As I see it, there are two conflicting requirements.

In case (a), where we're looking at the possibility of metalwork becoming energized by some faulty wiring or device, then we would want that metal bonded to ground to open the OCPD or trip the GFI as quickly as possible to prevent it becoming a shock hazard. But the book extract is clearly talking about Rp & Rx being in series, which would not be case in this scenario.

In case (b), where we're concerned with the metal being a return path to ground for somebody touching an energized conductor elsewhere, then bonding that metal to ground will reduce the overall resistance and make the shock more severe. The way the book describes the shock current as V / (Rp + Rx) means that this must be the case they're examining.

Yet they seem to be implying that by bonding the metalwork if the measured resistance is under 50K (or 25K) will reduce the risk, which would be true in the first scenario but not the second.

By the way, this supplementary bonding is in a general section, applicable to armor-grounded, PME, and local ground systems alike (the first two of these will not necessarily have GFI protection).

AFCIs? Oh boy... I'm kinda glad we don't have them over here. Not yet, anyway! [Linked Image]


[This message has been edited by pauluk (edited 06-11-2002).]