Is this fuse time-current curve wrong?

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.

• "the curves for fuses with >= 800mA asymptotically approaches a value that doesn't match their rated current" - I see no evidence for that; all of the graphs still have some slope to them. It's quite possible that if you would extend the measurements far enough up, you would actually end up below the rated current. This reflects the reality that a fuse used, say, 20% below its rated current can actually blow after some years. – marcelm Jul 18 '19 at 9:46

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.

• So I guess for slow-blow fuses they just didn't bother to test them over such a long period of time. The curves for the fast acting 0DAC/0ADAP fuses do go all the way to their rated current. – DKNguyen Jul 17 '19 at 20:07
• Yes, 10000 seconds is a long time. It would be interesting to go longer – Voltage Spike Jul 17 '19 at 20:08
• Yeah, sorry, got the graph wrong. I swear the I've see some that go to hrs, but I could be wrong – Voltage Spike Jul 17 '19 at 20:09
• I actually had problems like this in a product and the fuses weren't blowing fast enough. We had to switch to a lower current fuse. – Voltage Spike Jul 17 '19 at 20:11
• Yeah. Most of the time I need fuses it's to protect some power transistors from overcurrent but I think the fuses are too slow, even when you use fast-acting fuses and rate them a bit lower than the average current through the transistors (i.e. the current is expected to be peaky, but if it sits at the peaky level for too long it should blow). I could also oversize the transistors and have the fuse blow before the transistors are in danger but that's \$. – DKNguyen Jul 17 '19 at 20:14

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.