The PoCo doesn't supply a separate earth connection as in TN-S.
All services in the U.S. are arranged as what would be called TN-C-S or PME over here. The building's earth system is always bonded securely to the supply neutral, along with bonds to ground rods (or rings), water, gas, etc. In fact, at the main distribution panel the neutral busbar is bonded directly to the metal casing (isolated at sub-panels).
The NEC actually prohibits the use of TT systems which are common in Britain & Europe:
250.4 General Requirements for Grounding and Bonding. (A) Grounded Systems (5) Effective Ground-Fault Current Path. ..... The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path.
[This message has been edited by pauluk (edited 03-08-2003).]
Lyle, What is it that you call a Ground Ring? What we have over here in NZ, is if a certain size of Mains is installed, the corresponding size of Main Earth is installed also. Bonding with Large Buildings, is done with multiple Earth Electrodes and welding of Bonding leads to Rebar sections, by Exo-thermic means. With the advent of PE pipes, bonding of the water services, is not necessary.
Interesting point you make re PE pipes. However, it is common practice for the water supply to be plastic and the internal plumbing to be copper. In this situation we are required to effect a main bond.
Yes, I was aware that filling stations in the U.K. are prohibited from using the PME system, although I've never had to work in such an environment.
In the U.S., chapter 5 of the NEC covers "Special occupances" and it would appear to me that gas stations would be covered by article 514. I've browsed through it quickly, but I can't see anything in it which changes the standard American TN-C-S equivalent supply system.
I'm sure we must have some Stateside members who could provide some more details? Is there anything in any other NEC sections which might apply here, guys?
Lyle, We still use Copper water pipes, over here, like you said, we have to Bond and Earth all metallic pipework, makes sense though. I once saw a Plumber cut a copper pipe and recieved a shock between the two cut pieces, must have been a small fault in the wiring in the house, where we were working. He should have had a wire between both sides of his cut, but it gave him (and me) one H**l of a fright