Do You Trust Your Cheap Fuses?

When a fuse is fitted in a power rail, it gives the peace of mind that the circuit is protected. But in the case of some cheap unbranded fuses of the type that come in kits from the usual online suppliers that trust can be illusory, as they fail to meet the required specification.

[Andreas Spiess] has used just these fuses for protection for years as no doubt have many of you, so it was something of a shock for him to discover that sometimes they don’t make the grade. He’s taken a look at the issue for himself, and come up with an accessible way to test your fuses if you have any of those cheap ones.

It’s an interesting journey into the way fuses work, as we’re reminded that the value written on the fuse isn’t the current at which it blows but the maximum it’s intended to take. The specification for fuses should have a graph showing how quickly one should blow at what currents above that level, and the worry was that this time would be simply too long for the cheap ones.

In the video below the break, he looks at the various set-ups required to test a fuse, and instead of a bank of large power supplies, he came up with a circuit involving an 18650 cell and three one ohm resistors in parallel. The resulting 1/3 ohm resistor should pass in the region of 10 A when connected across the 18650, so with a 5 A fuse in that circuit and a storage ‘scope he’s able to quickly test a few candidates. He found that the cheap fuses he had were slower to blow than a Bosch part but weren’t as worrisome as he’d at first thought. If you have any of these parts, maybe you should take a look at them too?

33 thoughts on “Do You Trust Your Cheap Fuses?

    1. Your fan is burning fuses because you chose fast ones. For motors and anything with very high initial current spikes you should use identical but slow fuses. They’ll have the same ratings during use, but they can withstand much stronger initial inrush current surges.

  1. A cheap fuse in my car (a loong time ago) created a small fire. Luckily the fuse box was under the steering wheel and I detected it on time.

    That incident taught me to respect fuses.

  2. I’ve just tested all my fuses (better safe then sorry) and I’m pleased to say that they all worked like a charm. Only one problem, it seems you can use them only once.

    I’ve also read an article about unreliable safety matches, so I’ll test them tonight ( better safe then sorry).

    Tomorrow I hope to test the airbags in my car, although I’m not quite yet sure how to do that…

  3. Are the fuses being tested marked or purported to be “fast blow” or “slow blow”? In many applications, in addition to the amp rating, the reaction time does make a difference.

    1. There are three types, ultra fast, fast, and slow.

      The smaller blade type fuses are generally either fast, or ultra-fast for the micro types. The maxi fuses may be fast or slow.

    1. So does everybody.

      Not theft. The price of admission covers anything small enough to fit into a pocket. Fuses, fasteners.

      When I was a kid, they had ‘all you can carry out’ flat price days. Two big guys, rig a sling out of belts, cover a hood with two engines, a trans and every misc part you might every want. Dead lift it over the line. Good times.

  4. Drives me batty when people say “your circuit is protected” when talking about fuses … reality of the situation is something already went bang fizzle poop to blow it in the first place. Fuses are there to cut the power when something bad already happened, and hopefully prevent the situation from getting worse

    1. Yep, I was taught the same in embedded system design courses (among many): “The fuses exist to prevent a fire, and thus loss of human life, not to save your project from destroying itself.”

  5. Fuses tend to get problematic around their nominal value.

    I’ve never seen a fuse that doesn’t blow at double ore triple amps. But I’ve seen quite funny things when you put a constant load of 16A to a 16A fuse eg.

    I had plastic ones melting. I had also sand filled ones creating a strange sand and metal compound that finally made the crack open.

    1. Good manufacturers like Littelfuse have actual datasheets on this, they specify the ratings you should pick based on load / temperature etc… – TL;DR if you want a 16A load you should almost certainly not be using a 16A fuse anyway.

  6. You select a fuse based on it’s datasheet and the application including operating environment. UL and NEC have standards for fuses (UL248) and fuse holders (UL4248). IEC has their own similar ratings for fuses.

    Fuses are there to stop fires from wiring, so are circuit breakers.

    Motor starting fuse ratings are pretty well established. Most fuses have an opening time rating up to 400% of rated current. For fast blow fuses there is no minimum operating time above that, they can fail instantly or up to some MAXIMUM operating time. Slow blow fuses always have a minimum operating time so you can properly protect against nuisance tripping / temporary overload conditions.

    Note that fuses are typically spec’d at 125% of product’s current rating, but NOT > wire ampacity rating and in many cases this is required for products. This is to prevent nuisance tripping while not leading to fires. This is because nuisance tripping leads people to shunt out fuses and that leads to fires.

    Pay attention to fuse classes and maximum opening current rating. Some fuses are spec’d to open 10kA and are suitable to open branch circuits. Others are supplementary (UL248-14) and are only suitable to product a product’s internals and are not capable or reliably opening a 10kA surge without exploding.

    1. > Fuses are there to stop fires from wiring, so are circuit breakers.

      I’d love to have a fire that does all the wiring for me :-D

      Ok, jokes aside, the fast or slow characteristics are also important for selectivity. Imagine a medium slow fuse tripping immediately at 3x 16A rated current at the power outlet and a slow fuse tripping at 5x 10A rated current at your table saw. You may blow the main fuse without the saw fuse tripping. Occasionally that makes for an interesting troubleshooting.

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