# Fuses



## Mayhem

Let me pose this question to you:

What is the current rating of common household aluminium foil?

Don’t know the answer? Well, nor do I and more importantly, none of us should know because it should never be used as a substitute for a fuse! 

The reason I even pondered this question was because a friend of mine came to see me yesterday with a sprinkler timer he has been having problems with recently and had now stopped working. I asked what the problem was and he told me that it kept blowing fuses. So I opened the front access panel up and saw a 1A 3AG fuse wrapped in aluminium foil sitting in the fuse holder.

I asked what the hell he had done and was informed that he had run out of the correct size fuses, so he had improvised. He was actually rather impressed with his efforts and informed me that it worked for a little while longer than it had been. 

I then proceeded to open up the unit and inspect the board. What I found was quite impressive. The fuse had blown due to a short and by effectively bypassing the fuse the current draw caused by the short had heated up several components to the point that they not only failed, but lifted the tracks off the board. One resistor had actually caught on fire. All in all, given the chips on the board were quite probably damaged as well, he is up for a whole new unit. All this could have been prevented.

So, perhaps the question should be *what are fuses and how do they work?*


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## tenor_singer

Ah yes... the tin-foil bypass... lol,

I remember as a kid, my father would decorate our front yard's two HUGE (45') pine trees for Christmas. Each tree would hold thousands of 4-W lights... usually 100 to a strand. Our problem was that when you linked several of these strands together, the built in safety fuse at the male end of the strand would pop. His solution was to teach me how to cram aluminum foil into the fuse receptacle... I was 12 at the time.

The truly funny thing was that he couldn't figure out why the male ends of the strands kept burning out and why we kept blowing our fuse pannel in the basement. His solution there was to put in a seperate pannel and hard wire it to the studs of the original pannel. We then ran a couple of outlets outside from the new box which had 50-A fuses in it.

Come to think of it... I am happy to be alive and I am happy that exterior mini-lights became popular!


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## Mayhem

LAMO - I can see this being the next Christmas ad for VISA, with either Homer J Simpson or Chevy Chase as the lead!!


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## Inaki2

Now thaaaats scary...I'd love to post a pic of the system my house had when I moved in...I'll add one tomorrow in my profile pic thingie and pleeeaasseee check that out.


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## autophage

As to the actual question - a fuse is meant to blow. The idea is that if a part costing less than a dollar and that is easily replaceable blows instead of an electronic system that cost lots of money, it's a good deal. It's a protective measure.

When it blows, it means there's either a current spike from outside the system or a short/somesuch inside the system that could have led to voltages high enough to destroy the system. If a fuse is blowing repeatedly (and you know it's not due to external power spikes) you should get it in for a look/repair by someone licensed to repair it - elsewise you risk destroying the system/fixture and quite possibly hurting yourself or someone else.


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## propmonkey

our soundboard power supply had a fuse problem. they didnt blow instead other diodes blew. the fuses should blew before them. i remember in 7th using a gum wrapper for a strobe light fus(like in macgyver...).


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## soundman

To answer part two of the question; A fuse works by using a thin piece of wire, if the current draw is two much the wire gets hots and melts/blows/explodes saving the machine from further damage. Fuses unlike breakers are a one time use iteam.


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## zac850

Out of curiosity, how does a circuit breaker work? I understand perfectly well how a fuse works, and I sort of understand the concept of how a circuit breaker works, but how exactly does it work?


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## rgsw

to put it in lame mans term:

you have a electro-magnet close to a disconnector wire (i think) and the current flows through the electro magnetic and should anything get too big the magnets power gets bigger and pulls the rocker towards it cutting its supply.

its like a door bell apart from better.

have a look on the web or i'm sure ship will know exactly


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## zac850

That makes sense. I knew it was some way of cutting the power circuit (what's the correct term?) temporarily.

Cool, that makes sense.


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## Inaki2

Except that when the circuit is cut, the whole mechanism lock into place untill you reset it.


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## DMXtools

zac850 said:


> Out of curiosity, how does a circuit breaker work? I understand perfectly well how a fuse works, and I sort of understand the concept of how a circuit breaker works, but how exactly does it work?



The most common circuit breaker is thermal: a switch in series with a heating element. All the current for the load passes through both. A spring pushes the switch to the OFF position, a latch holds it in the ON position. The latch is made of two metals with different coefficients of thermal expansion, bonded together. As a result, when it gets warm, it actually bends, releasing the switch, allowing the spring to turn it off. The size of the heating element determines how much current it takes to generate enough heat to release the latch. There's also a time element - the temperature rise isn't instantaneous. At 110% of the rated current, it may take several minutes to an hour or so for the latch to release. At 200% it may still take several seconds.

