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