Fuses are some of the simplest safety devices in a modern vehicle. A fuse has one job: cutting the flow of electricity when current draw exceeds a safe limit. A fake fuse (that is, a fuse with an indicated current rating that is fake, and is lower than what the fuse will actually allow to flow without blowing) can cause damage or even fire in the event of an electrical fault.
Traditionally, automakers and standards bodies have worked together to make fuses simpler and easier to use. Automotive-style blade fuses are color-coded and marked with their current rating in amps. This makes it easy to select the right size fuse for the right circuit, and minimizes the risk of users fitting higher-rated fuses into low-current circuits. This is important, as a circuit or wiring that should only draw up to 5 amps could be damaged if 10 or 20 amps is allowed to flow, for example.
A number of reports have surfaced of cheap fuses that are incorrectly rated, posing a serious risk to safety. The insidious thing is that if you’re using these improperly labeled fuses, you likely won’t know about it until the damage is done.
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Way Off The Mark
A great example of the problem comes to us from Chistopher Smyth, in the video above. He’s an auto electrician in Loughbrickland, Northern Ireland. He took to TikTok to highlight how fake fuse ratings are causing trouble for the vehicles he works on. “Making people aware of an ongoing problem that’s causing thousands of pounds worth of damage,” he says . He puts the problem down to cheap replacement fuses coming out of China.
He demonstrates the issue by showing a stack of cheap fuses next to properly built examples. At a glance, all the fuses seem legit. They’re all marked with the right colors—orange for 5 amps, red for 10 amps, blue for 15 amps, and so on. The problem is that the fusible wire link inside is the wrong size for the rating. The fake 5 amp fuse appears to have exactly the same construction as the 30 amp fuse. What this means is that if a fault occurs, instead of blowing at 5 amps, the fuse can continue conducting electricity. This can damage the attached equipment or even lead to a fire.
Why is this a problem? Long story short, if you’ve got some small device that’s supposed to draw 5 amps, and it starts pulling 20 amps plus? It’s either on fire or will be shortly. When that happens, you want the fuse to blow and shut it all down. If the fuse doesn’t blow, you’re in a world of hurt. Christopher’s video shares a perfect example of this problem. He shows us a tractor that came to him after being fitted with one of these fake-rating fuses. When the tractor was operating normally, all was well. The fuse conducted electricity just fine and everything worked. But when a fault occurred, all hell broke loose.
“Fault in the headlights, [wiring] rubbing the chassis,” Christopher explains. “Cheap Chinese fuse, [it’s] really a 30-amps instead of a 15.” As the fault drew way too much current, the fuse should have blown. But it didn’t. The consequences were dire. “Went from the headlights, right up the loom, right up under here … burned all the wires, right back to the fuse box,” explains Christopher. He shows us the damage: wires in the loom melted together, and a massive multi-pin connector in the engine loom with a huge hole melted in it.
Repairing that damage isn’t cheap, either. Try finding a replacement loom for an old tractor. You know, the exact one that matches the specific 30-year-old model you’re working on. When you’ve given up, you’ll realize it will take endless hours to replace the melted wires and wind them back through the damaged loom. Oh, and don’t forget having to repin the connector. In contrast, if the fuse had blown as it should have, it would have been a one-hour job to fix the damaged headlight wire, at worst. Heck, you could have slapped some electrical tape on it and called it good.
Fuses 101
To understand why this is a problem, it’s worth discussing how fuses work. A modern automotive blade fuse is a simple device. It has two contacts that mate with a fuse holder. The two contacts are joined by a thin piece of metal or wire. This is the “fusible link.” This part is specifically sized so that it will melt and fail beyond a certain level of current draw. Small fuses have very thin fusible links. Heavy-duty fuses have thicker links so they hold up to more current flow before they blow.
The relevant electrical equation in this case is one we’ve seen before, in our discussions on 48-volt electrical systems. In basic terms, when electricity flows through a wire, there is a degree of power loss due to resistance. The amount of power lost is equal to the current squared multiplied by the resistance of the wire in question. This power is lost as heat. If that was too complicated, just remember this—the more current flowing through a wire, the more power is lost as heat.
Fuses are designed to blow at a certain current. This is achieved by making sure the fusible link is just the right size such that when the current exceeds a given threshold, the resistive losses to heat are enough to quickly melt the wire and break the circuit. Smaller wires have a higher resistance than larger ones, and they’re also easier to melt. Thus, lower-rated fuses have smaller wire links and larger-rated fuses have larger ones.
