It Was Just A Blown Fuse

BEARWALL that has nothing to do with gaming.

Has anyone ever told you this before?

“Oh, it was no big deal. It was just a blown fuse. I replaced it, we’re good to go”

Just so you know, that saying is a test.

A lot of things in life are tests, and it can be hard to recognize it when one comes around.

This post is in the way of a public service message for those of you that aren’t all too sure what “a blown fuse” means, and don’t want to look stupid or ignorant when someone tells you this in the future.

From now on, instead of nodding your head and walking away feeling vaguely worried, I’m going to arm you with science so you know what they’re saying… and what pointed questions to ask.

A Firm Grounding

Here’s the deal. If you’re reading this, you’re plenty smart enough and educated enough to understand what a fuse is, and what it means. You might just need a frame of reference.

Don’t panic. This won’t get technical.

You know your electronics runs on a power source we call electricity.

There are lots of technical terms used when discussing electricity, how it’s measured, how to calculate volts and amps, etc.

You don’t need to know any of that to live your life.

What you need to know is, how does it make that iPod spin out music, and can my iPod electrocute me if I drop it in water?

Quick answer: No.

We can functionally describe electricity as being similar to water. Water that is unaffected by gravity… but that loves finding a path to the deep, dark underworld.

What do I mean?

Let’s look at how water functions.

Water, when flowing, pushes things in front of it. The force of water pushing on things in it’s path can be used to get work done. The stronger the flow (or current), the more it can push, the more it can do.

Electricity works much the same way.

Picture a flowing stream or babbling brook. If there is a building on the riverbank, and that building has a waterwheel dipping into the river’s current, the force of the flowing water pushes on the paddles that are at the bottom, moving them forward, turning the wheel so that the next paddle dips into the current, and the rotation of the wheel continues, forever and ever, amen, ’til the river rises and the cows come home.

That waterwheel rotates on a shaft, and the shaft goes into the building, and what you get is a turning shaft inside a big building, powered by the flow of water. You can then attach stuff like gears and things, linkages and doodads, and get working machinery… powered purely by water. Triphammers, mill wheels, saws and drills and all sorts of stuff can be powered in this way.

Well, electricity is the same exact thing.

Except… instead of electricity flowing as water does, pulled down by gravity following the lowest surface it can find, electricity is special water that flows wherever it can find a conductive surface to carry it into the ground.

Electricity always heads for the easiest, simplest, fastest connection to the deep earth it can find. It follows the path of least resistance.

What is a conductive surface? Well, it depends on how strong the current of the electricity is, really.

Things like metal and water can be great conductors. Electricity touching metal will go straight to wherever the metal is touching the ground at the best point.

Rubber and the air can both be very good insulators, blocking the flow of electricity dead in it’s tracks. Plastic is pretty good at that, too.

But the more power, the more force, the more oomph in the electricity, the more resistance (or insulation) the electricity can overcome.

At high enough levels, the electricity can even jump through the air, conducting through the air itself to get to the ground. We call that an arc, and that’s some serious high power fry your ass mojo.

Why, if there is enough current in the electricity, YOU can be a conductor! You are a lot more conductive than the air, by the way. A LOT more conductive than the air.

Safety First

Let’s have a brief experiment to illustrate this point.

Say you take a metal knife, and you stick it in a wall outlet… the electricity will instantly see that if it flows through the metal knife, and then through your body, it can reach the ground through your knees where you’re touching it, and off it goes.

At this point, you will either get blasted away from the outlet because the electricity flowing through your body from your hand to your knees caused your muscles to spasm, OR you will get locked rigidly to that knife, taking the juice constantly, because your muscles all just convulsed and locked up.

This can be a fun experiment, because if your friend or loved one sees you there unmoving or unresponsive, they might run over to grab you and pull you away… and IF they are suddenly a better conductor (say they are in bare feet while you’re wearing jeans) than you are, now the electricity sees a BETTER conductive path of least resistance through them, and BOOM, they get zapped too.

Quick fun fact: In the Marines, when you’re going to work with electricity, we used to make safety devices. What these were, were long wooden sticks covered in rubber, with a metal hook screwed into one end and also covered with rubber. They were for when a Marine grabbed a live wire, convulsed, and you had to get them free without electricuting yourself. You could grab the 8′ long rubber-coated hook off the wall, and either hook them and drag them away or just whack them good with the rubber stick.

