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.
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.
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.
In my 50th year as a volunteer fireman, I wholeheartedly agree. Mhy story was my daughters 1st birthday party, also the 1st anniversary of our new house. There was a pop and the the stove went dead with the roast uncooked. I checked the breaker and it hadn't tripped. I turned off the breaker and pulled the stove out, removed the terminal cover since it was hardwired and turned the breaker back on. One side was open at the stove. Turned the breaker back off and pulled the panel cover. Lo and behold the screws on the wire were never tightened. Properly tightened and everything back together in a half hoiur, the roast was cooked.
I agree with the comment you made regarding the essential need for regularly scheduled maintenence of electrical distribution and utilization systems in facilities including infrared scans of all terminations.
To expand upon your recommendation, this work must be done when the electrical distribution system equipment (including but not limited to switchgear, switchboards, transformers, distribution panels, motor control centers, equipment or process control system cabinets or enclosures) have "dead front" (defined as intentionally grounded and not intended to conduct electricity under normal operating and energized conditions) covers have been removed as you indicated.
Removal of the dead front covers while any equipment is energized is an inherently dangerous and possibly lethal activity for the person(s) engaged in the operation.
Work is being done based on IEEE Standards which are being continuously refined, in the industry and and is addressed by OSHA to provide means to determine by calculation of the Arc-flash hazard for new and existing electrical distribution and utilizations systems in new and installations. The Arc-flash hazard classes address the different ranges energy that may exist due to the occurrance of an arc-flash at a specific location in an electrical distribution or utilization system. Arc-flash energy above Class 4 has no equivalent PPE because anyone within normal working distances from the arc-flash source cannot be protected by apparel. The blast pressure generated by the arc-flash having an energy greater than the upper limit specied for PPE Class 4 would probably be lethal.
The point I want to make is that whenever energized electrical equipment is opened, a serious hazard exists. There are four classes of Personal Protective Equipment (PPE Class 1-4) that have been defined for personnel to wear whenever they are expsed to a specific energy hazard level. Apparel specific to meet the requirements of each class of protection is available o allow workers to be in specified distances from energized equipment.
I repectfully urge anyone who is involved with the operation, maintenance, construction or modification of electrical distribution or utilization systems and equipment to determine the hazard classes for each electrical system with which you are involved before any proceeding with any operation involving exposed electrical terminations or equipment, especially those activities involving determining whether the equipment is energized or not, and making voltage, current or other electrical state (power, power factor, power quality, etc.) masurments.
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.
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.
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.
There is now IR-friendly glass that can be added to any cabinet to create a viewport so that the internal circuitry can be imaged by the thermal camera without opening the cabinet.
This is especially handy on Motor Control Centers that employ and interlock on the door. However, because of the increased engineering req'd and subsequent addt'l BoM cost, they are not common.
The thermal system explained by lynnbr2 is a good approach. There are inexpensive point and click systems that use a laser beam to determine the emissivity at a particular wavelength, and then a thermal sensor to measure the emission near that wavelength. These hand held systems are sold for consumers for use in kitchens for measuring the temperature of a pot, or a roast, or a pot roast... But when using a thermal approach one must have the circuit under significant power.
I cant' be of much help as I've never used any of these systems to discover overheated connections. The systems, like those provided by Omega, boast an accracy of about 1 degree C. This should be fine for line-of-sight instances. But for connections inside panels something like the glass covers mentioned by lynnbr2 might work.
Incidentally, the link to ElectroPhysics didn't mention anything about glass covers.
Great idea @lynnbr2. Anything that makes safety reviews and maintenance easier increases the likelihood that it will be performed. I like the idea of IR friendly glass which not only makes the checks easier, in many cases it means the checks do not require a process shutdown.
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!
I had a similar problem with our test station when I was in the Air Force. We were blowing the B phase fuse about once per day. First 45A fuses and then 60A ones. The Clamp-On Ammeter that we were only drawing 17A on each phase. It turned out water had gotten into the box and corroded the contact under the fuse clip. The fuse wasn't blowing, it was melting. After 3 more visits from Civil Engineering, and explaining Ohms Law, they replaced the box.
Hey, Mblazer, you may want to write up your story and send it along for inclusion in the Sherlock Ohms blog. We need at least 350 words. Simply explain the problem you faced and show how you solved it.
I am sorry if this comes of as a smart alec, but did you slap the guy who replaced blown 45A fuses with 60A? There are reasons for the sizes of fuses in any given circuitry and to just ignore that is a recipe for big trouble.
I had a similar experience while I was maintaining a large VAX / PDP-11 installation. Every Tuesday morning we would come in and one of the PDP-11/70s would be down. I puzzled over this for weeks, left a program running overnight that just wrote the time to one of the terminals every 5 seconds, and found that it was strikingly regularly crashing at 6:30AM plus or minus a few minutes. The next Tuesday I came in early and was sitting in the computer room at 6:30 - I heard a crash from outside and the front panel lights froze. I ran outside to see the garbage truck pulling away. The dumpster was about 10 feet from the main power box feeding the building. I opened up the box and could see signs of heat on one leg of the 3 phase power. The screw lug holding the wire was loose, and when the garbage truck would drop the dumpster, there was enough vibration to cause just enough of a power glitch to crash the one computer that we had plugged into that leg. Tightened the screw and no more crashes.
At one job, as the electrical engineer, I was asked to check into why one of the hydraulic pumps was usually "overloading". At this company it was standard procedure to use an ammeter clamped on one leg of a pump motor to set the relief valve, or adjust the pump compensator. This pump was one of two pumps arranged to assure that the big pumps always had a positive pressure at their inlets. They figured that the pump with the problem must have an excessive load, but the ammeter reading indicated less current on one phase.
The solution was to tighten the connection terminal screws in the motor starter. One loose screw was causing much more heat to be produced in the connection, causing the thermal overload device to shut down. Raising the trip current setting had just delayed the shutoff time. So with all connections tightened, I reset the overload trip point to the recommended value. The problem was solved and my reputation was enhanced.
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