# fuse ratings

#### mlkcampion

Joined Nov 16, 2006
59
Hey Forum
I have a fuse that is rated for 250v 4amp, what is
the rating for this fuse at 24vDC, i.e. what current
will it blow at?

Thanks
Michael

#### PointGiven

Joined Sep 10, 2005
7
It will blow at 4 amps.

#### mlkcampion

Joined Nov 16, 2006
59
Thing is the circuit is drawing more then four amps for hours, hence my
question on the subject?

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

Many fuses are rated to blow at the stated value after some time interval. The rating is a bit unrealistic is you want a fuse to pop immediately at the stated current. Most want to see 115% of the rated current befre they blow immediately. A circuit breaker will perform better that way.

Part of your problem is the lower voltage. Fuses fail when they are heated to the melting point by the power dissipated in the element. Power is the product of the voltage times the amperage. Your 4 amp 240 volt fuse will carry more current at 24 volts, as it does not get as hot.

#### mrmeval

Joined Jun 30, 2006
833
A 4 amp fuse blows at 4 amps. The voltage rating is there to guarantee there will not be an arc over. You can pick and choose the specifications on a fuse so that it will have zero delay in blowing or will allow small excursions over the rating, the difference between 'fast blow' and 'slow blow'

#### pebe

Joined Oct 11, 2004
626
Hi,

Many fuses are rated to blow at the stated value after some time interval. The rating is a bit unrealistic is you want a fuse to pop immediately at the stated current. Most want to see 115% of the rated current befre they blow immediately. A circuit breaker will perform better that way.

Part of your problem is the lower voltage. Fuses fail when they are heated to the melting point by the power dissipated in the element. Power is the product of the voltage times the amperage. Your 4 amp 240 volt fuse will carry more current at 24 volts, as it does not get as hot.
Surely the power dissipated in the fuse = I*I*R. That's not voltage dependant.

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

If you figure it by E*E/R, it is.

#### pebe

Joined Oct 11, 2004
626
Hi,

If you figure it by E*E/R, it is.
The voltage drop across the fuse(E) will be the same for both circuit voltages

#### beenthere

Joined Apr 20, 2004
15,819
Hi Pebe,

Good point.

#### mlkcampion

Joined Nov 16, 2006
59
Hey Forum
That discussion bout the power is exactly what i was looking
for, it was that which was bothering me!! Brilliant!!
Cheers for that!!!!

Michael

#### subtech

Joined Nov 21, 2006
123
Hey mlkcampion
If you will do a google search using the words "fuse melt data" you will get lots of info on different kinds of fuses and their applications. Bussman, Cooper, S&C and many others provide detailed data and "curves" to help you make the right choices in circuit or equipment protection.