Glowing Cable

Quite.

Indeed. Yes, I am aware of the non-zero resistance property. What surprised me was that a cable carrying a current much less than its rating should nevertheless glow so brightly in the infra-red. What I should have done was check other domestic high current circuits, and so it proved. The wires to the oven switch glowed as well. The cable to the kettle was not quite so bright.

#2⇑.

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The word "glow" may be overly fanciful, but I think we can agree that the colour yellow is chosen to be a representation of a warmer spot than purple or even orange. The cable was definitely radiating in the infra-red. The truth may lie in compression or expansion in the display of the range of temperatures, but it certainly surprised me.

The Flir should tell you what the actual temperature is, either by placing a cursor on the area your interested in or by reference to a temperature/colour chart (specific to the picture you're looking at).

You should then be able to compare with the ambient temperature and check that the temperature rise is what is expected for the type of cable and current being carried.

You are correct that the current is well within the capability of the cable:-

...depending on length and method of installation, of course, of which the forum knows zilch.

Bingo!

I agree with Rixter. Having done many IR inspections of electrical equipment in my professional career, it is important to have a temperature scale matched to the artificial color of the image. The difference between the dark purple and the bright yellow may only be a few degrees C, but the colors make it appear like a large temperature difference. If you adjust the scaling of the IR camera's display to a broader range, you will probably see that cable fade nearly into the background color.

Agreed!

Since the original images only show single wires, there is nothing to compare with. Here's an image showing several wires.

In this image, you can see six of the 9 cables in a 600 Amp 3 phase circuit (3 cables per phase). If you look closely, you can see a ring around each 3-cable set. Those rings are Rogowski coils (current transformers) connected to a power meter. The power meter indicated that each set of cables was carrying very close to the same current, so the heat visible in the lower set was not due to excessive current.

Closer examination shows that the hottest part is at the bottom right, where the cable feeds out of an old fuse block. Careful cleaning and tightening of the connection of the cable to the fuse block eliminated the problem.

Note that the orange color of the upper set of three cables is normal operating temperature; The brighter yellow to nearly white color (and the indicated temperature value) show abnormal heat.

It's an RTFM problem (unsubscribes)
Del

I am most grateful for all your advice, in particular for this simple experimental suggestion, which indeed showed the hand to glow almost as brightly as the cable. It has been a very educative session. I do have to mention (in relation to post #14) that my device is a small attachment for my smartphone, and did not come with an FM to R.

My photo in Post #17 was also taken with a FLIR One attached to my iPhone (6S+).

I do radiant electric floor heater design and the images don't look to be anything out of normal. You are probably not looking at anything beyond about 10 degrees difference between warmest and coolest in the image, based on what I have seen using the FLIR E6 or E7 for inspections. If you look at code ratings, you will see that most conductors are rated for several degrees temperature rise at normal operating current.

You have the answer, if you are still concerned, change the cable to a 4mm2 PVC SWA if this is a long run cable to the garage and outbuilding. And also, never plug stuff in and leave the trailing cable in a coil. Snake the cable but never coil a cable with any load on it.

Fsst-bang-trip-fire. Toasty!

https://www.youtube.com/watch?v=WSpTlAFRhYM

NB, May be difficult to understand, they speak American.