.
Get serious about power safety in the data center
IG was created in the early days of personal computing when electronics were highly sensitive to electrical disturbances. Large office buildings are great breeding environments for power disturbances and anomalies. The large motors on air conditioners and elevators, and their associated sudden large power draws for operation, inject significant spikes, dips and "noise" into the building's power circuits. Power line "trash" can also be introduced by some of the personal devices people often have connected in their offices.
All electrical equipment has safety grounding, but it was found that the grounding path historically used in buildings was just as subject to these disturbances as the primary power circuits. That path was, and still is, through the conduits and building steel, and is in parallel with the neutral conductor of the power circuits.
Isolated grounds are particularly useless in data centers. Every cabinet should already be grounded to the data center grounding system. Screwing a metal chassis into a grounding metal cabinet instantly violates any isolated ground that may have been installed. More egregious than rendering the IG useless, this potentially creates ground paths of different electrical lengths and impedances that can generate ground currents and problems with electronics. With poor data center grounding practices, you're creating the very problems you planned to mitigate. The best reference for data center power design and grounding is the Telecommunications Industry Association.
At the desk or workstation, running a separate ground wire from the power receptacle back to a separate grounding bar in each electrical panel insulating the ground bar from the metal panel housing and then running a separate grounding conductor from that ground bar back to the primary ground at the incoming service to the building reduced most power line disturbances to the point where they no longer affected sensitive electronics equipment. The IG circuits also terminated in special receptacles that kept the U-Ground pin separated from the metal strap that is used to mount the receptacle to the metal outlet box or workstation raceway. These receptacles are clearly identified by their orange color and a small triangle etched into the receptacle face.
The additional cost of all these special receptacles, and all the extra copper wire needed to connect them, is substantial. For the isolated ground to be of any value, the equipment plugged into it must use a power cord with a third U-Ground pin. Much of today's computer equipment does not, and if no U-Ground pin exists, there is no possible benefit from having an IG circuit. This is particularly true of laptop computers, tablets, smartphones and the like, which operate on direct current provided through a small transformer and rectifier, completely isolating the device from the power line. Think of it as a small uninterruptable power supply.
Even NEMA receptacles, European C-type receptacles and other equipment that do have a grounding pin do not need IG circuits today. It is good to have the receptacle grounds connected back to the power panel via a solid grounding conductor rather than just through the conduit, but that does not qualify the outlets as isolated ground, and further isolation is not necessary for any of the computing equipment commonly used in offices. If special grounding is required, the hardware's installation instructions should make that clear.
A dangerous isolated ground issue
An isolated ground is not a separate ground. Some manufacturers years ago erroneously called for separate grounds from their equipment to a separate ground rod that was not connected to the building grounding system. This is not only illegal, but extremely dangerous. One of the most devastating things that can occur in an electrical installation is a ground fault, which is essentially a very high-current short circuit. If the IG and building electrical ground are not properly tied together at the building power entrance, a ground fault can literally cause the earth between the two ground rods to boil, which could throw part of the basement slab up on the second floor. The potential power of a ground fault cannot be overstated.
Robert McFarlane is a principal in charge of data center design at Shen Milsom and Wilke LLC, with more than 35 years of experience. An expert in data center power and cooling, he helped pioneer building cable design and is a corresponding member of ASHRAE TC9.9. McFarlane also teaches at Marist College's Institute for Data Center Professionals.
Robert McFarlane asks:
Is your data center grounding design up to date?
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