Headlight Fuse is Melting, not breaking but MELTING!

[willmckeand]

Hey everybody, the headlights fuse on my FZS600 Fazer is getting extremely hot after about 30 seconds of having the lights on. So hot that it melted the plastic off of the fuse which then ran down into the fuse box.


I've re-wired the fuse to another fuse holder but I'm still having the same problem.  The ammeter showed around 10A at the fuse which explains why the 20A fuse is not breaking, just getting very hot.


Does anyone have any thoughts on what could be causing this? I have checked the majority of the wiring and the bulbs seem okay. I thought initially that water had caused it as the bike was out in the rain all day on the day the problem started, however it's been 4 days in the dry now and still no improvement.


Running out of ideas, please help!


Will

[andybesy]

What's the voltage across the wire?


And what's the resistance?


Andy

[willmckeand]

Andy, the voltage is 10.3V (engine not running and battery has been used a lot for tasting lately without charge)


The resistance is about 70 ohms although this changes as the fuse gets hotter, any ideas?


Thanks for your response
Will

[Motorbreath]

Silly question, but, have you tried another fuse?

[His Dudeness]

its probably a dodgy connection or wire causing higher resistance. try pulling the connectors in that junction box apart

[Motorbreath]

its probably a dodgy connection or wire causing higher resistance. try pulling the connectors in that junction box apart

I think that is not correct. Higher resistance=less current=less power=less temperature.

[His Dudeness]

its probably a dodgy connection or wire causing higher resistance. try pulling the connectors in that junction box apart

I think that is not correct. Higher resistance=less current=less power=less temperature.

or the alternator is supplying more current to overcome the higher resistance of a bad connection or bad wire so you're getting more heat

[Motorbreath]

An increase of available current would make no difference, to increase the power it would have to increase the voltage too. Also, the place where the bottleneck is should be the hottest, so if ithe fuse is the hottest it should be the faulty part.

[His Dudeness]

i thought heat was proportional to current squared times resistance so how can an increase in current even at the same voltage not cause more heat? also maybe the wire has more of a heat sink around it than the fuse so the fuse can't dissipate the heat as fast as the wire?

[Motorbreath]

The current cannot be increased for a given resistance and a given voltage. (I=V/R). To increase the current yo have to increase the voltage or decrease the resistance (or both). You can increase available current (for example, having a big car battery), but the actual current that will run through the circuit will be the same.

I do not know how the fuse can melt before break. But if another fuse do the same then probably the circuit is shortend to ground, by water or a peeled wire or other reason.

I hope I explain myself, English is not my first language.

[His Dudeness]

yes i see what you mean I=V/R. but if the resistance has increased due to a dodgy wire or connection then there would have to be more current flowing through the circuit to maintain the 12V wouldn't there? and if heat is proportional to the current squared and there's a higher resistance then you've got more heat. i'm only guessing here btw you probably know more about it than me

[richfzs]

you probably know more about it than me

Yeah, he does @pokefun

The 12V (nominal) is produced by the battery, that doesn't change (as the revs rise, the reg/rec keeps the voltage where it should be), so as Motorbreath says, the two variables are the current and the resistance - a dodgy/corroded connection increases the resistance, so the current drops. With a lower current, the power equation, P = I*V shows that the power also drops, with less power, you've got less potential for heat.

[scottyboy2203]

just to put my 2p in, excess heat heat in a wire is a sign of too much current being drawn through it. Since the voltage and resistance of the wire/alternator is fixed (within a small range depending on engine revs and heat etc) it's only current that can change dramatically. This is usually caused by something drawing too much, like a faulty bulb or short-circuit. The common mistake is to think that the alternator/battery produced too much power/current but its the other way round, the bulb draws the current it wants and the alternator/battery tries to provide it. So my point is that you probably have a short circuit between the fuse and the headlight or a faulty bulb that is drawing too much current and then melting the fuse. I've probably rambled on enough though... 


overheating = too much current

[His Dudeness]

you probably know more about it than me

Yeah, he does @pokefun

The 12V (nominal) is produced by the battery, that doesn't change (as the revs rise, the reg/rec keeps the voltage where it should be), so as Motorbreath says, the two variables are the current and the resistance - a dodgy/corroded connection increases the resistance, so the current drops. With a lower current, the power equation, P = I*V shows that the power also drops, with less power, you've got less potential for heat.

yeah i see what you're saying but if there's more load caused by a dodgy connection or just an increase in load like by heated grips or something like that would the alternator not produce more current to compensate for the increased resistance?

[richfzs]

there's more load caused by a dodgy connection or just an increase in load like by heated grips

these are 2 different scenarios.

