How Good Is Your Ground?

An article in EE-Evaluation Engineering, as passed along by a client. The article is by Vladimir Kraz, Credence Technologies, and J. E. Patrick Gagnon, Texas Instruments. Good piece.

My comment: the article is written from the perppective of a controlled environment (i.e. – semiconductor test, wafer fab, etc) where there is a lot of thought and investment in a good, controlled, ground plane / grid / system – and the goal is to get connected to this ground plane as solidly as possible. For a lot of the systems that I work with (i.e. – medical imaging) there is no deliberately conceived ground system – so things are a lot murkier. If one is trying to break up problematic ground loops / paths, the last thing one might consider is improving the system coupling to the outside world….

Oh, Just Use the Ground

I’m back from a site visit; an MRI system has been having issues. On site, I discovered that a large UPS, which apparently requires 5 wires (3 phase, neutral, and ground) has been connected with a short jumper or bond between the neutral and ground – since a neutral was not run to the device. This was with the sanction (in fact, at the direction of, via the installation manual) of the manufacturer (not a small or fly-by-night vendor, I might add).

The result: 8 Amps of current (presumably from the input rectifier filters) on the protective ground conductor. At another site, we measured 16 Amps of current on the neutral-ground bond (not all of which flowed to the source on the ground conductor, mechanical mounting and conduit connections presumably taking some of the current)

OK, folks, it’s Power Quality 101. YOU DON’T USE THE GROUND TO CARRY CURRENT.

I guess it shocks me (heh-heh, thats a pun) that in 2006 people are still doing the same sort of things that got the whole power quality mess started in the old days.

I got to drag out my leakage current and ground current meters and put into practice some of the things I wrote about in my last Power Quality paper: Leakage and Ground Currents: Measurement Techniques