Fuse failures

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T.M.Haja Sahib

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For the VI characteristic to be linear, the resistance would have to be constant.
Do you not agree that it changes with temperature?
And that the change is appreciable during the pre-arcing period as the element ruptures?
Do you agree that the resistance of a circuit comprising of its conductors resistance is linear?
 

kwired

Electron manager
Location
NE Nebraska
Do you agree that the resistance of a circuit comprising of its conductors resistance is linear?
He stated that the change in resistance vs the change in temperature is not necessary linear.

Some of the details of what goes on here is above and beyond what I am used to dealing with but I got that one easily.
 
T

T.M.Haja Sahib

Guest
OK.The temperature of the arc of the fuse on fault remains constant,when the arc is stable.
That is just yes or no?
 

Besoeker

Senior Member
Location
UK
OK.The temperature of the arc of the fuse on fault remains constant,when the arc is stable.
Can we deal withe the question I posed in post #65.
You have agreed that the fuse element will get hotter the more current it passes.
If it is hotter, will its resistance be greater?
That's just yes or no.
 
T

T.M.Haja Sahib

Guest
Can we deal withe the question I posed in post #65.
You have agreed that the fuse element will get hotter the more current it passes.
If it is hotter, will its resistance be greater?
That's just yes or no.

Yes.You are correct.
But Iam afraid you do not understand.
Please concentrate on post#66 to get back on track.
 

Besoeker

Senior Member
Location
UK
Yes.You are correct.
I was simply dealing with your point about the pre-arcing VI characteristic. Your comment that it is linear is simply not supported. I'm pleased that you have now acknowledged that.
But Iam afraid you do not understand.
Don't understand what?
You must know by now that my main fields of endeavour are electrical engineering and power electronics. Semiconductor device protection is a significant part of that latter field.
To that extent at least, I have had to know the i2t of both the fuse and the device that it is protecting. Data sheets give me that for the the device and I can deduce from that and ITSM how it will perform under conditions that differ from manufacturer's data. Like calculated Tj at maximum duty for the application......stuff...
The fuse operating characteristics I can generally get from the published values in BS7671 for BS88 fuses.
Of course that is a superficial description of my superficial understanding.

Please concentrate on post#66 to get back on track.
OK.
The temperature of the arc of the fuse on fault remains constant,when the arc is stable.
Since the arcing phase of rupture of the fuse element is usually not more than some ms, most in the field would consider that to be a transient rather than stable mode of operation.
Don't you?
 
T

T.M.Haja Sahib

Guest
Since the arcing phase of rupture of the fuse element is usually not more than some ms, most in the field would consider that to be a transient rather than stable mode of operation.
Don't you?
This thread deals with fuse failures i.e what happens when the arc of the fuse interrupting a fault tends to be stable.
 

Besoeker

Senior Member
Location
UK
This thread deals with fuse failures i.e what happens when the arc of the fuse interrupting a fault tends to be stable.

Here are some time-current characteristics for some fairly standard fuses.

BAO40.jpg


The fault current into our test bay is in the region of 40kA.

Under fault conditions, all of those fuses would operate within 0.01s. That is 10ms. Half a cycle of the supply.
I'd find it difficult to construe an event lasting half a cycle of the supply stable.
Wouldn't you?
 
T

T.M.Haja Sahib

Guest
Is your test bay well equipped to enable you to plot the VI characteristic of the arc of a fuse while it interrupts a test fault?
 
T

T.M.Haja Sahib

Guest
I dare say I could.
But, before you ask, I'm not going to.

Please go ahead.

Take into consideration the circuit resistance in series with the fuse.This may be taken into account while plotting the fuse arc VI charactristic.

Draw the circuit voltage as a horizontal line.

The combined fuse arc and circuit resistance characteristic may either intersect the circuit voltage line at two points (very rare occurance usually) or it may lie above it (very usual occurance).In the first case the fuse would fail catastrophically and in the latter case it would simply operate.In your test bay,I think you may observe either event by changing the circuit resistance.
 

Besoeker

Senior Member
Location
UK
Besoeker, you must have the paitence of Job.
Kind words, thank you.
I'm an old fellow and somewhat past my sell by date.
Maybe age makes a person more mellow. Or more grumpy. I've known of a few irascible old faarts. And some younger ones.
You probably have too. I'm Victor Meldrew, I am...:D
A grumpy old man...
 
T

T.M.Haja Sahib

Guest
A current limiting fuse if used at a fault current level lower than that it is designed for does not have its cut off function and thus it 'fails'.
 

kwired

Electron manager
Location
NE Nebraska
A current limiting fuse if used at a fault current level lower than that it is designed for does not have its cut off function and thus it 'fails'.

If available fault current is low then maybe a current limiting fuse is not needed in the first place.

Besides when there is fault current don't we want fuses to "fail":p
 
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