Previously, we discussed a method of finding which circuits on a given panel have neutral-ground (N-G) bond. Once you've identified the faulty circuits, repair them one at a time (verify by testing). We previously discussed how to do that in the control panel. Just apply the same techniques to each circuit until you've eliminated the N-G bonds. Then test again at that control panel receptacle to see that this problem is finally gone.
Although the N-G bond is one you don't want, there's another bond you do want — and it needs to have integrity throughout your load side distribution system. Metallic raceway, such as EMT, makes an excellent "ground" (equipment bonding) conductor. If assembled properly, this raceway is far superior to the green ground wire in handling high-energy transients. The reason is the difference in surface area.
Raceway must be installed in complete runs [300.18(A)]. If you're using raceway as an equipment bonding conductor, inspect to ensure:
- It's not connected to neutral (e.g., being used as the 277V return path).
- There are no breaks in the electrical path. These give you differences of potential. One of these could be zapping your circuit boards.
- Fittings are properly assembled and are not loose or corroded.