EEVblog 1377 - The Amazing UNPREDICTABILITY of Fuses!

How long does it take for your 400mA multimeter fuse to blow at 600mA?
Grab a chair and watch!
The amazing unpredictability of fusing current ratings at low overloads.

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[AppliedScience]

Very useful to know! Thanks for doing the tests. I never considered thermal hazard from a near-limit fuse.

[toddtempleton]

I always keep in mind Murphy's Law of Fuses: A $300 picture tube will protect a 10 cent fuse by blowing first.

[westinthewest]

For electrical installations here in the UK, overcurrent is considered separately as overload and fault current. For a multimeter, the usual reason for the fuse blowing is when you forget you're on the current range and you decide to measure the voltage of your car battery. This is when the fuse's I2t rating is important as it must limit that burst of energy before it fries the A/D converter or whatever components are unfortunate enough to be in the way.
I'm glad you were patient enought to run these tests, they've been quite an eye-opener.

[jkobain]

Dave: _Hi! Just a quick video…_
Me: _Looks at the duration…_

[MarcoTedaldi]

An older electronics engineer once told me more or less that lesson:"the only thing defined about a fuse is, that it should not blow at rated current"...
Another teacher told me, that it is quite futile to hope for a fuse to protect your semiconductors. It's only to prevent them from going up in flames. Because when your semiconductor has already burned out, the fuse is still in the process of getting it's temperature up... (compare a bond wire with a fuse wire, and you know one reason).

[Fix_It_Again_Tony]

Fuses were one of the biggest eye-openers when my team started to develop intrinsically safe hardware for explosive atmospheres. The standard (IEC 60079-11) automatically assumes 1.7x the rated current can flow continuously. Then, if you want your fuse to be used as a current limiting device on which safety depends you need to multiply the I_rated * 1.7 figure by an additional factor of 1.5. So the current assumed to flow through the fuse continuously is I_rated * 1.7 * 1.5. For a 400 mA fuse this is 1.02 A. It can make it really hard to meet the power derating requirements of all the downstream components when 2.55 times the rated current can flow. No parts can be run at higher then 2/3 the rated power if safety depends on that component.

It's a bit extreme, but you are trying to limit ignition energy so your product does not become a detonator in an explosive atmosphere.

This video is a great illustration of why the standard does not trust fuses. Thanks for sharing.

[reddev]

Kudos for sticking at this Dave! The SIBA was crazy! This video would have blown my mind back in the day when I thought a 300mA fuse would blow the instant the current reached 301mA. :D

[FalkBay]

I've seen many fuse holders on PV installations melted because of the heat that they dissipate. Thank you for this valuable information.

[kyleaudio]

Try using a frequency counter/timer on the time interval A to B function. Set it to common A and set your trigger levels so that it will start and stop when current is applied and when the fuse blows. This will also allow you to test the fuse's fast acting response time which will be in the mili-seconds.

Take a look at circuit breaker testers and protective relay testers. They can also be used for fuse testing. Companies like Megger/AVO/Multi-amp/Programma, Phenix Technologies and Doble/Manta/Vanguard are a few that sell this type of equipment.

[curtwuollet]

In power electronics, a blown fuse usually means an active device has shorted to protect the fuse.

[bumbixp]

Not may channels on YouTube where the audience agrees with: "that would be a fascinating video, tour of a fuse factory". :)

[jessicav]

I always figured that fuses really are just to protect against shorts and near-shorts. Perhaps similar to a spark gap...only for when things are going really wrong. It seems that the trip tolerance is even worse than I expected!

[Komma]

Very interesting video! Did not expect that they get that hot and still work

[sternis]

Very useful information! I've done some work with fuses in appliances, but I've never thought about this. Thank you!
One thing I learnt has also to do with "available current": If you design your fuse to protect your stuff, it's good to have a lot of current available. If you have a severe failure after the fuse, you're going to be at a very high current, which causes the fuse to trip really quickly (when you get up to that >3In).
Another interesting thing is that fuses sometimes will actually conduct a small amount of current after tripping. I've had loads of fun testing this where I used a fused voltage supply line both for power and for a signal. When I turned everything else off, the signal went high. I was very confused.

[mikeselectricstuff]

That Siba was marked FF - this would be faster than normal fast blow (Frickin' Fast ?)

[lazyjackass]

Not enough smoke was produced during the making of this video.

[SlykeThePhoxenix]

You know what they say "Anything's a fuse if you're brave enough".

[arielbriner]

Thanks for investing the time on this one Dave. Finally I have something to point clients to when trying to explain why designing fusing strategies is so hard!

[frankh.]

Dave, I asked the fuse question in your forum at the 14.02.2021 (Frank987). This video is the best answer ever. Super big thank to you.

[williamsquires]

It’s one of those new Schrodinger’s quantum fuses; they’re both blown and not blown at the same time. It’s not until you measure it on the continuity/diode-test range on your meter that you know for sure. 😉