rudi Posted August 15, 2009 Report Share Posted August 15, 2009 During the last rally we had an electrical problem. In the darkness suddenly we lost all lights on a very twisty road. We escaped without damage but had a vey scary moment. I could manage to "solve the problem" but without finding the real cause. Working on this now. It appears that the fuseholder has partly melted. I have a general question re. the electrics as I am not that of a specialist in this matter. How is it possible that a wire (I have an uprated circuit with 3 main cables from the cut out to a 3 fuse holder. All 3 fuses are 25 A. From there the power is divided over 3 main circuits.) can get that hot that it melts the plastic fuseholder without blowing the fuse of 25A first? The cables were not damaged. I guess that this particular cable is drawing to much power and therefore the cable gets hot. Or must there be a shortcircuit somewhere in the system to heat up the cable? What worries me most ist that this main fuse has not blown before the melt down of the fuse holder. Thx for any advise. Quote Link to post Share on other sites
Menno van Rij Posted August 15, 2009 Report Share Posted August 15, 2009 Well, perhaps I'm even less a specialist than you are regarding car electrics... but I've seen a situtation like this years ago in my own house (!) Only the smell of something burning gave it away: somehow a fuse wasn't seated properly in it's holder, thus started arcing, welding itself and the surround together, making a plastic/metal blob of the fuse holder. We were close to having the house on fire... It only happened after we had all appliances (secured by that fuse) working at the same time. As said, I'm really bad with car electrics, but perhaps this is pointer. Menno Quote Link to post Share on other sites
rudi Posted August 15, 2009 Author Report Share Posted August 15, 2009 Thx for the input Menno. I have installed a new fuse holder. Cables are connected properly and the fuse is sitting as it should. I have just fired up the car. This one cable only gets hot with lights and spots on. Both are relay wired. The fuse is getting so hot that I can't touch it, but it does not blow. So that leaves me with my initial general question. Why does a cable get that hot and why the fuse does not blow? Quote Link to post Share on other sites
ianc Posted August 15, 2009 Report Share Posted August 15, 2009 In normal circumstances, there should be almost zero resistance between all points on a given circuit other than at the load itself. The load might be the filament of a lamp (headlamps, sidelamps, trafficators etc.) or a motor (wipers, heater). However, if there is a bad connection (i.e. a significant resistance) along the route, then the voltage will be divided in the ratio of the resistance of the bad connection and that of the true load (i.e. the thing you really want to power - a headlamp filament, perhaps). Now, this means that heat will be generated at this bad connection and the load will not be working at full power (lamp is dim). If the bad connection is at the point where the end of the fuse fits into the clip of the fuseholder, then that is where heat will be generated (note - this phenomenon can occur also at any joints in the cable or at a switch, either because the link to the switch is not making a good connection, or because the contacts within the switch are dirty). Fuseholders, unlike headlamp bulbs) are not designed to dissipate heat, so the metal at the clip/fuse connection gets very hot and starts cooking the plastic of the fuseholder. The fuse does not blow because the current flowing is actually LESS than normal because there is greater resistance but the same voltage. The risk is greatest where current is high, and headlamps draw about 5 amps apiece, which means 10 amps through the fuse. If four sidelamps are powered via the same fuse, there will be additional current for those. Hope that helps. Ian Cornish Quote Link to post Share on other sites
Kiwifrog Posted August 16, 2009 Report Share Posted August 16, 2009 If I remember correctly the rated tolerance for glass fuses is between +/- 100% depending on the time so that it could take a long time for the fuse to blow. Looking at a manufacturers rated tolerance for 1.25 Inch fast blow glass fuses rated at 250V AC the open circuit times for different loads for fuses rated at between 0.1A to 30A are given below % Amp Rating Open circuit Times 100 None (ie will not blow) 135 60 Minutes Maximum 200 120Seconds Maximum That is to say a fuse rated at 25 A at 250V AC can carry 33A AC at 250V for 1 hour before it blows and will carry 25A forever without blowing which is why your fuse holder is getting hot. The fuse ratings are not given in DC but the AC rating gives 6250 W heating power at 25 A without blowing. As the fuse characteristic would be different under DC you can not make a direct comparison other to say that it would take a lot of current for the fuse to blow Needless to say check all the connections on the circuit that is heating up and clean up the fuse holder so that the resistance is reduced to a minimum then check each connection in the circuit with great care Cheers Alan Quote Link to post Share on other sites
