I think this is actually the limit for UK plugs- the outlet/wiring itself is rated for 20 amps, but the heat source of the fuse has to be limited to 1W so you get 13a, the fuse won't actually blow at all till 20a, but you're not allowed to draw more than 13a continuously, due to heat production in the fuse-13a is the point where it produces more than 1w of heat, and that's thus the limit. The plugs often come with a sort of paper stencil over the pins to show the correct wiring, it is important to remove this before use, or the fuse might catch it on fire in the event of fault.

This is also the case for circuit breakers. A short while ago there was a video from AvE where he was confused with that. This topic is often overseen electrical-engineering-portal.com/lv-circuit-breakers-essence

I have burned my fingers many times changing just blown fuses for the house.

I don't think it is hot enough to light your own poo on fire though...

I've seen PCB mounted fuse holders melting. The fuse was 5x20mm 10A slow blow,, sand filled ceramic tube type, the fuse holder was rated for 10A as well, and the nominal current was 9.2A trough the fuses. A lot of them started to fail after ~6 months.

That's very much the way Apple builds Macbooks. (Well, that and putting power lines on the connector pin next to data lines that go straight to the CPU with no ground pins in between, so if water gets in there it shorts power to the CPU instead of to ground. Burn the CPU to save the power circuit's fuse...)

Apple products cost a bit more where this happens 🤣. Ask Louis Rossman about that.

@@evensgrey , I remember a checker I had as a kid pushing a pen for the anti-missile system. I told him about a sheet I had which showed all the diodes on the relays were backwards. He told me, draw it as shown. If they catch it, we'll get to redraw the sheet. Holy shit.

WJAT HOW

Used to be common joke among DEC CEs; "transistor-protected fuse".

yep... just set playback speed to X2 ;D

guess im sleeping 34mins less this night :D

lmao I had same thought

I immediately paused and opened comments! Dave at his usual.

Quick for him maybe, not so much for us! Implying that is a bad thing though.

The fuses are there to protect the current measurement shunts, the over-range protection diodes around the uA/mA-range shunt resistors and the PCB traces. As long as those survive, there should be no way for sufficient voltage to make it out of the front-end and damage anything in the back-end.

I've ALWAYS decried fuses in favour of more sensitive and accurate electronics to protect equipment. Fuses are the path of last resort for the cheap minded. Cheap = prepare to lose your shirt because of bean counters. Just TWO laws of any worth rule in any engineering field: Murphy's and Sod's. They are NOT the same, contrary to popular belief. The former simply says: "If it CAN go wrong, it WILL"! The latter says: "WHEN it goes wrong, it will happen at the MOST inconvenient time, cause the MOST damage, MOST expense to resolve and the MOST frustration"! In over half a century of amateur and professional electronic practice I have never known BOTH to not be true together. Simple example: "a 1 1/4" glass fuse will be fine!" Result? Fuse blows, the resulting plasma shatters the glass, explodes over the circuit board and sprays molten metal across the board some of which is not noticed during repair. After repair, such "hidden" metal vibrates loose and eventually causes even greater damage. This time damaging the circuit board itself to not just char but be blown through. DOH!

@@boblewis5558 Doesn't Murphy's law apply to MCB's with their moving parts waiting to corrode and seize after a period of inactivity? A glass fuse could sit happily outdoors in a cloud of salty acidic mist for ten years and still do the business when needed. I share your confusion when I see newly-installed large expensive switch fuse boxes in commercial installations. They can cost more than £100 and be rated at 40A. No-one has been able to explain to me why a £3 MCB wouldn't have sufficed.

@@boblewis5558 , what's your favorite method of protecting circuits? have a favorite active over-current protection chip?

@@somedude-lc5dy nope. Take each case on its own merits and requirements but as an example: I used to design linear DC supplies with foldback current limiting. Extremely large 70v to 80v electrolytics would frequently be used on the output. Because of the foldback limiting there was absolutely zero requirement for any separate inrush (outrush?) limiting on the output. However the input side for high output amperage also required high value 60,000uF, high voltage 100v input capacitors that typically DID need inrush protection AND overvolt protection. Now the classic solution utilised is to use an NTC on the input and a MOV across the input AFTER a suitable fuse, usually HRC type. That's the cheap way but consumes power and does not auto reset nor allow for simple manual reset on overload. It typically means blown components blasting themselves over the board and sometimes all over the enclosure. Using a power resistor to limit inrush current is better but only during initial conditions, thereafter it's worse than an NTC. A less destructive protection solution is to use a suitable relay to short the power resistor after 30-50% charge on the caps is reached. The relay can be a pulsed toggle one taking zero power except whilst switching. Yes they exist, are only very slightly.more expensive than their more standard variants, and offer other advantages like enabling a thyristor crowbar input overvoltage and overcurrent trip that uses the relay (and or another) to isolate the input mains, the transformer output to the primary DC input supply and potentially to be either auto reset (more complex) or manually reset. One such supply I designed could be shorted directly across the output terminals, arc welding a copper bar to them, with no adverse impact on the PSU - linear remember - whatsoever! After removing the short the output immediately returned to normal withr the supply able to remain in the shorted state indefinitely with no rise in temp, in fact it would cool down in that state. Similarly shorting the DC input side was rendered safe via the input relay, as was any over voltage measured either at the mains input or the rectifier output. With some other minor tweaks even the transformer and bridge rectifier were protected and to a level of precision and speed completely impossible using fuses. Fuses are a cheap (all senses of the word) solution to a problem requiring a different level of thinking for quality of product and its electrical protection. MCBs are little better and certainly for their cost, inferior.

