I recently was told that grounds from different branch circuits in a box need to be isolated. I'd really like to understand the purpose of this. It doesn't really seem like it would contribute to either function or safety.
This also seems like it would imply that metallic conduit, which must be grounded can only have one branch circuit, regardless of size.
Now I know the latter isn't true, because NEC 250.122(C) gives an example of doing just that. Is there any way of explaining this discrepancy, or is this just one of those things I just have to accept at face value regardless of whether it makes sense.
(C) Multiple Circuits. Where a single equipment grounding conductor is run with multiple circuits in the same raceway, cable, or cable tray, it shall be sized for the largest overcurrent device protecting conductors in the raceway, cable, or cable tray. Equipment grounding conductors installed in cable trays shall meet the minimum requirements of 392.10(B)(1)(c).