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Electrical Troubleshooting Quiz

Jul 21, 2009 11:22 AM

Your facility has a small IT room that contains its servers. Each server rack receives power through its own rack-mounted UPS. The UPS event logs show frequent voltage spikes and repeated bouts of high THD (total harmonic disturbance). This room is on its own power transformer and panel (a separate transformer/panel supplies the lights). The receptacles are all isolated ground.

Using a UPS as a crutch instead of as insurance is tempting fate. You need to solve this problem before a UPS gets cooked. Where are the spikes and high THD coming from?

Your first clue is the use of isolated ground. The concept of "isolated ground" is actually referring to deferred bonding. It really has nothing to do with grounding (see NEC Art. 100 definitions). Essentially, you're insulating things so that you connect the supply end of the bonding jumper at a point near the source rather than the load (see IEEE Standard 142).

The typical IG installation is incorrect, adding problems that weren't there without it. In most cases where an IG is implemented:

• It's done instead of meeting the basic requirements provided in IEEE Standard 142.
• It doesn't solve any problems, even if implemented correctly.
• It's implemented in a way that allows potential to exist between metallic objects.

This last item is probably the underlying problem here. If you're seeing spikes on the UPS event logs and those UPSs are supplied by IGs, suspect bonding errors in the IG implementation. Refer to IEEE Standard 142 for correct implementation, or forego the IG altogether. Though the UPSs have protected your servers thus far, they aren't intended to bandage errors that violate applicable codes and standards. Your current situation may nullify your UPS warranty.

The spikes may be originating outside the room. Use a power analyzer on the supply side of the transformer to see what you're dealing with.

"Dirty power" may be coming from inside that room — whether also coming from outside or not. Look at the other loads on that panel. Typically, where there are computers there are also printers and scanners. Does the room have a laser printer? If so, run power from a different panel just for this kind of noisy load.

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