Melting wires

[dooscoop]

I am new here, so, Hello all.

I have a issue with my fuse box in my kit car.
I have been getting the car ready for the summer and started it up for the first time last week.
All was well for the first few runs, then i came to start it, turned the key to the first position and the fuel pump kicked in as usual then it went dead. so i turned the key again and there was nothing.
I turned the key again and there was nothing it was completely dead, first suspect was the battery, i tried jumping it and charging it but still nothing. Checked fuses, all present and correct.
I then checked the supply to the fuse box, that was fine, but from the box there was no supply to anywhere.
I took the box out and took it to pieces, this is what i found



As you can see the lower wire has got really hot and melted itself to another wire.
The hot wire was the supply to the light switch and the other white wire was the ignition switch supply.
The feed wire from the box to the switch was a little meted along the first couple of inches but only enough to deform it, not full on melted like the thicker wire in the box.
Anyone have any ideas why this could happen?
When i took it apart to replace the wires and connector block, which was also melted around the spade connector, i found that the feed wire on the outside of the box pulled out of the spade very easily. could this have caused it to arc, and heat build up or do i need to investigate further?

If you got his far, thanks for taking the time, i look forward to any replies.

 

[Marvo]

The two most common causes of melted insulation are poor connections and terminations and good old fashioned overload, from your photo I can't tell which one is the cause. Poor connections have a high resistance across them and they arc which causes heat to be produced locally so usually wiring damage is most severe at the termination or connection and gets less as you get further away. Usually the connection itself would show severe damage as it tends to get extremely hot whereas overload would usually affect the wire along its entire length and often more than one wire will be affected.

If overload is the problem then possibly one of the electrical components on the vehicle is faulty and drawing too much current or maybe too many components are being run from the same circuit.

The thing is that the fuse protecting the circuit should blow if it's overloaded. Fuses are always sized according to the current carrying capacity of the circuit wires and with a very low voltage system like a car the currents involved in even relatively small electrical devices are very large which make then especially prone to overload faults because it's current that causes the damage.

 

[dooscoop]

The thing is, there is no fuse for the lighting circuit, the white wire is a direct feed to the light switch from the main incoming power.
I am going to put one in the feed wire as a temp measure and look at getting a different fuse box sorted for next winters down time.

 

[Richard Burns]

If there is no fuse (there should be a fuse for almost all circuits) then the likely problem is overload, so the fuse may blow immediately.
Try and trace the fault before you power up again.

 

[Marvo]

Having fuse protection on all circuits is very important. Fault currents on 12 VDC circuits are very high because the wiring sizes are large and the battery can deliver enormous current and the likelihood of fire is correspondingly high in the case of a fault.

Fit a fuse before you do more damage, even if you install a temporary in-line fuse for now. You'll then need to trace the circuit that was burning and find what's causing the overload.

 

[dooscoop]

With the melting being confined to an area of 2" either side of the connector at the rear of the fuse box would it be wrong to assume that is where the problem is?
The lighting circuit is only basic, headlights, tail light and number plate light, no big spot lights or anything really to overload it.
I will fit fuse holder and then test some more.
Thanks for your help so far.

 

[Earthstore]

Lights are normally fused from the switch onwards, ie, lh dip, rh dip, lh main, rh main, side lights may also be sided too.
If you do put a fuse before the switch and have a problem, then you will have no lights at all..
Most modern cars are on relays to, for the headlights anyway.

It looks like you have a dead short somewhere, what is that conection through the metal with the piece of rubber? That could be where the problem is...

 

[dooscoop]

That is the main feed into the fuse box. It is not rubber it is some sort of plastic there seems to be a little melting in that area on the plastic.

 

[Earthstore]

Is the corresponding wire the other side of that white block connector melted too?

 

[dooscoop]

There is a spade connector on the outside of the box that takes the main incoming power, the two white wires and the brown were connected to this though the plastic casing with a pop rivet, You might have a better idea from this picture.

 

[Earthstore]

Is there another half to the white block connector, the one with the melted wires in?

Ps, the wiring does look very tidy for a kit car :yesnod:

 

[dooscoop]

Cheers, the box came as part of a complete loom kit.
There is antoher half to the white connector. The wire that seems to be causing the trouble is the bottom white which is the feed for the light switch.

 

[Earthstore]

But has the wire melted at all on the other part of the connector, the half not pictured?

 

[dooscoop]

Yeah the melting seems confined to the connector and the wire 2" either side of it, only the lighting wire is affected in the other half of the connector

 

[Earthstore]

It may just be a poor connection in that block, I somehow doubt it though, my advice would be replace the burnt wires,check for faults if you can, then see if they get hot quickly, if they do remove the power supply quickly and then you will need to find the fault.
The long and the short of it is it was all fine before wasn't it?

 

[dooscoop]

Yeah, the car has been running fine up till now.
I have swapped the wires and connector, i am also fitting a fuse to the light supply, i understand what you were saying about splitting the circuit but i think that is a job for the future, right now i just need to get it working again and check for faults.
At least with the fuse i will still have a car and not a pile of ash:laugh3:

I was going to swap the front half of the loom next winter for the vintage style cloth covered wire anyway so i can split the lighting circuit then.