Fuse didn't Blow! Nearly a Fire!!

[sailorboy1]

Originally Posted by MarkJ

The fuse holder is warm but not hot.


Why didnt the fuse blow if it was that hot? Or can it burn before it gets to 20 amps?

Heat is resistance and amps related. Fuses only care about amps and don't give a crap about heat.

Now I'm just going to stay out of it

[model 10]

So if you fuse for the wire size and not the device and you use oversize wire, you end up with pretty high amp fuses, right?

[btrayfors]

The "fuse to protect the wire, not the device" imprecation is not inviolable. It's a general statement which generally means "choose a fuse size so that it will blow before putting too much load on the wire".

However, there's nothing wrong with using a smaller fuse, and there are lots of places where this might be a good idea. Some of these are in charging circuits where you might well choose "oversize" wire to provide an absolute minimum voltage drop, but where you know the load will never approach the amperage carrying capacity of the wire.

Also, note that in most instances where a particular device specifies a fuse size, it's a very good idea not to exceed that size. An example is a small bilge pump where, e.g., the recommended fuse size might be 4 amps. If you were to use a 5A fuse instead, and the pump were to be stopped by an obstruction, the pump could well overheat and burn up creating a fire hazard before the 5A fuse would blow.

A couple of years ago we did a demonstration of just this case at a West Marine customer seminar. Sure enough, the pump began to burn up.

Bill

[sailorboy1]

Hell in my solar installation I have a 15 amp fuse "protecting" a #8 AWG wire and a 30 amp breaker "protecting" a #6 AWG wire. Neither "protection" has anything to do with the wire, it has to do with the device.

[Cheechako]

Originally Posted by jgbrown

The humor is in the idea that the boat has schematics that relate to it's wiring to begin with. We have 6 58' Meridians of the same series that we maintain, none have wiring that is more than 50% the same, including a pair that are one digit apart(constructed one after the other).
Extrapolate that to a possibly older boat with owner made modifications and probably best to figure out what the wire is doing and why.

Yeah, that was my haha. In smaller boats, probably the best diagram that the boat ever had was the most basic system... maybe three to 6 fuses and the starter circuit. Add years to that and well........... I've never had a schem included in a mono or a cat to 47 feet. These boats were only a few years old... but if they existed... they were long gone.
Of course as mentioned by TekNav... one has to be aware of the total loads you might be putting on one circuit. But then the real world is that all those loads are never on at the same time anyway... so I guess we all compromise sometimes..

[GordMay]

Originally Posted by Don L

... Fuses only care about amps and don't give a crap about heat...

Fuses blow due to heat, which (heat) is a result of current flowing through the resistance of the fuse element.
Any source of heat could contribute to temperature rise, which would affect (reduce) the current carrying ability of the fuse element.

[Dsanduril]

Fuses have two ratings; the fault current at which they open (in this case 20A) and the maximum current at which they will open safely (the AIC rating). If a current source can provide more than the AIC (say your big battery with very low resistance) it is entirely possible that the fuse will continue to pass current, either as an arc or by melting and turning into a solid wire.

Thus the ABYC recommendations as to fuse types at the battery, the recommended fuses have a very high AIC, which means they are capable of opening even in the event of a dead short of the battery.

If you need a smaller fuse to protect something else (wiring, device, etc.) you put it in series with the high AIC fuse. So, you put one high AIC fuse right at the battery, its job is to deal with the dead short situations. You then put the device protection fuse(s) between there and the device, their job is to deal with fault currents that do not represent a dead short (and to protect your expensive high AIC fuse by opening first if you are lucky). When you use this series type scheme your smaller fuses have to have an AIC rating that is greater than the maximum let through current (yet another fuse rating) of your high-AIC "master" fuse.