Oh no? Oh, hell yes.

Are you paying attention now?

Just to ease your worried mind, you should know that there are two kinds of electricity… direct current (DC) and alternating current (AC). The kind of electricity in your wall outlets and in your home is all AC, or alternating current. Think of it as special electricity that pulses instead of just staying strong and steady. It pulses so fast you wouldn’t notice it without special gear, but your muscles will know the difference, because if you get zapped by AC, the first pulse may lock your muscles up and cause them to contract but the next pulse will convulse you and blast you free.

Direct Current, now… that shit will lock you up, holmes.

Where do you mostly find DC (Direct Current)? Why you find it INSIDE a lot of pwoerful electronics like TVs, stereos, microwave ovens, motors, air conditioners, all that kind of stuff. AFTER where the AC power cord comes into the gizmo, goes through a transformer and some other stuffs, and gets distributed throughout the thingie as nice, smooth DC voltage.



Electricity is like water, it pushes stuff in front of it. It is supposed to start at, say, a wall outlet or breaker box. Then it flows through a conductive material, like metal wire, that is covered in a insulating material like rubber to keep it IN the wire, goes into a gizmo, pushes stuff around inside the gizmo to make it move and get work done… and then, believe it or not, goes right back out a second insulated metal wire and back into the wall outlet, return to sender.

It makes a complete circuit.

This is why, if you look at an AC power cord, it is two wires, each wrapped in rubber to isolate them from each other. One is the supply of juice TO teh gizxmo, the other is the return pipe FROM teh gizmo. They are commonly called the ‘Hot” and the “Neutral”, respectively. The hot is usually coated with black rubber, and the neutral is coated with white, when found in American wiring diagrams or inside a junction box.

You often also find a third wire. It is colored green inside gizmos, and it is called the ground wire.

Why? Because the ground wire does NOT carry any juice at all. None. It is dead as a doornail… and it is there to save your life.

The ground wire is attached to the deepest, darkest pit of black underground wetness there is anywhere near your house. It is THE favorite path for current to flow.

The ground wire is plugged into your gear, fixed to metal parts like the case… and is supposed to be a safety. If the hot or the neutral gets cut or shorted, instead of you getting killed by touching the metal case of your stereo, the power goes through the case, to the ground wire, and down to that inky it of blackness where all electricity finds it’s home instead.

It also provides a wonderful way of making sure you don’t get outside sources of electricity, like static electricity, interfering within your delicate electronics like your Xbox 360. If you zap the case, the ground wire bleeds the electricity off to ground so it never zaps the guts of the machine.

But what about fuses, you idiot!

It is normal to put a fuse in the wire at different points.

Breakers in your electrical panel in your house are, essentially, fuses too.

What a fuse is, is a wire designed to melt at a certain temperature, enclosed in a VERY insulative holder. It’s just the same as wire, but if it gets too hot, it melts. 

Fuses melt when they get too hot, and when that happens, no more path for the current to flow. Electricity stops flowing, because the wire just got cut. The gizmo stops working… because the electricity HAS to flow for it to push or otherwise make the gizmo do stuff.

So, if a fuse is designed to melt when it gets too hot, what causes it to heat up?

Electricity does.

More specifically, the amperage in the electricity.

What is amperage? 

You don’t need to know exactly what it is, but it can help to think of it like this.

Now, this is completely and totally wrong, and yet it may help. Professionals, if you think I’ve taken liberties before this, hold onto your hats. It’s all in a good cause.

When you see a sign saying # of volts, # of amps, think of it like this.

The amount of volts is the size of pipe the electricity is traveling in. The more volts, the bigger the flow of electricity can be, the more work it COULD do.

The amount of amps is the actual POWER, the push, the big honking wave that is flowing through the pipe, doing the actual work.

To complete this horrible analogy, the stuff that the electricity is pushing in whatever gizmo you’ve got? That is the resistance. The more it resists the amps trying to push it, the more amps you need to provide to get it to go.

Here is why you should care.

You could have 480 volts on the line, a huge pipe. But if there are only .2 milliamps in the circuit, an itty bitty amount of current, you can grab the bare wire in your hand and only feel a tickle.

