When inspecting electrical equipment with IR, there are obviously many connection points where high resistance can develop, and abnormal heating presents itself. We’ve all seen the pattern associated with increased electrical resistance; the localized heating and cooling off moving down the conductor or circuit part, but new thermographers may be missing some of the harder to spot areas where this failure mode is likely to occur.
Just as with other types of electrical connections, high resistance can develop in the crimped-on connections where cabling is terminated. There can be several various causes of increased resistance at crimped connections, including broken/missing conductor strands, insulation not trimmed correctly and ending up between the termination device and the conductor, or improper crimping technique. In this piece, we’re discussing what can happen in a properly terminated crimped connection over time.
This is more common when wires and lugs are terminated that are from different materials. This practice should be avoided whenever possible, and when not possible an anti-oxidation additive should be applied. Of course, there are plenty of times where this isn’t done. The usual combination is where copper conductors are terminated in aluminum lugs. When terminating dissimilar lugs and conductors, two scenarios are possible. As they are different materials, it makes sense that aluminum and copper have different physical properties. Among them is their respective rates of thermal expansion and contraction. As load varies on the connection, the level of heat varies. As the heat varies, the conductor and the lug will expand and contract, but at different rates. The result is physical looseness that results in increases electrical resistance.
The other scenario is what’s known as dissimilar or galvanic corrosion. This is an electro-chemical process wherein there is a transfer of metal from one side of the connection to the other. This process is hastened if there is a high level of ambient moisture. The result is an increase in electrical resistance, which again is not what we’re hoping for in a connection.
When inspecting electrical apparatus, high resistance connections are often found at the connection points between components, such as a wire to lug junction or where a terminating lug is bolted to a bus or to a distribution device. It’s a bit harder to detect anomalies where the crimped connection is the point of high resistance. Narrowing the Span setting in the imager and moving the level slowly up and down will help pinpoint the areas of highest energy. When this type of problem presents itself, the suggested corrective action should be to inspect the hardware used and ensure that the terminations are re-made properly, using an antioxidant treatment.