Voltage rating on fuses

[Randall]

I got a late night call from a studio. They had a blown fuse on their Ampeg B-15 bass amp while tracking a session. All they had were 250v rated fuses, but the one they took out was rated at 125v. They wanted to know what to do. I told them to go ahead and use the 250v fuse, and that without looking at a schematic, it was probably what was in there originally.

Which brings me to a question I have often wondered about, what is the difference, and why?

[Jazz P Bass]

The fuse voltage rating is the 'flash over' voltage that the fusable device can withstand.

When the fuse fails, the element can evaporate or in the worst of cases, literally explode inside the fuse element enclosure.
(Ceramic withstands the 'explosion' very well.)
This can lead to splashing of the molten metal, possibly leading to arc over where the fuse does not fully open.
The 'flash over' voltage rating indicates at what voltage the fuse will open & stay open in a failed state.

[MarkusBass]

I think that only current rating is important. This is explained for example here:
How does a fuse blow at its current rating, regardless of voltage? - Electrical Engineering Stack Exchange
Voltage rating is important for a short moment where the circuit is switched on. During this short moment the fuse has to withstand the voltage (without arcing, or any other type of failure). But, later the voltage drop on the fuse is close to 0V. As an example you may imagine using a 120V rated fuse for 10kV circuit. It will arc and most probably will fail (no matter what is the current rating).
In you case you replaced 120V rated fuse with 250V. It should work correctly. The reversed situation would be bad: using 120V rated fuse with 230V circuit. In this case the fuse may fail because it is not rated for 230V.

Mark

[Leo_Gnardo]

Which brings me to a question I have often wondered about, what is the difference, and why?

No worries at all using 250V fuses in a 120V circuit. The voltage rating has to do with the likelihood of current arcing across the metal bits left in the fuse once it has "opened". If the gap is too short, current could continue to jump that gap and what good is the fuse if that happens? So a 250V rated fuse, theoretically, is "safer" than 125V.

What you want to avoid is using 32V rated fuses meant for automotive use, sometimes called SFE fuses. The hassle with fuses rated for AC line use, many 10 amp and up have ceramic not glass housings, and you have to break out the ohmmeter to find whether they're blown or not. Typically the auto rated fuses are glass and often they find their way into audio and other gear run off AC lines. In spite of all the warnings I've yet to find even a 32 volt fuse that failed to work in a 120 V situation.

Now as to that B-15, I'd keep a weather eye on that to find out what's stressing the fuse. The last thing you want to happen is to wreck a PT or OT. They're very expensive, not always available. OR you can do the mad scientist thing like me and melt the bad tranny out of its housing and replace it, a scene to be avoided if at all possible. Failing filter caps, bias cap, and/or output tubes are the main suspects.

[R.G.]

It's also important to recognize that breaking AC is easier than breaking DC because no more than 1/2 cycle later, the power line drops the voltage to zero as it inverts. DC rated fuses are rated to interrupt the DC that never goes to zero, so generally a dual AC/DC rated fuse will have a lower DC voltage rating.

There is a wear-out mechanism in fuses. If the fuse is operated near its rating at power on, the surges will heat it and cause it to mechanically flex. This is something you can actually see in clear glass fuses. Over many cycles, the mechanical bending can weaken or snap the filament near the end cap and make it open.

Finally, it is a very good idea to fuse the secondaries of an expensive or hard-to-replace power transformer. The AC mains fuse is not there to protect the power transformer. It's there to prevent fires. It can be tough to size the AC mains fuse correctly to prevent fires but still stop the PT from burning out. Secondary fuses can be set much more closely to the actual load needs and catch a load condition that would damage the secondary but not pop the AC mains fuse.

[Dave H]

There's another fuse rating to consider, the 'rupturing capacity' or the maximum current the fuse is designed to interrupt. Glass fuses typically have a rupturing capacity of only about about 35A whereas ceramic HRC fuses can safely interrupt thousands of amps. Glass fuses should really only be used on the secondary side of the power transformer where the maximum fault current is limited to less than 35A by the transformer winding resistance. Ceramic HRC fuses are safe to use on either the primary or secondary side of the PT.

[R.G.]

Good point. All fuses have a maximum voltage and current they can interrupt. I wonder if the use of glass fuses on the primary side of a device is allowed because the breakers in the distribution panels themselves will interrupt on currents that are near-infinite.

[Randall]

Very informative answers, per usual. I also wonder about mechanical fatigue on small filament fuses, such as the vibrations they endure being in a bass combo that has wheels that no doubt has been pushed over bumpy sidewalks, etc to and from gigs. That would be my suspicion here.

[Leo_Gnardo]

Very informative answers, per usual. I also wonder about mechanical fatigue on small filament fuses, such as the vibrations they endure being in a bass combo that has wheels that no doubt has been pushed over bumpy sidewalks, etc to and from gigs. That would be my suspicion here.