If you lick a 9 volt battery, getting your tongue on both prongs at once and feel the electricity flow across your taste buds from one pole to another, it won’t blow your ass up because the amps are very low.

But if you grabbed that same 480 volt wire, and there were 20 or more amps on there… if those 20 amps of force decided to flow through YOU as the fastest way to get to the ground, if YOU became the “path of current flow”, then you can die, cooked from the inside out, with your feet blown off and still steaming in your boots.

I’m not kidding around here.

What makes a fuse melt?

Amps of force performing work, pushing through things that offer resistance, generate heat.

If there is too little wire to handle all the amps flowing through it, that wire will, literally, melt.

The reason you have circuit breakers in your house is to prevent you plugging in too many things on one circuit or loop of wire, drawing a SHOTLOAD of amps through the wires in the walls of your house to power all that crap, melting the wires buried in your walls and setting your house on fire.

The circuit breaker is a fuse, designed to trip out or ‘break’ when it gets too hot… and capable of being reset. It trips when there are more amps flowing through it than the wires attached to it are capable of handling.

Circuit breakers are designed to be reset, on the assumption you know enough to unplug stuff from the appropriate outlet when one pops. Old school power panels had actual fuses that you had to replace… and many skilled and brilliant electricians would replace them, all right. With copper pennies. Sigh.

So, pop quiz because you know the answer now. What does a blown fuse mean?

It means that something got so hot it melted a piece of wire. It melted a piece of wire that was designed to melt for a reason; to protect something else from getting damaged from too much force/amps/electricity/power.

So now we come to the main event.

If a fuse blew, it didn’t do it out of spite, or vindictiveness.

That fuse blew because something somewhere else went wrong, and the fuse melted to protect your valuable shit, or even your life.

Why your life?

Because the most common place to stick a fuse is right where the wire comes into your gizmo from the power cord plugged into the wall. If that fuse melted, something somewhere in your gizmo suddenly decided to suck so much juice out of the wall it melted a wire… melted that wire before it melted something else. Or tripped a breaker in your power panel.

Or shorted right through you, blowing off your feet.

So if someone says to you, “Oh, it was no big deal. It was just a blown fuse. I replaced it, we’re good to go”, the very next question you need to ask is, “What caused the fuse to blow?”

That is the test.

To know that a blown fuse is not the problem, a blown fuse PROTECTED you from the problem.

What caused the fuse to blow? Because if all you did was replace the fuse, what the hell is stopping whatever it was from causing it to blow again?

What if the reason the fuse blew, was that there is water somewhere inside the gizmo. Electricity likes to find the easiest path to ground, right? And water makes for a good conductor. the electricity doesn’t want to do work, it doesn’t want to flow through any resistance, it’s always looking for the easiest way out.

So there is water, and sometimes the gizmo moves, the water flows, touches somewhere that has electricity, and the electricity says “Ah HAH! I can bypass almost all this other shit, flow right through the water, take a shortcut, and go through this here control knob, through that person’s hand, down their arm, and ground myself on the metal arm of the chair. YAHOO! FREEDOM AT LAST!”


Or maybe, and god this is common, maybe you’ve got a motor that is powered by electricity in your gizmo. Like your car. Or your air conditioner. The motor is physically moving, spinning round, from the force of electricity pushing it.

It takes a lot of amps to physically move a motor. Lots more than your iPod needs. Rule of thumb, if the electricity has to get a motor physically moving, it’s got a LOT of juice running through it. Moving parts take power.

The motor has all this power running through it, some insulation starts wearing away, or the bearing that lets the shaft turn nice and smooth starts binding up making the motor use a LOT more power to get that shaft to turn, and the heat from the increased amp draw builds up.

The fuse blows. It gets hot and melts, protecting your motor from turning into slag.

If this is caught right away, the motor can usually be fixed. Maybe by something as simple and easy as putting a bit of grease or oil on the bearing that the shaft turns on, reducing how hard the motor has to work.

But what do I see all the damn time?

“Fuse blew, I replaced it and got the device back in service.”

“What caused it to blow?”

“I dunno, probably just a power spike.”

“Nothing else went down, and the lights didn’t flicker. Go check it out, find out why it blew.”

