In training across the United States, and even abroad, I have found that this failure to follow manufacturer’s torque specifications has proven to be the source of equipment failures, industrial arc flash accidents, and house fires.
As a firefighter, I was introduced to this fact by the Indiana State Fire Marshall as I assisted him on an investigation of a mobile home fire. A contractor had been updating the main feed wiring with new pedestal mounts and feeders to 100 mobile homes in a large mobile home park. The source of this fire turned out to be a recently-installed 200 amp panel board with its aluminium service wires seriously under-torqued. The required torque spec was 20 foot-pounds (250 inch-pounds), and the connections were only tightened to around 12 foot-pounds (144 inch-pounds).
I was a young firefighter and licensed master electrician, and I was fascinated with the array of investigative tools the marshal had at his disposal to determine causes of electrical failures. The methods to discover causes of fire due to electrical failure range from colorimetric analysis to something as simple as back-checking of torques. In this particular case, the panel main conductor lugs were back-torqued and found to be only half of the required torque value. When copper wire is used, colorimetric analysis is often able to determine an exact cause, but even with aluminum, things like electrical spatter pattern(s), melting conductors versus insulation burn-back, and a host of other clues, can nail down an exact cause of the electrical failure.
In this case, the neutral and one of the hot leads from the outside supply disconnect switch showed obvious signs of arcing, with some spatter around what was left of the conductors, and subsequent post-fire melting of the remaining conductors, as shown by the aluminum that had run down the left hand edge of the cabinet. The neutral was completely missing about 5 inches of its 4/0 conductor.
The contractor was subsequently cited for the fire, and his insurance company was required to pay a large sum for the complete loss of the mobile home and for the pain and suffering of the family. In addition, his electrical license was pulled. I’m sure this was not what he expected to have happen when he accepted the original contract.
The NEC (2017) requires that “listed and labeled equipment shall be installed and used in accordance with any instructions included in the listing and labeling” in 110.3(B). A new addition in 2017 goes further in requiring “where a tightening torque is indicated as a numeric value on equipment or in installation instructions provided by the manufacturer, a calibrated torque tool shall be used to achieve the indicated torque value, unless the equipment manufacturer has provided installation instructions for an alternative method of achieving the required torque.” (NEC 2017 110.14(D))
The International Association of Electrical Inspectors published an article in July-August 2010 and in January-February 2015 stating that their research indicated that only 25% of connections performed without a torque wrench were within +/- 20% of the manufacturer’s recommended torque value. This statistic is confirmed over and over in my classes when I ask the class, “How many of you have AND use a torque wrench or a torque screwdriver to make up your electrical connections?” The average response in my unscientific survey is 5% or less – and unfortunately, usually closer to 1%.
Of course, I then ask the students how many of them actually READ installation instructions, and their answers point to the lack of torquing. Almost no electrician or engineer actually opens the paperwork and reads the instructions, and even those who do will confess that they did not FOLLOW the instructions because of lack of proper tools (calibrated torque wrench or torque screwdriver).
The going joke is, “If tight is good, then tighter is better. Tighten it until you hear it start creaking!” This usually results in either a stripped-out thread on the tightening lug, or a damaged (over-torqued) conductor, which subsequently can lead to failure. These failures, unfortunately, can be catastrophic in nature, especially when the improperly-torqued wire is a ground wire feeding a swimming pool, which could easily result in a shock or electrocution. We simply cannot afford these types of errors in our industry.
Please take the time to read the manufacturer’s instructions, understand the various requirements, and of course, follow-through. Torque specs are often listed on the manufacturer’s literature, on the actual lug or terminal, or even on the front or side of the circuit breaker.
Here are a few examples: