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Am I reading this diagram correctly? Top x-axis translates to "rated current for fuse". Bottom x-axis translates to "constant current through fuse in A". y-axis translates to "melting time in s".

Looking at the line for 10A fuses that would mean that the minimal ampere needed to melt a 10A fuse is about 18A which would take 10^3s (~16.6m). Can the current be lower if time is longer?

Diagram taken from here: Schmelzsicherung

Time till trip vs. current

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    \$\begingroup\$ Your conclusion is correct, but the definitions a bit wrong. There is no "top axis". The numbers on top have nothing to do with the grid, they simply indicate the rated currents of the corresponding curves \$\endgroup\$
    – Maple
    Commented Aug 5, 2021 at 17:36
  • \$\begingroup\$ I see it's just a labelling for the lines, thanks for pointing it out... \$\endgroup\$
    – CuriousIndeed
    Commented Aug 5, 2021 at 17:37
  • \$\begingroup\$ Why? The 2 after 10^1 on the x-axis refers to 20? Since the x-axis is logarithmic. Following where 2 intersects with the line for 10A we find a value between 10^2 and 10^3?? \$\endgroup\$
    – CuriousIndeed
    Commented Aug 5, 2021 at 17:47
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    \$\begingroup\$ you are correct, I've misread the power number, its too small for my eyes \$\endgroup\$
    – Maple
    Commented Aug 5, 2021 at 17:51

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Looking at the line for 10A fuses that would mean that the minimal ampere needed to melt a 10A fuse is about 18A which would take 103 s (~16.6 m).

Notice that the curve is vertical at that time. The fuse will last indefinitely at that current as thermal equilibrium will normally have been established by then (103 s). They don't bother to go out past 1000 s as it would give no additional information and would reduce the resolution of the graph.

Can the current be lower if time is longer?

We usually look at it the other way around. If current is lower then the time will be longer.

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  • \$\begingroup\$ So even If I run an appliance over a 10A fuse drawing 18A constantly it will never melt? I'm normally using circuit breakers where the maximum tolerated overcurrent is 10-15% depending on manufacturer \$\endgroup\$
    – CuriousIndeed
    Commented Aug 5, 2021 at 17:35
  • \$\begingroup\$ @Maple, we're following the green line down from the 10 A fuse. Drop vertically from the 10 on the top axis down to the bottom axis and you'll see that it intersects at around 18 A. \$\endgroup\$
    – Transistor
    Commented Aug 5, 2021 at 17:53
  • \$\begingroup\$ @Rubus, there will be a reliability factor in there and hence the graph is supplied so you can work it out. \$\endgroup\$
    – Transistor
    Commented Aug 5, 2021 at 17:54
  • \$\begingroup\$ @Rubus if it's a hot day it could melt \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Aug 5, 2021 at 17:59
  • \$\begingroup\$ @MathKeepsMeBusy, I'm referring to the 1000 s in the quote from the OP. I'll clarify. \$\endgroup\$
    – Transistor
    Commented Aug 5, 2021 at 21:39

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