D
Den
- Jan 1, 1970
- 0
Group:
I'm going to embarrass myself now.
I've been thinking about a problem too much, and now can't see the wood for
the trees.
Can you just check this for me - I've been thinking about this too much, and
got myself confused.
Let's ignore, for the moment, that many fuses will allow more than the rated
current to flow through them.
I think that a fuse limits the amount of *current* through it. It does this
buy popping when the power (I^2.R) exceeds a limiting factor - since the
resistance of the wire in the fuse is constant, the factor is the square of
the current. This means that the fuse will pop at a given current,
regardless of the voltage. Despite this, fuses are rated in terms of
voltage and current. I assume that the voltage rating is a safety thing -
correct?. So, a fuse rated at 250V/2A will pop whenever >2A crosses it,
regardless of the voltage - correct?
Where I got myself confused (in two different ways!) was here:
1. A fuse (say) 3A rated at 230V will allow 690W of power to be drawn before
it pops. But 690W @ 115V is 6A. Therefore a 3A / 230V fuse is the same as
a 6A / 115V fuse; and
2. A fuse (say) 3A rated at 230V, implies the fuse has a resistance of
76.66Ohm. But 76.66Ohm @ 115V is 1.5A. Therefore a 3A / 230V fuse is the
same as a 1.5A / 115V fuse.
I sort of know (err hope) that 1 and 2 are wrong but am having difficulty
explaining to myself why! Can someone (gently) put me straight.
Cheers
Den
I'm going to embarrass myself now.
I've been thinking about a problem too much, and now can't see the wood for
the trees.
Can you just check this for me - I've been thinking about this too much, and
got myself confused.
Let's ignore, for the moment, that many fuses will allow more than the rated
current to flow through them.
I think that a fuse limits the amount of *current* through it. It does this
buy popping when the power (I^2.R) exceeds a limiting factor - since the
resistance of the wire in the fuse is constant, the factor is the square of
the current. This means that the fuse will pop at a given current,
regardless of the voltage. Despite this, fuses are rated in terms of
voltage and current. I assume that the voltage rating is a safety thing -
correct?. So, a fuse rated at 250V/2A will pop whenever >2A crosses it,
regardless of the voltage - correct?
Where I got myself confused (in two different ways!) was here:
1. A fuse (say) 3A rated at 230V will allow 690W of power to be drawn before
it pops. But 690W @ 115V is 6A. Therefore a 3A / 230V fuse is the same as
a 6A / 115V fuse; and
2. A fuse (say) 3A rated at 230V, implies the fuse has a resistance of
76.66Ohm. But 76.66Ohm @ 115V is 1.5A. Therefore a 3A / 230V fuse is the
same as a 1.5A / 115V fuse.
I sort of know (err hope) that 1 and 2 are wrong but am having difficulty
explaining to myself why! Can someone (gently) put me straight.
Cheers
Den