You recently took a job at a plant with high rates of equipment failure. Your new boss said solving this is a high priority. Being the ace troubleshooter that you are, you head out to the production floor to look at the equipment and talk with the operators.

On some enclosures, you notice odd rainbow-colored splotches and "hatchet marks" in some places. You ask an operator about this, and she replies it doesn't bother her. "What bothers me," she says, "is I get a shock every time I use the drinking fountain."

What's the problem?

The operator's comment explains the funny coloring and "hatchet marks." Those marks are grooves where metal vaporized due to flashover between the equipment and something nearby. The colors are due to heating, from circulating currents in thin metal, and/or by the heat of flashover.

Flashover is a consequence of differences of potential between metallic objects. Those differences can exist only where Art. 250, Part V bonding requirements are violated. If connected to your equipment grounding (bonding) conductor (see the Art. 100 definition), this bonding creates a "drain" system for undesired current.

Fix these code violations and equipment failure rates will drop dramatically.