angelfj Posted August 16, 2009 Report Share Posted August 16, 2009 IFuseholders, unlike headlamp bulbs) are not designed to dissipate heat, so the metal at the clip/fuse connection gets very hot and starts cooking the plastic of the fuseholder. This is not rocket science! The goal here is to deliver rated voltage to the load, which in this case are the headlamps. Even a perfectly designed system will experience a small voltage drop between the battery and the load. A properly designed fuse holder should have at least the same conductivity as the fuse/conductor/for which it was designed. Otherwise it would be contributing an unacceptable amount of series resistance. This would of course assume some acceptable level of voltage drop (total circuit less than 5 percent). So its nice to start with something like 12.5 - 13 volts and then size the wires and loads for proper voltage. If the fuse/fuse holder is truly too hot to touch , perhaps 140F, something is very wrong with this picture. Theoretically, the temperature rise of a conductor under ideal conditions would be perhaps a few degrees not 40 degrees as this case suggests. I suspect a really poor connection where the wire connects to the fuse holder and/or where the fuse clip "embraces" the fuse. the latter connection deteriorates with time and thermal cycling, to the point that the insulating plastic will melt and fires are not uncommon. I would also expect fairly dim lamps, but this wasn't reported. Quote Link to post Share on other sites
littlejim Posted August 17, 2009 Report Share Posted August 17, 2009 Just read Ian's explanation and can now see that the lousy high resistance connection at the end of the fuse turned into a 10 amp (wouldn't blow the fuse) radiator. 120 watts enough to melt the plastic. Quote Link to post Share on other sites
Menno van Rij Posted August 17, 2009 Report Share Posted August 17, 2009 Just read Ian's explanation and can now see that the lousy high resistance connection at the end of the fuse turned into a 10 amp (wouldn't blow the fuse) radiator. 120 watts enough to melt the plastic. ... and how my house nearly caught fire... Menno Quote Link to post Share on other sites
choc-ice Posted August 18, 2009 Report Share Posted August 18, 2009 I suspect a really poor connection where the wire connects to the fuse holder and/or where the fuse clip "embraces" the fuse. As he said. I don't know what sort of fuseholder you have, but it's well worth taking it apart and giving it a good clean with a brass wire brush or a fibreglass pencil. You say that you were plunged into darkness once so there's a bad connection somewhere. You mention you've got 3 fuses of 25A for your lights, can you explain which lights are on each circuit? 2 headlights on dip or main beam are less than 120W so that's 10 Amps, a 15A fuse should be plenty. Quote Link to post Share on other sites
ianc Posted August 18, 2009 Report Share Posted August 18, 2009 The resistance of a lamp filament when cold is considerably less than when it is incandescent, which means that although a pair of headlamps rated 60 watt will draw approximately 10 amps when fully alight, the inrush current will be considerably higher. For this reason, I would install a 20 amp fuse for a pair of headlamps. Ian Cornish Quote Link to post Share on other sites
rudi Posted August 19, 2009 Author Report Share Posted August 19, 2009 My self build circuit is based on Dan Masters Powerblock. The 3 fused big cables coming from the cut out are not for feeding the lights directly but to feed all electrics: one goes to the ignition switch, one to all electrics that are "non ignition powered" and one that feeds via a power relais all other when the ignition is on . It was the latter that got so hot the fuse holder melted. The fuse holder is a modern blade fuse holder and was/is new. The wires are fearly new as are the fittings. So a bad connection there is not expected. The fuse only gets hot with spots on, but as said does not blow. Quote Link to post Share on other sites
rudi Posted August 19, 2009 Author Report Share Posted August 19, 2009 forgot to mention that even when the fuse is hot the lights are visually giving the same light power both spot and head lights Quote Link to post Share on other sites
stuart Posted August 19, 2009 Report Share Posted August 19, 2009 I think I would then look at the light units themselves as they are the only things on that circuit. Stuart. Quote Link to post Share on other sites
rudi Posted August 19, 2009 Author Report Share Posted August 19, 2009 This evening I took an additional 4th big cable from the cut out, to feed the power relais. So I have now from the cut out over a modern 4 blade fuse holder Cable 1 : to ign. switch (than a cable from the ign. switch to the power relais. See Dan Masters power block) Cable 2 : to power relais (30to 40A) that feeds all electrics that are ignition powered (new) Cable 3 : to all electrics that are not ignition powered Cable 4 : to nr 30 of the relais of spot lights, head lights, horn, OD, fan, fuel pump. (With 3 big cables before this one was also feeding the power relais The 25A fuse of cable nr. 4 still gets hot when both spots and headlights are on. I have not tried if it also gets hot with e.g. spots and OD on. A 10A fuse blows when both lights are on. But that is normal I guess as both spot and head lights draw more tha 10 Amp. (18,5A or so). I also found the power relay to get warm. Any ideas? Rudi Quote Link to post Share on other sites
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