Using thermal imaging to determine both the cause and severity of a hot spot is not always easy. Just because you see a problem does not necessarily mean you have located the heat source. In most all cases, thermographers are only detecting heating that has migrated to an area where they have direct line of sight.
In this infrared tip, read about one particular situation where the temperature rise seen by the thermographer on the outside surface of a bus duct was hundreds of degrees cooler than what was actually occurring inside on the bus at the point of failure. The difference is stunning and stands as a stark reminder of why we should never dismiss small temperature changes on components with large thermal gradients.