Re: Electrical Puzzles - more

[Glenn Merrell <gmerrel@domain.elided>]

Hello Peter,
	I would hope that the fusing in your fuse block is there to protect the wire
leading from the fuse block to the various accessories. That is the normal
function of a fuse.  Wire current carrying capacity is sized to carry at least
125% of the maximum current load of the device, and the fuse is (should be)
sized to protect the wire from drawing more current than it is designed to carry
before it burns up and catches your car on fire.  This 125% is a minimum value
for safety.  he wire may be larger.  By adding a larger fuse, you risk burning
up the wire harness.
	The second reason that the fuse is more than four times the actual operating
load amps to the headlights is is different than the initial current surge from
the lamps being switched on.  First initial current to an inductive device like
a lamp is like a shorted circuit for a millisecond or two.  This initial
"inrush" of current is not enough to heat up the fuse to burn it out.   In other
words, the wire to the headlights should be sized to carry 125% more than the
rating of the fuse so the fuse breaks before burning up the wire, and the fuse
is sized for the full maximum rating of the device.

Just some basic electrical standards stuff, not my opinion, and I would not
speculate buy adding more stuff on the circuits.
Regards,
Glenn  Merrell
Triumph Stag Register USA VP

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PGAHOWELLS@domain.elided wrote:
> 
> Hi all
> Puzzle 1
> Why is there a 25 Amp fuse for each headlamp and a 10 amp fuse for each dip
> lamp.
> In UK they are all the same bulbs, 55watt. divide 55 by 12 Volts =  4.58 Amps.
> Thus 10 amps should be enough for both main and dip.
> Puzzle 2
> On the cover of the fuse box we see Headlights Main Beam 1
> then Headlights Main Beam 2
> then Headlight Dip Beam RH
> Headlamp Dip Beam LH
> Why use a 1 and 2 convention in naming the main beams, and a RH LH convention
> to name the dip beams?
> 
> I have just wired in a rear Fog Lamp to my 1971 Car, hence my current (ha ha)
> preoccupation with things electrical.
> I wired the fog lamp so that it comes on when I put the Master light Switch to
> the FOG position. Trouble is the headlights go out because this switch
> position is meant for Front Fog lamps. However on the relay board I have
> installed an extra relay powered by the fog lamp wire and this pulls the
> headlamps back in by drawing power from the spare overdrive fuseway to the
> spare blade on the headlamp (Dip) fuseway.
> I think it is quite a good arrangement. I hope you do too!
> Peter H