You'd some sort of digital over current protection circuit for 1mA of precision.

@@simontay4851 and a fuse to protect it.

but doesnt that make sense?? isnt supposed to blow at 300!! what about the poor multimeter internals?!?!??

I think that the main reason to use them is low price and simplicity. Anything that needs more sophisticated protection seems to get a breaker these days.

They are mostly there to prevent other wires from overheating and causing fires or sparks.

Right. Actually more likely to damage the meter itself with a fuse than without since as soon as it blows the sense amp sees the open circuit voltage across itself, which can normally never happen. You could protect it I guess with a high impedance voltage clamp at the cost of attenuating the input.

And a fuse is also a shunt resistor as well! I have seen a simple circuit that compared voltage on a glass fuse with a reference in order to decide when to trip electronic protection. It appears that on low currents (like below 1/2 of the specified sustain current of a fuse) the resistances are stable enough for many applications and the fuse itself acts as an additional line of defence to protect the wiring in case the electronics does not trip.

@@Kirillissimus That's a pretty neat idea, actually!

And every circuit has a fuse, whether you put one in or not.

Perhaps that's the real reason for the variation between them - differing amounts of magic smoke placed within the tube

Only at >4In

F and T are German - Flink and Trage. M is Mitteltrage

While I have you both here. Do you know where I can get fluke parts in the UK. I need the input terminals for my fluke 89v, I can only find one selling 1 all in from the usa it's 60 squid fo a £15 part

@@UberAlphaSirus Contact Fluke?

@@perfumedmanatee6235 I would guess it means "Fast and Furious" for not having any arc suppression inside but who knows.

Except that when you measure it, there is a small probability that measurement alone will blow the fuse.

You could put a bunch of fuses in series and let people bet on which one blows!

@JM Coulon resistors in series, First is.

Big Clive did Resistor Roulette!

@@bigjd2k A second game! Progress on the casino. 🤣

excellent idea, i will be a habitual gambler

Yes, would be fascinating. Would blow (pun intended) a few hundred bucks in fuses though.

@@EEVblog that would be a cool video!

@@EEVblog Could you do it with a roll of fuse wire?

It's like those machines that have a switch that activates a robotic arm to turn the switch off again

I know there are designs that use fuses as precision current shunts, as the resistance is very well controlled for such use.

GPIB ;)

Amazing. Couple of years ago I designed a custom UPS for video surveillance and network equipment. To keep it simple and safe I used regular fuses on each line with according nominals. And it's only now that I realized that when I got some "mysterious" voltage drop due to those fuses. It actually worked just fine, but I could not explain a weird difference in voltage between UPS meter with the one that the router shows in its software. "Trap for young players" as Dave says:)

Can be slow blow types as well, so surges don't blow them.

and..... at the going down of the Sun we will remember him..... LEST WE FORGET he was a good fuse he was loved by everyone Never did anything wrong UNTIL THE END LOL he shall be remembered

Yes, a FET + real fuse would be best. FETs fail short.

@@simontay4851 they can fail short but generally the reaction time is microseconds and it’s well within the MOSFET’s SOA. The only reason to add the additional fuse would be if you needed some amount of fault tolerance but even then I’d recommend a different solution. Most battery packs have only eFuses (really discharge control). Fuses degrade especially if the load is drawing high peaks that aren’t necessarily shorts and over time it will oxidize and blow in a nominal setting. Even the NASA fuse de rating guide is complete BS (someone I know tracked down the original writer of the spec who claimed such) which is why we don’t design them into high reliability systems.

Once you got the normal noise/drift variation beeps out of the way, yeah, that would work. I think the Keysight meter has programmable threshold values IIRC.

Yes and No. This is Lowest, This is Highest. Now what would be a great feature Is a Window. beep higher, beep lower.

@@EEVblog Actually a proper "burned fuse" detection should be easier. Just measure the voltage across the fuse, when higher than 2V, Beep! Also it will beep just before is blows up, if enough current! Maybe you can save it, if you are fast enough!

@@EEVblog You could put a multimeter in continuity mode so it keeps beeping constantly. When it stops beeping, you know it's done. Kind of like Homer Simpson's "everything's OK alarm". kzfaq.info/get/bejne/bdx5naWlyau1gnk.html

Or just a logging meter. Then look at the log for when it dropped. It's not like Dave doesn't have plenty of those.