It is hard to do but don’t do it

Check out these 300mm long cables that came out of some gear that we recycled!

image of three short power cords with a label saying "WARNING DO NOT OPERATE UNLESS CORD IS FULLY UNCOILED"

Yep! Like everything else on the internet it is for real. Got to be because it is in full colour!

I managed, with some difficulty, to get three tight coils out of  it.  The label is a complete waste of time, and I think some of the other ones I came across did not have the labels.

Some of you are probably wondering why you see these warning labels on cables, especially extension leads and retractable vacuum cleaner leads. It is due to the temperature rating of the cable.  Most cables used on appliances are PVC sheathed and PVC has a maximum temperature rating of 105°C.  If the cable is coiled up and has a load plugged into it it will heat up more than if it was uncoiled.  How much the cable heats up depends on how tightly it is coiled, the current though the cable, the resistance of the cable, the air flow (if any) though the coiled cable, and the ambient temperature.

We can do some maths on it.

Total resistance of the cable (300mm long with two conductors of 0.75 mm2):

R = ρ L/A = 1.68 x 10-8 x (2 x 0.3)/0.75 x 10-6 = 0.013 Ω

Power dissipated at the maximum loading of 10 amps:

P = I2 x R = 102 x 0.013 = 1.3 W

That does not equate to a lot of heat.  My head is spinning with all of this maths so I am not going to do any more to figure out the temperature rise. I know from experience that 1.3 W is not going to give much of a rise in the cable temperature.

So I guess the labels were slapped on the cables by the production staff when they were made in the factory because they are put on the longer ones.  They did not consult the engineers about whether they were needed.

Or are we in a really risk averse world?



About Alan Liefting

Alan Liefting is the founder, a shareholder, and the Managing Director of Ecotech Services. The views expressed here are his own and do not necessarily represent those of Ecotech Services Ltd.
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