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# fuses and short circuit?

Discuss fuses and short circuit? in the Electrical Forum area at ElectriciansForums.net

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#### portokal

Welcome to ElectriciansForums.net - The American Electrical Advice Forum

we have 2 fuses with the same break value10 A..power supply, and one load on the end(bulb or thermal load).Now..it happens a short circuit on the bulb(thermal load).My questions are?.....which fuse break first and why??,,,fuse near to the bulb or fuse near to the power supply??.........
The order of circuit elements are>power supply,wire,fuse,wire...fuse and bulb(load.).....
I speak for ideal same fuses.I need math explanation of circuit flow(direction),...the energy flow from power supply to the load.if we have a short circuit on the load..which fuse break first..I say again..i speak for math(laboratoric) ideal model and i need math explanation from some electrical engineer.

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#### WarrenG

It would be difficult to say unless you had all the relevant info for the fuses in place.

Different manufaturers have different fusing factors so for example two different fuses at the same break value could peak at different times and blow first, irrespective of what position they were in?

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#### DanBrown

Its called ''discrimination''...
Ideally the fuse closest to the load should always be designed to blow first.

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#### Guest123

Hey.

As the man said different types of fuses different characteristics. All may have 10A overload but blow in different ways i.e HRC, slow blow maybe glass or ceramic.

Desription should be on fuse. It's unlikely that they would be exactly the same type, as has already been mentioned there would be no discrimination, which we spend most of our lives trying to achieve between breakers, RCD's etc.

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#### marshr02

Academically, if the fuses have identical characteristics, then the 'energy let through' means that the fuse nearest the supply would blow first - in that energy is flowing towards the load.

Even though current is a.c. (assumption) the energy flow in purely resistive cct is towards the load...

#### bonjovi

##### -
Arms
has anyone given a maths explanation no

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#### randyrat

The fuse nearest the load will go first, assuming they areabsolutely identical in characteristics. The short occurs at the load, and so this is the point at which there is the first acceleration of current flow, and therefore the fuse nearest to this point will see overcurrent first.
If you could see the ac waveform, at the point of short it would reduce slightly in peak voltage value. As V=IR, so therefore I=V/R, and when a short occurs, R decreases giving a corresponding increase in I, blowing the fuse. The time differential is going to be so small it would be unmeasurable between first fuse and second fuse seeing the change.
In practice, fuses are never quite the same, and the break current of each is always slightly different....

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#### portokal

Thanks to all for the answers..i get it.These is one of the best forums for electricians.bravo.

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