5*20mm 230V Glastube fuses?

[hantto]

I'm just wondering if one can use a 230V fuse in a for ex. 12V system? and would the fuse still burn out at the same current as in 230V, or does it get higher. (What i mean is if you put a 100mA 230V fuse in a 12V circuit, would it still burn out if the current passes 100mA?)

 

[kinjalgp]

The fuse can work for any voltage. Its the current through the fuse that blows it and not the voltage across it or the voltage of the whole circuit.

 

[hantto]

Thanks for making it clear!

 

[bogdanfirst]

well, i never understood why they type on the fuses the voltage...
it doesnt matter what voltage u use in the circuit, a curent passing trough it, larger than the value writen would blow it.....

 

[mechie]

Fuse Ratings

Try putting 10kV across a 240v fuse and you will find out why there is a rating :shock:

It has to be able to cope with the resultant arc and create a gap big enough to break it. some fuses are sand (silica) filled to bury the arc and so cope with a higher voltage than would otherwise be possible in a small fuse.

 

[bogdanfirst]

well i thought of this, but why would u use 2 kv for a 240v fuse, i referred to voltages wich were somehow near the nominal voltage

 

[herbymcduff]

I would not know why you would use a 2000 volt fuse in place of a 240 volt fuse with the same current rating. My only guess would be for safety reasons. :roll: :? :?

the 2Kv one would probably be ceramic or something or that sort, hince the 240 one would be glass.

I would just say, don't go lower on volts, but stay in at least a 200 volt range above.

 

[bogdanfirst]

well, ya, the high voltage ones are maybe in glass and filled with something like small grain sand in it, ore some are in ceramic cases, i saw some in a store and some guy said he wanted one for a car fan....funny no......?

 

[Phasor]

Just a comment on the sand-filled ones: They are generally known as HRC, or High Rupturing Capacity fuses. When the fuse blows, the heat from the arc actually melts the sand into a glass-like substance, which improves the breaking capacity. Typical breaking capacity for a small (100A!) fuse is roughly around 80kA, I think.

 

[bogdanfirst]

im not sure i understand this........"Typical breaking capacity"
i heard this notion b4, but i didnt understand clearly what it means....
maybe u could give me a better explenation..THNX

 

[mechie]

Fuse Ratings

My test multimeter has a current range, protected by a 440mA fuse.
The fuse is rated to blow at, typically, 440mA to protect the meter but can cope with 10kA.

If I connect the meter across the mains when set to amps range (STUPID ) there will be a finite time that the meter will carry a high current - way in excess of 44mA - this will be limited by the circuit impedance (mains supply, test leads and meter - guess at 1000 A ?). There will be molten copper where the test lead probe tips were. There will be an audible 'pop'.

When the meter designers chose the fuse (I hope) they will have considered the above and rated the fuse to safely break the circuit. The moten copper from the probe tips could be forming a growing arc now, due to the high temperatures causing the air to ionise.

My meter is now history but I survived to buy a new one :wink: (sounds like I tried it?).

HRC = high rupture capacity - now called ...
HBC = high breaking capacity
Typical breaking capacity = current that will eventually blow the fuse (could take a second or so)

FF = super-quick action (for semiconductor protection)
F = Quick Acting (normal fuses)
M = Medium (slower than normal !!!)
T = Time Lag (Slow-Blow)
TT = Super Time Lag

FF will blow in 1mS with 10 (TEN) times its rated current.
TT will last nearly a second with 10 (TEN) times its rated current.

All this for a thin bit of wire !

 

[herbymcduff]

Re: Fuse Ratings

My test multimeter has a current range, protected by a 440mA fuse.
The fuse is rated to blow at, typically, 440mA to protect the meter but can cope with 10kA.

I'm not understanding. If your fuse is rated to blow at 440ma, then how can it cope with 10000 AMPS :!: :!: :!: :!: :!: :!: :!: :!: That is sure alot if current, and if I was that little wire, I wouldn't want to find out what 10000 amps was. I would try to blow at 440mA.

So that is what I do not understand. Is my terms off, or what?

 

[mechie]

Fuses continued

If a FF (super-quick action - the fastest type) fuse will last for 1mS with 10 times its rated current then it will be seeing 440mA*10 = 4.4A. During this time it will be generating enough heat to melt the wire.
Extrapolate a bit and the fuse could last for 0.1mS with 100 times its rated current etc.

A fuse requires time to melt - the more extreme its overload the quicker this will be - but never instantaneous. It MUST therefore be able to cope (not explode into a fireball) with much higher currents than its rated working current.

Expensive equipment and safety-related gear (testmeters et al) will have HRC fuses, my homebrew power supply has a cheap LBC glass fuse.

 

[herbymcduff]

so you say, at 1mS it can handle 10 times its rated at 440mA. So that would be 4.4A

At .1ms its 100 times, so that would be 44A
At .01ms its 1000 times, so that would be 440A
At .001ms its 10000 times, so that would be 4400A
At 100ns its 100000 times, so that would be 44000A
and so on right?

 

[mechie]

Fuse curves

All I meant to say was that a fuse Can handle currents in excess of the value printed on them. They are simple bits of wire, not precision delay devices. Their reaction times are non-linear.

The attaced graph will give a feeling for their resposes (sorry its poor quality scan).

 

[herbymcduff]

thanks for clearing me up.