Blowing Fuses & Exploding Circuit Panels

I once worked as an engineer in a small firm that fabricated glass capillary tubing. The process involved graphite furnaces mounted on towers. Silica glass tubing preforms were inserted into the top of a furnace, and thin glass tubing was drawn out the bottom.

One day, one of the 30-amp fuses supplying power to the furnace driver circuit opened-circuited. The driver circuit used phase-controlled SCRs (silicon-controlled rectifiers), and so it was natural to first check them for shorts.

No problem there -- all were fine. Next, we checked the furnace. Nope, no shorts there, either. Reasoning that it was just a power surge, I replaced the fuse and stood by as the unit was brought back up to temperature. Current monitors showed everything to be well within range, so I assumed it was simply a power surge from the outside lines, and I returned to my office.

About two hours later, the fuse again opened-circuited. Returning to the production area, I performed the same checks again, this time with considerably more rigor. The blown fuse was one in a three-phase circuit. It was hot. The other two were only warm. This contraindicated a power surge.

Replacing and restarting now was out of the question. I checked, rechecked, and checked again. Finally, it dawned on me that a loose connection inside the fuse box might be generating the excess heat. That excess heat could flow into the fuse through the copper connections. We shut off the main power to the fuse box and removed the cover. Indeed, one of the wires was a bit loose. Tightening it solved the problem.

Years later, when working as a research physicist (not an engineer) for a different employer, an electrical explosion and fire in a main panel took the life of an electrical worker. The story was very similar, except that it involved circuit breakers rather than fuses. Again, a breaker kept opening, even though the amps were within spec. Repeated resetting of the breaker eventually led to the explosion. Later, with power off in my labs, I examined our internal control panels and found numerous loose wires, which I tightened to spec. The importance of checking all such connections on a regular basis cannot be overstated.

This entry was submitted by Don Schmadel and edited by Rob Spiegel.

Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.

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