This morning, at around 9 h 20, I was listening to the radio when all of a sudden a loud boom was heard, followed by silence only interrupted by the tick tock of my wind up wall clock. My house is fed from a small feeder going from the main road to my transformer (I'm the only customer on that one) and then further back to two other houses and a workshop on a separate transformer. The fuse at the main road connection was blown, as well as the one on the transformer at the end of the line (feeding all others except me). Of course, since the fuse at the main road was blown, I had no power despite mine being intact.

I called the power company immediately and after a bit of "voice jail" I got the message that my report was filed despite not having to speak a word in the telephone. At 10 h 10, they showed up and only replaced both fuses and everything works so I guess that whatever caused the fuse to blow had either burnt up or was no longer there. I live in a coastal area and we had very heavy rain last night after quite a few days of relatively dry weather and fog. Might have been salt related.

My question is : I was watching the worker replace the fuses from the holders. Now this is a non-conductor tube into which a fuse wire is manually inserted and tightened. Just before hanging the fuse holder on his hot stick to reinstall it, he cut a portion of the excess of fuse wire. With a fuse, does the wire length matter or is it only the thickness of the wire ? Just wondering if the utility fuses only come in a few types and the length is adjusted for the current limit or if it is in fact the thickness of the wire that matters and there is a type for every current limit. Maybe the excess wire is only there to facilitate threading it in the holder.

Anyway, I obviously now have power again. I guess I'll be resetting the microwave and range clocks now.

Hello Sol the length of fusewire really makes no difference the reason why utility HV fuses are longer is because it enables the manufacturers to add arc suppression mechanism into the fuse holder. The length of arc at say 7200 volts might be several inches where as at 110 volts it will be  under 1 inch

That's what I thought. Thanks.

Use similar fuses at work in some of our transmitters and our 4160V substation.If the fuse is reusable-you replace the expended parts-the fuse wire and wire spring.The fuse renewal kits are marked in amp and the voltage it can be used on.For ours,5Kv and the fuse amp values.Some fuses are one shot-like any other standard fuse.The substation fuses are like 400A-When renewing one of these you have to pull quite hard to "cock" the fuse spring in its holder-the fuse link has spring tension on it so it will spring free when blown-so there will be no arcing in the fuse.Cocking one of these is sort of like cocking a crossbow!

It surprises me, it is even possible to repair such fuse in the field.
When voltages go above few kV, the fuses need quite elaborate arc quenching and that means way more consumables (at least e.g. the quenching sand fill, but with that I would not expect more than 5..10kV) and more complicated process needed to renew the fuse elsewhere than in a specialized shops (where you may guarantee the arc quenching fill has no voids or so).
Of course, you may get any voltage by just making the fuse long enough, but then you face huge power dissipation, so reliability problems (oxidizing contacts, spontaneous blows,...), not speaking about rather impractical physical sizes (just 400V can make arcs nearly 0.5m long, when high short circuit currents are involved).

The cut out fuses in question here are similar to the ones in this article. The fuse holder is about 30-50 cm long (I don't remember, I've only seen them up close once). There is indeed a spring that keeps tension on the fusible element. Part of the length of the fusible element is in a cardboard tube, probably infused with some kind of arc quenching substance. When the fuse blows, of course it releases the spring which does two things : it separates the fusible wire at the break point and also modifies the overall length of the fuse holder so it no longer fits in the fuse fixture. The holder then falls and swings on the lower hinge assembly.

If linemans are supposed to replace just the wire, it looks quite scary to me: Normally the arc quenching components are for just a single use only (as once they operate, they become spent and/or damaged), so need to be replaced together with the wire. And I can not imagine to do that in the field in any other way than replacing the whole tube as a complete cartridge (so the replacement one has not only new wire, but it is thoroughly inspected and refilled with the arc quenching material as well; I can not imagine do this properly somewhere in the height on a pole, exposed to elements, with live wires nearby).

Here similar looking fuse assemblies are used to protect the HV input of the transformers, but there the complete fuse cartridges are replaced once they blow. It then depends on the holder design, if the blown cartridge remains seated (and only the indication cap get ejected; that is in the simplest installations), or if it automatically swings "open" (the newer systems) or if it ejects the blown cartridge completely (the old systems). In any case, the main functionality, so the ling melting and arc quenching does not depend on any of the indication mechanism (even when the indication cap get stuck on the cartridge, the circuit is already interrupted). In fact the indication mechanism starts to operate only after the current is already interrupted (even the indication cap)
But the truth is, here the lowest high voltage distribution lines (those I'm used to) are using 22kV, which could be yet another "league" regarding the fusing, arc quenching requirements and so on.


If this contains just a wire just in the air (although in a tube), what is the reliability of such fuse system?  Wouldn't it just draw the arc along the nearby pole and so set it on fire?

Medved don't forget when an HV fuse blows usually a circuit breaker further up the line will open momentarily as well so in fact all power is disconnected the breaker then recloses and because the fuse is now opened and physically disconnected no arc can restrike

It could be.
Definitely at least the 22kV fusing was long time quite big pain: It is already high voltage (so long arc problems,...), yet supposed to work on remote locations, so without any supervision and with only minimum inspections (the higher voltage installations always start/ends in substation with at least somehow qualified crew; the 22kV lines ends on transformers in the middle of nowhere, with no one kilometers around). Their failures to operate properly were the major sources of infrastructure breakdowns.

