A fuse question

[hylaphone]

Hi folks, I am working on a small run of standalone, single tube preamps. I've always used panel fuse holders in the past, or fused IEC sockets, but with limited real estate in this layout I am considering mounting the mains fuse inside the chassis, a la:

Is this a bad idea, or violation of some safety standard? Is a mains fuse necessarily "user serviceable"?
I perused an online copy of UL standards for A/V and Musical gear, which addresses fuse links accessible "by hand" i.e. - without tools. But it doesn't mention fuses mounted internally.

[catalin gramada]

Hi.As time is a safe protection device should be a standard ISO reglementation somewhere. otherwise we can just speculate. If will be by me I not agree to mount the fuse access at outside just for two simple reasons- first. If a fuse blow it is absolutely necessary to check the cause which produce it then to replace. second- the user /owner should never have access to replace it.this should be a qualification tech work. A fuse is a component of a circuit.never leave it to have easy access for anyone. Fuses sockets are classified by dimensions/ size. There are many fuse values. fuse types/ fast.temp.fusible element could be in air. sand or whatever...That means responsible choices for the same socket size. Easy mistakes could be done. that means to not allow access for anyone.
An output fuse could be useful when the equipment demand a special start.up protocol. If you start.up protocol failed cause the right order of operations was not respected the protection act. Have not sense to open the equipment for that. You know in this instance you did some wrong. so you know the cause.and can change it from outside by means a special designation fuse.

[R.G.]

Yes - what he said!

Not having a fuse externally available invites the user to open it up and muck it up to replace a fuse. You really want the correct value to go back in if a fuse blows, so putting it where the fuse can be accessed externally and having the right value printed on the case by the fuse holder is smart.

I don't know of any standards requirement for a mains fuse to be externally accessible, but IMHO it's a good idea.

It occurs to me that I ought to separate the AC mains fuse from fuses for other purposes. The AC mains fuse is not there for protecting the insides of the equipment. It's there to prevent fires and electrocution when something has already gone badly wrong inside the box. There may be other fuses inside for other purposes, like protecting transformers and such. IMHO it's OK to have those be accessible from inside the covers only.

[catalin gramada]

Just thinking... One of the most used electronic device is the PC. That should be under standard rigors of 21st century.I wonder how many external fuses it have...I have to admit I never checked for the rest of my new domestic electros eg.TV. ..etc
An instrument amp/preamp think is classified as domestic use product.no matter guys do professional work. Cannot be classified as industrial one.so the same standards could be applied
We have protection classes today.that means the electro device was designed to be safe by application no matter the fuses are in or out.
Btw.R.G. do you have the courage to replace a blown fuse without investigation care to know what was going on ? cause I don. t. That is a task for a little educated peoples at least.not for anyone

[hylaphone]

Good points, guys.
I am looking at it like Catalin, if that fuse blows it should be for reasons demanding expert eyes.
In this case the tube is internal as well, a component which will be "handled" much more often than the fuse.
I think of all the studio rack equipment with tubes mounted internally...
Needless to say, there is a bleeder circuit so no B+ is present by the time inside of chassis is exposed.

[R.G.]

Btw.R.G. do you have the courage to replace a blown fuse without investigation care to know what was going on ? cause I don. t. That is a task for a little educated peoples at least.not for anyone

Actually - I do have the courage to replace a blown fuse - exactly one time.

That is because there is a failure mode in fuses that lets just the fuse fail. The active element in a fuse is a precisely-sized filament of metal. The metal melts at a specific temperature, and has a certain resistance per unit area and times the length of the element. Fuse makers calculate the heat rise of the filament precisely, and adjust the length, area, and melting temperature of the metal filament to make the fuse NOT open at its rated current. Above that, somewhere the rated current, it will melt and open. But every cycle of power on/off causes the metal filament to heat up, and expand its length a little. This can in some situations cause fatigue cracking on the ends where it is firmly connected.

So fuses can actually "wear out" by repeated heat/cool cycles if they run near their max current.

And because of that, I will replace a blown fuse with another exactly-the-same rated fuse once. If that one blows, it was not just the fuse.

[catalin gramada]

Hey.didn't know a fuse have a life span.Normaly though the choice regards the right type including high currents transient condition in respect with temperature aso.Oversize till 50% is also common practice. But you talk about limited cycles. that.s something new for me.Thanks for info

[R.G.]

It's a subtle thing. Fuses can last for many, many years, or only a few years depending on how hot the internal filament gets, and whether it's a simple straight wire, or coiled, or zig-zagged so it flexes more easily. The mechanical side of me thinks that some fuse makers may make the ends of the filament curved so that it does not change cross section at one point so the stress of flexing isn't concentrated there.

It was quite a revelation to me when I was informed about this, too. The biggest effect is when the fuse heats very hot and expands the maximum amount before breaking but does not quite break, then cools down. Many cycles of this can make the metal crack and break. But it's a complicated mechanical issue and hard for me to predict.

So I'll let any fuse application have one replacement before suspecting enemy action. You know - once is an accident, twice is a coincidence, three times is enemy action.

[J M Fahey]

Of course if you are worried about fuse mechanical failure you might consider one of these high tech options

[trobbins]

Nowadays fuses have an i2t inrush versus number of events typical curve. Different amps imposes different i2t stress. So there is a nominal service life expectation for a particular fuse and amp.

A fuse could blow due to accidently using a higher mains AC voltage.

The iec fuse switch socket combos usually have space for a spare fuse, so the user can replace with exactly the same fuse.

[Jazz P Bass]

Circuit breakers also have a limit as to how many times they can be 'whacked'.

[Enzo]

Even technicians get old and grow weak.

[hylaphone]

Even technicians get old and grow weak.

...and their positions often filled internally.

Of course fuses have a lifespan, but in the 20+ years I've played through amplifiers, never once have I had to replace one which simply wore out..

[g1]

I've replaced fuses that corroded inside at the end connection but didn't look bad outside (yes an autopsy was performed ). Not sure if that counts as worn out.
Sometimes a freak spike will hit at the exact point of the cycle that puts the fuse over the edge. Sometimes someone will have used a fast blow instead of slow that holds up for years then gets a surge that is too much. Just a few more things to go along with RG's statement about trying a replacement fuse 'just once' but no more.

[J M Fahey]

...and their positions often filled internally.

Of course fuses have a lifespan, but in the 20+ years I've played through amplifiers, never once have I had to replace one which simply wore out..

Just curious: you never had to replace an open fuse which looked good and where a new one did not blow
Weird.

I have, at least a few times, not every week of course but enough to remember them.