12 & 240v Fuses

[robbieg]

Can someone settle an argument. If you use a 13amp 240v fuse in a 12v system will it blow when a 12v appliance drawing more than 156w (ie 12v x 13amps) is connected to it or does it require a 12v appliance to draw more than 3120w (ie 240v x 13amps) before it will blow.

Probably very simple to those who understand these things but we got through a bottle of red discussing it & still(!) couldn't work it out..

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[bedouin]

Fuses are rated for amps so it will blow at 13amps whatever the voltage.



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[VicS]

I believe I am right in saying that a domestic fuse will pass the rated current indefinitely without blowing, needing rather more to blow it. By contrast a car type 12volt fuse is designed to blow at the rated value, so there is a subtle difference between the two.
There are also quickblow and time delay fuses just to further complicate the matter.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[Colin_S]

Disregarding the fact that fuses will mostly need more than their rated current to blow, the fuse should blow on amps and not watts, i.e. 13a at any voltage.

Remembering back to my physics lessons at school, the size of electrical cable is dictated by the ampage that runs through it. If you take the example of the high tension cables that run along elctricity pylons, they would need to be vastly thicker if they were carrying 240v instead of the 1,000's of volts they run at if they were supplying the same overall wattage.

If that doesn't make sense then blame that Australian product who's current TV advertising campaign requires dodgy hairdoos to drink in the sun!


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[Leisure27]

I too have tried to get my head round this. The answer as has been said is that the the 13 amp fuse will blow at 156w on a 12 volt system but will blow at 3120w on a 240 volt system.

But if a fuse blows at a given amperage then I still don't understand why the fuse I was replacing had the voltage stamped on the side. Unless it is some indication of physical size?

Brendan.

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[MainlySteam]

<<<why the fuse I was replacing had the voltage stamped on the side>>>

It is only a rating from an insulation/safety point of view - in a similar way that a switch may be rated for 12v but not 240v. Most of the common glass fuses (3AG) are rated at least at 125 v, but in countries with 230v ac they are always available at 240v rating.

The fuse can be used as long as its voltage rating exceeds that of the application it is in ie a 240v 1 amp fuse can be used in a 110v application but a 110v 1 amp fuse should not be used in a 240v application. Both the 110v and the 240v 1 amp fuses will, however, blow at 1 amp or above (amount above and time to blow depending on the characteristic of the fuse itself).

If the same fuse is put into a lower voltage 12 v application the minimum current that it blows at will be unchanged.

However, for the same downstream fault causing the fuse to blow, the fuse will blow quicker, and maybe more explosively, in a higher voltage application than in a lower voltage application (because for the same fault the fault current will be greater, that being driven by the higher voltage, and the greater the current over the fuses current "blow" rating the faster it will blow).

John

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[bedouin]

In fact fuses come in different "flavours" such as quick blow, anti surge which have different characteristics as to how quickly they blow at various levels of current. A typical mains fuse is unlikely to blow at much less than twice the rated current however long it is sustained and even at 3 or 4 times rated current it can still take 2-3 seconds to blow

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[lumphammer]

A 13A fuse will stand 13A indefinately, I don't have the tables to hand but from memory the length of time to blow will be anything from hours to milli-seconds depending on the fault current. The fault current will depend on the cable cross section area and the length.
As the fuse is there to protect the cable, you will need it to blow, before the fault current melts the cable.

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[cindersailor]

If you think of electricity as a flow of electrons through a conductor (eg. copper) the voltage (V) can be thought of as a measure of how much energy each electron is carrying (its potential). The current is the number of electrons passing through the circuit and is measured in Amps (1 Amp = approx 6.25X10 to the power 18 electrons per second!). The power of an electrical devise is measured in Watts (W) and is quite simply calculated by W = VxA which is easy to understand if you consider that each electron gives up the amount of energy it is carrying to the device as it flows through it. The ability of a given conductor (eg. copper) to carry a current is dependent on its dimensions, the thin wire in a fuse will heat up, due to the resistance (another story!), as the amount of current (No. of electrons per second) increases until it reaches its melting point. It is the number of electrons flowing, not the amount of energy each is carrying, that results in the heating effect. So it is easy to see that the rating of a fuse has everything to do with current, but is unrelated to voltage.

