OT: Testing fuses

A variac is a variable transformer, no? Does yours have a current limit function of some sort??? Or do you mean a power supply with a current limit?

Rick C.

That would be a valid test. Fuses should pass the specified current continuously and only blow when the current in higher. How quickly they blow depends on how much the current is above the nominal current.

I did a search on " fuse specifications " and the first web site was littel fuse and has lots of information. Although fuses should carry the nominal current, they recommend using a fuse at 75% of its rated current.

Dan

These blew within about a second.

Sounds like good advice. In my case, the feed resistance of the wiring loom is 1.5 ohms and the bulb at the end is 3.5 ohms (cold) so it's very much borderline.

1R5||3R5 at 13V gives about 13A, but that will come down quickly as the filament heats up, I'd imagine. I suppose I should have measured the current under working conditions rather than the resistance, but it's only just occurred to me. This often happens. Sigh...

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AC and DC RMS amps heat the fuse element the same so should blow at the same current.

Fuses can get some heat sinking from their socket, so might blow at lower current if you were to connect to them some other way.

ebay sells a lot of garbage.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

What physical size or type of fuse? Original fuses are slow blow perhaps? There are many types and sizes:

Each type has a time-curve. For example: If you shove 10A though a 10A fuse, it will NOT blow. However, if you shove 15A for 10 seconds, 19A for 1 second, or 40A for 0.1 second, it will blow. Slow blow fuses are sometimes specified for loads with high inrush current or capacitive loads. Test your fuses to see how well they match the curve for their specific type of fuse.

My guess(tm) is you bought some floor sweepings (rejects) on eBay or the original fuse is a slow-blow fuse.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Just the plain standard ones like this:

That's what I thought. I just needed confirmation from someone like your good self who knows what he's talking about.

These are going 'phut' too fast for that to be an issue, fortunately.

Yeah, and I buy a lot of it.

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Thanks. Methinks that would be this type: Argh... no time curve for the ATOF series. I would guess(tm) that it's the same for the smaller fuses as below:

Yup. ATOF like you say.

Yes, and we've had confirmed instances of fake Chinese fuses for our 240V domestic mains installations turning up in some high street shops. Looks like fuses along with voltage regulators have to join my list of 'don't trust, test' devices. Probably fewer than one person in a thousand would bother testing fuses, having neither the time nor the equipment to do so (and probably also not knowing of the necessity for so doing).

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Found the time curve. It's on the data sheet: The curve looks almost identical the the smaller fuse. There's also a time-current table on the right side of the page. For 10A fuses: Percent of Poof rated current min max 135% 0.750 to 600 sec 160% 0.250 to 50 sec 200% 0.100 to 5 sec 350% 0.080 to 0.5 sec 600% -.--- to 0.1 sec Nothing like a huge 50:1 to 200:1 tolerance range for when the fuse decides to blow. With that level of inaccuracy, why bother testing? I wouldn't be surprised if your Chinese fuses met the Littlefuse specs.

I was told that India supplied the counterfeit goods for the UK market, while China supplied the US.

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Jeff Liebermann     jeffl@cruzio.com 
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Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

The thermal mass of the tungsten filament in an incandescent lamp isn't large but the turn on time is finite - about 100msec.

If your load is an incandescent bulb a slow blow fuse might be a good idea.

Even quick blow fuses don't blow fast enough to protect semi-conductors.

--
Bill Sloman, Sydney

Also from the Daily Mail and Russia Today

--
Bill Sloman, Sydney

Why bother? These are most likely counterfeit and unlikely to have any consistent value.

Send them to the recycling depot.

You don't seem to understand electronics very well, yet you post a lot of negative political crap here, who are you - a Russian?

John

Yes, that /is/ a pertinent observation and question.

A sockpuppet would never give a trustworthy answer, of course.

He's a Brit and has been a regular poster for many years, intially as Eeyore.

He's never had much sense, and should be seen as gullible, rather than as a Russian - or any other kind - of troll.

--
Bill Sloman, Sydney

** Huh ??? 1.5 ohms of wiring resistance means bulb current will never exceed 10amps. ** The two resistances are in SERIES across your battery!!!.

Those Ebay "10A" blade fuse are blowing with a load current that starts at 3 amps and falls rapidly.

Betcha they are actually "1A fuses", mislabelled.

Automotive fuses, like many low voltage types are made from almost pure Tin - makes them slow acting and with low voltage drop.

.... Phil

The only way to test a fuse is to overload it and see if if blows. Take the value of the fuse and add 50% to the current draw. In other words, if the fuse is a 10A, run 15A thru it. If it blows, it's a good fuse. If it dont blow, throw it away. Do this with every fuse in the box. Then save the ones that blew, because those are the good ones.

** Not true if the fuse blows at less than 10 amps.

I have a Variac and a 240V to 10V toroidal tranny capable of 20amps short term - gets used for checking fuses and breakers from time to time.

.... Phil

You're clearly a force of nature with persistence like that, Jeff. ;-)

I was aware that there was a bit of leeway with such fuses, but you're right: it's staggering. There's a common misconception among 99.9% of folk that at the very instant a fuse exceeds its rating, it goes phut. Only an ideal fuse does that and an ideal fuse doesn't exist, sadly. Given the giveaway price of these things I suppose figures like you cite are about all we can reasonably expect.

I don't know for sure, but I very much doubt the fakes business is that clear cut. Maybe they're *all* made in China and the Indians just sell 'em on at a small premium since India is more trusted (or more accurately, haven't acquired the same dire reputation as China).

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