When Circuit Boards are Toast, Part 8

Aug. 8, 2011
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

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.
If you repair those four items and still have a mismatch, use a power analyzer to look at:
  • Power factor.
  • Harmonic content.
  • Waveform shape (for distortion and other anomalies).

Voice your opinion!

To join the conversation, and become an exclusive member of EC&M, create an account today!

Sponsored Recommendations

Electrical Conduit Comparison Chart

CHAMPION FIBERGLASS electrical conduit is a lightweight, durable option that provides lasting savings when compared to other materials. Compare electrical conduit types including...

Fiberglass Electrical Conduit Chemical Resistance Chart

This information is provided solely as a guide since it is impossible to anticipate all individual site conditions. For specific applications which are not covered in this guide...

Considerations for Direct Burial Conduit

Installation type plays a key role in the type of conduit selected for electrical systems in industrial construction projects. Above ground, below ground, direct buried, encased...

How to Calculate Labor Costs

Most important to accurately estimating labor costs is knowing the approximate hours required for project completion. Learn how to calculate electrical labor cost.