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To fuse or not to fuse? That is the headlight Q!

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sultanoswing

sultanoswing

Jedi Hopeful
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I'm putting some 7" Cibies on my 71BGT. I'll (at least initially) be running standard 60/55W H4's.

I'm planning on creating a relayed sub-harness, but there seems to be debate about to fuse or not to fuse?

I WAS planning on running two fat (4mm) brown wires from the alternator (85 Amp Bosch) post and placing a 30 Amp inline fuse into each side, then running this into two 40 Amp 5 pin relays, from here to a 4 fuse blade-box (15 Amp for high beams, 10 Amps for low beams), then out to the lights themselves.

However - it seems to me there is debate about the safest way to fuse things. One main (albeit fat) fuse can blow, resulting in total loss. I'm trying to avoid this by running seperate high and low beam main feeds. I appreciate a 30 Amp blow is a low possibility and that fusing is of course designed to reduce the wiring harness from totally frying. The guy who wrote THIS PAGE is dead against fusing headlight circuits.

I guess what would be better would be to wire things so that each main feed supplied a left and right side only - that way a mainline blow would result in loss of one side only, but I can't quite figure out how to do this without introducing uneccessary complexities and wire taps!

What are your collective thoughts??
 
My thoughts have nothing to do with headlight circuits specifically but wiring in general. The first rule I follow is to fuse to protect the wire. This means if your wire is rated at 40A, you can put a 40A fuse in seies so that the wire would not overheat if more than 40A flowed thru. You can overrule this slightly by reducing the fuse size if your known load is less than 40A. This means that if your known load is 26A, you could fuse at 30A even though the wire is rated at 40A. A lot of times wire is sized not only for the amp rating but also the voltage drop so a smaller fuse is OK.
Obviously, you want to fuse as close to the supply voltage as possible. This way, if a wire gets pinched and shorted along the route, you will blow the fuse and not melt the wire.
As far as fuses in your headlight circuitry, All cars are fused there for safety reasons. If you size your fuses properly, you shouldn't have problems.

---------------------
1979 MGB
 
Well... I have a relay panel and I have a fuse in the main supply line (feeding both high- and low-beams). I guess there are some reasons why you might not want to fuse the circuit but I feel more comfortable that the fuse is there.

I agree with CraigFL that properly sized wire can be conservatively fused and you should never have a problem. Regardless of the decision on the fuse(s), feel comfortable that the Lucas switch on the dash will no longer be carrying current to the halogens. Adding those relays has improved the safety of your behind-dash wiring AND will prolong the life of the switch.
 
I just read the piece on why not to fuse headlights and I can't agree with the individual at all. I think his example is a little short sighted. ( I found a lot of his mods a little overkill and not to my taste ).
Fusing is used to protect circuits and wiring. The size of the fuse is dependant on the operating load and the inrush load of the circuit and the amperage capacity of the supply wire.
I always seperate the headlight wires from the original harness wiring and seperatly fuse them (Low has a seperate fuse from High in my harness as well as the fog lights).
I work in automation and in control circuits the positive and negative lines as individually fused and almost every input and output is independantly fused.
This is for protection and isolation.
I know there are a lot of competing theories on wiring but I also know the headlight rewiring reduces the load on the harness behind the dash and improves the lighting output of the headlights.
Safer and less smoke from the dash always a good sign.
 
To add my $0.02, I put relays in the high and low beams in my Midget. I put one fuse in the high beams, and one in the lows, so at least if I lose one set the others might still work. The relays were a definite improvement though; the headlights are brighter, as are the rest of the lights, and my wipers keep running at a decent speed if I have the lights on too.

-Dave
 
Re: To fuse or not to fuse? That is the headlight

Hello SoS,
I would not go your route but rather fuse each individual filament seperately. (As I have done with my Triumph 2000 saloon [plus relays]).
I don't believe that you can use automotive fuses and get accurate industrial level protection, unless, of course, you use industrial quality fuses, and I would hate to lose headlights at night so suddenly.
I do have a battery isolation switch that I can reach from the drivers seat and would prefer to have some smoke and cable damage to total blackness.

