On Fri, 17 Jan 2003, Russell McMahon wrote:

*>I'd feel quite safe doing this but would also expect it to be frowned on by
*>somebody. FWIW here in New Zealand flexible cords are not meant to be run in
*>such a manner as if they effectively fixed. It is however a very common
*>practice in practice.

Common and dangerous. The rule is because the current capability is for
free air operation. When you fix it up it is not in free air. As you know
a 20A cord run at 20A will be quite hot (hot enough to self destruct after
a few hours if left coiled on a drum). Also the insulation and tensile
strength is mechanically not up to taking the stress from hardpoints
(cable nails, ties, kinks, hard 90 degree corners etc, especially when
heated by the current passing through it). This is very literally 'playing
with fire'. If you have to do this at least use outdoor cable (rubber
insulation, as used on power tools etc) and overrate it as much as you can
afford. The worst part is, that I have come to the conclusion that most
'fire retardant' plastics become much less fire retardant if exposed to
heat over a length of time. Something happens to the plastic and it burns
great after the heat treatment is applied for a long enough time. In other
words, the overstressed cable may be much less safe than you think over
time. Time here is days or weeks or more at 75-80C, like in a roof crawl
space, behind hot ovens, etc.

I find that normal household distribution implements are about 95%
efficient (if of good make). This means that an extension cord and plug
with a 2kW load on it will itself dissipate up to 100W (I am talking about
a longer extension cord). Also copper cable is subject to thermal runaway.
Would you put a 100W soldering iron against a wooden panel or wallpaper
and leave it on unsupervised ?

Peter

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