It’s worth noting that fuses do not operate in an on/off digital sort of manner. An excessive current flowing through a fuse takes a certain amount of time to heat it up and blow the fusible link. The higher the current is above the fuse’s actual rating, the less time this takes. Standards organizations account for this, mandating certain performance curves for fuses. For example, Eaton builds blade fuses that meet the ISO-8820-3 standard, as per the datasheet. This standard mandates how quickly a given fuse should blow for a given current level.
This is where things can get a bit confusing. A 10 amp fuse has to allow 10 amps to pass indefinitely without failure. Beyond that level, it should eventually burn out and break the circuit. It’s not really possible to build a simple fuse to work at 10 amps but burst at 10.1 amps. Instead, there’s usually a bit of leeway. For example, as per the graph above, the Eaton 10 amp fuse will last forever at 10 amps. At 12 amps, it’ll burn out in around 10,000 seconds—just under three hours. At 13 amps, it’ll blow in 100 seconds. At 20 amps, it’ll blow in under a second.
The basic idea is that a 10-amp fuse is suitable for a piece of equipment drawing up to 10 amps. If there’s a fault with it, or a fault in the wiring, that would tend to draw significantly more current and pop the fuse pretty quickly. The most common fault is usually some kind of short circuit, which can see incredibly high currents flowing—in the range of tens or hundreds of amps. In those cases, a small fuse will blow in a tenth of a second or less.
The problem with fake fuses is they don’t adhere to these curves. When everything is working correctly, you’ll never notice they’re not up to spec. They’ll conduct electricity just fine. But when there’s a fault and amp draw goes up, hell breaks loose. Instead of the fuse blowing and shutting everything down, it allows current to keep flowing, come what may.
Let’s explore an example. You’ve got some cheap LED running lights running on a 10-amp circuit. The lights themselves only draw about 8 amps at maximum. You’ve fitted a 10-amp fuse, and you’ve run them off wire and connectors rated for 15 amps, adding some margin to be safe. The problem starts when your cheap lights get wet in the rain. The electronics in one of the lights fails and the short circuit is pulling over 20 amps. A properly functioning 10-amp fuse will blow nearly instantly to stop the flow of current and spare the electrical system from damage. You can then look at the system, diagnose the fault, and replace the damaged light.
A fake fuse changes all that. Imagine our LED lighting scenario above, but now that 10-amp fuse is actually a 30-amp fuse in disguise. Despite the fault, 20 amps keeps flowing. The failed light starts smoking heavily, and your wiring is getting hot. The connectors you used are at risk of melting and fusing together. It’s very easy to end up with a car on fire in circumstances like these.
Fundamentally, the fuse is supposed to be the weakest link in a circuit. It’s supposed to burn out in a very controlled manner to keep the rest of your wiring and components safe. If it doesn’t pop, something else will. Maybe many things.
It might sound like an extreme example, but it’s really not. Christopher’s video above shows just how bad a fake fuse can be. One short circuit can destroy an entire loom, or see your car burning to the ground. If you’re lucky, you’ll smell something burning and turn off the right switch in time. But in the event your cabin starts filling with smoke, how confident are you that you can de-energize the entire vehicle quickly enough to avoid disaster? It’s for reasons like these that race cars have kill switches, but seldom few of us fit them to our daily drivers.
I Want To Avoid This
If you’re reading this, you’re probably wondering how to avoid this problem. It’s a bit of a sticky situation, truth be told. Still, there are easy ways to avoid bogus fuses. There are also moderately difficult ways to check the ones you have.
The easiest way to avoid this problem is to only use good-quality fuses. If you’ve got a new car, you can probably trust that your automaker specified high-quality fuses from a legitimate supplier. If you’re replacing fuses, you should buy name brands from stores that sell genuine parts. It’s pretty hard to go wrong with Bosch fuses from a big-name retailer, for example. Buying fuses from random sellers on the Internet is less trustworthy. If you’re getting fuses from eBay or Amazon, what you buy might be more questionable.