Alec
 
Re: To fuse or not to fuse? That is the headlight

I have two fuses, two relays, and new wire from the power to the realy and to the headlights. The only old wire in the system is though the switch to the relay. Both of my new cars have fuses on the headlight wiring, why not my MG. I went to relays after the headlight switch failed. It is not good to pull a load through the switch and it is not good to have unfused wiring. Renew your grounds for the headlights when you install the wiring. The best wiring will not work without a good ground.
 
Re: To fuse or not to fuse? That is the headlight

An option to fuses is to use self resetting circuit breakers which are readily available. At least, if the breaker trips, it will come back on in a few seconds.
D
 
Re: To fuse or not to fuse? That is the headlight

/ubbthreads/images/graemlins/savewave.gif

HI Piman, You been reading them Lucas Books again??---Keoke /ubbthreads/images/graemlins/laugh.gif
 
Re: To fuse or not to fuse? That is the headlight

Good debate - keep 'em coming!

piman - I am planning on fusing each filament seperately...it's the trunk supply that I'm wondering about.

Dave R - good thoughts - although a theoretical problem with circuit breakers is that they reset without you nec. knowing there was/is a problem which needs fixing. Also - even a few seconds of darkness could be hazardous! (not that I'm THAT paranoid about this!). I must say, I'm leaning towards my orginal plan at this stage of creating a seperate, relatively isolated (by relays) headlight sub-harness.

SoS
 
Re: To fuse or not to fuse? That is the headlight

I just read the article on the V8 site and...well... the statements made against fusing headlights borders on the idiotic. I've never owned a car that didn't have the headlamp circuit(s) fused - and I've never seen one accidentally blow when I was driving.

His assertion that a fuse might blow because of a "bounced wire" (or something to that effect) leads me to believe he needs to concentrate on increasing the quality of his wiring - not solving the problem by not fusing the circuits! In fact - the past few new cars I've had all have had independant fuses for the right and left side for redundancy against the "problem" he describes.

Furthermore - having been the victim of a "DPO" of an airplane who thought it was "OK" to not fuse a radio circuit I can tell you that a single relatively low-amperage circuit, when shorted, can lead to quite a mess. A single, small wire can quickly turn into a red hot burning filiment that conveniently starts to burn anything it's touching. At least in a car you can pull over and run from the fire, but then again losing a single headlamp beats rebuilding after an electrical fire any day.
 
Re: To fuse or not to fuse? That is the headlight

[ QUOTE ]
I'm trying to avoid this by running seperate high and low beam main feeds. I appreciate a 30 Amp blow is a low possibility and that fusing is of course designed to reduce the wiring harness from totally frying. The guy who wrote THIS PAGE is dead against fusing headlight circuits.



[/ QUOTE ]


this guy is NUTS.. He is describing how to rewire a complete car on his site, yet his only argument for not fusing the lights is in case a wire grounds out while doing some 'spirited' driving???

I've rewired several cars, and it turns out I have NEVER had a wire momentarily bump into a ground ever.. I've had my share of sanfu's, like pinching the harness installing an engine, and crossing many wires within the bundle. I was certainly glad I had fuses then!!

There should be NOTHING in your vehicle that is not appropriately fused or switched. The relays that you are adding, while good in one way, already introduce a point of failure for your lighting circuit.. they CAN go bad.

Put them in, you'll have brighter lights.. Use wire at least one size large than you expect your draw, and a fuse that is at least 5 amps above your expected maximum draw. It's an easy guideline to follow.. Wire and fuses are cheap compared to tracing down a bad harness.

Sounds like you are on the right track!!
 
Re: To fuse or not to fuse? That is the headlight

/ubbthreads/images/graemlins/savewave.gif Well SOS, Then just get some of those nonresetting
circuit breakers they got little red tops that jump up and tell you just where the problem is.---Keoke /ubbthreads/images/graemlins/laugh.gif
 
Re: To fuse or not to fuse? That is the headlight

My relays are between the fuse box and the front radiator panel. The distance between the power supply and the headlights is very short. Just make sure that your wiring is solid and that there are rubber bits around each hole in the body work where the wires pass. The chance of failure for a good system is very low. The more fuses you have the more connections you have put into the system. Each connection is a point of potential failure. Try to keep it as simple as possible.
 
While I run 80/100 H4 lamps without fusing them, I understand your desire to protect the circuit. If you want to be safe, consider the way a couple of my old VWs were wired. Each side had it's own fuses. There was one for the left low beam, one for the left high beam, one for the right low and one for the right high.

Maynard
 
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