Odd value 20mm 700v 250mA Ceramic fuse finding

[OwenM]

Fuses are not only rated for current (performance) and voltage (safety) but also for breaking current capacity (safety) , where required.
https://en.wikipedia.org/wiki/Breaking_capacity

So, with that in mind the breaking capacity of the supposed 700v fuse I had (if it was truly 700v) would have to have been designed to have a much higher breaking capacity than that of the 250v fuse, in order to have managed it's 700v / 250mA rating, right? If so, given that they say small circuit breakers and fuses have a breaking capacity of 'as small as 85amps' does that mean that for fiddling with amps breaking capacity is usually not something we need to worry about? Is it more of a household switchboard supply consideration?

[Helmholtz]

So, with that in mind the breaking capacity of the supposed 700v fuse I had (if it was truly 700v) would have to have been designed to have a much higher breaking capacity than that of the 250v fuse, in order to have managed it's 700v / 250mA rating, right? If so, given that they say small circuit breakers and fuses have a breaking capacity of 'as small as 85amps' does that mean that for fiddling with amps breaking capacity is usually not something we need to worry about? Is it more of a household switchboard supply consideration?

Breaking capacity is not related to voltage rating. I don't know where the 700V rating is coming from and I don't know the breaking current capacity of your fuse.
I don't think a high breaking capacity is required for tube amp measurements, but a meter manufacturer cannot know where his meter will be used. I'm sure there are safety standards for meters but I'm not familiar with them.

My professional DMMs use 400VAC high breaking fuses.

[OwenM]

Breaking capacity is not related to voltage rating. I don't know where the 700V rating is coming from and I don't know the breaking current capacity of your fuse.
I don't think a high breaking capacity is required for tube amp measurements, but a meter manufacturer cannot know where his meter will be used. I'm sure there are safety standards for meters but I'm not familiar with them.

My professional DMMs use 400VAC high breaking fuses.

Ok I think I've gotcha, the fuse voltage rating is the rating below which a manufacturer can say it will not arc when it blows. The breaking capacity is the current at which an arc in a broken fuse will occur, but that current can only be pushed by a voltage (unrelated to fuse voltage rating), so different voltages will yield different breaking capacities in a fuse. The maximum current you can draw from the wall in a short circuit situation must be lower than the fuses breaking capacity for a given voltage or it will arc in an unsafe way, a higher voltage short-circuit situation will make the breaking capacity of the fuse lower.

So at 220v my 250v blown fuse will not arc from voltage. Given the max current from a UK socket is about 13A, if my fuse breakdown voltage was rated at 30A at 220v the fuse would not reach it's breakdown capacity and all is ok.

If I was plugged into a current source that could give me 40A max at 220v, my fuses 250v rating would still not cause an arc because of voltage alone BUT the 40A at 220v would exceed the breakdown capacity of the fuse and cause a problem.

Is that roughly right?

[Helmholtz]

Ok I think I've gotcha, the fuse voltage rating is the rating below which a manufacturer can say it will not arc when it blows. The breaking capacity is the current at which an arc in a broken fuse will occur, but that current can only be pushed by a voltage (unrelated to fuse voltage rating), so different voltages will yield different breaking capacities in a fuse. The maximum current you can draw from the wall in a short circuit situation must be lower than the fuses breaking capacity for a given voltage or it will arc in an unsafe way, a higher voltage short-circuit situation will make the breaking capacity of the fuse lower. Is that roughly right?

High breaking capacity may be required even at relatively low voltage. When e.g. accidentally shorting an ecap charged to 50VDC or higher with an Ammeter, the surge current may exceed 100A. Depending on capacitance the stored energy may cause a standard fuse to explode.

[OwenM]

High breaking capacity may be required even at relatively low voltage. When e.g. accidentally shorting an ecap charged to 50VDC or higher with an Ammeter, the surge current may exceed 100A. Depending on capacitance the stored energy may cause a standard fuse to explode.

OK cool, yeah factoring in the quick discharge of an ecap as the current source in a hypothetical situation helps me understand the more practical concerns of the fuses breakdown capacity. Thanks for the info!

[vintagekiki]

The fuse in the DMM serves to protect the DMM from improper use or from overload when the DMM is used as an ammeter.
Fuse F10A in DMM protects the ammeter range of 10A and fuse F250mA in DMM protects the ammeter all range of 200mA and less.
If the DMM is used properly, fuses can almost never blow.

The fuse F250mA in a DMM usually blows if a measured current greater than 200mA, or if inadvertently measure voltage when DMM is in position an ammeter or ohmmeter. Since the ammeter has a low resistance, and if it is inadvertently connected in parallel to a voltage source, the ammeter for the voltage source acts as a short circuit and a huge current will flow through it.
That's comes the fuse (protect) which burns out as soon as the current exceeds the nominal value and thus protects the DMM.

As for the overvoltage protection when the DMM is used as a vmeter, there is no protection of the instrument, and therefore it should not be used to measure higher voltages than predicted, because there is a possibility of a spark jump over the measuring resistors in the DMM.
Voltages higher than 1000V can be measured with any DMM with a special measuring HV probe, and with special attention.
Measuring voltages higher than 1000V without a special measuring HV probe puts DMM users at risk.


https://www.electronics-tutorials.ws/category/blog

[vintagekiki]

https://www.youtube.com/watch?v=DX-NoKuBzmk
Using Ammeters & Voltmeters

[J M Fahey]

Guys, (Helmholtz and Owen M), PLEASE stop quoting the immediately above post, and to boot in full.
Thanks.

[Helmholtz]

Guys, (Helmholtz and Owen M), PLEASE stop quoting the immediately above post, and to boot in full.
Thanks.

I take the liberty to decide from case to case and I don't like shouting.

[g1]

Sometimes if I don't quote the post I'm replying to (because it's the directly previous post), someone gets a post in between because I'm not quick enough. Then I have to go back, and include the quote so my post has proper context.

[vintagekiki]

Sometimes if I don't quote the post ... ...

And I have a problem with speed in response, it seems to be (at least with me) a generational problem.
These younger generations as if they had come from a quickly moved game of chess.
In most cases, instead of quoting, it is enough to quote 3-4 words relating to the quotation and end the quote with a few dots ... ...
The meaning of the quote is not lost, by clicking on the address you can see the post being quoted. It is quite tedious for the reader, when the post being quoted has almost half a screen page and more.