Splitting up wiring with new fuse box - what fuses to use?

[soe8m]

In the first post it is also about an additional electric fan and electric waterpump. Only the lights as in my wrong example are a "solid" load in a car.

Jeroen

[Jon Tilson]

Its all good...

We are all on the same page and no need for anyone to blow a fuse over it all...


(Sorry couldnt resist)...

Jonners

[MIG Wielder]

Tin I think you meant resistance is constant - so as voltage drops current drops too and the lights dim because
the power dissipated in the filament drops.
A 60W bulb doesnt always do 60W. Its a nominal rating.....It might do 60W if it has 12V and draws 5 A.
If the volts drop to 10 - You can check the sums...
12 volts = 5 A x R, so in this case R = 2.4 ohmes.
So when the volts drop to 10 say the current drops too...
10 = Q A x 2.4, so Q = 4.1666 amps
So the power drops to 41.6 watts and your bulb is dimmer...
Jonners

I think I'd like to take issue with this .......
First of all the incandescent bulb problem.
Incandescent bulbs are definitely not constant resistance. Because they are made of a thin tungsten filament the resistance varies enormously with current. Particularly as when cold the filament is at 25 deg C (ish) and at full brilliance it is at ... lots and lots of Deg C.
Bear with me because this is very relevant to fuses.
Here are some measurements I have done on a Dolomite main-beam headlamp unit and a regulated power supply.

dolly hlr.jpg (42.34 KiB) Viewed 707 times

What does this tell us ?
1. You get 50W at 12.5 V.
2. At 10V across the headlamp you are down to 70% of power. So if you drop 2V in your wiring and earthing then the brightness is going to be well down.
3. I've calculated the d.c. resistance at each voltage point and you can see that the filament resistance varies from 3.1 ohms hot to 0.75 ohms when cold. Quite a variation.
4. So the relevance is that with a good 13.5 V supply the peak current for a few mS will be 13.5/ 0.75 ohms = 18 A per headlamp. So the fuse has to cope with this peak surge current.
I'd like to come back on this motor problem as well later.
HTH,
Tony.

[Carledo]

Tin I think you meant resistance is constant - so as voltage drops current drops too and the lights dim because
the power dissipated in the filament drops.
A 60W bulb doesnt always do 60W. Its a nominal rating.....It might do 60W if it has 12V and draws 5 A.
If the volts drop to 10 - You can check the sums...
12 volts = 5 A x R, so in this case R = 2.4 ohmes.
So when the volts drop to 10 say the current drops too...
10 = Q A x 2.4, so Q = 4.1666 amps
So the power drops to 41.6 watts and your bulb is dimmer...
Jonners

I think I'd like to take issue with this .......
First of all the incandescent bulb problem.
Incandescent bulbs are definitely not constant resistance. Because they are made of a thin tungsten filament the resistance varies enormously with current. Particularly as when cold the filament is at 25 deg C (ish) and at full brilliance it is at ... lots and lots of Deg C.
Bear with me because this is very relevant to fuses.
Here are some measurements I have done on a Dolomite main-beam headlamp unit and a regulated power supply.

dolly hlr.jpg

What does this tell us ?
1. You get 50W at 12.5 V.
2. At 10V across the headlamp you are down to 70% of power. So if you drop 2V in your wiring and earthing then the brightness is going to be well down.
3. I've calculated the d.c. resistance at each voltage point and you can see that the filament resistance varies from 3.1 ohms hot to 0.75 ohms when cold. Quite a variation.
4. So the relevance is that with a good 13.5 V supply the peak current for a few mS will be 13.5/ 0.75 ohms = 18 A per headlamp. So the fuse has to cope with this peak surge current.
I'd like to come back on this motor problem as well later.
HTH,
Tony.

I'm not in a position to question your figures Tony, BUT every "modern" car i've dealt with which has one fuse per filament on the headlights, ie most of them, that fuse is a 10 amp. And mostly they carry 60/55w 472 bulbs. Which does not gell with your 18 amp max load figure. Just saying!

Steve

[Jon Tilson]

I did add the proviso to my comments with the phrase "second order effects". Obviously resistance is
temperature dependent and all metals have a temp coefficient for resistance.

Bulb filaments are made of a material which is both robust and has a fairly low coefficient of temperature.

Clearly Tony's experiment has shown that that the secondary effect is larger than I thought.

Jonners

[tony g]

Tony in your example you state 13.5/ 0.75 ohms = 18 A per headlamp whereas the 0.75 ohms was for the lower voltage so based on the higher voltage you tested at (no 13.5 in there) with 3.13 ohms the current would be 4.31 amps. Have I missed something?

Tony

[MIG Wielder]

Tony.

