Electrical Troubleshooting Quiz, July 12, 2011

Maintenance is buying too many of a particular circuit board as a replacement part. During your initial fact-finding, you discover the following:

• The four main production lines each use this board in six places.
• About 90% of the failures occur in the same two particular places in each line.
• At each cabinet using this board, the 120V supply power waveform is severely distorted.
• At one line, you measure 2,917 ft of wire run to its supply panel.
• Each cabinet is grounded per the manufacturer's installation manual.

Armed with just these facts, what should you recommend?

That is a long run, making excess voltage drop an obvious issue. After verification with an AC resistance tester, you should recommend:

1. Moving the supply transformer/panel closer to the load.
2. As part of this move, upsize the conductors from the NEC minimums.

This should fix the voltage distortion problem. But if not, this needs to be done anyhow. Perform waveform analysis at each affected cabinet, before and after.

Are the production cabinets actually "grounded" (per NEC Art. 100 definition)? These must be bonded, not grounded. Grounding here allows undesired currents to circulate.

Next, do a survey to identify high harmonic sources for mitigation and/or isolation from these cabinets.