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.
I’m not sure the “Why?” question is answered in the electrical course. If it was, I don’t recall it.
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”)
Beyond the service disconnect the neutrals (grounded conductors) and grounds (grounding conductors), typically, should be separated from each other and the neutrals isolated.
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.