One thing I think we could do without: filament fuses. Some may argue and I'd like to hear why, but I generally jump the connector with some nice fat copper wire @ 16 ga solid generally.

I have yet to see one case where a filament fuse opened in response to a fault condition, but hundreds where the fuse opened due to fatigue, or else it sat there & fried itself due to failing connections to its circuit board clips.

Related: Fuse or not I've seen other cases where the filament was connected to a circuit board with handy push-on connector, and plastic melting on the pushon insulator, the metal heating due to poor connection from corrosion plus insufficient current handling especially at switch-on. The big 300W Fender and Sunn bass amps are prone to this, and I'd look out for it on modern SVT's too. I find a nice fat land I can solder to and "land" the filament wires there, no further problems.

[R.G.]

One thing I think we could do without: filament fuses. Some may argue and I'd like to hear why,

OK, that would be me, I guess. Not really an argument so much as a technical rationale.

I have yet to see one case where a filament fuse opened in response to a fault condition, but hundreds where the fuse opened due to fatigue, or else it sat there & fried itself due to failing connections to its circuit board clips.

That would be because true faults are rare and the fuses are not well sized, as well as the general design of circuit board clips not being all that good.

PCB designers in general don't do all that well at designing high current paths, and in particular high current paths with connectors. I think this says more about the PCB design than the concept of fuses.

But, as I said, true filament wiring faults are rare. The time when filament fuses will really save your bacon is when a filament fault does a mild overcurrent of the heater winding - perhaps 300% or so, not infinite, or not wire-resistance limited - and the heater winding gets too hot and causes an internal breakdown inside the power transformer. In this case, you lose the power transformer, which is the single most expensive part in the amp.

Again, not the most common occurrence, but very costly on the times it does happen.

[teemuk]

what is the difference, and why?

A 125V fuse can reliably interrupt rated current flow at maximum 125V, a 250V fuse can reliably interrupt rated current at maximum 250V.

So the rating is about reliable tripping of the fuse, one that does not result to arcs, flames, molten metal and glass, explosions or the fuse failing to open circuit.

I think that only current rating is important.

No. Both ratings are important.

But, later the voltage drop on the fuse is close to 0V.

It is close to 0V only when the fuse is intact. When it actually trips you have a considerable voltage potential across the terminals and if the voltage rating isn't up to it the fuse may trip unreliably or in worst case will not trip at all.

I told them to go ahead and use the 250v fuse, and that without looking at a schematic, it was probably what was in there originally.

What if it was a B+ fuse and expected to interrupt 400 - 500 VDC...? Like with any fuse you can fit a nail or any piece of metal there and the device will "work".... but the main point is what happens when the fuse is supposed to do its job.

[g1]

I told them to go ahead and use the 250v fuse, and that without looking at a schematic, it was probably what was in there originally.

What if it was a B+ fuse and expected to interrupt 400 - 500 VDC...? Like with any fuse you can fit a nail or any piece of metal there and the device will "work".... but the main point is what happens when the fuse is supposed to do its job.

Are you aware of any fuse in north american tube amps requiring any fuse rated higher than 250V ?
He's asking about using a 250V fuse to replace a 125V fuse, perfectly acceptable.

Even if he had asked about replacing a 250V fuse with a 125V one (which he did not), we are talking about guitar amps. In case of arc-over of a blown fuse, what's going to happen? It will blow "again".
Yes, you may have added a few milli-seconds of current. Trying a good replacement fuse and having it blow again would also add a few m-secs of current.
I realize when speaking of safety it's better to speak in broad terms to cover all electrical issues, but sometimes we need to be practical and discuss guitar amps.

[teemuk]

Are you aware of any fuse in north american tube amps requiring any fuse rated higher than 250V ?

Yes. Take PRS 25th anniversary amp for example, which calls for 3AG 500mA 440v fuses to the HT circuit. In practice all amps with fused HT circuits should have fuses spec'd accordingly to those high circuit voltages.

He's asking about using a 250V fuse to replace a 125V fuse, perfectly acceptable.

That is an acceptable replacement. Acceptable is not blindly assuming the fuse was originally rated for 250V.

In case of arc-over of a blown fuse, what's going to happen? It will blow "again".

Yes, in worst case it indeed will. Say, it might arc from terminal to terminal, sustaining the current flow resulting to serious damage. I've seen inproperly rated fuses that have blown "again". They were a blob of molten glass and metal and had charred everything in close vicinity to them. I've seen one "blow" by a violent explosion with flames involved. In worst case the fuse "blows", sustains an arc, which then melts the -entire- fuse violently (not just the fuse wire within the envelope), sets the amp in fire, which further sets everything else in fire. Fuses are a safety device, if you wish to ignore that aspect by all means do, but don't make rest of us suffer from it.

[Gregg]

One thing I think we could do without: filament fuses.

I'd rather not use them on the bias winding.