In the cabinet where your circuit boards have been turning to toast, do you have a receptacle fed from the same panel that supplies the electronics in that cabinet? If so, this allows you to check the panel that supplies the cabinet without having to remove the panel cover (or even find the panel).

Take these two voltage measurements: line-to-neutral (L-N) and neutral-to-ground (N-G).

A low L-N voltage could be due to:

  • Voltage drop (conductors are too small for the length of run).
  • Voltage loss (for example, through poor connections).
  • Overloaded branch circuit.
  • Low panel voltage.
  • Any combination of the above.
The N-G measurement can help you sort this out. The hot and neutral conductors have impedance and current, which is why you have voltage drop. If these conductors are the same length and size, the voltage drop on them is the same. If you have a drop of 1V on each conductor, then for a 120V supply you’ll have 118V at the receptacle.

Assuming your system is correctly wired and bonded, you'll see the other 2V on L-G. Why? Because current doesn't (normally) flow in the ground conductor, it has no voltage drop. This means it acts like an extended test lead from your DMM to the NG bond.

In Part 8, we'll see why this is useful.