Blowing some fuses

[bdunham7]

I recently received some fuses from mwb1100 to be tested to see if they were reasonably functional.  They were very low cost 5x20mm ceramic tube fuses from Aliexpress.  They are stated to be "fast blow" and are general purpose, not specifically for multimeters. They are supplied without any further specifications or data sheet.  I tested them using a Fluke 5220A transconductance amplifier aka current calibrator.

https://www.eevblog.com/forum/beginners/budget-multimeter-brands-that-use-commonly-avalable-fuses/msg5766839/#msg5766839

https://www.aliexpress.us/item/2261799814281678.html

For some guidelines about how to test them I used the specs from the datasheet for the Littlefuse 216 series.

https://www.littelfuse.com/assetdocs/fuse-216-lead-free-datasheet?assetguid=69cb9c39-1497-4ecc-b09a-c0168520688e

I only have 4 of each, 0.25/0.5/1.0/10 ampere ratings so I wanted to try to determine whether they were rubbish or not without blowing too many of them.  Knowing the fuses have different ratings by different agencies and that there is no consensus about whether the rating means continuous with a safety margin and if so how much that margin is, I decided to test them at 100% for an hour and then 120% for 15 minutes.  Presuming they all survived that, I'll test them at 210% for 30 minutes to make sure they blow.  If not, then 275% and 1000%.

The first thing I noticed was that the smaller 3 fuses had much lower resistance than the Littlefuse spec whereas the 10A one was approximately the same.

0.25A     92mR
0.50A    102mR
1.00A     88mR
10A        ~7mR

These readings were later confirmed with a second set of fuses, including that the 0.5A fuse measured a bit higher than the 0.25A fuse.  The three smaller fuses are confusingly or suspiciously similar.

The smaller 3 fuses were run at 100% for 1 hour, then 120% for 15 minutes.  To minimize the heat load on my calibrator I skipped the first step on the 10A fuse and just ran it at 120% for 15 minutes.  Burden voltages were as follows:

0.25A    28mV
0.50A    57mV
1.00A    118mV
10A       ~200mV but falling slowly to 170mV over the 15 minutes.  The fuse got quite hot and I confirmed as best I could that the voltage didn't include the drop across the holder clips.

So far no fuses were blown, so I then went to the 210% step where I anticipate that the fuses should eventually blow.  I used a DSO and a current clamp (for the 10A) or a 1R power resistor (the others) to measure the current and give me an accurate timing of the fusing.  I did them in reverse order.  Here are the fusing times.

10A (21A)         2s
1A (2.1A)          6s
0.5A (1.05A)     fuse did not blow after 30 minutes  (uh-oh)
0.25A(0.525A)  fuse did not blow after 30 minutes

So now I went to the 275% step where I think most people expect a "fast blow" fuse to pop pretty quickly.  Littlefuse says "3 seconds max" but this isn't their product so...

0.5A (1.375A)   fuse did not blow after 4 minutes
0.25A(0.6875A) fuse did not blow after 4 minutes

And now 400%...

0.5A (2.0A)    2s
0.25A(1.0A)   fuse did not blow after 4 minutes.

And 1000% (!)

0.25A (2.5A)   790ms.

Opinions on what that all means?  Are these fuses a deal or are they rubbish?  I have 3 more of each fuse, what other tests would be helpful?  The original discussion was about multimeter fuses but I'm not sure how that is relevant.

[thm_w]

Sounds like garbage fuses. I would source from LCSC or somewhere else instead if you want cheap reliable.
You can find some youtube videos showing similar results with automotive fuses, where they were just all 10A or 15A.

Grabbed a random 0.5A fast blow here, 0.5R, blows at 1A in under a second or so. The filament should be thin as a hair.

[tom66]

A major automotive parts store around here was caught selling fake 12V fuses (ATOF type).  You could tell they were fake since every single one was stamped to have exactly the same width conductor out of a simple metal plate.  My guess is that they would all be designed to fuse at 30 amps or so, eliminating nuisance blows, but creating a serious risk of a vehicle fire or damage to a wiring harness if a short did occur.

I am now extremely careful where I source fuses from since it's rarely possible to see if a fuse is fake, ATOF is a nice exception since they are usually (though not universally) transparent.

[mwb1100]

Thanks for doing this testing - a lot more work was put into it than I expected.

Looks to me that the higher ratings performed OK even if not great in terms of breakage time.  The lower rated ones not so much.  I'd guess that it's a bit more difficult to manufacture the lower ratings  because the tolerances would be tighter: a small absolute variance in wire diameter translates to a larger percentage variance.

I'd bet that they are the same quality of fuses that you get in the cheapie meters (if you get any fuse at all).

If I blew a fuse in one of those meters, would I replace it with one of these?  Probably, since I'd figure the replacement would probably perform about the same as the original.

[floobydust]

Secret sauce (quality-wise) must be the fuse wire - the thickness and alloy mix.
You'd have to inspect the wire to see if it's junky or simply the wrong thickness to begin with.

[edpalmer42]

More info on fuse testing here:

https://www.eevblog.com/forum/beginners/testing-fuses-with-bench-power-supply-cv-vs-cc/

[aeg]

Might the smaller 3 fuses all actually be identical ~1A fuses but with different ratings stamped on them?

[mwb1100]

Might the smaller 3 fuses all actually be identical ~1A fuses but with different ratings stamped on them?

Definitely.  However my guess is that they are supposed to be different but slop in the manufacturing process affects the smaller rated fuses to a greater extent than the larger rated fuses (ie., it's easier to make the larger rated fuses).  And QA is very loose.

[unicornio]

I recommend everyone to use spanish fuses, they are the best in the world, while everything burns, they remain immovable, stable, robust, and in perfect condition...they always use some other component to survive, they are the best...;-)))