Link - page two, starting bottom left column and moving into the middle column on why it most certainly DOES break the NEC.
The two are connected (here at least) at the main breaker panel to protect 2-prong appliances, offering a path to ground if there is no 3rd prong on an appliance... the problem with bonding them at the bus bars is that:
A) You create new ground planes for every circuit in the house... without a common grounding point, you end up with different potential between different wire runs. Now, this naturally occurs anyway, since there is no effort ever to make sure every run in the house has the same resistance from end to end... but this aggrevates the problem.
B) From a safety standpoint, when a ground is activated (say a hot wire slips out of a screw on an outlet and hits the metal box)... in a correctly wired system, that voltage will be carried back to ground at the box, incorrectly bonded neutrals could provide a "least resistance" path back the other way, putting voltage back into the house on the neutral wire - now white & ground have a potential difference of 115VAC as well as black & ground... what's so bad about that? The black (hot) leads are protected by circuit breakers. Neutrals are never, EVER protected. Sure the faulted breaker will eventually trip, but maybe not before you get hurt. Also, looking at feeding back voltage through a neutral wire, you end up with yet another fun AC fact... the potential difference between two different hots is 240VAC, which can be lethal.
Bren R.