I'm not in a position to question your figures Tony, BUT every "modern" car i've dealt with which has one fuse per filament on the headlights, ie most of them, that fuse is a 10 amp. And mostly they carry 60/55w 472 bulbs. Which does not gell with your 18 amp max load figure. Just saying!
Steve

Hi Steve, Good point. To get the details of why a 10A nominal fuse (per filament) is fine for this sort of application with a peak surge current of 18A , I got the precise fuse design data off the data sheets of a major manufacturer and supplier of the modern style of ATOF 32 V fuses.
http://www.littelfuse.com/~/media/autom ... 014_lr.pdf
On pages 12 /13 there are quite comprehensive graphs relating fusing current with the time the current is applied. So for the 10A fuse it will actually pass 100A but only for 15mS ; 50 A for 50mS .... and running up the graph 18A for 1 second. This is plenty of time for the bulb filament to heat up and get up to working temperature and for the current to drop down to a nominal 4A at 12.5V. Yet it allows a realistic margin for blowing under real fault conditions.
I don't have access to the warm up time of a bulb filament but I would guess it would be there in 100mS.
Perhaps somebody with a storage scope could measure this ?
Thanks,
Tony.

[MIG Wielder]

Tony in your example you state 13.5/ 0.75 ohms = 18 A per headlamp whereas the 0.75 ohms was for the lower voltage so based on the higher voltage you tested at (no 13.5 in there) with 3.13 ohms the current would be 4.31 amps. Have I missed something?

Tony

Hi Tony, I only tested up to 12.5 V because that is where the power supply ran into current limit. What I was trying to illustrate is the peak surge current you get with filament bulbs. And this peak, worst case surge current - that probably only lasts 100mS - is the worst case highest battery voltage divided by the lowest bulb filament resistance when switched on from cold. And then the current rapidly drops to 4A at 12.5 V . So with 13.5 V , yes the current would be about 4.31A as you quite rightly say. ( maybe a little less as the filament would be even hotter ). Or about 58 W.
T.M.

[Jon Tilson]

Maybe there is some justification in not fusing bulbs at all....

If the filament blows do you need a fuse?

Jonners

[GTS290N]

No, but if the wiring to lamp is compromised (chaffed against metal perhaps) you'll wish you had fused them!
So pick a fuse of a sensible size and rating (how fast it blows under a particular in-rush current) and you should have fault protection without nuisance failures,
btw, why don't cars use mcbs? Mains mcbs have a whole range of types from A to D to cover most eventualities, are much more accurate and are re-settable...

[Mahesh]

I agree on the rcd's, unfortunately all the ones I found are home consumer box size.

[soe8m]

btw, why don't cars use mcbs? Mains mcbs have a whole range of types from A to D to cover most eventualities, are much more accurate and are re-settable...

Using these in automotive would cause too much resistance from the current fuse manufacturers watts no go good for their business.

Jeroen

[tony g]

btw, why don't cars use mcbs? Mains mcbs have a whole range of types from A to D to cover most eventualities, are much more accurate and are re-settable...

Using these in automotive would cause too much resistance from the current fuse manufacturers watts no go good for their business.

Jeroen

[gmsclassics]

I always find it fascinating how on this forum a topic can grow and follow a perhaps completely different path from the first post. Anyway, back to the beginning.

Yes, Jon was correct, the short in the overdrive was worn wires where these wires exit the gearlever half way down. These have been replaced and all is working again, fortunately without having to remove the tunnel.

I did take the opportunity to make changes to the fusing arrangements on the car. The original fusebox is now used exclusively for the auxillary circuits (those not through the ignition) to which I had already added the feeds to the radiator electric fan relay. This so it can continue to operate after the ignition has been switched off. The main power to the EWP controller is also from here, although it has its own in-line fuse.

I have also installed a neat 10 outlet fusebox for the circuits through the ignition. It has LEDs that illuminate if a circuit is on but the fuse has blown. I have split up the five green wires from the original loom so that each is fused separately. I think I have identified everything each one controls:
1. Heated rear window (6 - 15A)
2. Wiper parking (motor?) (7 - 10A)
3. Wiper and indicator switches, washer, brake lights, heater fan (9 - 10A)
4. Reversing lights and overdrive (8 - 15A)
5. Voltage stabiliser (10 - 10A)
There are also feeds to other additions such as fuel pump, warning lights, gauges, rev limiter and change light (4 & 5, both 15A). The sensor wire to tell the EWP that ignition is on is at position 2 - 10A) Positions 1 & 3 are not used at present. Fuses have been guessed at really so time will tell if they are correct.

photo (3).jpg (256.43 KiB) Viewed 626 times

Now I’ve done all this if a fuse blows, the answer as to what is wrong will be much easier to find, but hopefully I will never need it!

Geoff

[MIG Wielder]

As promised here are some measurements I've done on a Dolomite windscreen wiper motor.
The armature looks like it has a d.c. resistance of 1.2 ohms.
So this means the peak start-up current at 12.5 V will be 11.25A.
More importantly the stalled current will also be 11.25A, such as when the wiper blades are stuck to the windscreen when its frosty or when they are obstructed by snow.
In this state the motor dissipation will be 12.5V x 11.25A = 140W.
So the fuse here needs to blow at 11A .
From the fuse data I reckon a 7.5 A fuse would be O.K. here.

Dolly ww V2.jpg (94.33 KiB) Viewed 597 times