Fuses

No need to apologise for adding some humour – I just wanted to ensure that it was clear that the voltages were AC and DC and that my question was clear.

Good work on making it sound feasible on the first read.
 
(doh! forgot to login before replying :oops: )

Here's my thoughts. I'm probably going to prove that the less you know about something the easier it is to (mis)understand. ;)

Anyway, here goes:

I'm going to use an equation I learnt a long time ago: V=IR (Ohm's Law voltage = current x resistance).

For a 240V 10A fuse we get: 240 = 10 x R, giving R a value of 24 ohms.

For the 12V 10A fuse it's: 12 = 10 x R, making R = 1.2 ohms.

Now, if we take this last result and shove it back into the equation in a 240V circuit we get:

240 = A x 1.2, which makes A = 200 amps.

In other words the fuse won't blow until we stick 200A through it at 240V.

You'll notice that I've ignored the AC/DC aspects of this (sorry Angus), that's partly because I'm not sure it's relevant.

Of course I could be completely wrong, but to the unenlightened I think this is a plausible explanation. At worst I'll be told why I'm wrong, so I'll learn something new (and that's why I joined CB in the first place).
 
There are many different fuse types, some DC rated fuses are not as sensitive to overcurrent as AC rated fuses. And some have a time delay factor so that they don't blow due to startup voltage surges. So a DC rated fuse might actually be letting 2 to 5 times as much current flow without breaking the circuit.
 
The AC fuse has more "stress" put onto it becuase of the current always switching directing. It goes for and back 50 or 60 times a second (might be different depending where you are). This causing the fuse to heat up more since its like rubbing to sticks together at one point.

The DC fuse deals with direct current meaning the current always goes in one direction so the easiest way to blow it would be to put too much current through it. This would be like pushing something on castors, the faster you push it the more of a chance it has of falling over.

If you want to get all technical about it, then dimwatt is on the right track with the Ohm's law thing with V=IR (voltage = current x resistance). The more resistance it has, the more it would heat it due to the power loss and eventualy blow. Some fuses have a power rating on them as well as a resistance so you can use P=IIR (power = current x current x resistance) to get the total current that can flow through.
 
an arc fault occurs when the current jumps from one cable to the next, usually its between two cable halfs that have been broken. These give off a small disturbance in the power line. When the arc-fault breaker detects the disturbance, it shuts off.
 
This one had to be my favourite:

dominicgross - taken from [url said:
http://www.blue-room.org.uk/index.php?showtopic=5731&st=0]While[/url] doing a tour in sweden we noticed that all the 13amp (euro style) plugs had had nails put in the fuse holders!

When asked the reason for this they answered - 'We have good earth'...???

I will be in Sweden and Denmark next month, so I will keep my eye out for such novel uses for nails!

Recently received this explanation from a friend of mine:

As for the question - current is current, irrelevant of the voltage that it is at as far as a fuse is concerned. IE if you take the 10 amp 240 volt fuse and slap it into the car and then try to pull 20 amps it will go futt.

Where the difference is, is a 240 volt fuse is capable of interrupting the current flow of a higher potential than the 12 volt jobby without exploding/shattering and generally f**king over your fuse holder.

As for the heat thing, you generally find that the car fuses (read low voltage) are not as well made (read cheap) as the 240 volt ones and as such tend to give off more heat and when used in a screw in type fuse holder they have a higher contact resistance (more heat) as they generally do when used in the clip-in varieties that have larger contact areas and so on.

The main reason though is the afore mentioned ability to break higher voltages, as, when the fuse goes splat it really does go splat leaving a fine coating of copper all over the inside of the glass/ceramic tube. This allows the possibility of conduction if you then add ionized gas (created when the fuse goes futt) to the formula and say an inductive load.

Now the other problem you get is most fuse holders that you come across will tarnish with time equalling more heat which equals faster tarnishing which equals more heat which... You get the picture.

And there is the other problem, the users. The amount of time that I have seen 20 amp car fuses come out of dimmers begs disbelief. This is a good way to really f**k things up because not only do you toast the fuse holder you toast the cable as well.

240 volt fuses tend to be made of ceramic or a high temp glass to withstand the high temperature of a piece of copper/nichrome turning to liquid and the HRC variety then add a sand fill to snub out any arcing if it occurs.
 
rgsw said:
to put it in lame mans term:

I know this is awfully off topic, but it's "Layman's terms."
 
I think another part of a fuse I didn't see mentioned isn't just to protect the internal hardware of a device but also to provide a safty. As you said in the one you looked at something had cought on fire. I don't know where you or he lives or anything but if the entire unit had done so on dry grass or something flammable he could have ended up with alot more serious damage than a broken sprinkler timer.

As another question, we use SCRimmer Stik, they are just basically portable dimmers. Each of them has 4 sockets for a light and each has its own fuse. We have been having this problem where a fuse will blow but then since it is glass will break inside the Stik and basically make it useless. (The Metal end stays in the bottom). This isn't a trational just push in type fuse with 2 holders its more like a hole and you drop it in. Are there any such things as fuses with high temp plastic on the outside (with metal rings on the ends and eveyrthing) that might stop these from breaking when we try to get them out?

~Nick
 
I don't know what all this means so if anyone wants to explain it please feel free to. I looked on the EDI website for <A HRef="http://www.edionline.com/downloads/P155-SCRIMMER_STIK_0205.pdf>SCRimmer Stiks </a> and I'm pretty sure it's one of these:
"
Surge Current -- 250Amp Peak, 1 cycle
or more likely,
Fused Output -- Four 6.25 Amp or two 10 Amp 3 AG Ceramic Fuses.

The Stiks that we use have four fuses in them.

Thanks,
Nick
 
Sorry in that last thing, I messed up some HTML somewhere in the link that I made and it turned the whole end of that into a link. It goes to the online product info for the SCRimmer Stiks that we use.
 

Users who are viewing this thread

Back