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When Circuit Boards are Toast, Part 3

May 24, 2011 1:31 PM

The main purpose of Chapter 3 wiring method requirements is to prevent the melting of conductors. Some of these requirements also reduce or prevent power quality issues that damage equipment. Look for violations of:

• 300.3(B). Running conductors (current-carrying, neutral, and bonding) from the same circuit in different raceway. The electromagnetic fields generated in these conductors will tend to cancel each other out. However, if run in separate raceway, those fields don't interact and thus don't cancel each other.
• 300.3(C). Running different system conductors together without ensuring all of them have an insulation rating at least that of the maximum voltage applied to any conductor. A classic example is running signal wiring in with 480V power wiring, consequently smoking input circuits.
• 300.10. Allowing discontinuity of metallic raceway systems. Where the resulting differences of potential are great, the flashover can provide a lethal shock hazard to people and/or destroy equipment. Where the differences are moderate, the consequence is the circulation of undesired currents that can easily cause semiconductor failure.

Once in place, these violations are hard to detect and may be expensive to correct. But we go back to the main goal of Chapter 3: to prevent the melting of conductors. If circuit boards are failing because of Chapter 3 violations, then you may find one smokey morning that power quality is the least of your problems. Part 4 of this series will deal with other aspects of finding the sources of circuit board smoke.

 

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