“Okay.” *very grumpy*

A week passes.

The motor ‘burns out’, from too much heat because instead of greasing the motor bearings, the jackass replaced the fuses and didn’t ‘waste his time ‘troubleshooting the core problem.

I look inside at the fuses, wondering why the $20 fuses did not pop, protecting the $3000 motor from melting by blowing first, like they were designed to.

I see that the fuses, which are supposed to blow if the electrical current flow exceeds 20 amps, have been replaced by 30 amp fuses.

It takes a lot more heat to blow a 30 amp rated fuse than a 20 amp rated fuse. If the amps never rise above 30 amps, the wire inside will never heat up enough to melt.

But that motor sure did love the extra amps that drove it far harder than it was ever designed to, at a temperature it’s wires weren’t designed to handle. Wires melted, or maybe even the motor windings.

Meltdown. $3000 motor burnt to shit. Repairs and rewinding will probably cost about $1200.

Oh wow, but at least those $20 fuses are still in great shape, and the tech that decided to swap 20 amp fuses for 30 amp fuses so he wouldn’t have to keep replacing them when they blew over and over?

Well, at least he had some piece and quiet for that week.

Wrapping this up

Now you know what a blown fuse really means. It means more juice, more power, more amps, more OOMPH just went through the thing than it was designed to safely handle, and the fuse blew before something SERIOUS happened. Read: expensive or dangerous.

If you simply replace the fuse, you are giving whatever it was a chance to do it again, shocking the system and risking damage from the fuse melting too SLOWLY to stop the big jolt of power from going through and doing it’s damage to the sensitive guts of your gear first.

If you replace the fuse with a BIGGER fuse, what you’re doing is saying, “I don’t like to live safely, or to save money. Fuck it, let the motor burn, just as long as it stops bugging me by popping all the time.”

Yes, a spike of power from the source can cause a fuse to blow or breaker to pop. A lightning strike on the main supply coming into your house, etc.

But if it did… you should have seen lights flicker, or had some other indication than just one thing popping a fuse.

At the very least, I hope that now you will feel confident whenever you are talking to someone about your car, or stereo, or air conditioner, or circuit breaker, to call them on the carpet if they feed you that old “It was just a fuse” line.

Today, it was just a fuse. Tomorrow, it’s the water pump, or the fan motor, or the overhead crane drive, or whatever it may be.

Or something compound in your car. I don’t care what it is, if it’s compound, it’s money.

This may not have helped you, but by God I’m glad to get that off my chest. Freaking idiot techs, I swear I’m going to start using the Big Safety Stick™ to give them a current test they won’t soon forget..

21 thoughts on “It Was Just A Blown Fuse

  1. Heh, I’ve heard this analogy several times and when I started out my career (sp?) I tried to use it. It never made sence to me until the day I relised that the push of the current is actually a suck 😀

    Retired wow:er


  2. In my head, it’s more like the width of the pipe is the resistance, not the voltage. With a high resistance (a narrow pipe) you need a bigger push to squeeze a bigger amount of current through. In this case the voltage is the push, and the current is the wave that gets pushed.

    When you put in a bigger fuse, the pipe is bigger and that lets a much bigger wave through with the same push, which is why it’s dangerous.

    It’s all the same words that you used, just in a different order. I suppose that what with them all being one-over the other it makes very little difference in the end. The important message was crystal clear – always carry a wooden stick in case you need to beat your loved ones. (That’s right, isn’t it?)


  3. Here’s my Favorite Tripped Breaker story.

    I work as a Stagehand on the Las Vegas strip. Specifically, a stage electrician.

    Theater shows have lots of lights, on lots of dimmable circuits (hereafter referred to as Dimmers) and we pull down a lot of power. The show I was working on when this story took place was powered by 13 400A 3 Phase 120/208V panels.

    Every day before a show we run a light check to make sure we don’t have any burned out lamps. One of my techs was onstage with a controller, calling up groups of dimmers at a time. To this day I don’t know why, but he decided to type in the following command line:

    1-1000 @ Full Enter

    That’s Channels 1 through 1000 (each Channel could have multiple 20A Dimmers assigned to it) at 100% in 0 time.

