fusing current 1

[WhyDoYouAsk]

#14 AWG copper melts at 166A, but to calculate the I²T rating I need to know how many milliseconds it takes to melt at this current.
I know there is a link to this info but I've lost it.
Can anyone help?

 

[7anoter4]

The conductor has to reach 1084 degrees C [1983.2 F] that means about 12 sec [may be more if there will be some cooling [12 sec is not a short time so will be a heat dissipation to the outside]

 

[WhyDoYouAsk]

Thanks for your quick reply.

Is this a calculated value?
Do you have links to measured or calc'd values? The formula for fusing current is well known but I couldn't find time durations.

This would put the I²T value at 300,000 A²sec.

 

[Matsukaze]

Google "Onderdonk equation" and you should get some helpful information. Unlike the Preece equation for fusing current, Onderdonk's equation considers the time the current flows.

 

[WhyDoYouAsk]

Thanks; I'll put this formula in a spreadsheet and crank some numbers.

 

[7anoter4]

The relation is taken from IEEE 80 for bare grounding copper conductor:
Sqr.mm=I*SQRT(A/B*t) or:
t[sec]=(Sqr.mm/I)^2*B/A
A=alpha*Ro*10^4/Tcap
alpha=0.00393 ohm/Co for copper
Ro=1.724
Tcap=3.422
B=ln(1+(TA-TB)/(K+TB))
TA=1084 Co[copper fuse temp.]
TB=40 Co[ambient]
K=234 Co for copper.
I=0.166 KA
Sqr.mm=2.08 for 14 AWG.

 

[Clyde38]

WhyDoYOuAsk,
Try this..
Something I've had laying around for some time.

 

[7anoter4]

Very well Clyde38!But is the same thing as in my post except that it is in xls and use circular mils.The fusing time is the same [about 12.7 sec for 166A and 14 awg].

 

[Clyde38]

7anoter4,

WhyDoYouAsk (Electrical) 22 Nov 08 18:13
Thanks; I'll put this formula in a spreadsheet and crank some numbers....

Just thought I'd save him the trouble. . .

 

[cuky2000]

Clyde38,

Well deserved an star for the valuable information posted.

For application other than short duration for fusing bare conductor without static wind, adjustment may be required to account for the following effects:
* Conductor insulation on heat disipation for no bare conductors.
* Adiabatic or heat dissipation effect(include wind speed)
* Constant vs variable thermal capacity per unit volume
* Symmetric vs asymmetric (DC decrement factor, X/R ratio)

 

[waross]

When I was young, an old inspector gave me a reference from an old text in regards to using bare copper wire for fuse links. By a quirk of memory I still remember that #14 wire was rated at 140 amps when used as a fuse link. That would correlate with your figure of 166 A quite well. We expected fuse links to hold for more than 12 seconds at rated load.


Bill
--------------------
"Why not the best?"
Jimmy Carter