In Part 7, we started looking at the line-to-neutral (L-N) and neutral-to-ground (N-G) voltage measurements at a convenience receptacle in the cabinet where circuit boards were turning to toast. So why are these measurements useful?
As the circuit load increases, so does the current (per Ohm's Law). Voltage drop increases as current increases. So if a branch circuit is overloaded, voltage drop increases. Voltage drop is something you calculate based on circuit current and voltage, and conductor length and gage size.
Using a DMM, you can also measure the voltage drop. If your system is wired and bonded correctly (and connections are good), then your N-G measurement will be the voltage drop. Subtract this from your nominal voltage, and the result should match your L-N measurement. If it does not, you have current flowing in the ground conductor.
You’ll need to fix that problem before continuing. Here are four things to look at:
- Ground rods attached to equipment somewhere on this branch circuit.
- Improper N-G bond (usually at the transformer).
- Bonding deficiency, resulting in circulating currents.
- Neutral and ground swapped one or more times somewhere on the branch circuit.
- Power factor.
- Harmonic content.
- Waveform shape (for distortion and other anomalies).
