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I have found this sheet about fuses and it makes zero sense to me - a fuse rated 30A will only blow at 50A?

Does that mean I have to choose a lower value, like ~20A for my <30A application?

this

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    \$\begingroup\$ You choose a fuse based on what you are trying to protect. \$\endgroup\$
    – Andy aka
    Commented Apr 5, 2023 at 12:25

2 Answers 2

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Fuses are rated at the current that they will not blow. They will often also have curves that indicate the conditions at which they are guaranteed to blow. In between, there is no guarantee. e.g. A 30 A fuse may blow at 35 A after a long time.

So, due to part tolerance, your plan to use a smaller value fuse will not work.

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  • \$\begingroup\$ that is what i thought. the temperature, pressure will affect the fuse parameters. \$\endgroup\$
    – Ri Di
    Commented Apr 5, 2023 at 13:00
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Fuses are rated according to their normal current, not the trip current.

From Wikipedia

Rated current IN
A maximum current that the fuse can continuously conduct without interrupting the circuit. ...
Time vs current characteristics
The operating time is not a fixed interval but decreases as the current increases. Fuses are designed to have particular characteristics of operating time compared to current. A standard fuse may require twice its rated current to open in one second, a fast-blow fuse may require twice its rated current to blow in 0.1 seconds, and a slow-blow fuse may require twice its rated current for tens of seconds to blow.

The curves you show explain that yes, the fuse doesn't blow until it has had a certain amount of current for a certain amount of time. And specifically, a 30 A fuse should never blow at 30 A.

This stops the fuse blowing every time an inrush current briefly exceeds the normal operating current.

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