Voltage ratings on fuses

[nevetslab]

I was in the process of picking up some hardware for this KT88 Power Tube Test Fixture....present item is an IN-Line Fuse Holder that I was going to place in series with the rear panel HT Plate and Screen Test Points, where the fuse would be in series with 700VDC and 400VDC. Only there's a voltage rating on the fuse of 250VAC. Period. Or, 32V, in the case of Automotive Fuses.

Trying to find WHAT IS THE ACTUAL Voltage Rating on a fuse? Is what I'm attempting to do NOT a sensible or actually Dangerous thing to do? I saw on one of the in-line fuse holders, it had a 500V rating. The more traditional ones made by LIttelfuse and others are 250VAC rated.

The thought of having Full Current Potential behind what's on that rear panel Test Point...at Lethal Potentials make me want to have some protection.

Thus far, I haven't found anything on Littelfuse' website as to what the ACTUAL Maximum Voltage Potential is for a given fuse product line, such as a type 312 Fast Blow. I figured I'd place a low current fuse in line so nothing would happen connecting my DMM to that test point, but a fuse WOULD blow should some mistake be made, as all the test points on the rear panel are Tip Jacks.

Any thoughts??

[eschertron]

Are these fuses between the PSU and the tube, or between the tube and the test jacks? If the potential difference between the ends of the fuse is essentially zero Volts under all normal operating conditions, I'd not worry. When a short presents itself, the fuse is going to fail. Hopefully the fuse is going to fail in a way that reduces the possibility of damage or harm!

What's the possibility that this fuse arcs over? Isn't that the real max voltage concern?

[trobbins]

Perhaps take one step back and clarify why you need a direct connection of plate and screen voltages to something external like a DMM. If those points are for just voltage monitoring then I'd recommend inserting a 100:1 resistor divider, so that any external connection is just 1/100 of the voltage (ie. not hazardous and not prone to short circuit). I do that for my maintenance connection socket I typically add to an amp. I tweak the 100:1 divider to obtain a verified accuracy on the external meter (ie. to allow for resistor tolerances and DMM loading).

[nevetslab]

Scrap that idea of the direct connection of the plate and screen voltages there to the test point. Once again, not thinking clearly. I'll instead make a 1000:1 divider, so 700V is read as 700mV, and so on. And, as you've done, tweak the resistors to get the accuracy. 1M and 1k and tweak them to get the 1000:1 divider, and 10M DMM loading will have little effect on the result. Removes the hazard at those test points as well, as they're too close to the floor of the cabinet, which will have ground plane on it. I'll probably have to stake in some more Turrets to mount the divider or use the spare single-row Turret Boards I have.

It did interest me in what the actual voltage rating of common fuses are. I never did find any documents on that from either Littelfuse nor Bussman.

[Enzo]

The voltage rating on the fuse tells us what they guarantee it can safely break. As to what the actual voltage at which they flash over, I doubt you will find that. I have to say that in all my years using the common 250v fuses in HV circuits, I have never seen one flash over.

[trobbins]

Littlefuse have an app note indicating what level of 'secondary side' ac voltage is acceptable for a 250Vac rated fuse. The normal 250Vac fuses aren't designed to break dc current so imho only use them in secondary side ac windings (eg. a CT), or more benign dc circuits. I'd also recommend that more thought on protection is needed if your view is that a fuse is 'needed' in a HT DC feed.
https://www.littelfuse.com/~/media/e..._guide.pdf.pdf

[Jon Snell]

Always use a sand filled ceramic fuse for HT as the sand extinguishes any arc at voltages up to 1000volts. Bear in mind electricity will hold an arc across 1mm gap in dry air for 1kv. 5kv will arc a 5mm gap.
Voltage ratings are a law unto themselves but we are not talking Medium Voltage (12kv up to 63kv) we are talking low voltage or E fuses.
Be aware that many panel mounted fuse holders are not safe at voltages over 100volts due to the construction allowing a person to touch a conductor whilst inserting a fuse.
Fleabay Chinesium fuse holders do not conform to most safety standards.
I cannot find the faulty fuse holder anymore but I had a Yaqin MC10L overpriced Chinesium 'HiFi' amplifier brought in for diagnosis a few years ago.
It turned out, the mains transformer primary had burnt up and the fuse had not blown. The fuse holder conducted current without a fuse in it! A manufacturing defect.
Quality Control is non existant for these manufacturers. You get what you pay for in this World.

[J M Fahey]

The voltage rating on the fuse tells us what they guarantee it can safely break. As to what the actual voltage at which they flash over, I doubt you will find that. I have to say that in all my years using the common 250v fuses in HV circuits, I have never seen one flash over.

You were lucky.

Flashover depends 1000% on available current, you need a lot of it to sustain the arc alive (like on an arc welder, exact same principle)

Arc welders work with lowish voltages, think 35 to 60V or so, but at 160-250-500 Ampere ratings, which of course are imposible iside a transformer fed tube amp, now the mains plug *can* supply high current in a gross short, so fuse ratings refer to that.

I fear more interrupting a "lowish" Mains voltage (110 to 240V) short . than, say, a 800V in a tube amp.

Although for a second capacitors can supply "as much current as you want", hundreds of amperes if necessary, the principle behind Capacitive Discharge magnetizers ... which are VERY dangerous.

Another example of low voltage but high current arcs appears in Lithium batteries.
Fire and explosions come not only from internal "resistive" losses but electrode to electrode arcs whichb are way hotter.

[Enzo]

Me and a zillion others too. Most amp makers use regular fuse for their B+. Maybe it isn't "safe" but they all do it.

Yes I know I myself preach that getting away with something doesn't make it a good idea.