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Infrared (Thermography) and Ultraviolet (UVigraphy) inspection technologies are being used to assess the condition of operating electrical components and installations. Thermography detects temperature changes and is proportional to the square of the current drawn. Ultraviolet UVigraphy, on the other hand, detects existing electrical discharges and is directly proportional to voltage. Both technologies are complementary, hence in the absence of current no heat develops, regardless of voltage. Yet, voltage is prerequisite for partial discharge, corona and arcing, regardless of current. 

Both UV and IR inspections are classified as NDT – Non Destructive Testing technologies and as Remote Sensing Visual technologies. Both technologies must have a line of sight to the inspected object in order to observe temperature or UV radiation.  Ultraviolet cameras are looking at shorter wave lengths, while IR cameras are looking at longer wave lengths. The combination of both inspection technologies is beneficial as it covers a wider spectral range and provides more details.  As such, both technologies are required for accomplishing  a comprehensive surveys.

Left: Example of corona activity not detectable with infrared imager (white circle on thermogram is location of corona). Large white spot in background is the moon. (courtesy of Brady Infrared)

Right: Corona/dry band arcing detected on ceramic insulators (courtesy of Brady Infrared)

IR detection has very limited ability to detect corona, since corona emits very little heat. Experience shows that only advanced stages of corona, which are in fact arcing, can sometimes become visible to IR. For IR to see corona it might be possible to carry out inspections at night, when the components cool down, and even then the temperature difference is slight and hard to detect.  

IR and UV technologies cannot be used under the same conditions, neither ambient nor electrical. IR inspection relies on current, the higher the current (lower voltage) the more valid is the test, UV inspection,on the other end, relies on voltage. The higher the voltage (lower current) the higher is the UV discharge.  

No discernible heating of strands or conductor vs. corona detected and displayed clearly on of broken strands (courtesy of EPRI)

IR Image – no hot spots on the loose connection between the bus bar and the bushing vs. Corona that is easily displayed on the loose connection between the bus bar and the bushing (courtesy of Brady Infrared)

Detects ΔT – Hot Spots Not Corona!
Detects Corona
Current Dependent Heavy Line Loading is needed
Voltage dependent (e-field) No line loading needed
Solar radiation and hot environment can mislead
All lighting & weather conditions
Detection occurs usually at progressive stage of degradation
Detection at Earlier of Degradation (i.e. Commissioning)

Infrared (IR) detection technology, while ideal in identifying hot spots in electrical power systems, has limited value when it comes to detecting corona activity. Experience shows that only advanced stages of corona can sometimes become visible using an infrared camera.
 – James Brady

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