A magnetic breaker is similar, but substitutes an electromagnet for the heating element. At slightly more than the rated current, the electromagnet becomes strong enough to release the latch The advantage is that the time lag is much smaller than that of the thermal type. In many applications that's also a disadvantage - when a tungsten filament first sees power, it can draw many times the current it will draw when it's hot. Motors, when they're first starting also draw extra current. The time-lag of a thermal breaker allows lamps to light and motors to start - before the heating element is hot enough to release the latch, the current has fallen back to the normal range.

A ground-fault breaker combines both a thermal and a magnetic component. The thermal element operates as a normal thermal circuit breaker. However there is also a magnetic element. Rather than seeing the full load current, however, it includes an electronic circuit that continuously compares the current in the hot and neutral lines, looking for a difference of only a few milliamps. If more current is flowing through the hot than the neutral, it must be getting back to the power plant a different way... possibly through the body of a musician who touched a microphone and his electric guitar at the same time. Sensing that small difference, the electronic circuit energizes the electromagnet, tripping the breaker very quickly, hopefully before the musician fries onstage.

John


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## zac850

Which do theater dimmers usually use?

Thanks for that, it makes a lot of sense.


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## DMXtools

zac850 said:


> Which do theater dimmers usually use?


Typically they'll use the thermal type, so they can handle the starting surge of tungsten-filament lamps. There's no need for the added expense of ground-fault protection because theatrical lighting fixtures are typically not meant for hand-held operation and, if properly installed, are well-grounded in any case. An exception would be follow-spots. If dimmable at all, however, the dimmer is internal. Some high-priced units may have built-in ground-fault protection. If not, it's best if the follow-spot is plugged into a ground-fault protected outlet if possible.

John


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## Mayhem

Recently I saw a dimmer that had melted most of the fuse holders that protect the outlet for each channel. 

The reason the fuses didn' blow, which resulted in the subsequent damage to the fuse holders and some of the internal wiring was because the wrong fuses were used.

The correct fuse for this dimmer is a 240V 10A ceramic fuse and someone hade used 12V 10A "car" fuses. 

So - what is the difference and why would this happen?


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## ship

Blown Fuses: Over loaded Fuses will have a clean window as the wire has broken without much heat build up (it wears down.)
Short circuited Fuses have a darkened window as the wire as heat vaporizes the wire in a short time.


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## Mayhem

Yes - but the question is what is the difference between a 12VDC 10A fuse and a 240VAC 10A fuse?

The 12VDC 10A fuses did not blow and as a result, caused damage to the fuse holders and wiring.


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## darkfield

Mayhem said:


> Yes - but the question is what is the difference between a 12VDC 10A fuse and a 240VAC 10A fuse?



That's easy. 228VDC.


Uh, and for say 500w, something about 41+ amps. I wonder why the fuse went out so spectacularly.


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## Mayhem

Not sure that I can follow your answer. 

The comparison is between two 10A rated fuses:

The first fuse is a *DC* rated fuse, 12VDC 10A

The second fuse is an *AC* rated fuse, 240VAC 10A

The dimmer is a 12 channel 2.4K (per channel) 3 phase dimmer that uses 240VAC 10A fuses for the individual channel protection.

The question is why is it that when (incorrectly) fitted with the DC rated fuse, the fuse did not blow, where the correct AC rated fuse would. Or simply put, what is the difference between AC and DC fuses (given the same amp rating)?


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## darkfield

Sorry, I was just trying to add some humor. (Then as an afterthought threw in quick abuse of Watt's law, in my hurry skipping over AC/DC factors, and the technical issues you are interested in.)

I'll stop bothering you now.


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## Mayhem

No need to apologise for adding some humour – I just wanted to ensure that it was clear that the voltages were AC and DC and that my question was clear.

Good work on making it sound feasible on the first read.


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## dimwatt

(doh! forgot to login before replying  )

Here's my thoughts. I'm probably going to prove that the less you know about something the easier it is to (mis)understand. 

Anyway, here goes: 

I'm going to use an equation I learnt a long time ago: V=IR (Ohm's Law voltage = current x resistance). 

For a 240V 10A fuse we get: 240 = 10 x R, giving R a value of 24 ohms. 

For the 12V 10A fuse it's: 12 = 10 x R, making R = 1.2 ohms. 

Now, if we take this last result and shove it back into the equation in a 240V circuit we get: 

240 = A x 1.2, which makes A = 200 amps. 

In other words the fuse won't blow until we stick 200A through it at 240V. 

You'll notice that I've ignored the AC/DC aspects of this (sorry Angus), that's partly because I'm not sure it's relevant. 

Of course I could be completely wrong, but to the unenlightened I think this is a plausible explanation. At worst I'll be told why I'm wrong, so I'll learn something new (and that's why I joined CB in the first place).


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## darkfield

There are many different fuse types, some DC rated fuses are not as sensitive to overcurrent as AC rated fuses. And some have a time delay factor so that they don't blow due to startup voltage surges. So a DC rated fuse might actually be letting 2 to 5 times as much current flow without breaking the circuit.


