Best practice for fuses

[TheBeastie]

I am planning to re-do the wiring for my lighting - second time but older and (marginally) wiser now! I want to find best practice for fusing power lines.

This time I plan to put the relays on the rear bulkhead and run the wires to the lights from there. It means much shorter lengths of wire permanently supplied with juice but drawback is longer run to lights so will use a heavier than normal cable to try and counter voltage drop.

I currently have the relays on the N/S front wing towards the front but need to move them in order to position solenoids for my winch. I have looked on the engine side of the bulkhead but can see nowhere with enough room for three 70A relays.

I clearly need to fuse the live feed from the battery to the relay but does it also make sense to have a second fuse on the line running to the lights? My thinking (possibly flawed) is that if there is a fault between the relay and the headlight then the second fuse will blow first and thus protect the relay. Does this make sense?

Any views on this will be very much appreciated.

Thanks

Malcy

[Puffernutter]

Normally the wiring will go up in smoke to protect the fuse

I assume you are talking about the live feed to the relay from the battery, which should be fused and the "switched" feed from the relay to the headlight? With two fuses, one will certainly blow on a short circuit, which one is anybody's guess as each fuse will have some manufacturing tolerance. In my personal view, just fusing the battery feed to the relay is sufficient, anything else introduces more places to get poor contacts and then intermittent operation! (Lucas Rules!)

Cheers

Peter

[nickwilliams]

Wot Peter said!

[GW8IZR]

If you fuse the feed to all of the relays, which you should if the wiring to those fuses is long and passes through bulkheads and grommets, then you rate the fuse appropriate to the combined total load.

Each filament dipped L&R and main L&R ( thats four fuses) should then be fused at the rating for that load .

That way if a filament or wire fails blowing a fuse it doesn't plunge you into sudden and total darkness.

If the main feed to the relays shorts to earth unexpectedly it wasn't a safe and proper installation ;-)

HTH

[TheBeastie]

Paul

What you are suggesting is quite different I think. I was always "taught" to fuse back as close to the battery as possible so I have put a fuseboard on the rear bulkhead that takes short and sort of small diameter but proper battery cables back to the battery, then gives me six fused outputs. I had planned to take one fused feed to each of the three relays, one for dipped beam, one for full beam and one for the spotlights - so each pair on separate fuses.

I then planned to run two wires off the output blade of each relay and run one to each side.

So logically it means if the full beam fuse blew I would still have dipped, but I am not fusing each of the six lamps individually.

Do you think this is a planning error?

From my reading of your post you suggest I should put a "secure" unfused feed (say 39A cable) to each relay and then fuse each of the six output lines from the relays so that if a fuse goes I only lose one lamp - is that correct?

[GW8IZR]

So if you look at the original diagram for the lighting it could have been made much simpler, one fuse for all the lights etc, but the designer elected to add separate circuits - the reason being to make the vehicle more resilient and safer to use. What you propose is fine and will work and its probably how lots of LR are modified.

Trouble only comes if a filament pops when your going round a bend on a dark lane *(note) Filaments can go dead short for a few mS as they blow which can pop the fuse. If that happens both your main or dip will fail. For the sake of an extra fuse after the relay you can maintain three lamps instead of two. So ideally you have four relays, I do but some might argue that's another failure point. Take a look at what LR did, two contacts on the switch to separate the circuits.

*(note) other failures like wires chaffing or dropping off - but with a proper instal that risk is low.

Your fuse box is ok as it is assuming the feed to it doesn't go through any panels - if it does you should really fuse it before it exits the battery box. Or you do what LR did and make sure the cables cannot chaff rub or cut, make sure you use good cable and ideally a plastic sheathing, grommet on each hole and cable supports to stop the wires moving.

On this tablet I can't see if I've tried to answer all the points till I press 'post' so if I've missed anything or not made sense just shout :-)

[TheBeastie]

Thanks Paul, I think that does and can see the logic.

I have tried to put it on paper using Visio (may have got sympbols wrong and the colours are simply to distinguish the route) and I have omitted the ground for the relays for simplicity.

I have already got three 70A relays and would struggle to find space to fit a further three so I think I am restricted to running both sides of each filament on one relay but I now do see the merit in separately fusing each line. I think I will also take the live feed from the battery to the relay from the existing fusebox since it is already there and I have three feeds currently doing the three light pairs on the MkI wiring. I just now need to find room to fit another fusebox to manage the six line feeds to the lights.

Working on basis of 55W per filament that would suggest using a 7.5A fuse (possibly 5A but suspect that is cutting a tad fine) for each of the six feed lines and 15A fuses in the existing fuse box to supply the relays.

Will get there in the end!!!

[GW8IZR]

That look about it on this little display but as the spots are only auxiliary lighting I would only fuse that as one circuit. At end of day you would still have main beam if the spots failed.

The big relays you have selected should be nice and resilient so three is fine.

10A fuses I use for each circuit as they pull a lot of current from cold. 20A for the spots.

HTH

[GW8IZR]

Ideally it would be:

Supply --> fuse. --> switching device --> filament--> return (-ve)

For each filament

As you only have one switch separating the circuits after the switch is the best you can achieve.

Now with your setup in the drawing the switching devices are unfused. You *should* protect them with a fuse.

The failure mode is likely to be physical I.e. melted relay, broken in an accident etc.. so it will be a high fault current if it happens. A big fuse capable of taking the full load current of the fuse box would be best.

Yes its a bit of a nuisance all these additional fuses to take care of a "what if.." scenario

[TheBeastie]

Thanks everyone. Order now placed with VWP and will start as soon as it arrives.

Will try and remember to give an update once all finished.