LUCAS FUSE RATING

[BLOWNMM]

G’day all
Because the lighting circuits on a MM are not fused, along with the fuel pump and ignition coil, I decided to fit an inline blade fuse holder in the wire no. 36 from A1 on the control box to A on the lighting switch. The fuse is close to the light switch and accessed by removing the centre grill panel. The circuits fed from this fuse are those from the lighting switch being the main beam at 10 amps., parking lights 0.4 amps, tail lights 1 amp, no. plate light 1.0 amp, head and side warning lamps 0.1 amp, instrument lights 0.75 amps, This gives a total of 13.25 amps. Also from A on the lighting switch a feed goes to the ignition switch which feeds fuse A3 – A4. This feeds the coil 2.0 amps and the fuel pump 2.0 amps from the live side of the fuse being an extra 4.0 amps. From the load side of the fuse is fed the trafficator at 2.5 amps, stop lamps at 3.0 amps, DR2 wiper at 2.0 amps and fuel gauge say 0.5 amps giving an extra 8.0 amps.. This would give a total loading on the new fuse of 25.25. In view of the fact that not all circuits were likely to be energised at the same time I decided to use a 25 amp fuse in this new fuse holder. Recently a problem occurred with the left trafficator and resulted in the new 25 amp fuse blowing. The fault with the trafficator was finally traced and fixed, however if I had not fitted the 25 amp fuse it would have then relied on the 35 amp fuse in the A3 - A4 fuse holder to supposedly protect the wiring by blowing. The wiring to the trafficator is 14/.012” (14 strands of wire .012” dia.). This is equivalent to 1.022 mm. squared which Lucas rate at 7.0 amps and Narva or SCA rate at 10 amps. It is possible for a 35 amp 3 AG fast blow fuse to not blow for up to 20 seconds when carrying 200% of it’s rated current. Imagine the 10 amp trafficator cable carrying 70 amps for 20 seconds! No bloody way! The circuits connected to the load side of the A3 –A4 fuse are trafficators 2.5 amps, stop lamps 3.0 amps, DR2 wiper motor 2.0 amps and fuel gauge estimated 0.5 amps, giving a total load on the A3 – A4 fuse of 8.0 amps. These circuits would almost certainly be fed by 14/.012 cables (10.0 amps) and are also supposedly protected by a 35 amp fuse which has the ability to carry 70 amps for up to 20 seconds! Because I have added indicator lights (blinkers) to my trafficators, these are 21/5 watts front and rear with the filaments wired in parallel, it results in an extra 4.33 amps drawn from the A3 – A4 fuse resulting total of 12.83 amps. The best I could come up with was to replace the 35 amp fuse in the A3 – A4 holder with 15 amp fuses. This is still not perfect but will provide the 12.83 amps required and afford infinitely better protection for 10 amp cables than the 35 amp fuse recommended by Lucas. I have attached 2 links, one from Lucas showing cable sizes and ratings and another from Littlelfuse showing the Electrical Characteristics for 312/318 Series Fast Acting Fuses.
Hope this is of value
Cheers Bob
https://fromtheframeup.com/uploads/Luca ... rcuits.pdf
http://www.littelfuse.com/data/en/data_ ... 12_318.pdf

[bmcecosse]

It's a big mistake to have a single fuse in the lighting circuit - sudden total lights failure at speed is not funny - I know from experience in my rally car (it wasn't a fuse blowing) . The Ignition/fuel pump circuit is also not fused for good reason. By all means fuse each headlamp filament individually if you really must - the side lights are fused anyway in later cars. The 35 amp fuses supplied as standard on a Minor are easily capable of protecting the circuits. They are quick-blow and will drop immediately in the event of a short circuit. You are making your wiring needlessly complicated.....and more complicated = more trouble. it's fine as it comes from the factory!

[IslipMinor]

Bob,

As Roy has said, the standard glass 35A fuses are 'quick blow' and I am pretty sure that they are rated for 17.5A continuous current, with a short term peak of 35A (the same 200% in the fuse document you attached).

In the event of a momentary short-circuit the standard 35A fuses blow instantly.

For your calculated A3-A4 fuse load of 12.83A, I doubt that an automotive 15A glass fuse (7.5A continuous) will last very long, but try it and see. Just make sure you carry a spare 35A fuse just in case!

[BLOWNMM]

Hi Richard
According to the Littlelfuse data I attached, a fuse up to 35 A. carrying 100% of its rating it will last for 4 hours minimum. This is what I based my choice of using a 15 A. in the A3 -A4 holder. I will give it a go at 15 A. and also carry spares of higher rating.
Will report back after trying for a while.
Cheers Bob

[IslipMinor]

Hi Bob,

I am pretty certain that the glass fuses used in automotive applications are rated for continuous use at 50% of their maximum capacity. There is a separate Littelfuse catalogue that refers to automotive aftermarket applications, and covers automotive 'Full Glass' or 'FG' fuses, but does not give any real current or fuse characteristic data.

http://www.littelfuse.com/products/fuse ... fuses.aspx

On a different website I have seen the FG glass fuse rating described as the 'blow' current, which would sort of fit with my belief that the continuous rating is 50% of the maximum? Maybe??

Can only try it and see the results.

[BLOWNMM]

Hi Richard
Don't know about the fuses being for automotive use or otherwise. I will do some research into that. The other thing on my side with 15 A. choice is the duty cycle. If doing city driving this may be about 50%, but if open road work probably more like 10%. On top of that I rarely take my car out in the wet so this saves 2 amps from the wipers. Will see what happens.
Cheers Bob

[IslipMinor]

Hi Bob,

Did a bit more research and edited the previous reply!