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Glowing Cable

04/10/2019 6:15 AM

The previous owner of our house, a professional electrician, installed a circuit to the garage and outbuilding using SWA 4 core 2.5mm2 cable, presumably brought back from work. I have had several doubts about the quality of workmanship in the house, so I thought to check the junction boxes with an infrared camera while charging the car. To my surprise the junction box was not particularly warm, but the cable was glowing. Even the Tesla-supplied type 2 charging cable (coiled in the second picture) was glowing a bit. I was charging at 15A, well within the ratings of the cables, and neither felt warm to the touch. Is this normal?

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#1

Re: Glowing cable

04/10/2019 6:42 AM

Any cable will dissipate heat while current is flowing within it, as cables are not superconductors with zero electrical resistance.

British Standard 7671, for example, gives full protocols for correct cable sizing wherever the standard is applicable. There is no information as to the applicability of this standard in the location of the cable in question.

<...Normal...> invites assessment of a number of installations, including this one, to see whether this particular one is commonplace or part of a deviation to the standard against which it was installed, which this forum is not in a position to carry out.

So, if in doubt, consult a qualified local Electrician to investigate and report.

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#2
In reply to #1

Re: Glowing cable

04/10/2019 6:57 AM

Just to clarify for the OP:

"Any cable will dissipate heat while current is flowing within it" - the heat will show up as an orange glow, using an infrared camera.

The time to get worried is if it glows like that in visible light!

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#3
In reply to #2

Re: Glowing cable

04/10/2019 7:04 AM

Quite.

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#9
In reply to #1

Re: Glowing cable

04/10/2019 11:49 AM

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.

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#10
In reply to #9

Re: Glowing cable

04/10/2019 12:17 PM

#2⇑.

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#11
In reply to #9

Re: Glowing cable

04/10/2019 1:09 PM

It is not "glowing"...
A computer program is assigning a colour to correcpond to it's temerature.
If the software was changed to display dark blue or black at that temperature you would presumably be happy?
Del

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#12
In reply to #11

Re: Glowing cable

04/10/2019 4:42 PM

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.

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#4

Re: Glowing Cable

04/10/2019 8:45 AM

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:-

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#8
In reply to #4

Re: Glowing Cable

04/10/2019 11:36 AM

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

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#5

Re: Glowing Cable

04/10/2019 9:24 AM

When I first read this title I thought the cable was glowing like a filament making visible light, not the invisible light of infrared. Many FLIR displays will automatically change the colors used for certain temperatures. They do this so when the camera is looking at a scene with only a fraction of a degree difference across everything in view the user can still tell if they are looking at the right thing. If you add a hot mug of coffee into the scene I expect the automatic gain controls will adjust the color spectrum so the mug is visible but the junction box and cable will just blend into the background.

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#6

Re: Glowing Cable

04/10/2019 9:27 AM

It looks like it's glowing, but the thermal camera measures differences in temperature and the software in the camera color codes these different temperatures. The color scale auto ranges, depending on the range of temperatures in the image. Without a calibration scale, the image only gives a relative indication.

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#7
In reply to #6

Re: Glowing Cable

04/10/2019 11:05 AM

Bingo!

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#13
In reply to #6

Re: Glowing Cable

04/11/2019 9:42 AM

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.

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#17
In reply to #6

Re: Glowing Cable

04/11/2019 12:05 PM

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.

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#14

Re: Glowing Cable

04/11/2019 9:59 AM

It's an RTFM problem (unsubscribes)
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#15

Re: Glowing Cable

04/11/2019 10:28 AM

Hold your free hand near the cable and see which is brighter yellow.

It's a question of scale

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#19
In reply to #15

Re: Glowing Cable

04/11/2019 3:45 PM

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.

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#20
In reply to #19

Re: Glowing Cable

04/11/2019 7:03 PM

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

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#16

Re: Glowing Cable

04/11/2019 11:01 AM

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.

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#18

Re: Glowing Cable

04/11/2019 12:43 PM

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.

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