More lights

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

In message <UPMAIL13.199710191930300458@classic.msn.com> "Ian J Haseltine" writes:

> I'm probably going over old ground here but -

Yup.

> A 100w lamp draws 8.3A and a 130w lamp draws 10.8A.
> The headlight cables on my quattro appear to be 1.5mm. Cables of this size
> should be able to carry a continuous current of around 13A, but as there are
> no relays in the circuit, all the current would have to pass through the main
> light switch which judging by other Audi switches would have a very short life
> if subjected to that sort of load.

Don't even think about it.

The resistance of copper wire increases with age.  At first,
spectacularly and fascinating to watch.

(Trust me - my first intended career was designing cables.)

Copper is a wonderful metal.  It is usually processed 'cold' - rolled
from cast billets into rods.  Delivered to the cable manufacturing
plant, it's usually in the form of 500kg coils of this 'rod', about 1cm
in diameter.  It's washed, dipped in acid, spray washed, dipped in
soluble lubricant, and then drawn down to a working size.  As it's
drawn, it hardens and needs to be annealed - this is usually done 'in
flight' by passing HUGE electrical currents through it and then spraying
it with ice cold lubricant.  At the end of the process, there are (VERY
roughly speaking) three states in which it is left:

a) 'Flexible' (or 'flex' for short) - a soft and highly conductive
   state used for appliance leads where there will be continuous
   movement throughout its lifetime.  This is the grade used in cars,
   though for automobile use it tends to the hard side.

b) 'Cable' - e.g., as used for house wiring.  Designed to be put into
   place and not moved again during its lifetime.

c) 'Self supporting' (or other terms) - self-describing; a grade so
   hard it will cheerfully support its own weight plus some insulation
   for its lifetime.  Uncommon these days - the old telegraph wires are
   gone, and BT uses a soft-drawn stock with a laid-up steel support
   wire.

As you go from a) to c), the wire's resistance increases.  As it ages,
the wire moves in that direction, too.  'a' becomes 'b', and 'b' becomes
'c' - although the latter is a slow process.  Heating it (e.g., by passing
a current close to capacity) doesn't help - unless you can provide for
rapid forced cooling of the entire wiring harness.

IMO much of Audi's 1980s wiring is underspecified - age does not help.

--
 Phil Payne
 phil@isham-research.demon.co.uk
 Phone: +44 385302803  Fax: +44 1536723021  CIS: 100012,1660