Author Topic: Fuse Question  (Read 961 times)

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Offline bostonmanTopic starter

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Fuse Question
« on: February 05, 2020, 03:18:11 am »
I'm having a work related problem regarding fuses.

It's regarding maximum A^2 * S

I'm given two columns, the first has the title Peak Current (A) and one of the values is 30. The second column has Maximum [indefinate integral] I^2 * t , A^2 * s and has a value of 90.

I understand the peak current being 30A, but don't understand the second column with regards to 90 being A^2 * s.

I looked on LittelFuse, but didn't find anything directly explaining this. Basically I don't understand the connection between the indefinate integral, A^2 * s, and what it means with the value of 90.

Can someone help end my confusion?


 

Offline helius

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Re: Fuse Question
« Reply #1 on: February 05, 2020, 04:57:31 am »
The main parameters that are relevant to fuses are current capacity, arc breaking or interrupting current, speed, and energy absorption.
The current capacity is the current that the fuse will pass without blowing over any amount of time. It's the intersection of the current diagram with t=∞ and is notated in amperes.
\$ \int I^2 dt \$ is the area under the curve represented by the current squared on the Y axis, with time on the X axis. The idea is that this function looks like a hump rising up from (near) zero at a certain point, and then decreasing back to zero since the circuit is broken. Power is proportional to current squared: \$ P = I^2 R \$ , so the area under the curve is power times time, or the energy the fuse is rated to absorb. I would expect this parameter to be in kilojoules.
 

Offline SG-1

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Re: Fuse Question
« Reply #2 on: February 05, 2020, 05:42:47 am »
I2t (Amperes Squared Seconds) This is a value obtained by multiplying an effective current squared by the time of flow of the current in seconds. It is not a heat energy value, but represents heat energy for comparison purposes. Some common uses are to determine fuse selectivity and to select current limiting fuses that will limit this value to be compatible with the withstandability of semi-conductors that have an I2t rating.
Advice is a dangerous gift, even from the wise to the wise.
 

Offline bostonmanTopic starter

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Re: Fuse Question
« Reply #3 on: February 05, 2020, 01:23:26 pm »
Quote
∫I2dt is the area under the curve represented by the current squared on the Y axis, with time on the X axis


So to write it correctly, am I integrating I^2t dt or I^2 dt?

Also, if I integrate, I get I^3/3 * t +C. What value do I use for C?

 

Offline bostonmanTopic starter

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Re: Fuse Question
« Reply #4 on: February 11, 2020, 09:22:43 pm »
Quote
So to write it correctly, am I integrating I^2t dt or I^2 dt?

Also, if I integrate, I get I^3/3 * t +C. What value do I use for C?


Just realized, I didn't get clarification about this.
 

Offline rstofer

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Re: Fuse Question
« Reply #5 on: February 11, 2020, 10:09:09 pm »
Quote
∫I2dt is the area under the curve represented by the current squared on the Y axis, with time on the X axis


So to write it correctly, am I integrating I^2t dt or I^2 dt?

Also, if I integrate, I get I^3/3 * t +C. What value do I use for C?

To evaluate C, you need an initial condition.  Just evaluate the expression for t = 0 and see what happens to C.  Usually, you will have an expressions like f(t) = {the integral stuff} + C and C = f(0).
 


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