By Scott Black
Thermography has always been the exclusive domain of the professional thermographer. Cameras were hard to use, fragile and so prohibitively expensive that their purchase could not be justified for occasional usage. When it came to thermography use in plant maintenance applications, a staff or a contract thermographer would conduct thermal surveys and produce reports with their findings. Experts at deciphering the hidden meanings behind the images, their findings would be passed on to the appropriately qualified party for action. Once the problem was fixed, the thermographer would be called upon to perform another check to ensure that the repair was properly executed and the true cause of the red flag.
Thermal Cameras have now become much more affordable, easier to use and rugged enough for everyday use. They are now finding their way into the hands of a new breed of user – one less specialized and less trained in the art and science of thermography. Electrical, HVAC/R and plant maintenance professionals alike have been “dying to get their hands on one of these thermal cameras” to add it to their problem solving arsenal. With the mounting pressures they all face to keep things running, having the right tool in their kit can easily save tens of thousands of dollars in unscheduled shutdowns and loss of productivity. Having a thermal camera in their kit will give them control over the diagnosing element. It will help them zero in quicker on the problems and allow them to instantly make use of their current clamps, multimeters, and 3 phase analyzers to confirm, troubleshoot and ultimately fix the problem or potential problem.
Infrared thermography has been proven to be an awesome tool for fast, comprehensive, nonintrusive and safe examination of thermal patterns. Heat is known to be an excellent indicator on the operating condition of electrical components and the thermal camera shows the heat patterns. Thermograms do provide a lot of information. But to fully benefit from the use of this new tool, the user will need to get familiar with some of the basic principles that govern the taking of thermal images.
Concepts such as emissivity, reflectivity, apparent and emitted temperature, heat transfer, distance to target and target size are the building blocks of thermal imaging and will, in one way or another, affect all your readings. A general understanding of these will be necessary for all new operators.
The latest generation of thermal cameras has been designed to be very easy to operate and will display images right out of the box. Nevertheless, do take the time to read the manual very carefully. It is a must to be able to produce good and meaningful images. If it comes with free training, take full advantage of the offer. It was produced with the assistance of professional thermographers and they will be sharing their invaluable experience with you.
With a thermal camera, misdiagnosing can happen easily and be costly. Read a temperature higher than it really is and expensive parts get replaced for no reason. Err on the low side and a problem gets overlooked that ultimately causes an unplanned shutdown or worse.
Once the process is mastered and the readings are accurate, the user will then face an important question: At what temperature is it a problem?
There are a number of factors and considerations to determine at what temperatures measured on electrical circuits would constitute a problem. NETA severity guidelines are an excellent starting point for determining whether there is a problem or not.
The International Electrical Testing Association provides guidelines, shown to the right, that aid in determining the degree of severity of an electrical problem due to the “apparent” temperature measured with an infrared camera on an electrical circuit. These temperature band areas were developed to quantitatively assess thermal anomalies in electrical equipment. It is important to note that these temperature assessments are for “direct” temperature measurements only.
NETA Infrared Temperature Considerations:
For example, if the apparent temperature measured on an electrical circuit is 35°C O/A (over ambient) and the temperature compared to an adjacent, similar current-carrying component is 27°C O/S (over similar), then by using the table to the right, this thermal anomaly would be considered a “Deficiency” and should be corrected as soon as possible or as time permits.
Possible Deficiency – Further investigation required
Probable Deficiency – Repair at next available shut down
Deficiency – Repair as soon as possible
Major Deficiency - Repair Immediately
Major Deficiencies, whose detected temperatures are greater than 40°C O/A, typically have temperatures compared to similar current-carrying components greater than 15°C and should be repaired immediately. In situations where there are no similar current-carrying components to which to compare, simply use the O/A reading. There are other severity criteria listing cable sizes with associated maximum current and temperature rise. Transformer types and their size relating to their maximum allowable temperature tables are also available. Coils, disconnects, fuse disconnects, over-current devices, connectors and bus ducts have decision-made severity criteria guidelines associated with them. These vary with manufacturer and industry. It can, at times, be very confusing as to what severity selection criteria to utilize.
Often these “standards” were derived from hazardous instances that formulated the temperature decision segments of these tables. These tables should be utilized not as hard-and-fast rules, but rather as guidelines to acceptable temperature rise decision severity criteria.
It is important to also consider the importance of the electrical circuit on which a thermal anomaly is detected. Hospitals, for example, have life support systems that must be maintained with ZERO tolerance for interrupted power. Thermal anomalies detected here are very critical in nature. Office buildings with computer centers, government installations, airport communications facilities and other buildings have important areas as well.
The bottom line: Any unexpected temperature rise in any controlling electrical circuits is of concern. It represents a loss of electrical power that should be delivered to the circuit load. The severity criteria decision for your industry should be evaluated as to the best overall benefit to your application.
Other category descriptors pertinent to your application and plant preventative maintenance procedures can be substituted in the table provided. It is important to have a quantifying procedure as part of your preventative maintenance schedule at your facility.
Know What You Don’t Know
There are times when knowing how to operate your equipment and applying what you have learned on IR theory will not be enough to let you find the problem. Just like the sound level meter operator who at times turns to an acoustical engineer to assist him in reducing noise levels, and just like the x-ray technician relies on the radiologist to read the x-rays, there are times when you will have to call upon a professional thermographer for assistance. Asking a professional for assistance is never a sign of weakness. It is simply making the best use of all available resources to ensure that things keep on running smoothly.
Thermal Imaging cameras are powerful diagnosing tools and all can benefit from their use, as long as one knows what to look for and what they are looking at.
About the Author:
Scott Black is a Product Manager at Extech Instruments, A FLIR Company.