Fuse characteristics

[morqthana]

Trying to get my head round this...

If I'm reading it right (??), then, subject to a bit (not too much) of ± due to line thickness, and by-eye interpolation, this says that at 6A, a 3A fuse should blow after about 90ms - 10 minutes.

But at 26A, a 13A fuse is between 4.5s and nearly half an hour.

Why aren't they the same, for the same multiples of rated current?

I can see why there would be a range, but if the X axis was "multiple of rated current" and not absolute values, why shouldn't there be the same shape of range for all fuse values?

 

[JohnW2]

Trying to get my head round this...

If I'm reading it right (??), then, subject to a bit (not too much) of ± due to line thickness, and by-eye interpolation, this says that at 6A, a 3A fuse should blow after about 90ms - 10 minutes. .... But at 26A, a 13A fuse is between 4.5s and nearly half an hour.

Agreed.

Why aren't they the same, for the same multiples of rated current? .... I can see why there would be a range, but if the X axis was "multiple of rated current" and not absolute values, why shouldn't there be the same shape of range for all fuse values?

Interesting question - but, other than for the application of 'probable common sense', I suppose there is not necessarily a reason to assume that fuses of different ratings will have similar operation times for the same "multiples of rated current", is there?

 

[morqthana]

That "other than" is a pretty big one. Yes, I would have expected the characteristics of a particular type of fuse to be common to all ratings. I'm sure when I looked at how MCBs behave they are all the same no matter what the rating?

 

[JohnW2]

That "other than" is a pretty big one. Yes, I would have expected the characteristics of a particular type of fuse to be common to all ratings. I'm sure when I looked at how MCBs behave they are all the same no matter what the rating?

Agreed - my suspicions/intuition would be the same as yours, but I can't be certain.

Those curves are easy enough to produce, so it's hard to believe that they are showing something which is not true, whatever the reason for that?

 

[morqthana]

I'm not suggesting they are wrong. More, I guess, that the whole "specification" would seem to be "wrong".

I was only looking because I was thinking about using panel mount rocker-switch thermal breakers as alternatives to FCUs or grid switches and fuses, and I was wondering if 15A ones would be equivalent to 13A fuses.

And from the makers data

I'd say yes.

 

[JohnW2]

I'm not suggesting they are wrong. More, I guess, that the whole "specification" would seem to be "wrong".

Fair enough, but I'm not sure what you mean. If what the curves are showing is correct then, as you pointed out, the fusing currents of 3A and 13A fuses are not related to currents which are 'multiples of the rating' in the way one would expect - in which case there must be some explanation for that?

I was only looking because I was thinking about using panel mount rocker-switch thermal breakers as alternatives to FCUs or grid switches and fuses, and I was wondering if 15A ones would be equivalent to 13A fuses. .... And from the makers data .... I'd say yes.

I can't disagree with your reasoning. However, if this related to an 'electrical installation' would the use of such devices be 'compliant' with anything (if that mattered to you)?

 

[morqthana]

It would be compliant in ways that mattered to me.

 

[JohnW2]

It would be compliant in ways that mattered to me.

... and me, but ......