When inspecting electrical equipment with IR, there are obviously many connection points where high resistance can develop, resulting in abnormal heating. We have all seen the pattern associated with increased electrical resistance. The localized heating and cooling off moving down the conductor away from the termination or component, 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 conductors are terminated. There can be 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. These are examples of improperly made crimp connections. In this piece, we are discussing what can happen in a properly terminated crimped connection over time.

This is more common when wires and lugs are terminated, that are made from dissimilar 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 when this is not 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 dissimilar materials, it makes sense that aluminum and copper have different physical properties. Among them are 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 a physical looseness that results in increased electrical resistance.

The other scenario is what is known as bimetallic or galvanic corrosion. This is an electrochemical process wherein there is a transfer of metal from one side of the connection to the other. Breaking down the surfaces, resulting in accelerated corrosion (which is the primary concern) 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 are hoping for in a connection.

When inspecting electrical apparatus, high resistance connections are typically 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. Due to the lower emissivity of the materials, it can be 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.