Re: Lighting questions

[quk@isham-research.demon.co.uk (Phil Payne)]

In message <19980808185250.12339.qmail@hotmail.com> "scott miller" writes:

> And wearing nomex!  I dunno, I'm pretty sure 14 Ga is good for 15 amp
> applications (which means a lot more than 15 amps), I have a chart back
> home.

I used to draw them.

This is an issue a little like the timing belt issue.  We use a rule of
thumb of nnnnn miles - but it's much more complex.  David Sutton's
_EXTREMELY_ expensive event with the S1's cam belt was after the car
had covered a few hundred miles.  But after eleven years?

It's very much the same with copper flexible.  As it's drawn, physical
changes are forced upon its molecular structure.  Tensile strength and
resistance both increase, as does hardness.  Multi-stage drawing
machines are the most wonderful contraptions you can imagine - they
draw in 1/4" cleaned copper rod at very low speed, pull it through dies
to reduce the diameter, and pass it over a slightly larger pulley to
another die that does the same thing and passes it over an even larger
pulley ...

You can imagine.  It comes in at inches per second and comes out at
n miles per hour.  Along the way it passes between special annealing
rollers that 'flash' heat it - immediately afterwards, it passes
through a dual-purpose cooling and lubricating spray.  And ever
onwards.

The final product is a drum (more correctly, n drums where n is the
number of wires required to make up the load-carrying cross section
from the drawn cross section).  It's laid up, then covered with PVC,
then painted, etc.

>From the moment the last annealing is done, the molecular structure
starts to revert to a preferentially high resistance form.

That's the problem.  The 'charts' assume, for flexibles, a life of
around five years.  For cables (as built into houses and nailed to
rafters) the assumption is 25 years.

The acceptable load given for, e.g., 14 gauge, is based on a 5-year
life.  At significantly more than this, you either have to replace the
old 14-gauge with new 14-gauge, or you have to take other measures to
circumvent the problem.

--
 Phil Payne
 Phone: 0385 302803   Fax: 01536 723021
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