Quality Assurance Engineering

Environmental Test Chamber Explosion

By J. Bart Henthorn

J. Bart Henthorn and his team were responsible for creating an early-design computer-controlled Traffic Light Intersection System in 1977. The entire system was a giant shed-like metal structure that was loaded up with wiring racks, power supplies and one mid-sized box that contained all the real brains.

Environmental Test Chamber Blew Up The assembly department was responsible for the brains. They would solder the parts to the main PCB then wire everything up into a "Finished" unit. It was handed off to Quality Assurance (QA) for the final burn-in and test.

The QA department was located along the parking lot side of the building with two large roll-up dock type doors. The Environmental Test Chamber was the centerpiece of their building and certainly their pride and joy. Its combination of heaters, coolers, water-misters, dehumidifiers and a sealed door made it perfect for testing each and every unit under the expected range of outside conditions. Although typical build runs were between 5-10 units this one was big enough to hold 40 at once.

As a last minute effort to get a rather large order shipped, the QA department grabbed up every unit sitting near the "Ready for Test" rack to get them ready for an accelerated test.

A bunch of things went wrong. Those old computers in the test chamber used a lot of power. Common filter capacitors were 2" to 4" in diameter and were often used in pairs. Because of their large size, they also had a safety plug designed to allow internal pressure to escape rather than build up (except when you incorrectly attach the wiring lugs to the capacitors so they trap the plug in place). The transformers used the wrong color wires which created some unexpected results turning your basic step-down transformer into a rather nasty snarling step-UP transformer. The elevated voltage then ran through a diode bridge and directly onto two giant electrolytic caps with their safety plugs blocked.

The lead QA Engineer finished plugging the last unit into the AC mains inside the ETC and left the chamber. He sealed the door and then threw the main breaker to bring all the outlets (and devices) alive. How long does it take to walk to the front office and tell management that "they're in test right now and we WILL ship today"? About the same time as it takes a seriously overloaded cap to reach critical internal pressure. Let's just say it took him less time to make the return trip!

All I remember is seeing a cloud of dust BLAST right through the doorway into our lab. Most of the ETC survived however the pressure-sealed door was a little off-kilter. The observation window facing our lab entrance was laying in pieces scattered in a rough trail to the doorway. And the rest of the QA Team? Carefully pondering other vocations.

We didn't ship that day.
J. Bart Henthorn lives in central North Carolina. He has been married for 36 years and has three children. Most of his professional career was spent straddling the gap between mechanical and computing systems. Somewhere along the path from wide-eyed discovery to "it'll be ready Friday", he came across the Jameco catalog. Bart really found his true calling flying around the Americas, teaching medical professionals how to best use their new high-tech tools.