When Circuit Boards are Toast, Part 9

Suppose you're taking neutral-to-ground (N-G) voltage measurements at a convenience receptacle in the cabinet where circuit boards were turning to toast. How do you know when these measurements indicate there's something wrong?

If you see a volt or two, you have no worries (much higher than that is a bad sign). But if you see 0.10V or less, you may have found the smoking gun. Why?

Because such a low number indicates the presence of an N-G bond. Such a bond on the load side of the service allows undesired current to circulate. It may take some sleuthing to find it, but here are some key places to look:

• At the 120V supply panel. Ensure neutrals and grounds aren’t cross-connected.
• At that receptacle. To isolate this problem, lift the neutral wire from the receptacle and measure N-G on the incoming wiring. If you still see the N-G bond, it’s upstream. Check the other equipment on that supply panel.
• If the problem disappears, it’s at the receptacle or on other connected load(s) in the cabinet. Ensure the receptacle doesn’t have an N-G jumper. If the supply conductors are stranded rather than solid, careless workmanship may have produced a nearly invisible bond. Look closely.
• If the receptacle and upstream are good but the bond persists, follow the incoming power to each termination point.