“But Lewin!” you cry. “How can I check my fuses, to see if they’re safe?” There are a few ways to go about that. You can do a simple check, as Christopher demonstrates in his video above. He took some known-good fuses from a brand-name supplier and looked at the fusible links inside. He noted that the fusible links got thicker with higher current ratings. He then visually compared these with the fake fuses. When he saw a 15-amp fuse with a link of similar thickness as the genuine 30-amp fuse, he knew something was wrong.
This visual inspection is useful if you’re experienced, but it’s not a surefire thing. Particularly as different fuse manufacturers can use different geometry for their fusible links, which makes comparison difficult. There are ways to directly test your fuses, but it gets a little complicated.
Electronics YouTuber Andreas Spiess decided to investigate this problem in a video released earlier this year. He used a large lithium battery as a power source, as it is capable of delivering high currents. He then hooked up a circuit between the battery, a fuse, and a programmable load. If you haven’t heard of a programmable load, it’s a piece of electronic test equipment. Among other things, you can set it up to draw a certain amount of current for testing power supplies and other circuits.
Andreas used the load to test fuses at various current levels. He then used an oscilloscope and multimeter to monitor the performance of the fuses quite accurately, including timing how long it took the fuses to fail. For example, he tested different brands of 5-amp fuses at a current of 10 amps. He found one Chinese no-brand fuse that took 3 seconds to blow. This was far too slow compared to the international standards that companies like Eaton hold themselves to, but at least it blew. However, he found two other no-name fuses that failed in 0.5 seconds or less—within typical specifications. Bosch fuses, manufactured in Slovenia, universally hit the specs, too.
The same testing was applied to higher-rated fuses with increasingly high currents. Andreas found that one pack of no-name fuses was worse than the other two, but all fuses reliably blew within a few seconds at double their rated current. Not great, but okay. If you want the most reliable fuses, name brand is probably the way to go.
It’s worth noting that this testing method is destructive. It also doesn’t tell you if all your other fuses are okay. If you test one fuse in a batch, it’s a good indication that the others are probably up to spec, but it’s not a promise. If you do go to all this trouble, it’s still worth doing a visual inspection of the other fuses in a given batch.
Testing in this manner is a lot of work, and involves expensive equipment, too. Andreas recommends a simpler “dirty” method. He suggests getting an 18650 lithium-ion battery, most of which can deliver over 20 amps without issue. Voltage isn’t important, as it’s current that heats up and pops a fuse. He then wires three 1 ohm resistors in parallel, creating an 0.3 ohm load. In circuit with the lithium-ion battery, this creates a draw of about 10 amps. Putting a 5-amp fuse in this circuit should pop it in a matter of seconds.
Andreas’s idea is that when you buy a batch of cheap fuses, you should pull a couple out and test them in this manner. If they blow fast, the batch is probably okay. If they blow slow, or don’t blow at all, the entire batch should be discarded. This test does come with a risk. If the fuse doesn’t blow quickly, the battery could heat up and catch fire. You’ll want a way to shut the circuit off quickly if that’s the case, lest you have a small fire in your driveway. Never do a test like this inside, due to the risk of smoke and fire.
Be Alert (But Not Alarmed)
How common is this problem? That’s hard to say. There are definitely bad fuses out there. As we saw above, auto electrician Christopher Smyth is running into these on the regular. The Autopian has contacted Christopher for further comment on the matter. In any case, he’s not the only one seeing this in the field.
YouTuber Louis Rossman found some concerningly poor fuses from Amazon late last year. His findings were concerning. He found some fuses that failed to blow even at four times the rated current. For a 2 amp fuse, he found it took a full 10 amps to actually get it to blow. He notes the lack of policing on the Amazon marketplace to be a real problem for parts like these.
It’s not a new problem, either. An Australian YouTuber known as BradsXmasLights found some really dodgy fuses on eBay back in 2015. He noted that the plastic on the cheapest fuses tended to melt under typical fault currents. Generally, a fuse should blow quickly enough that the heat only melts the fusible link. It shouldn’t be heating the plastic case to the point it deforms or burns. To a degree, using cheaper plastic could also be playing a role here.
The problem mostly appears to affect blade fuses. However, as the most common type in the automotive space, that could just be because they’re the most widely used. While I’m yet to find evidence of this problem with other types of fuses, that’s not to say it’s not happening. If you’re seeing it, let us know.
When it comes to fake fuses, I’ll say this. They’re definitely out there. But they’re not everywhere, and they can be avoided.