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## falcon

The AC fuse has more "stress" put onto it becuase of the current always switching directing. It goes for and back 50 or 60 times a second (might be different depending where you are). This causing the fuse to heat up more since its like rubbing to sticks together at one point.

The DC fuse deals with direct current meaning the current always goes in one direction so the easiest way to blow it would be to put too much current through it. This would be like pushing something on castors, the faster you push it the more of a chance it has of falling over. 

If you want to get all technical about it, then dimwatt is on the right track with the Ohm's law thing with V=IR (voltage = current x resistance). The more resistance it has, the more it would heat it due to the power loss and eventualy blow. Some fuses have a power rating on them as well as a resistance so you can use P=IIR (power = current x current x resistance) to get the total current that can flow through.


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## koncept

can some one also explain how arc fault circuit breakers work??


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## falcon

an arc fault occurs when the current jumps from one cable to the next, usually its between two cable halfs that have been broken. These give off a small disturbance in the power line. When the arc-fault breaker detects the disturbance, it shuts off.


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## ship

http://www.blue-room.org.uk/index.php?showtopic=5731&st=0 

While not directly related to the discussion, this one fits and is a really good read. Also a good forum to read if not take part in.


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## Mayhem

This one had to be my favourite:


dominicgross - taken from [url said:


> http://www.blue-room.org.uk/index.php?showtopic=5731&st=0]While[/url] doing a tour in sweden we noticed that all the 13amp (euro style) plugs had had nails put in the fuse holders!
> 
> When asked the reason for this they answered - 'We have good earth'...???



I will be in Sweden and Denmark next month, so I will keep my eye out for such novel uses for nails!

Recently received this explanation from a friend of mine:

As for the question - current is current, irrelevant of the voltage that it is at as far as a fuse is concerned. IE if you take the 10 amp 240 volt fuse and slap it into the car and then try to pull 20 amps it will go futt. 

Where the difference is, is a 240 volt fuse is capable of interrupting the current flow of a higher potential than the 12 volt jobby without exploding/shattering and generally f**king over your fuse holder. 

As for the heat thing, you generally find that the car fuses (read low voltage) are not as well made (read cheap) as the 240 volt ones and as such tend to give off more heat and when used in a screw in type fuse holder they have a higher contact resistance (more heat) as they generally do when used in the clip-in varieties that have larger contact areas and so on. 

The main reason though is the afore mentioned ability to break higher voltages, as, when the fuse goes splat it really does go splat leaving a fine coating of copper all over the inside of the glass/ceramic tube. This allows the possibility of conduction if you then add ionized gas (created when the fuse goes futt) to the formula and say an inductive load. 

Now the other problem you get is most fuse holders that you come across will tarnish with time equalling more heat which equals faster tarnishing which equals more heat which... You get the picture. 

And there is the other problem, the users. The amount of time that I have seen 20 amp car fuses come out of dimmers begs disbelief. This is a good way to really f**k things up because not only do you toast the fuse holder you toast the cable as well.

240 volt fuses tend to be made of ceramic or a high temp glass to withstand the high temperature of a piece of copper/nichrome turning to liquid and the HRC variety then add a sand fill to snub out any arcing if it occurs.


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## Lisa

rgsw said:


> to put it in lame mans term:



I know this is awfully off topic, but it's "Layman's terms."


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## SketchyCroftPpl

I think another part of a fuse I didn't see mentioned isn't just to protect the internal hardware of a device but also to provide a safty. As you said in the one you looked at something had cought on fire. I don't know where you or he lives or anything but if the entire unit had done so on dry grass or something flammable he could have ended up with alot more serious damage than a broken sprinkler timer. 

As another question, we use SCRimmer Stik, they are just basically portable dimmers. Each of them has 4 sockets for a light and each has its own fuse. We have been having this problem where a fuse will blow but then since it is glass will break inside the Stik and basically make it useless. (The Metal end stays in the bottom). This isn't a trational just push in type fuse with 2 holders its more like a hole and you drop it in. Are there any such things as fuses with high temp plastic on the outside (with metal rings on the ends and eveyrthing) that might stop these from breaking when we try to get them out?

~Nick


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## koncept

Yes, there are high temp fuses. What is the rating on the ones you are currently using?


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## SketchyCroftPpl

I don't know what all this means so if anyone wants to explain it please feel free to. I looked on the EDI website for <A HRef="http://www.edionline.com/downloads/P155-SCRIMMER_STIK_0205.pdf>SCRimmer Stiks </a> and I'm pretty sure it's one of these:
"
Surge Current -- 250Amp Peak, 1 cycle
or more likely,
Fused Output -- Four 6.25 Amp or two 10 Amp 3 AG Ceramic Fuses. 

The Stiks that we use have four fuses in them.

Thanks,
Nick


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## SketchyCroftPpl

Sorry in that last thing, I messed up some HTML somewhere in the link that I made and it turned the whole end of that into a link. It goes to the online product info for the SCRimmer Stiks that we use.


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