It is quite common for thermographers to come across strange or confusing patterns when conducting IR inspections. This is true for most applications for IR, whether you’re inspecting a building, a motor, a roof, or electrical equipment. When inspecting the latter, you may have come across a pattern similar to the one in the images below and wondered, “what’s going on here?”

An amperage reading confirmed that amperage across all fuse groups is even. Generally, one would expect to see a fairly uniform pattern across the fuse groups if there are no differences in amperage. So why is the camera detecting differences in temperature across the fuse groups? Differences in the fuses themselves, whether in all three phases or just one phase, can cause these patterns to appear even when there are no faults present.

First, let’s handle the obvious. Are the fuses correctly sized? All too often an improperly sized fused is placed in a circuit out of convenience. What if a different type of fuse, of the same size, is placed in a circuit?  Different types of fuses can produce strange thermal patterns. Some of these types include:

1. Dual Element Time Delay
2. Slow Blow
3. Instantaneous Fuses
4. Renewable or Repairable Link

The internal designs in the various types of fuses are different, causing each type of fuse to exhibit a different thermal pattern. Another important factor influencing the thermal pattern on the fuse is the quality of the fuse. Is it a high-end fuse or low-end? The lower-end fuses still meet the rating requirements, but higher-end fuses are made with better materials and stricter quality controls. Material and construction factors that influence thermal patterns include:

Body materials: The thickness of the material and conductivity of the material used in the fuse body will affect what we see thermally. If one end of the fuse has a thicker wall than the other end of the fuse, we can look to Fourier’s law of conduction which states; as the thickness of a material increases, the amount of conductive heat transfer decreases.

The centering of the elements in the fuse, or lack thereof, can cause thermal anomalies to appear. In higher-end fuses, the manufacturers take greater care in making sure the elements are centered properly. With many lower-end fuses, this is not the case. If the elements are placed too close to the wall of the fuse body when built, this can cause a “hot spot” to appear on the outside of the fuse.

Filler materials: The amount and type of filler material present in a fuse will affect what we see thermally. Filler materials are used to quench any arcing that may occur inside the device when the fuse “blows” or opens the circuit. There are a number of different materials used as filler and the amount of filler required to safely quench an arc only needs to cover the area in which the arc occurs. Higher-end fuses will fill the entire fuse body with filler material, lower-end fuses may use just enough to cover the link (arc) area, leaving an air space at the top of the fuse (Image 1 above).

The position of the elements or segments in the fuse body will affect what we see thermally. This is usually more apparent in dual-element, time-delay fuses. If the components within the body of the fuse are not centered, this can cause varying thermal patterns on the surface of the fuse (Image 2 above).

Renewable/Repairable links:

The above images are a few of the various renewable/repairable link fuses. These types of fuses present the thermographer with additional “unviewable” or hidden connection points, that can be misinterpreted as fuse holder/spring clip faults, when in fact these can be internal connection faults, if the components are assembled incorrectly or improperly torqued.

As thermographers, if we don’t know what we’re looking at, it makes our job that much more difficult. Never be afraid to ask questions. What could be a fault indication on one fuse may be a normal pattern on another type of fuse. If we don’t ask, we won’t know, and that’s an easy way to end up with egg on your face.