ECN Forum Forums Code Related Discussion NEC & other Code issues Sub-Panel

We work at quite a few campus style facility's and all of them are presently or are in the process of switching to fiber for all connections between buildings.

They will use the fiber for voice, data, video security etc.

Many are also using fiber for these systems in the buildings as well, with Cat 5 or 6 filling in the gap to the last device.

Bob, parallel paths on the supply side of the disconnect are _known_ to cause problems. I suspect that the reason that they are permitted on one side and prohibited on the other side is a combination of 'balance of risk/benefit', jurisdiction, and economics.

At the level of the distribution network, multipoint grounding is the norm. This makes the earth itself a big parallel path, and we have had stories in this forum of problems caused by this. I can't find the thread, but I recall an EC who was trying to figure out how a customer was getting shocks from a hose tap outside his house. The piping was copper, the electrical system properly grounded. But there was apparently an Earth voltage gradient between the house grounding electrode and the patch of soil next to the hose tap.

But on balance, I expect that multipoint earthing substantially improves overall safety because of things such as lightning discharge.

At the level of the secondary side of the transformers, we have stories of plumbers getting shocked cutting pipes, because the metal water distribution network was causing a very good parallel path between separate structures sharing the same transformer. My guess is that if the NEC could eliminate this risk, it would...but multiple buildings sharing the same transformer secondary are the domain of the POCO, not the NEC.

At the level of multiple services on the same building, this is a parallel path that is explicitly permitted, but we are talking a very short parallel path (the separate ground/neutral bonds in each disconnect) with everything grouped together, which should eliminate most of the risk. At some point you must always have parallel paths; the combined neutral/ground bar in a residential panel is _not_ a single point, and if you look closely, you will see parallel paths there...but for the purpose of the NEC it is considered a single point.

-Jon

Jon I know I presented the question like I did not have a clue but I do have some idea.

The fact is that these parallel paths on the utility side do cause problems and have been the subject of much litigation.

Try a www search using "stray current cow".

Or

"stray current pool"

Here is some info
http://www.mikeholt.com/documents/strayvoltage/pdf/ElectricalShockShower1-25-99-1-27-00Rev1.pdf

We may get to a point that the cost / benefit balance may change.

Bob

I have been catching up on the reading since my last post on this subject. I was wondering since there are people that disagree which is better, or is it preference, running 4 wires (one being the EGC) and using the grounding system from the original main panel or 3 wires and starting a separte grounding system at the new service panel?

I would say, if there is ever a chance that you will have a parallel path, go with the 4 wire.
If this is just a remote shed out in the yard that will never get connected in any way to the house, 3 is fine, perhaps even better since "ground" is a local thing.

edit to add;

If you are running 6ga and driving a rod you are actually making your GES better.

[This message has been edited by gfretwell (edited 12-27-2005).]

I have been told by a couple of different electricians that began their careers in Europe that the neutral is never bonded at the customer service unit / Service Disconnecting Means. I don't know if I understood them correctly; one spoke French as his first language and I speak only English; and I never got to ask if there is a bonding, grounding, earthing conductor run with the service conductors. Does any one here have accurate intelligence on that issue.
--
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison

That is fascinating. I hadn't even considered thinking of that. I don't have much to add to this thread other than "thanks" for making this much easier to understand than reading it in a book.
--Ron

okay so im still confused...? IS there any diagrams that show this .

Which this out of this involved thread are you interested in.
--
Tom Horne

We have many temporary metal frame (Tents) structures on the project. We are driving a ground rod bonding the structure and then going into the panel and connecting to the ground bar with a #6 GEC. This is in addition to the EGC run with the phase conductors.