Sputtering System Grounding

[djr3203]

I am working on a project where we are installing a sputtering system. The manufacturer is calling for us to drive (2) new ground rods (not bonded to the building ground electrode system) to ground the vacuum chamber. This ground will be in addition to a safety ground coming from the panelboard to the sputtering system power supply. At first I thought this additional ground electrode had to be bonded to the building's ground electrodes; however, people are quoting NEC article 250.54, Auxiliary ground electrodes, and saying this allows the ground rods to be NEC compliant. After doing some research it appears this article will allow this installation (even though most of the information found online makes me think this article is to allow ground rods at remote light poles). Is this installation NEC compliant?

The most important question of all: Even if this installation is NEC compliant, is it a good idea? Wouldn't any ground faults, lightning strikes, etc. cause a potential difference between the two ground systems which could cause issues?

DJR

 

[Skogsgurra]

Code, articles or whatever. Physics says you are right. But, then again, you are in TT territory (I think) and that's where anything can happen.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[jghrist]

250.54 allows extra ground rods to be connected to the equipment grounding conductor, not just to the equipment. The difference between this and a normal grounding electrode is that you don't have to comply with the bonding requirements to the grounding electrode system. The auxiliary rod is still connected to the grounding electrode system through the equipment grounding conductor.

That being said, if the auxiliary ground rod is connected to the vacuum chamber ground, and the equipment grounding conductor is connected to the vacuum chamber ground, then the auxiliary ground rod is connected to the equipment grounding conductor. If somehow, there are two independent equipment ground points, then it would not be NEC compliant to connect one of them only to the auxiliary ground rod. This would use the earth as a ground-fault return path which doesn't comply with 250.54.