Switches are pretty widely utilized in electrical applications. They can go by any number of names, depending on the particular application. In utility applications we hear terms like “air breaker” or “cutouts,” and in industrial applications you might hear “disconnect” or “safety switch.” Here’s the real deal however; if it’s a mechanical device that operates for the purposes of interrupting the flow of electrical current, it is by definition a “switch.” All switches have elements in common, despite how they might appear.
The anatomy of a switch is simple, generally speaking. There will be contacts, both stationary and movable. The movable contacts are the parts that pivot away and break the mechanical connection with the stationary contact, which as is implied by their name do not move. On the movable contacts there is a connection point where the contacts can pivot. That’s pretty much it – two contact surfaces that interface mechanically and the pivot point that allows the movement.
Just in its normal operation a switch is subject to thermal changes. As the load varies, current through the switch will vary, and with this variance in current there is a variance in the temperature of the switch. This thermal variance causes expansion and contraction, and over time this normal cycle can increase the electrical resistance through the device. It is for this reason that switch inspections can be so valuable. Their normal operation eventually lends itself to thermal failure, even in optimal operating environments.
The tough part though is that switches are necessarily made from metal and left bare, since finishing the surface would detract from the relatively low electrical resistance of the switch material. Bare metal, as we learn in The Snell Group’s Level I Thermographic Applications course, is highly reflective, and thus not capable of giving us reliable temperature measurements. Since switch inspections are so important, but tough to inspect, what is a conscientious thermographer to do? Well, in a word; inspect.
Despite the switch surface not lending itself to reliable temperature measurement, a thermal pattern can still be discovered related to the increased electrical resistance of the connection points. In the images to the left, patterns are plainly visible even though radiometric measurement of the surface temperatures would be unreliable due to the low emissivity of the surfaces in question. This is what is known as “qualitative analysis”. To grossly over simplify, think of Sesame Street. Remember the segment where they would split the screen into four sections, and in three of the sections there might be a kid bouncing a ball, and then in the fourth section a kid was jumping rope? There was a song that went along with it…”One of these things is not like the other …” This is the essence of qualitative analysis; discovering discontinuities. In the images to the left, the discontinuities are clearly visible, and temperature data is of lesser concern.
So, when it comes to switches, worry less about the ability to derive a temperature value, and look for what seems out of place. Switches are an excellent opportunity to find anomalies, so inspect them carefully, and try to choose which thing is not like the others.