In Marc's case, I don't think there was an overcurrent situation, so no reason for the fuse to blow. It looks like the connection had high resistance (probably corrosion from life at sea). This in turn leads to a reduction in current flow (so no blown fuse) but turns the connection point into a heater which melted the plastic. If there had been a load on the other side of the fuse it is possible that the voltage drop would have caused the current to increase and the fuse to blow, but solar panels are a current source, so the current does not rise in this case, it just keeps flowing at a steady rate until the voltage drop causes it to start to fall as it moves past the knee of the I-V curve.

[sailorboy1]

the fuse blows (burns out) due to too many amps though a metal with a given loading carrying capacity of resistance (the effect of the amps, not the heat)

but if you just heat the fuse it does "blow", because it cares about the effect of amps though it (ie the amps), not the heat


and yes if you heat the fues the resistance goes up, but we are talking practical application to boats not electrical theory class, do people go to the store looking for a fuse for 20 amps at X degress, or just at 20 amps

[Dsanduril]

Don,

fuses "measure" that current by how much heat is developed, so they do care about the environment in which they operate. The manufacturer's assume that the fuse can discard the heat they generate (from resistance) to the surrounding area at a standard rate. The table below is an example for one type of fuse.



If you look at this table, a fuse at 140F will open at 85% of it's rating, and in 70% of it's nominal time as compared with its rating at "room" temperature. For a fuse installed in an engine room in the tropics it would not be unusual at all to see these operating temperatures, and 20A fuse suddenly becomes a 17A fuse.

[thinwater]

As Mainsail pointed out, fuses only protect against one failure mode; to many amps for the wire. In fact, it is quite possible to get significant sparking and a fire from an insulation failure (either a cut, abrasion, a sharp spot somewhere...) without even getting anything very warm, far below rated capacity. Perhaps half of the electrical fires I have seen in plant operations were from arcing, not over current, and most from over current were localized, inside a motor.

Good connections and good chafe protection are critical where there is motion.

No, the melted fuse holder is not so surprising. The corroded spade is probably correct; some good contact grease would probably have helped. I'm obsessive about No-Oxid or something equivalent in damp locations.

[sailorboy1]

Originally Posted by Dsanduril

Don,

fuses "measure" that current by how much heat is developed, so they do care about the environment in which they operate. The manufacturer's assume that the fuse can discard the heat they generate (from resistance) to the surrounding area at a standard rate. The table below is an example for one type of fuse.

I'm pretty sure I've said as much, but I lose and everyone else wins.

Please search for:
I ain't no expert
I'm an idiot and lair
Things I think and do that are wrong

we are just talking freakin sailboats, not nuclear submarines (been there and done talk and we did just called it a freaking nuclear submarine and not a space scuttle)

PS - I have it on good authority that Mark has already fixed his problem so this is now a good old CF free for all and I don't really mind being the target

[noelex 77]

The fuse holder must have had a fair bit of resistance, probably for some time.

On the bright side, having now removed the defective connection, you should be getting a few more AHrs

[Dsanduril]

One thing to remember in a solar installation, the fuse is only intended to protect from too much current running from the battery to a shorted wire/component on the solar side, and not the other way around.

A panel capable of putting out 14A will never put out more than that, it is a current source. It may put out less if the light level is lower than the rated level or if the voltage is higher than the rated voltage, but it simply won't put out more than 14A. So, there really is no point in putting any kind of protective device with the thought that the solar panel will overload the system. Instead, the protective device is only there to prevent withdrawal of too much power from the battery, as might be caused by a short of some kind. Thus the protective device must be able to handle a dead short of the wire when fed by the battery being used.

[bill good]

This is a good lesson on using the wrong part for the job because it looked OK for the job. How many other parts have been added to the boat over years that could cause problems. Mark the lessons are never over but sometimes it does come back to what is best for the job. Another mistake is not having a switch to turn off the panels if something is heating up.

Regards Bill

[nimblemotors]

See, I knew solar panels are too dangerous for a boat, they can cause fires! Unsafe!
You should use diesel instead.