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# difference between interrupting current and normal current in fuses

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#### mamech

##### Full Member level 3
hello

I am in need to choose a suitable fuse for an application, but when I read about fuses, I found some details that I do not understand.

there is a term called "interrupting current" or called also "breaking capacity" , and it seems that this is the current that the fuse will be damaged if you pass this current in it.

there is another term called "current rating" or called "nominal amperage value", and it seems that it indicates the nominal current that is supposed to flow in the fuse in normal conditions.

I am confused now.
in past, I thought that when I see a fuse, I look at the number on it, so when I find it is written "5 A", then this meant to me that this number is the max current that can flow, so if the circuit draws 5.1 A the fuse will damage.

now I do not know what is the number written on a fuse means?
and what is the relation between current rating value and interrupting current value?

and more importantly, if I am trying to choose a fuse, if my load may draw current from 1 A to 15 A, what are the supposed values of "current rating" and "interrupting current" of the fuse to protect this load from damaging?

thanks for help

The Breaking Capacity is for surge currents that exceed average. To protect against unwanted surge failures that result in thermal failure, you want the fuse to fail before the device. So a breaking rating of 10x normal fuse rating would be normal for a low Breaking Capacity. Typically fuses also have time delays so the fusing time is rated by I²t for some value of standard benchmark. Fuses are not precise enough to trip 2% above the rating.

So the rated current is the average it can withstand and the breaking current is the value designed to break for a short time such as 5 ms which will be at least 10x the rated current.
This delay varies from SMD fuses to glass tube to slow blow which has a high Breaking Capacity as opposed to a low Breaking Capacity for a fast fuse.

mamech

### mamech

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thank you very much. this is exactly what I need to know.

but I have 2 questions

1) the number written on a glass fuse for example, it indicates the nominal current? or the breaking current?

2) regarding fusing time. I heard about kind of fuses that are used to make protection of home appliances against lightening shocks. I heard that its fusing time can reach to some nano seconds so they can stop the lightening shock (which is in range of nano seconds too)

what is the name of such kind of fuses?

thank you very much. this is exactly what I need to know.

but I have 2 questions

1) the number written on a glass fuse for example, it indicates the nominal current? or the breaking current?

2) regarding fusing time. I heard about kind of fuses that are used to make protection of home appliances against lightening shocks. I heard that its fusing time can reach to some nano seconds so they can stop the lightening shock (which is in range of nano seconds too)

what is the name of such kind of fuses?

Have you calc. how far does current travels in a nanosec. ?

1) normal max
2) lightning is 10 microseconds typ duration and break current prevails but may not protect device
no such nanosec fuse.
maybe you read about static discharge in a small gap

hello

there is a term called "interrupting current" or called also "breaking capacity" , and it seems that this is the current that the fuse will be damaged if you pass this current in it.

If a physically small fuse is subject to a huge overload, then the fuse wire inside it is turned into a conducting gas and current continues to flow, even though the actual fuse wire has broken.
If you compare a 500A, 600V HRC fuse with a 13 A 230 V. First of all HRC is high rupture current. So the 13A fuse would be about 25mm long by 6mm diameter. The 500A fuse is huge, about 150 mm long and 50 mm diam. This is because the fault current on a domestic supply rarely exceeds 5 KA, whille on a 440 V supply it will be over 100 KA. So for a 2 1/2 increase in voltage rating the 500A fuse is 6 times longer. It will also be filled with sand. This is to stop the gas plasma forming by conducting the heat away.
Frank

mamech

### mamech

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