The dodgy connection (assuming its corrosion) just means its an increase in resistance, so lower current - the scenario I described before.

extra load (such as heated grips) will pull more current out of the battery (and reg/rec combination) yes, but that's perfectly normal for the accessory that's being switched. And it's not what is happening in the OP scenario (I dunno what the problem is there tho!), he has a fault of most intriguing origin !!

A dodgy connection could be a fault to earth - and then there's very low resistance, and hence a very high current, with the attendant heat and melting wiring & fire risk - and that's why we use fuses, they're not really there to protect the accessory, but to protect the rest of the bike.

What the OP problem is, I dunno - the fuse should be doing its job long before the plastic melts

Will, is the heat definiately coming from the fuse, its not something else right next to it?!? Cant recall whats next to the fuse box, and its WAY too cold to go take a lok in hte garage 

[His Dudeness]

ok

i thought that a bad wire would cause an increase in load in the same as say heated grips cause an increase in load.

[unfazed]

Have you modified the headlight?

[ponkster]

As some one said earlier in the thread heating is due to increased resistance or load if the bulbs are standard and in good order I would suspect a poor conection - Its a simple enough circuit so I would check and clean the conections and put in a new fuse.
 
You can never rule out the PO factor ( previous owner) - my wiring was getting real hot when on full beam - on inspection the main beam on one bulb had a wire soldered on it linking it to the un used dipped fillament - way too much load - again strangly didnt pop a fuse but the bulb connectors started to melt!

[willmckeand]

Hey everyone, thanks for all of your responses, I'll change the bulbs and then do a more thorough check of all the connectors and wires in the circuit.


@unfazed
I do have a modded headlight but the problem still occurs when I disconnect the wire, it even does it when just the side/tail lights are on so I'm not sure it's an issue.
Thanks

[JZS 600]

http://www.aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/ATM_Specs.pdf
 
Here you go, all you need is a multimeter set on DC amps in line with the circuit and you see what's happening.

[Motorbreath]

ok

i thought that a bad wire would cause an increase in load in the same as say heated grips cause an increase in load.

Yes, the heated grips are in parallel, so they decrease the resistance of the complete circuit. (current runs both thorugh the original circuit and the new grips circuit)

As some one said earlier in the thread heating is due to increased resistance or load if the bulbs are standard and in good order I would suspect a poor conection - Its a simple enough circuit so I would check and clean the conections and put in a new fuse.
 
You can never rule out the PO factor ( previous owner) - my wiring was getting real hot when on full beam - on inspection the main beam on one bulb had a wire soldered on it linking it to the un used dipped fillament - way too much load - again strangly didnt pop a fuse but the bulb connectors started to melt!

As said before, resistance reduces heat. As you describe it probably you had both H4 filaments enlightened, that caused the excess of heat. Your fuse couldn't break because there wasn't more current thorugh the circuit, actually there was a bit less. The normal headlight mod we all do increases current as both bulbs are in parallel (but just one H4 filament), but still it is not enough to melt the fuse or anything else.

Again that strange word "load". I do not understand what you mean, but it seems it causes confusion. I do not know even how to translate it to Spanish when talking about electrics.

http://www.aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/ATM_Specs.pdf
 
Here you go, all you need is a multimeter set on DC amps in line with the circuit and you see what's happening.

It seem fuses apply the 80% rule. It means a 20A fuse can melt if it has 18A running all the time? But he measured only 10A already, mayby he is wrong.

Hey everyone, thanks for all of your responses, I'll change the bulbs and then do a more thorough check of all the connectors and wires in the circuit.


@unfazed
I do have a modded headlight but the problem still occurs when I disconnect the wire, it even does it when just the side/tail lights are on so I'm not sure it's an issue.
Thanks

What mod? Did you replace the fuse?

[JZS 600]

The fusing element must not hinder the normal circuit path, but it must also overheat and melt in response to excessive current (overcurrent). In simplified terms, the melting function depends upon the power expression of P = V * I, where P is the power in Watts. By algebraic substitution with Ohm's Law, the expression becomes P = I² * R, showing that a linear change in current produces an exponential change in power dissipation, which is useful in a fuse. But how quickly should the fuse respond? Fast? Slow? How fast or slow? The capabilities can be summarised with a TCC (Time/Current Curve). A logarithmic scale is typical, with current shown on the horizontal (x) axis and time on the vertical (y) axis. The TCC in Figure 1 is illustrative of a very fast 1A fuse. First, we observe that no fuse of this type can fail from overcurrent until at least 1.25A is flowing, and none is guaranteed to fail until 1.5A is achieved, or 150% of nominal current. A factor of 1.5 is rather optimistic, and many real-world fuses are more in the range of two, representing the safety factor that must be allowed when a fuse is specified.

