Who are the electrical inspectors

Inspections of electrical systems are part of everyday life for every electrician. The thermal imaging camera has proven itself as an aid for years. Not to be confused with common night vision devices that amplify residual light. Where electricity flows, it gets warm, especially at contact resistances or defects. The measurement takes place from a safe distance during operation. Defects can be seen months to years in advance. So there is time to repair as planned. With regular IR inspections, negative developments can be identified early with a high degree of certainty and costs can be saved because only really necessary repairs are made. Experience has shown that defects are more common in new and older systems. In between there is a few years of calm. As always, the devil is in the details, that is to say, the handling of the thermal imaging camera. The operation is learned quickly and there are always colorful pictures. Background knowledge about important physical relationships is appropriate. Not everything that is warm has to be defective. Terminals are regularly loose or fuses have to be tightened.

Since IR cameras work far above visible light, there are sometimes amazing effects. "Long waves" with 7 ... 13 µm are currently standard. They are less sensitive to reflections from short-wave radiators (sun) and are therefore ideal for inspections outdoors. The sun is problematic because it leads “IR laypeople” to make mistakes. Important! The camera measures surface radiation. One can easily through smoke, but nowhere to look in or through. Exceptions are foils, which are often used as inexpensive IR windows. Unfortunately, it has so far been impossible to look through plexiglass covers or glass. They are opaque from ~ 2 µm (the manufacturers of IR cameras work on sensors that work in this area). Faults in encapsulated systems, distributions, switches, etc. often appear on the housing or are visible at the outlets. It takes some practice here.


To practice an infrared inspection
A clamp meter should not be missing next to the camera. In the case of high and medium voltage systems, the instruments may have to be consulted. Reliable information can only be obtained when electricity flows. In my experience, there is a maximum of 5 ... 15 minutes left after switching off a system. Actionism is rarely necessary when locating a “hot spot”. After all, the system has usually been running like this for a long time. Contactors, chokes, power packs, resistors, but also cables and terminals with high loads are operationally warm (measure current if necessary). Bare metal surfaces have very low emissivity and, depending on the degree of soiling and wear, strongly fluctuating emissivity and regularly show temperatures that are far too low in the thermal image. Busbars, bare metal terminals and cable lugs are difficult objects. Where they are close together, they shine and look noticeably warm (cavity radiator effect), but are hardly warmer than the bare parts. Bare metal surfaces can look cold in the IR image, but in reality they are critically heated (left image).

In addition, there are reflections of warm, but also cold objects, which can be recognized by the fact that they “wander” with the camera when the position is changed. The busbar in the picture on the left seems to be cold. However, the stickers for the conductor designation show a noticeably high temperature! The rail is at this level in real life, but appears cold in the thermal image due to the low emissivity (~ 0.1ε)! Plastic or painted surfaces such as cable sheaths, coated busbars, stickers, etc. can be measured without any problems. The effects apply to all IR thermometers (hand-held pyrometers) with a laser point / circle, the display of which, following the advertising, likes to blindly trust becomes. In terms of object signal, they are very similar to IR cameras.To thermal imaging
To speed up the inspection, many thermographers use an isotherm in the image, which highlights areas above an adjustable threshold temperature in color. For this purpose, an acoustic alarm can be triggered on some devices. It is advisable to display the IR image in grayscale with a transparent isotherm in order to still recognize the structure underneath (bottom left image). The method is particularly suitable for systems without Plexiglass covers.
Standard for IR (bolometer) cameras are handy, uncooled, long wave cameras with FPA chips (Focal Plane Array, 7-13 µm, 30-50 Hz), with 20,000 - 80,000 image / measuring points (picture below right). Scanners are also available that scan the scene line by line. You need about a second for this, which speaks against freehand recordings. The resolution is 0.015 ... 0.2 ° C, with the lower value delivering significantly better images at low temperatures. The weight is 0.7 to 3 kg, the battery life depending on temperature and monitor usage 2-3 hours. The IR images are stored in the camera or on standard media. Some devices allow the parallel recording of IR image and photo as well as voice commentary for each image. All manufacturers offer report and analysis software with which the images can be edited afterwards. Wide-angle optics are very beneficial for electrical inspections.