Hot spot detection—what to look for

Thermal imaging

Capturing accurate temperature measurements of electrical equipment with a thermal camera can be complex until you know exactly what you’re looking for. One of the things to understand is that because electrical components are mostly bare metal, emissivity is low, and thus temperature measurements can be unreliable.

Emissivity (ϵ) is the ratio of how well a material radiates infrared energy, compared to a perfect radiator. Emissivity values fall between 0.0 and 1.0. An object that measures 1.0 is considered a perfect radiator and is called a "black body".

In the real world, there are no perfect radiators, and materials vary in how much less than perfect they are. This is one complication (among several others) that makes it difficult to use infrared technology to conduct quantitative inspections that require obtaining accurate temperature measurements. For this reason, many thermographers choose to conduct qualitative inspections where they focus on apparent temperature difference between comparable equipment under comparable loads, or the same equipment under comparable loads over time.

A simple illustration of this is this image of a hand with a ring on it. You can see a difference in the thermal image. The ring appears to be much colder than the hand, yet the ring is a similar temperature to the hand. Therefore, although the two objects are at the same temperature, they are radiating different amounts of infrared energy.

Despite this, electrical anomalies can be relatively easy to detect if you know what you’re looking for. The simple fact is that heat is a byproduct of normal operation. Electrical circuits with current flowing through them generate heat. So, when you inspect an electrical component it’s often hot. The important thing to determine is what kind of hot is it? Is it due to normal heating or abnormal overheating?

It’s the thermal pattern that’s important in detecting electrical system anomalies. The lion’s share of abnormal heating in electrical system components is caused by abnormal electrical resistance on a contact surface.

This increased resistance could come from:

• Phase on phase short
• Winding to winding resistance imbalance
• Insulation breakdown

Notice the pattern. The area of the highest thermal energy is at the connection point, and the circuit gets cooler the further away from the contact point. This thermal signature is most often associated with increased surface resistance at the contact surface. The greatest amount of heat is generated at the point of resistance and then it conducts away from its point or origin, resulting in the telltale “trailing away” pattern.

Understanding emissivity in thermal images

Emissivity varies by surface condition, viewing angle, temperature and spectral wavelength. Most nonmetallic materials are efficient radiators of energy. Human skin is close to a perfect radiator with an emissivity of 0.98. A polished copper surface is at the other end of the spectrum with a value of 0.01.

Most infrared cameras have the ability to change the emissivity setting, so if you know the emissivity value of the material you’re inspecting, you can make an adjustment in the camera to get closer to the actual surface temperature. However, if the emissivity of the material is less than 0.60, you should not expect to be able to obtain an accurate temperature reading using infrared, and even if it is higher than that, other factors may affect your temperature reading.