bus bar ties

[Kenneth Rentz]

I inspected a house today. That had one main 200 amp handle and two sub panels. All of the panels had the neutral and ground buses tied together. I'm not an electrician but I thought this was not allowed. What is the problem with this and why is it bad. We were always taught that only the main panel should have the neutral and ground buses tied together.

[grusk]

http://nachi.org/electricalcoursereleased2006.htm

[mnahrgang]

I'm not sure the "Why?" question is answered in the electrical course. If it was, I don't recall it.

[Speedy Petey]

Rather than try to explain it I'll give credit to the NEC Handbook commentary:
(keep in mind, the "grounded conductor" they refer to is more commonly know as the "neutral")

One major reason the grounded circuit conductor is not permitted to be grounded on the load side of the service [except as permitted in 250.30, 250.32(B)(2), and the four exceptions to 250.142(B)] is that, should the grounded service conductor become disconnected at any point on the line side of the ground, the equipment grounding conductor and all conductive parts connected to it would carry the neutral current, raising the potential to ground of exposed metal parts not normally intended to carry current. This could result in arcing in concealed spaces and could pose a severe shock hazard, particularly if the path is inadvertently opened by a person servicing or repairing piping or ductwork. Even without an open grounded conductor (usually referred to as an open neutral), the equipment grounding conductor path would become a parallel path with the grounded conductor, and there would be some potential drop on exposed and concealed dead metal parts. The magnitude of this potential difference would be determined by the relative impedances of the equipment grounding path and the grounded conductor circuits. Not only would the equipment grounding conductor path be affected, but all parallel paths not intended as equipment grounding conductors would be affected as well. This could involve current through metal building structures, piping, and ducts. The requirements of 250.30 and 250.32(B) have been revised in recent editions of the Code to prohibit the creation of parallel paths for normal neutral current.

[Larry D. Kage]

Originally Posted by krentz

I inspected a house today. That had one main 200 amp handle and two sub panels. All of the panels had the neutral and ground buses tied together. I'm not an electrician but I thought this was not allowed.

Beyond the service disconnect the neutrals (grounded conductors) and grounds (grounding conductors), typically, should be separated from each other and the neutrals isolated.

Originally Posted by krentz

What is the problem with this and why is it bad. We were always taught that only the main panel should have the neutral and ground buses tied together.

I pulled this definition from somewhere:

When neutrals and grounds are bonded (connected) together, the return neutral current will split (not necessarily equally) and run on parallel paths through the grounding and neutral system back to the main panel and up the neutral to the transformer. This parallel travel can cause unbalanced conditions in the system because the current remaining in the neutral will not counterbalance the current in the hot wire. In some cases, neutral current travel on the grounding system and other metal systems, such as piping or ductwork, can be a shock hazard.