Infrared thermography has been used to inspect electrical systems since the 1960s. Even with all the limitations of the technology (i.e. issues with low emissivity, large thermal gradients, and resolution just to name a few), it is still one of the primary testing tools for electrical applications and is now routinely used to successfully inspect electrical systems. Still, environmental variables and equipment limitations can make inspections challenging, especially when it comes to outdoor components. We will examine a few of those issues in this tip.
Thanks to the growing awareness of NFPA-70E, there are more professionals who are now wearing hardhat/face shield combos while performing thermography. This initially presented a problem as many infrared cameras, up until a few years ago, had only an eyepiece for viewing. Anyone who has ever tried to conduct an inspection with an eyepiece and a face shield in place knows that this approach really does not work well, if at all. Changes to camera design have helped in recent years and today most models have an LCD screen.
While this was a nice fix to the face shield issue, it created a new set of problems for anyone trying to view the LCD screen outside in direct sunshine or even on cloudy days. Luckily for us, some models are bringing back the eyepiece in conjunction with an LCD in an attempt to address both issues. Additionally, the LCDs are improving all the time so they are providing better viewing in bright conditions, and those with just an LCD display now have access to sun visors and hoods provided by the manufacturers that they can affix to their thermal imager to help reduce glare.
The sun also poses another problem when inspecting outdoors. It’s hot--and so is everything else it shines on in a substation. Darker colors heat up even more as they absorb a greater amount of the sun’s broad spectrum of radiation. As such it is many times difficult at best to perform an infrared inspection during the daytime on outdoor components. Couple that with the low visibility of the LCD screen and you’ve got a good case for scheduling outdoor electrical scans in lower light conditions such as in the evening or early morning. The drawback to this approach, however, is the possibility of encountering lower electrical loads. If that is the case, a cloudy day might be best, but as we all know, a lot of this is dependent on the weather and schedule which does not always work in our favor.
Another hurdle in the path to outdoor electrical scanning excellence is distance to target. If you are a service provider, and access to your customer’s substation equipment is limited, you may find yourself outside of the substation fence trying to image targets at some significant distances. But even if you are a utility thermographer and inside the fence, your infrared equipment may not be suitable for the application. As such, it is important to know the resolution capabilities of your thermal imager when distance is a factor.
When it comes to an infrared camera, there are two types of resolution that the thermographer must understand: spatial and measurement. Spatial resolution (also known as IFOV) is the smallest thermal anomaly your camera can see at a given distance. Measurement resolution (also known as IFOVmeas) --an example of which is included in the two images above--is the smallest target you can measure the temperature of at a given distance. With a 320x240 detector array, it is typically 1/3 of the spatial resolution distance (i.e. you must be 3x closer to the target). As seen here, depending on your distance to the target, the accuracy of the temperature measurement is impacted.
The 1/3 rule for measurement is a good general figure, but in Level II we teach you not only how to calculate what your camera is capable of detecting, but also offer a lab activity where you discover both the spatial and measurement resolutions of your system as we have seen actual performance versus a specification in a datasheet to be quite different.
Emissivity issues are certainly in play when conducting outdoor inspections. Components are usually bare metal, making it difficult to nearly impossible to capture accurate and repeatable temperature measurements. Additionally, accurately reflected background temperatures can be a challenge to measure, making both seeing low delta-T hot spots and measuring accurate temperatures of the hot spot to be challenging. Given this situation, it is therefore often easier to perform a qualitative analysis and show contrasts between the thermal anomaly and similar components rather than trying to measure actual temperatures, while keeping all the limitations in mind.
Of course, even with all of these challenges, great results can be obtained when using infrared thermography inspections. There are many important reasons why we still want to conduct outdoor electrical scans as this can be some of the most critical equipment supporting the continued operation of your facility. Just be sure to keep these issues in mind when you perform them.