Single Point Ground

I guess I didn't quite follow what you said--are you saying that before they hooked up the PDU (which I presume is Power Distribution Unit), the resistance from the frame of their new cabinet (and any ground bus within it) to a "real" ground was infinity, and after they connected the (cables from the) PDU to their cabinet, that resistance was effectively zero--and they were surprised?

If so, I guess I'm like you, I'd be scratching my head trying to figure out why they were surprised. I mean, I assume the cables from the PDU to their cabinet included a ground wire.

When we did what I sometimes call a tree-type grounding system, what we typically did was use the ground in the power cable only to ground the cabinet (as in "their new cabinet") and we installed a ground bus within that cabinet that:

• was used to ground the shields of signal wires leaving that cabinet
• was grounded to an "isolated ground" (isolated from the power ground) (the isolated ground was carried around the entire facility, as was the "power / dirty" ground, but they were (at most) interconnected at only one location (I'm a little hazy on that memory--I think in some facilities they were never interconnected--they went to separate ground wells / beds)

I guess the point of my last little ramble (starting with "When we did what ...") is that, if they had / expected a similar system (that is, with what should have been an isolated signal ground), then I can see the reason for their surprise.

Sometimes even when we carefully specified that "isolated ground bus" to be isolated from the cabinet, we found that it was actually not isolated for various reasons--either intentionally because somebody somewhere thought it had to be grounded to the cabinet, or accidentally by virtue of things like bolts directly from that bus to the metal of the cabinet.

Yes, some older documents we have compare it to a tree structure. And yes, they were actually surprised.

I am shocked and dismayed when we actually have engineers and electricians come into our building and will argue with me over the legality, safety and functionality of our grounding system. I don't know how many ways to say without offense that they simply need to refer to the NEC. They all act as if I came up with some wacky invention on my own.

I had a cable "technician" come to my house once and tell me that the reason my T.V. didn't work was that my box was hooked up all wrong and it'd never work that way. It took forever to explain their job to them.

Interesting--thanks for confirming my understanding.

I've had my share of long conversations about grounding--I think that's one thing I won't miss.

Going back 40 years, I was trouble shooting a micowave system. I found adjacent racks with one grounded to one side of the building and the other grounded to a second side. The equipment was grounded only to the racks. there was an intermittant 20 to 60 volt DC voltage difference between the two grounding points. A common ground point corrected the noise problems.