Fuse failures

Status
Not open for further replies.
Besoeker said:
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?
Click to expand...
Do you agree that the resistance of a circuit comprising of its conductors resistance is linear?
 
T.M.Haja Sahib said:
Do you agree that the resistance of a circuit comprising of its conductors resistance is linear?
Click to expand...
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.
 
OK.The temperature of the arc of the fuse on fault remains constant,when the arc is stable.
That is just yes or no?
 
T.M.Haja Sahib said:
OK.The temperature of the arc of the fuse on fault remains constant,when the arc is stable.
Click to expand...
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.
 
Besoeker said:
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.
Click to expand...

Yes.You are correct.
But Iam afraid you do not understand.
Please concentrate on post#66 to get back on track.
 
T.M.Haja Sahib said:
Yes.You are correct.
Click to expand...
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.
T.M.Haja Sahib said:
But Iam afraid you do not understand.
Click to expand...
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.

T.M.Haja Sahib said:
Please concentrate on post#66 to get back on track.
Click to expand...
OK.
The temperature of the arc of the fuse on fault remains constant,when the arc is stable.
Click to expand...
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?
 
Besoeker said:
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?
Click to expand...
This thread deals with fuse failures i.e what happens when the arc of the fuse interrupting a fault tends to be stable.
 
T.M.Haja Sahib said:
This thread deals with fuse failures i.e what happens when the arc of the fuse interrupting a fault tends to be stable.
Click to expand...

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?
 
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?
 
Besoeker said:
I dare say I could.
But, before you ask, I'm not going to.
Click to expand...

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.
 
John120/240 said:
Besoeker, you must have the paitence of Job.
Click to expand...
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...
 
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'.
 
T.M.Haja Sahib said:
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'.
Click to expand...

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
 
Status
Not open for further replies.
Top