Chinese fuses

Sorry guys, ignore that; it should have gone to a different group as it's UK voltages and whatnot. Apologies!

While some people seem to think this is a facebook replacement for geriatric American right-wing gunslingers, it is actually an international group about electronics.

So fuses are on-topic, even with UK voltages and Chinese fuses.

As for the original question, I too would have thought that a 5A fuse would blow in a 2 kW heater - it should be drawing 8A. Maybe your heater is not working fully?

Well this

shows a 20A fuse with 30A lasting for ~1000 seconds. And looks to take ~40A to blow in 10 seconds.

George H.

Fuses are mostly intended to protect against shorts.

A nominal 5A mains fuse might well survive 2-3 minutes @ 10A load as it is designed to survive a working lifetime at up to 5A.

These curves are for high power fuses but show the general principle -

Slow blow or quick blow makes a difference too for transients.

It is surprising how far you can push things. Our local church has destroyed more than one extension lead by plugging two 3kW kettles into them with most of the cable still on the reel. Both kettles boil OK the first time around and the fuse blows only when the cable melts inside with a live to earth or live to neutral fault.

All the VH extension cables now have thermal cutouts in them.

Regards, Martin Brown

So you get more for your money when you buy Chinese >:-} ...Jim Thompson

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How do you test every fuse ?

Jamie

Measure it's resistance at a few temperatures/currents?

George H.

You can measure the resistance of every fuse so you know they are not blown and have a resistance that is the same as some fuses you test to see the amount of current at which they open.

Or you could do something like a Bruceton analysis. Does not test every item, but does test batches.

Dan

Right, an 8A load on a 5A fuse will take a long time to blow. If it ever blows. Tolerances may allow it to not blow at all.

Martin's later post has a site with good plots. There are many others. He points out the fuse's primary job: to open in extreme circumstances, and to NOT open in more ordinary circumstances.

If you need a sensitive and predictable "fuse" blowing threshold, use a proper electronic fuse. I can post a good circuit if you don't find one. They're not expensive or very complicated.

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 Thanks, 
    - Win

The only electronic current limit I've used is a resistor turning on a bipolar Vbe drop... and then doing something with that... I've seen crowbars, (and read your crowbar section.)

A two terminal device that you could stick in series someplace with limited voltage drop would be nice.

George H.

** This U-tube is on the same topic and shows actual tests on genuine and fake Chinese BS1362 fuses.

Warning:

The presenter's delivery style is one many insomniacs would appreciate.

.... Phil

** You need to test an least one example to destruction and see if it blows at an appropriate current and time for the fuse type and with no anomalies .

If the above test is passed, you can then check similar examples at a somew hat lower current and see if self heating is consistent.

I use a Variac and toroidal transformer combination for such things - the t ranny has a hand would secondary supplying up to 3V at 35amps or more. By m onitoring the primary current and using the off load voltage ratio, the sec ondary current is easily computed.

Thorough testing would include a test of breaking capacity, which is not so simple to do at home.

.... Phil

I was surprised that the 15A fuses that were inconsistent, and exhibited other problems such as smoking and breaking, were clearly labeled "Bussmann", while the better ones did not seem to have any visible markings.

Usually fuses and circuit breakers are designed to hold rated current indefinitely, and are only guaranteed to open above 135%, or 20 amps for a

15A device. At that point, opening time can range from several seconds to hours, and can be affected by ambient temperature, air convection, and the thermal conductivity of the clips and wires.

For circuit breaker testing, trip time for a delayed trip is often measured at 3x, where trip time is typically 10-30 seconds, and instantaneous trip (less than 3 cycles or 50 mSec) may occur at 8x-15x. Here is my video of the testing of a molded case circuit breaker (MCB) using my PI-250 test set:

This video shows the ampacity of magnet wire, using an earlier prototype of the PI-200 test set:

Paul

ws at an appropriate current and time for the fuse type and with no anomali es.

Sure that makes sense. I never tested any fuses. We just buy good ones an d use them. These,

(BTW does anyone know what causes the slope change at ~1-10 seconds? Different cooling mechanisms?)

The only fuse problem I've ever had is when someone thought the fuse had bl own on an instrument and replaced a slow-blo with a fast blo.. and then it blew almos t every time on power up.

George h.

ewhat lower current and see if self heating is consistent.

tranny has a hand would secondary supplying up to 3V at 35amps or more. By monitoring the primary current and using the off load voltage ratio, the s econdary current is easily computed.

so simple to do at home.

(BTW does anyone know what causes the slope change at ~1-10 seconds?

Thermal conduction to the end caps and fuse holder, I expect.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
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hobbs at electrooptical dot net 
http://electrooptical.net

Oh, so 1-10 seconds is the thermal time constant of the fuse. that makes sense.

The lower slope (short times) is 2:1... An energy, power (R*I^2) times time.

George H.

Yup. In a normal fast-blow fuse, heat is generated uniformly throughout the fusible material so no thermal conduction occurs at short times--the only place there's a significant temperature gradient is at the ends. (And the surface, of course, but the alpha is much lower there.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

And the fundamental thing is that R increases quite rapidly with rising temperature so once the thermal inertia of the fuse body is overcome the filament at the middle gets to melting point quite sharpish.

I have known 13A fuses in UK mains plugs run so hot as to partially decompose the plugs plastic shroud without actually blowing!

3kW fan heaters at 240v really stress nominally 13A fuses.

Two 3kW kettles at once make them run really hot (but for long enough to significantly discolour the plug).

Regards, Martin Brown

That would be impractical to say the least! I just test *one* at random out of every batch and hope they're all about the same.