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.
I can vouch for several occurrences of "hot" spots in facilities wiring systems. The supply voltage surely doesn't have to be in the kilovolt range either. I've seen dedicated 208/240 (delta / wye) lighting panels explode. The problem with these panels is NOT ONLY a possible loose lug on the busbars, but also excessive harmonic currents due to the inductive ballasts of large quantities of fluourescent fixtures in place. These i²r losses can come back to haunt a good lighting load design.
Personally, I've experienced similar scary moments at home. Since the air handler is situated in the attic crawlspace, separate from the circuit panel, the code requires a disconnect at that location. One day I happened to be up there inspecting things, when I decided to remove the cover from the 2-circuit circuit breaker disconnect. Aluminum cable supplied the load (heat strips for wintertime use). One of the poles of the circuit breaker was blackened due to heat treating because of the oxidation of the aluminum conductor & resultant i²r losses. The insulation within about 2" of the lug was evaporated. It was a lucky thing that the electrician dressed the conductors in such a manner that they were not resting on each other, OR there would have been an "interesting" series of phone calls ....... first to the fire department to put out the roof fire, and then to the insurance company for a loss claim. I was lucky that there was plenty of slack in the supply cable, so I replaced the mini-panel & doused the lugs & conductors with heaping gobs of NO-ALOX, anti-oxidizing compound. Been fine ever since, and ever year I instruct the A/C service person to check the connections in that disconnect!
We used an IR camera the same way on a 3 phase 13.8kv line feeding our military facility. At every insulator on the 1/2 mile run, I had a guy take a monthly photograph. We were working next to the ocean and salt residue would build up on the insulators decreasing the resistance, causing heat, and eventually burn through. Fixing these was a nasty job and we had to post a guard at the power transfer switch. But using the camera, most of the time we would kill the power and simply clean the insulator using a pressure washer. It would seem like insulators are perfect, but a few meg ohm resistance (cable insulation/insulator) could have knocked the site off line for days. Nasty stuff that HV power is. I hate it. Explains why I love TTL so much.
At regular intervals (I think it was yearly) a PM task for Panel Imaging would be created. The contractors would be contacted and would come and take Infrared pictures of all the Electrical panels (including panels containing control circuitry such as motor drives and PLCs).
They would then visually analyze these pictures to locate any "hot spots" These hot spots could be caused by a loose connection that just needed to be tightened or a failing component, etc.
Checking the torque of electrical terminations is a normal part of scheduled electrical maintenance of industrial machinery. I don't know that I would check every low voltage connection, all of the high voltage connections (those that could likely heat up by being loose) should be.
Years ago I worked for a company that contracted a service to image each electrical panel in the plant with a FLIR camera. This process was part of their Preventative Maintenance (PM) program and was designed to identify these types of failures before they interrupted production. FLIR camera prices have fallen over the years making this type of PM much more affordable.
Happens all the time. There have been numerous articles on this site about such situations. I know I have "fixed" plenty of problems where it was just a loose wire. It is tragic that someone was killed by the circuit breaker situation. Repeatedly doing the same thing is inviting a problem. Thankfully, it is not usually so deadly.
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