If you’re really concerned about this problem, the answer is simple. Buy only name-brand fuses from genuine retailers. If you’re worried previous owners have put junk fuses in your car, it’s no biggie. You can source some genuine high-quality fuses for a few bucks and swap them all out, and sleep soundly in your bed.
In any case, fuses are not something to skimp on. You might keep a few extra bucks in your pocket in the short term, sure. But for $10 or $20 saved, you could be out thousands of dollars if the worst occurs. A fuse is like insurance. It pays to invest in a policy that’s going to cover you properly when trouble strikes.
Image credits: BradsXmasLights via YouTube screenshot, agrispark86 via TikTok screenshot, Andreas Spiess via YouTube screenshot, Eaton, NAa
Not automotive….but my neighbor was having issues with the slow blow fuses on his home air conditioner blowing, particularly when it was very hot outside (and you need your A/C the most). He “solved” the problem by wrapping the fuse in aluminum foil. A few months later he had an A/C technician over replacing his outside unit, and an electrician updating the power box that services the A/C unit. I was chatting with the electrician, and he commented that it was lucky the house hadn’t burned down.
I suspect that, to some extent, this is intentional. The typical car owner will simply replace their fuse rather than troubleshooting the underlying problem — and when that fuse blows, blame the fuse manufacturer.
One way to keep people from rating your parts “one star. blew immediately” is to dramatically overbuild them. Which is, of course, a problem but guess what? No one-star reviews.
I wonder where Chinese EV manufacturers source their fuses from? From asbestos in toothpaste to nuclear waste in drywall it all comes courtesy of Chinese manufacturers. But I’m sure the makers of cheap China EVs source only quality parts. Simple nothing from China is any quality.
EV Manufacturers probably followed Tesla’s lead and eliminated fuses.
This is why I only bought fuses from parts stores- if there’s an issue there’s at least someone to go after.
The other reason for the curves in the current/time tables for fuses is inrush current. When you first turn on an electrical device, it uses more current for a certain amount of time until it reaches steady state operation. If the fuse tripped at the same current across any time scale, you’d either have nuisance tripping of fuses, or fuses that wouldn’t protect the circuit. Allowing more current at small time scales allows for the inrush. There are also fuses with different curves depending on what they are protecting (slow blow or fast blow fuses, for example).
Sorry I’m more of a inAC/DC fan.
One should not buy safety related (or safety adjacent) equipment from cheap no-name Chinese resellers. This includes most everything on Amazon and everything on eBay.
Who knew?!
(I knew.)
Or Temu. When you can buy a 50,000 piece toolkit for a buck fifty it’s time to walk away.
With fuses, you’re not really paying for the cost of the fuse, you are paying for the quality control and reputation of the manufacturer plus the integrity of the supply chain.
One thing that is not readily apparent is that some fuses are manufactured with mechanical stress on the fusible link so that when it heats up, it pulls itself apart. Some fuses even have little springs in them to help tear the fusible link apart when it weakens.
Arc-flash is probably not a big deal in ICE cars, but in EVs I imagine it could be a very big deal.
Fauxses?
Interesting. Now a note for home owners.
Each time you pop a circuit breaker, the setting drifts down a bit. I’ve had to replace a couple when they kept tripping randomly to solve a problem.
Just tie a wire around it to keep it from tripping
What the hell, dude???
The CORRECT way is to superglue the breaker in place.
WRONG. The ACTUALLY CORRECT way is to replace all those pansy breakers with something any MANLY MAN has in large supply, good ol .30-06 cartridges. That way you know when you tripped a breaker, and the bullet opens up the wall to give you a head start on your troubleshooting.
And you can patch the damage with a handyman’s secret weapon. Duct tape.
In the old days of home fuses you could fix this problem for a penny. But thanks to the government and safety features a penny isn’t enough anymore
Huh, turns out my long term habit of leaving the junkyard with a pocket full of OEM looking fuses has been the correct choice! I always hated interrupting a project because I didn’t have the right amperage fuse on hand, so I’ve built up a decent collection in this manner.
Now I’m thinking of re-writing a System of The Down song.
I’m crusing from the yard with a pocket full of fuses.
<scratching>
I’m crusing from the yard with a pocket full of fuses.
<scratching>
These people haven’t seen an oil pan since their mothers wore diapers.
I REALLY don’t have a good way to re-write THAT line from Rodeo.