That certainly looks like the same basic arrangement.
The NEC rule 250.36 seems to be limiting the system to 480V and above, whereas the French document is talking about it for all LV systems. (Note that according to the document the IT system is
required in French operating theatres.)
(4) Line-to-neutral loads are not served.
This seems to be adding an extra restriction which isn't mentioned in the French paper either.
Maybe the NEC reasoning is that if no line-neutral loads are served then there's no need to distribute the neutral on branch circuits and there's therefore less chance of the neutral being grounded out by a fault. Sound plausible?
By the way, the relatively new European designations such as IT, TT, TN-C etc. might be a little confusing to Americans, so perhaps I should explain the system:
1st letter indicates the grounding arrangements of the neutral at source: T means solidy grounded, I means insulated from ground or grounded via an impedance.
2nd letter is the grounding arrangement at the building: T=separately grounded to earth, N=grounded to the neutral.
Subsequent letters indicate whether the neutral and protective ground paths are separate or combined, or a combination of both.
Thus in the U.K. all public LV supplies have a grounded neutral, so the first letter will always be T. We have:
TT = Building grounded just to a local rod.
TN-S = Grounding to neutral point of xfmr, but ground path is kept separate from the neutral throughout its length (i.e. a 5-wire distribution system).
TN-C-S = Building ground is to the incoming neutral, but separate from that point onward.
Under this system, a normal American residential or commercial wye supply would also be classed as TN-C-S.
[This message has been edited by pauluk (edited 06-23-2002).]
