Newsgroups: sci.engr.electrical.compliance
Subject: Re: Fuse ratings
References: <[email protected]>
"gab"  writes:

> Hy guys,
> the question is: why fuses are rated with voltage values too?
> Isn't it enough to indicate rated current?

Just the current is not enough for fuse rating. 

The votlage rating the the fuse tells at what voltage 
that fuse can stop the current flow when the current 
rating exceeds the current limit. 
If you use a fuse at higher voltage than for what it 
is rated for, then when the fuse blows it coudl happen 
that instead of current stops flowing, there will be 
a a strong arching inside the fuze that keeps the current 
flowing even through the fuse wire inside it has burned. 
Then the voltage on the circuit is withing fuse voltage 
rating, it can stop the current flowing.

Voltage rating is not the only thig that needs to be 
considered. There is also a thign called 
"current breaking capacity" that tells what is the maximum 
current that the fuse can safely stop. 
If you have a short circuit and power supply with higher 
current output capacity than the fuse current breaking 
capacity rating, what can happen is that the high current 
keeps flowing even the fuse is burned, because the high 
current can keep up the arch... This will definately 
do all kinds of harm, including excessive current still 
getting to your circuit and fuse heagin very radipdly 
(even causes some fuses to "explode". 

So when selecting fuses you need to look first the 
current rating you need for your application. 
Then you need also look at the fuse has high enough 
voltage rating and enough "current breaking capacity" 
for the application. The needed "current breaking capacity" 
depends on the application heavily, for example on normal 
mains powered equipment that is plugged to wall outlet 
you don't need very high current breaking capacity, 
because the normal sort circuit currents are not 
very high on normal outlets, and there is a second 
fuse on the mains panel bereaking current and catastrophic 
short circuit situatuions (the fuses on the mains panel 
hav much higher "current breaking capacity", they are 
designed to allow to stop the full short circuit current 
available from the power company to your house in away 
or another). 


> In fact I can use a 250V fuse also on a 400V supplied circuit.

250V fuse protecting a circuit that operates at 400V is 
not a good idea...

Can you describe more your situation. 

> Any suggestion?
> Thanx
> BR
> 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/