Figure 1 - Possible TCC for a 1A fast-acting fuse

Second, note that the TCC is a region representing unavoidable variations, since a thermal melting mode cannot be precise with real-world materials. Here, one fuse might pick up at 1.25A and fail, i.e. clear the fault, in about ten seconds, and another fuse may not clear until 1.5A flows for ninety seconds. The gap between the nominal current rating and the minimum trip is also unavoidable, as a minimum trip too close to the nominal rating would subject the fuse element to large, thermally-induced mechanical stresses during normal operation. The curve characteristic is maintained in a family of fuses. A 10A fuse of the same family would yield a similar curve, but shifted right, with an initial pickup from around 12.5-15A. For any fuse, three data points are particularly interesting to a hobby user:
 

• Voltage Rating: When a fusing element separates under load, an arc can strike in the gap. Current flow is sustained by the arc, and the element may weld back together for a second break, or the arc may persist and continue melting the element outward. The voltage rating specifies the maximum voltage for which clearing can be guaranteed. The voltage rating must account for all possible failure conditions including the limits of the dielectric medium (type and degree of physical separation between ends), vaporised metal products formed in the chamber, and any support structures that may allow the arc to "track". AC ratings are usually higher than DC ratings since the zero-crossings accelerate the arc collapse. Fuses intended for 12V DC automotive electrical systems usually have a 32V DC rating. The AC rating is often 250V, but this cannot be assumed if data are not available to prove it.
  
   
• Current Rating: The nominal rating is the maximum current the fuse can carry continuously. A correctly specified fuse should have a long service life, but an overrated fuse may not protect the circuit if the required safety factor current exceeds what the circuit can source in a typical failure mode.
  
   
• Timing Characteristic: Smaller fuse styles will specify "fast acting" (FA) or "slow blow" (SB, or Time Delay) and not provide a TCC. Fast fuses are intended for applications where any failure condition must be quickly interrupted. Time delay fuses are useful when a momentary overload may legitimately exist for a short time (motor or transformer startup) and cause nuisance tripping of a fast fuse.

A nuisance (fatigue) failure occurs when the fuse has been aged by numerous operation cycles, or the fuse is operating slightly above its nominal current rating, or the fuse is subject to abuse such as inrush spikes or even mechanical vibration. Metal fatigue from aging is normal, but the latter cases can indicate a system design problem or an insufficient fuse rating. A fuse that is being run slightly above its current rating, or stressed by an inrush, can often be observed to move inside the cartridge. It may survive a number of cycles but it will eventually fail.

[JZS 600]

Or just give everything a good going over with contact cleaner   

[Fazerider]

This sounds like a poor connection, probably to one of the blades of the fuse.
Check the connectors are clean and tighten them... it should require a noticeable amount of force to shove the fuse in. If that's not the problem then the cable, where it joins to the connector may have corrosion or a few broken strands, in which case you'll need to remake the end.
The problem is indeed too much resistance where there shouldn't be.
As others have been pointing out, that will result in less power... but only from an overall point of view. If the contact resistance to one end of the fuse is, say a tenth of an ohm, that will lose a volt across it if the current is 10 amps... the lights will then be working from a volt less, but the current won't reduce by much as a result... even if they were simple resistors it would only be a 9% drop (in fact they're non-linear resistors... they'll be a little bit cooler and yellower so their resistance will also have dropped a bit).
So the headlamps are dissipating a little bit less power, but the poor old fuse is now trying to get rid of 10 watts (resistance x current squared = power) as a result it gets hot enough to melt the plastic.

[clayt74]

This sounds like a poor connection, probably to one of the blades of the fuse.
Check the connectors are clean and tighten them... it should require a noticeable amount of force to shove the fuse in. If that's not the problem then the cable, where it joins to the connector may have corrosion or a few broken strands, in which case you'll need to remake the end.
The problem is indeed too much resistance where there shouldn't be.
As others have been pointing out, that will result in less power... but only from an overall point of view. If the contact resistance to one end of the fuse is, say a tenth of an ohm, that will lose a volt across it if the current is 10 amps... the lights will then be working from a volt less, but the current won't reduce by much as a result... even if they were simple resistors it would only be a 9% drop (in fact they're non-linear resistors... they'll be a little bit cooler and yellower so their resistance will also have dropped a bit).
 
So the headlamps are dissipating a little bit less power, but the poor old fuse is now trying to get rid of 10 watts (resistance x current squared = power) as a result it gets hot enough to melt the plastic.

Have to absolutely agree with Fazerider,
I am assuming when you measured 10A current the fuse was removed and your meter was across the fuse contacts?
10A is about right for a good loom with 2 off 55w bulbs, not sure how you measured 70 ohms tho??? 2 bulb filiments should measure about 0.7 ohms!!!
sounds like you were measuring the additional resistance of poor contacts between the fuse and fuse holder which as Fazerider says is responsible for the volt drop and excessive heat across the fuse.
only other cause is that the fuse itself has excessivly fatigued and has built up its own internal resistance (rare but can happen)