I've had a meltdown (electrical - not the kind a 4yr old has)

[skiday]

I have been fitting a new dash top to my 2+2 (that's another story) and so had to reconnect the wiring for the map light (which is now a low energy led striplight & much brighter). I previously added a simple circuit with switch so that I could cut out the map light when the door is open for a long time when valeting or working on the inside of the car. Instead of using the 'carefully working out which wire is which' method, I used the 'This is the only male I can see and this is the only female I can find so they must go together' method. Unfortunately the wire coming from the door switch and the power for the map light are both purple. As soon as I connected into the power instead of the switched earth, my little circuit became an element and proceeded to burn off its casing very rapidly. I couldn't uncouple the joint as it burnt my hand , so I leapt out and disconnected the battery (had to find a spanner first). Yes, yes, yes, I know the battery should have been disconnected in the first place, but even then it would have started burning as soon as I connected it. Lesson learnt, but it has left me wondering - all the fuses (bar the horn #3 @ 50 amp) are rated at 35amp as per the little plaque on the drop down panel. Isn't that very high? On a modern car most are very much lower and virtually none are over 30amp. Even household mains is only 13amp max and you can run a washing machine on that!, so do all the fuses really have to be 30amp? I know that all the fuses are for multiple things, but even so, it seems very high to me, and I have replaced a lot of the bulbs with LEDs. The fuse that covers the horn relay and brake lights (#6) has gone a few times, but the others could perhaps be lower? I'm thinking a fuse should have blown before my wires become toaster elements. Has anyone rewired their car with a modern fuse board? The fuse board is getting crowded anyway having added security and central locking (which are both fitted pre-fuse and then fused in-line).

P.S. I'll tell you about the remote central locking later. It works great.

[Durango2k]

Lets talk about Watts here: Watts = Volts x Amperes.

Now, households, fused at 16 Amps in germany: 220 Volts x 16 Amps = roughly 3.3 kiloWatts = 3300 Watts

Then, cars: 12 Volts and 35 Amps: 12x35 = 420 Watts.

See the difference....Electrical power is converted to heat 1:1 in an electrical heater as you built :-), because a short is just a very low resistance, hence it forms some kind of a heater....

There is another law to explain it easier: Volts = Resistance x Amperes. Or U=RxI.

If you now say 12 Volts is constantly delivered / kept up by the battery, then RxI is always 12.
If you now reduce R to almost zero, as you did shortening them (a bulb is a small resistor, by the way, a limited small resistor...), then of course I has to grow.

If R becomes zero, then I becomes infinite which is, well, quite large. Enormously large in fact.

So, if you have a short, the huge I produces a lot of Watts, see above. Which burns your fingers, because a small wire and a lot of heat mean a very high density of heat, say like on an oven plate.

Now, why did the fuse not blow ? Well, if I stayed below 35 Amps, and in a certain time (it takes time to heat and blow a fuse...), then the wire burns up, not the fuse.

Carsten

[christopher storey]

We have been over the question of the various methods of rating fuses so many times that I hesitate to post again, but since you have raised the question, I will do so. The Lucas glass fuse ratings were the blow ratings i.e. the maximum transient current that the fuse would carry. Modern plastic fuses are continuous ratings, i.e the maximum continuous current the circuit is designed to carry , and very roughly speaking blow at about twice the nominally rated figure. So broadly speaking a modern plastic 17 amp fuse is a direct equivalent for an old 35 amp fuse , 7.5 amp is equivalent to 15 on the Lucas scheme etc.

The second point is that modern electrical looms carry huge numbers of circuits compared with cars of the 1930 to 1970 era, and the wires on individual circuits as a result are much thinner to keep weight down. Consequently, the wires are only capable of carrying a much smaller current without overheating, and this is why modern cars have a multiplicity of circuits each with a lower rating than the circuits of the past

I hope this helps

[288gto]

Sorry to hear about your mishap Skiday. This is the very reason I decided to upgrade the fuse boxes. As Christopher mentioned above there is also some fuse rating info later in the thread.

viewtopic.php?f=9&t=10289

Hope you get it sorted.

Simon

[MarekH]

This is a good time to take the old fuses out one by one and put a meter (set to amperes) across the fuse holder, so that you can then switch each fan, light, relay, etc, on and off individually and write down how current each "appliance" uses.

You can then use a more appropriately rated fuse in each slot.

As a general matter, modern cars already do this and they also site the fuseboard close to the battery. This means that all of the wiring after then fuseboard is protected as well as the circuits. The etype has a much simpler and less safe setup, whereby the high current carrying wires all of the way from the battery to behind the dash are unprotected and the circuits themselves have little sensible protection. Add to that, the ignition side is not fused at all.

You could have avoided the meltdown by quickly shutting both car doors and making sure that the maplight switch was off. This would have cut the circuit quicker. (The circuit would still have had battery 12v on it but the earth back to the battery would have been open circuit.)

Consider fitting a battery isolator switch.

kind regards
Marek

[64etype]

I'm very interested in your remote door lock implementation. By the way, there are quite a number of modern relay and fuse panel upgrades documented both here and on Jag-Lovers. The primary issue is where to put them. I built a remote distribution center and located it in the vacated voltage regulator position after a generator to internally regulated alternator conversion. Seven 30 amp mini relays and 13 mini fuses. All very compact and unobtrusive...but not for the purist. The original wiring looms are unmolested...all done with a few extra wires, reassignment of wire function, and some short jumpers. Wires in the original loom now serve primarily as low amperage signal feeds from switches to the relays. A second remote fuse/relay panel, which services the starter, electronic ignition and air conditioning kit is hidden behind the glove box (removable false back).

[cactusman]

Always always disconnect the battery before working on the electrics....terminal that goes to the chassis FIRST. If you have modified something then reconnect the battery via a 15 ish amp fuse first. If the fuse immediately pops you know you did something dumb. If not connect as normal and check your mod works...original poster was very lucky....

That said we have all fiddled with the wiring at some point with the battery connected I will bet!!