I worked for consulting engineers who analyzed accidents from fires, floods, explosions, and auto accidents. Suspect culprit devices ended up in our lab, where we carried out forensic engineering on what was left in order to determine the root cause of what happened.
A large refrigerator had caught fire and created $250,000 of smoke damage to artwork. The insurance company wanted to know the cause before settling the claim. The senior electrical engineer said that we'd inherited a bit of a shambles, since there were bits missing and other things that didn't seem to belong (holding up two wires with burned off insulation and partially melted terminals).
At the time, the terminals were vaguely familiar to me. We read up on manufacturer's manuals to familiarize ourselves with the devices to figure out cause and effect of the damage. It suddenly struck me where I'd seen the melted terminal parts. They were parts of a microswitch. Back in Scotland, I worked for Honeywell in its microswitch and meter division troubleshooting production, and among the detail parts were terminals for microswitches. I add this because "connections" plays a significant part in troubleshooting.
I went along to the senior electrical engineer's office, with the burnt out wires, and told him the melted ends were microswitch terminals. We promptly went out to the lab, rummaged around in the other door recesses, and out of the ashes found the other microswitch. Now the events began to fit together.
A nurse had opened the lefthand refrigerator door to get baby formula. Later, she smelled smoke, saw black smoke pouring up the stairwell across the artwork, and called the fire brigade. Our report suggested that the microswitch internals collapsed when the nurse closed the door, short circuiting the terminals; the insulation started to smolder and, although it was fireproof, created enough smoke and soot to do the damage.
The short-circuited microswitch didn't draw enough current to trigger the circuit breaker, and the continuing smoldering condition did all the damage.
A ground fault indicator (GFI) somewhere in the circuitry might safeguard against a recurrence.
Tell us your experience in solving a knotty engineering problem. Send stories to Jennifer Campbell for Sherlock Ohms.