    Would it surprise you to know that one theater lamp, which normally draws around 5.5A at 120V, uses a whopping 80A when slammed to full. (I’ll leave it to Bear to explain Inrush Current)

    7 of my 13 panels lost power. But here’s the thing, none of the 400A breakers tripped. Oh no, we tripped a massive breaker deep in the bowels of the hotel that fed all 7 sub-panels. It took us 20 minutes just to find it, and when we did it took authorization from the VP of Engineering to reset it.

    Everyone except a couple of the Hotel guys were instructed to leave the room when they reset the breaker. From the other side of a cinder block wall, in a corridor that was literally buzzing with the sound of the voltage coming in and being distributed out to the property, resetting that breaker sounded like a shotgun blast.

    We got everything back online just in time to let people in for the show. The tech in question, egg very much on his face, had nothing to say to me for about three days.


  4. I work around industrial pipelines (the big 20-50 inch lines). When building those lines we occasionally cross under existing power lines. We have to ground said pipe during construction as it sits under those power lines in the open due to a nifty effect called induction. Go google it.

    Left without grounding, a piece of 42 inch pipe under a 300 kv line will build up about 3 amps in around 2 hours (that’s without touching any wires). In 24 hours, the pipe will build up as much as 22 amps. This depends a bit on humidity and other conditions, but that’s enough to actually blow off body parts.

    The threshold for “feeling” an electrical current is only 0.001 amps. It takes 0.1 to 0.2 amps to kill a person depending on the circumstances.

    It’s always “worth it” to take extra care when working around electrical components or current.


  5. Back in the dark days I used to work on them “large” Disk drives. You know the 80megs that were about 3 foot high and took a 120v twist hubble. What I hated was finding the blown fuse. They used ceramic fuses. yuck. We all went out and found the same type of fuse with the glass around it. All the better to see them with.

    On a side note both fuses and breakers do wear out. I know it’s odd but when you replace the fuse and watch the dang device work for an hour or 2, and you have checked for shorts what conclusion can one come to. Did have one breaker trip. Would not reset if one’s life depended on it. Pulled it from the panel and shook it. Made a nice noise from the inside.


    • Yup.

      One time, working on a gripe on an EA-6B, Airman *censored* and I discovered the circuit breaker was bad. Read open (infinite ohms) in both positions. When we went to order one, the supervisor gave us grief about circuit breakers “don’t go bad”. I persisted, we replaced the circuit breaker and fixed the gripe. I set the bad circuit breaker in front of the supervisor with a meter and said “check it out”. He did, and said “holy sh*t, that’s the first time I’ve seen one go bad.”


      • Many circuit breakers work by having solder-style meltable contacts, and after the contact melts from the amp draw, the spring releases, the breaker ‘pops’ open, and the solder, released from the source of heat hardens again, resulting in a resettable breaker.

        Now, anyone that knows what solder is like knows that yes, eventually, if a breaker gets tripped enough, it ain’t gonna be happy. Melting material loses some to vapor each cycle IF it tripped from overload.


  6. This actually taught me a lot that I didn’t know about fuses. How cool is that? Great job on the blog, keep it up!


  7. Putting a fuse in thats over amped is one of the worst things you can do as a tech for the reasons described. Stupid shortcuts people take kill me. When I was in the navy I figured that 1/3 of my electronic power plant issues were inflicted by my teammates. If the fuse blows more than once its time to troubleshoot. I think my favorite one was when a mechanic pulled a fuse using needle nose pliers thinking the rubber on the handles would protect him. He was wrong.


    • I am certainly no mechanic, but isn’t that why the fuse kit for my car comes with a nifty all plastic FUSE PULLER?


      • Worst part as a navy mechanic he had no reason to pull a fuse as he’s supposed to get someone qualified to avoid things like this. The fuse exploded, he got a nice shock (450VAC so it can kill you) and a bunch of smoke got in his eyes so he was blinded for a day. Buddy had to lead him around the ship.


  8. Your analogy about the water reminds me of learning Chemistry at school.

    Year 2: So, this is how the atoms in the periodic table connect together to make molecules….

    Year 3: What we told you last year was a lie. It’s actually called valencies, and this is how it really works…

    I can understand why they did it, but they lost my trust. And now a group of people who didn’t take third year science “know” something wrong.


