Bench Examinations of a Fire-Damaged Well Site Control Panel and Two Circuit Breakers (2018 and 2019)

Some questions are answered on a bench instead of at a scene. These two examinations, from 2018 and 2019, show how that works. Client details are confidential.

A control panel that burned the day after start up

In 2018 a supplier of oilfield control panels asked us why one of its panels caught fire at a well site in North Dakota. The panel had been installed and commissioned one evening with normal readings, and the next morning field staff found it dead, with the door open and fire damage inside that had already gone out. It was the first panel built for this supplier by a new panel shop. More than one hundred earlier panels built to the same drawings by another shop had worked without trouble.

We were brought in after the site work was finished, and no photographs of the site were available to us. We examined the panel in a warehouse after it had been shipped to Texas. The fire damage was confined to the inside, in three areas. Two were cell ruptures in the second and third capacitors at the top of the capacitor bank, with no spread to nearby parts, and the diode bridge tested normal. The third area showed longer fire exposure, with damage decreasing away from the bottom of the AC surge protector. Its 30 A breaker was tripped. The ground terminal of the surge protector was heavily oxidized and had sagged, while its three metal oxide varistors tested normal, with no swelling. The only arcing in the panel was between two 480 V conductors where the fire had burned through their insulation, which explained two blown fuses in the disconnect outside the panel.

A surge protector is built to pass brief surge currents to ground, not to carry a fault current for an extended time. The damage at its ground terminal showed it had carried current before it overheated, at a level too low to trip its 30 A breaker, most likely because the equipment ground path back to the transformer had a higher resistance than normal. Capacitor cells rupturing at two separate spots, away from the fire, also fit an overvoltage, and a transformer in another enclosure on site had reportedly been damaged as well. Weather records showed no lightning at the time, and we found no recalls on the surge protector. Our opinion was that an overvoltage on the AC supply was the probable cause. What caused that overvoltage could only be answered by a study of the electrical system at the site.

Two breakers on the test bench

In 2019 an electrical distributor sent us two 100 A molded case circuit breakers for trip time tests. The first carried 80 A without any sign of trouble and, at 100 A, tripped in 82 seconds, a normal result. The second tripped in 33 seconds at only 80 A, and before it did, it made audible arcing sounds, gave off smoke from inside the case and ran hot. For safety we did not test it at 100 A. We marked one of its terminals with tamper evident tape so its condition could be tracked, and kept both breakers available to be returned to the client.

Why the bench matters

A part on a bench can be measured, opened and photographed one step at a time, in a sequence anyone can follow later. We record the condition of each item before anything about it changes. Our engineering lab supports this kind of work, including non-destructive X-ray imaging when a part should be seen inside before it is opened.