This is just a general question, I was wondering for what reason ground and neutral bus bars must be separate in sub panels. I've seen situations where someone ran a sub panel with 3 wires (hot+hot+neutral), drove a new ground rod at the subpanel, and left the two bus bars bonded in the sub, with no visible problems.

I've also seen a situation where the electrical company installed a new breaker panel adjacent to the meter outside of the house, which fed the panel inside the house, but left the bus bars in the breaker box in the house bonded.

Why is this incorrect?
Thanks

I don't suppose, "Because I says so" will suffice.

Honestly, this aspect of grounding is difficult to explain to laypeople.

Put simply, there is to be only one point of connection where the neutral is grounded and this is at the main circuit breaker.The neutral conductor is a current carrying conductor.

The equipment ground conductor is reserved for fault current, and the design is to force all fault current back to the point of grounding. This is to insure the impedance of grounding is kept low to allow fault current to rise high and fast to operate circuit breakers and fuses.


You may have a meter with a main breaker outside, neutral is grounded there, the panel that would be just inside, in lay terms, would be considered as the "subpanel' and the neutral and ground must be separated here, beyond the main breaker.

That makes sense, especially about the fault current. Thanks.