Xjyuk

I'm looking at the datasheet for the Bel 0ADKC /0ADKP fuses.

In it is this:

These curves don't seem correct. Notice that the curves for fuses with >= 800mA asymptotically approaches a value that doesn't match their rated current.

Is there some assumption I am unaware of? Or is this graph in error?

This isn't the case with other curves that I see, but those were fast-acting fuses and these are time-delay fuses.

For comparison these are the curves for the fast-acting Bel 0DAC/0ADAP fuses and the curves do approach the rated current.

They don't match the blow current. The graphs are the current that the fuses blow at (not the rating with one number). The reason for this being is it takes temperature to blow the fuses and then the fuse material must break down. This process does not happen instantaneously, its a phase change process. So the longer you dwell above the temperature, the more it will break down. If the current is exactly at the point of breakdown, it could take a long time to break down the fuse material down.

To really blow a fuse, you must go beyond the blow current, the more you go beyond the blow current, the faster it will blow.

It's also why currents close to the blow current should not be approached for long periods of time (like between 700mA and 800mA) as the curve probably also extends to this region but was not tested.

Have a look at Littlefuse's Fuseology application guide where much of this is discussed.

The graph Y-axis is in seconds. 10,000 s is < 3 hours. A year - and I would be selecting fuses for life measured in years rather than hours - is about 32 Ms so if you extend the Y-axis by three decades (100k, 1M, 10M) and extrapolate the curves you will be close to the rated current.

Relevant highlights from the Littlefuse application guide:

CURRENT RATING: The nominal amperage value of the
fuse. It is established by the manufacturer as a value of
current which the fuse can carry, based on a controlled set
of test conditions (See RERATING).

Under the heading of Nuisance Tripping we can read:

For example, one prevalent cause of nuisance opening in
conventional power supplies is the failure to adequately
consider the fuse’s nominal melting I²t rating. The fuse
cannot be selected solely on the basis of normal operating
current and ambient temperature. In this application, the
fuse’s nominal melting I²t rating must also meet the inrush
current requirements created by the input capacitor of the
power supply’s smoothing filter.

The procedure for converting various waveforms into I²t
circuit demand is given in the FUSE SELECTION GUIDE.
For trouble-free, long-life fuse protection, it is good design
practice to select a fuse such that the I²t of the waveform
is no more than 20% of the nominal melting I²t rating of
the fuse. Refer to the section on PULSES in the FUSE
SELECTION GUIDE.

In addition when selecting a fuse you should note:

RERATING: For 25ºC ambient temperatures, it is
recommended that fuses be operated at no more than
75% of the nominal current rating established using the
controlled test conditions. These test conditions are part of
UL/CSA/ANCE (Mexico) 248-14 “Fuses for Supplementary
Overcurrent Protection,” whose primary objective is
to specify common test standards necessary for the
continued control of manufactured items intended for
protection against fire, etc.

It's an interesting subject but a read of the linked article will be worthwhile.

Yes those are correct. Fuses pretty much don't blow at their rated current.
I remember testing 12A fast fuse at 15A and it took a few minutes.