Sorry, physics lesson over, its a slow morning!

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[andyball]

a household 13A fuse will not blow at 13A....it may get warm though, & at higher currents very hot, still without blowing....it will behave just the same in a 12V circuit.

some auto fuses are actually marked " xA continuous- 2xA blow" which seems about right for non-specialised fuses


Worth checking what you've got, since I've noticed many near-fires under motorcycle seats due to fuses overheating (but not blowing) due to the incorrect rated fuse being fitted by the owner. eg, place a 10A blow fuse in a circuit carrying say 8 or 9A & it will get very hot. A quality fuseholder will cope with this, but those with plastic covers etc can & do melt alarmingly, all without blowing the fuse.

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[William_H]

Consider then how a fuse works. The current running through the resistance of the fuse wire causes it to heat until at some point the wire actually melts and the circuit opens. Thicker wire has lower resistance and a greater ability to disipate the heat so higher current rating. A longer fuse wire has more resistance but also more ability to dissipate the heat so the length of the fuse wire does not matter very much. A really short piece of wire would introduce less resistance to the circuit and so be ideal, however when the fuse wire melts there is a tendency to arc across the gap caused. So at low voltage say 12 volts there is not much tendency to arc across the gap when the fuse blows but at 240 volts or higher the arcing becomes quite significant. If the arcing melts more of the wire no problem because the gap gets bigger, but if the wire melts to the ends and the arcing continues until there is an arc between the caps then you have a fire in the fuse box. So the fuse must be rated for voltage as being its ability to quench an arc by consuming the fuse wire (after the fuse has initially opened from excess current). You can imagine then that a long fuse would have a higher voltage rating and a short fuse would have a lower voltage rating regardless of current rating or fuse wire thickness, though there is probably more to it than that.
Incidently AC has less tendency to arc than DC because the current and voltage drop to zero 50 times per second not that that dropping to zero always stops an arc if there is lots of ionised air around but would be less of a problem than same voltage DC hence the AC rating on the fuse.
So there is food for though for a few more bottles of red . good luck regards will

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[Guest]

Can I ask a question ?

I sued to cut foam wings for radio control models ...... we used to use the Nicrome wire from mains electric heaters. Pull wire out of ceramic tube, stretch it straight and then cut a 1.5mtr length. This was then secured across a 'long-bow' style frame. Each end had a lead going of to connect to a 12V source...... one end to +ve, other to -ve.

Now my question is ..... as a 240V item - it glowed red hot and produced the required heat...... approx 1kw per wire. Now connecting it to the 12V - it doesn't glow red-hot but only produces less heat - but enough to melt through foam - its enough to give skin a nasty as well !!

But shouldn't the wire glow / actually fail if connected to 12V source if amps or elctron si only reason ?

Sorry if its a stupid post of mine.


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[bedouin]

Re: Can I ask a question ?

You probably used proportionally more wire for the 12V than you had for 240V.

Had you had 1/20th of the length you would have expected to glow in the same way. Using more wire = greater resistence = less heat (power/heat is inversely proportional to resistance).

Varying the length of wire used would change the amount of heat generated - so have the length and you get double the heat, what is more that heat is distributed over a shorter length so in effect you get 4 times the heat per unit length of wire.

It's not quite that simple as once you get to that sort of temperatures the resistance of the wire is non-linear, but the general principle is good



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[tome]

Re: Can I ask a question ?

Nigel

The resistance of the wire is fixed (per length) and the power (ie heat given off) is proportional to the square of the voltage applied across it.

Therefore if you reduce the voltage from 240 to 12 you will only develop 1/400th the power for a given length.

Still plenty enough to burn!

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[brianrunyard]

Fuses are current rated not wattage rated, so yes it will blow at the same current regardless of voltage.

<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard