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
To get more insight on this matter, I spoke to Chistopher Smyth, the man behind the video above. He’s an auto electrician operating his own business 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.
How common is this problem? It’s obviously difficult to pin down exact numbers on how many fake fuses are circulating around the world right now. Christopher, though, is seeing this on the regular. “I would find them in at least one in every five vehicles,” he says.
From what Christopher has found, it’s very easy to accidentally come across a batch of bad ones, too. “The worst source for these vehicles, it’s hard to say but you’ll definitely find them on eBay, Temu, or AliExpress,” he explains. “And you’ll also find them in car shops, they just seem to be everywhere!” He’s currently planning to launch his own brand of fuses so that his customers can buy them knowing they’re the correct rating.
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
I bought one of those big variety pack of fuses so long ago and still have it. It was part of the everyday carry in the back of a Mk2 or Mk3 Golf, especially when you liked to tinker with audio systems and various lighting. The pack I have has to be 15-20 years old or more at this point. I don’t think there was the fake fuse market in effect way back then.
“Voltage isn’t important, as it’s current that heats up and pops a fuse.”
No no no.
You are controlling current with voltage across a fixed resistance (V=IR) to dissipate a certain power level in heat to melt the fuse (P=IV).
I’m just gonna leave this here…
“This is a quick and simple way to test fuses—but only if you know what you’re doing.”
my buddy: I keep blowing my 8amp fuse on my old bimmer
me: put 20amp and forget about it
i have not heard from him since
Way back in the day, when I was first starting my career, I was responsible for maintaining all the tools and machines in a wiring harness plant. I had one machine that consistently blew its fuse every few weeks. So instead of actually fixing the issue, I simply replaced the fuse and returned the machine to service.
I’m not sure the point of this anecdote, but I don’t get a relevant opportunity every day to talk about fuses.
I think they call that job security. If you fixed all the problems, you’d have nothing left to do for work
Mechanic here. I buy my fuses in bulk from Rock Auto, where you are not getting counterfeit parts and a pack of 10 fuses costs 1/3 of what a pack of 5 does at Autozone.
I just checked all my fuse kits and luckily am not fucked. All the different amperages have different thickness fuse elements. Phew!
I’ve personally had this happen to me on a customer’s vehicle. The car came in with the interior lights not working. Found the dome lamp fuse blown, replaced the fuse and it instantly blew. I disconnected an inline connector in the trunk and put in another fuse. The fuse didn’t blow. Thinking I’d isolated the fault, I started taking the trunk apart. A minute later the whole inside of the car started filling with smoke. I disconnected the battery and started taking apart the interior to assess the damage. The wire going from the fusebox, up the a-pillar into the headliner had completely melted, embedding itself into the plastic and taking out all the surrounding wiring.
Pulled the fuse out and found it was a cheap knockoff. The original problem, whomever had the headliner down last ran the screw for the visor through a dome light wire. Had to replace a BUNCH of wiring and the interior A-Pillar bezel, but was able to save the headliner.
What I’ve noticed since then, if I see a fuse and the rating # is only embossed and not painted, I throw it out immediately. This seems to be a common thing with the knockoff fuses. Also, if there’s any question, replace it.
“Dirty” is spelled incorrectly in one of the images.
I found a spare Genuine Lucas Fuse from the 70’s in my MGB! I tested with my multimeter and it hasn’t blown. I don’t plan on actually installing it on the car though.
You’re lucky it didn’t burn down your multimeter
I keep it out of the house, just in case.
How sure are you that it’s a Lucas fuse and not simply a 1/4-20 bolt?
Hahahaha
If you’re having trouble wiring up an accessory and worried about melting wires/fires, dont worry! OEM’s still screw this up on the regular. Looking at you GM and BMW blower motor wiring engineers!
Good to know. I have couple of assortments of automotive fuses I bought from eBay, I should probably give them the heave-ho. Only one of them is in use in my current car, for a dash cam I wired in.
It is surprising to me how much money people will spend and how many risks people will take to save money.
Many old British cars had only a couple of circuits with high (30, 40, or 50) amp fuses in each. When that crappy wiring failed and you needed your Lucas lights to shine, you must channel your inner MacGyver. This article is not complete without measuring the current carrying capacity of these improvised roadside fuse standards:
copper penny (both classic and modern)
6d nail
aluminum gum wrapper
.22 bullet
Local motorist shot by his own car. Story at 11:00.
Chinese paper clips here.
The classic audio-visual fuse.
Shit! Now I’m totally spooked trying to remember every fuse I’ve replaced in the last decade.
Great article!