  9. A friend had a (then VERY expensive) 19″ CRT fritz on him, and he wanted to see if he could figure out why, so he asked a friend of his that worked on electronics how. “First, don’t. CRT’s can have upward of 50kv stored in the tube. Second, if you feel you absolutely have to, write out your next of kin on something and wrap it in plastic, then put it in your pocket so the firemen can identify your corpse more easily. Then get yourself some 18″ fiberglass tools that you can operate with one hand, and keep the other hand in the pocket you put the list in to make sure you don’t lose it.” (Note; inaccuracy in the numbers or units involved in how much power a CRT can store is mine, not his. This was quite a while ago…)

    My friend decided that things that came with that many dire warnings probably wasn’t really worth it…

    I once replaced a fuse in my old Beetle with a paperclip. In my defense, it was 3am, I was out of that size fuse (Beetles used open air fuses, a flat solder wire stretched over a tiny plastic frame that would fit into the clip. No glass bottle around it, and in Florida the melty part tended to corrode and break just from age and vibration, so I usually kept a load of them around) and the fuse was for the headlights so it would have been just to get home when I’d replace it with a real one.

    About five minutes later, I notice the light under the dash was on, which was odd because the doors weren’t open, and more importantly, I’d never had a light there before. Pulled over and looked; the paperclip was glowing a merry yellow. I grabbed the loop I’d left sticking out (Had bent the center into a handle to put it in) and tossed it out the window onto the road, then used up some words because the bit that wasn’t glowing was still pretty hot. So, the stupidity was self punishing. 😛

    (Ending: Replaced the paperclip with a SMALLER fuse, just because I didn’t want to spend the night in a VW Beetle next to a back road, and figured that it was worth a try. It held, oddly… and when I replaced it with the normal fuse when I got home, it worked fine and I never had another problem with it. )


    • Heh. Vaguely similar vehicular issues with the 20+-year-old Isuzu pickup I used to drive. It cut out on my one night and the next day the mechanic told me the fuse had blown and I could either carry spares or they could put the time into looking for the cause, which would be more immediately costly and could take a while. The spare fuses served me well for a couple recurrances until things reached a state where the fuses were frying just from trying to turn the thing on. At least then it was easier for them to find where the old wiring had frayed and was shorting out so the actual problem could be fixed.


  10. a=c/s!
    That was the most entertaining learning-about-electronics-for-idiots I’ve ever read in my life.

    ….1 mol e- is 96, 485 c!



  11. I’ve seen a 120/208 3-phase 400 amp main service disconnect with the fuses replaced by copper busbars — in an elementary school. I was doing the walk-through on the building prior to the formal bidding process on a renovation and we were looking at the mechanical room — The print note read “Existing 400 amp main service to remain”.


  12. Nice rant.

    I love the water analogy, i’ve used it before myself though on a different scale. (battery or generator = pump, switch = valve, etc.)

    I can also vouch for taking a big charge hurting like hell. Another tech violated my red tag back when I was Navy. He put power back to a motor controller I was in the process of changing out the contact pads on.

    Melted mah ratchet, burnt mah hand and knee, and launched me off the catwalk for a nice bumpy fall down onto another piece of equiptment. Lucky for me it wasn’t running. I’m one of the few I know that tripped a 30A breaker across thier chest and managed to walk away.

    Actually I ran, holding a “safety stick”, chasing the asshat that closed that breaker.


  13. Hey, at least it was 30a. I’m old enough to have lived in a building with “real” fuses, or at least fusebox, which the landlord filled with pennies.


    Or there was the brilliant young man who decided to shunt a fuse with a length of 10 gauge copper wire. Fortunately his car had no passengers when that one paid off.

    Nice rant, BBB


    • My favorite, I mean absolutely favorite, was the tech I knew that had a big ass fuse blow, you know the kind, about 3/4 inch around and 4 inches long for a main circuit breaker, and he replaced the fuse with….

      a cut piece of copper water pipe. He cut the copper pipe, and he stuck in in place across the two contacts, and flipped the switch.

      His excuse?

      “We couldn’t afford to have the machine down during production”

      Well, he sure as hell got to find out what the lead time was on a 25 HP exhaust motor, and it wasn’t “30 minutes or less or your pizza is free”


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