T
T.M.Haja Sahib
Guest
Do you agree that the resistance of a circuit comprising of its conductors resistance is linear?
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kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
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.
Do you agree that the fuse element will get hotter the more current it passes?
That's just yes or no.
T
T.M.Haja Sahib
Guest
Yes.But I am afraid you miss the point.
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?
That is just yes or no?
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
Yes.You are correct.
But Iam afraid you do not understand.
Please concentrate on post#66 to get back on track.
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.
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.
OK.
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
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.
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?
I dare say I could.
But, before you ask, I'm not going to.
T
T.M.Haja Sahib
Guest
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.
Which part of "I'm not going to" don't you understand?
John120/240
Senior Member
- Location
- Olathe, Kansas
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...
A grumpy old man...
Johnmcca
Senior Member
- Location
- Mid-Hudson Valley, NY
But full of experience, and you have a great way of letting us not so schooled understand.
Something I appreciate greatly.
Something I appreciate greatly.
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
- Occupation
- EC
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"
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