And the breaker aid you are describing may indeed improve the fusing reliability (in most cases it is able to act), but it can not be relied on (a long, lower power, so higher ohmic branch from high loaded line may limit the short circuit current so, the substation still does not sense anything abnormal because of the high typical load), so the fuses should really be working independently.
Since 1930's, there were at least 5 or 6 fundamentally different concepts of how to handle the arcs, only the first few ones relied on something else than what is just inside of the fuse cartridge tube itself.
Some designs were quite interresting: The fuse link was actually very short (just a cm or so; so it featured very low power dissipation), but it only held together the piston against a tension spring. When the ling blew, the spring pulled the piston back, drawing an arc. But at the same time the piston pushed up a liquid fill from the bottom section of the tube and that liquid then was the main arc quenching medium. It seemed to work very reliably (short link means very low dissipation, so the fuse remained cold, so no material degradations), but there was one significant weak point: If the fluid leaked away due to some sealing fault or crack, the current continued to flow and yet there was nothing anymore to quench the arc once an overcurrent occurs. Therefore this concept was abandoned even before WW2...
Today the most common is just a metal wire or stripe (with higher current both in parallel - the stripe carries majority of the current, the wire holds the indication cap on and triggers the release mechanism if it breaks) inside a ceramic tube, filled by an arc quenching sand. First the sand can not leak that easily (once the crack is so big the sand may go out, usually the whole tube disintegrates and so breaks the circuit that way; or if a loaded, but naked wire remains there, it is not cooled anymore by the sand, so it melts and breaks the current; because there is not yet an overcurrent, just the length and the still remaining sand in the lower section are enough to break the circuit).

In the UK there is also can earth leakage detector at the feeding substation end so if a fault like lightning or a tree branch on the line the earth fault breakers do there thing. Sometimes this is in addition to fuses sometimes instead of

The fuses in question here are for 7200V. There is an ejector spring that effectively shortens the fuse holder when it operates, and the top end slips out of the top clip, so the holder hangs freely. An intact fuse wire holds it in place. As Hannah mentioned, there is also an automatic recloser circuit breaker upstream that operates and momentarily shuts off power further quenching the arc. In this case, I believe this is what happened as my mother, who lives about 2km away reported a 3-5 second power outage at the same time.

Here's a youtube video showing the replacement of the fuse element. In real applications, a hot stick would be used to remove and install the fuse holder, however.

The ground fault detector responds only to the short circuits towards ground, but not on e.g. transformer failure (that causes short circuit between phase wires, so towards ground everything remains balanced).
These detectors have I think one automatic retry attempt (after some second or so), mainly designed to recover the service, when lighting strikes directly to the wiring and causes an arc in the overvoltage arrestor spark gaps.

And indeed, 7200V is 1/3 compare to our common 22kV, that is significantly lower, so maybe the strict requirements needed for the 22kV are not that much necessary for just the 7.2kV, the fuse length appear to be very similar (around 0.5m or so), so the 30% voltage coluld mean just the tube to be enough to quench the arc (by the way I doubt the inner tube is really a standard (cellulose based) cardboard as you described, I would rather guess for some either Mica or Asbestos based material, both are or were used for cardboard-like looking materials, but they are highly heat resistant).

I have a fuse like that somewhere that my father gave me when he was still working for the utility. If ever I find it (I'm cleaning the basement now so I might just come across it) I'll post a picture. The tube does look like cardboard but is probably some other material. I believe different ratings have different colours on the tube. I doubt it is asbestos as the linesman working here yesterday was handling them with bare hands. He only wore gloves when removing and installing the fuse holder in the pole with the hot stick.

The one shot-non renewable MV fuses here contain a sandlike material.Indeed in the renewal kits the fuse wires and spring are in a cardboard like tube.It is lined with some sort of arc quenching material.Its been a very long time-many years since I dealt with the 400A 5Kv renwable fuses.They blow in only very rare and severe OL-this case a shorted regulator transformer in one of our CEMCO transmitters-was shorted to case.We also have capacitor fuses that use like 20Ga copper wire and the spring system-they are in a cardboard tube-these are being replaced with the nonrenewable fuses.They are on the filter caps in the GE transmitters.When those old ones blew a button on the end of the fuse pooped out.You put a ground stick on the cap-BANG-as you discharge the cap-leave the stick there and replace the fuse.The caps are in the 12Kv,15Kv DC power supplies in the transmitter.The CEMCO's have MV "grasshopper" fuses that sometimes blow.These are open-just two loops soldered onto a peice of the 20Ga fuse wire.The AEG transmitter has grasshopper fuses on its HV caps-these haven't blown YET-the caps are working at 30Kv DC.This has killed one guy already-so great care is taken when working inside this transmitter.Grounding hook is your friend!!!

Could it be something like this?
Did you see if the smoke was brown? That is caused by copper vaporizing. It is possible that some birds in your area use the transformer as a roost and the weight of all of them on something caused it to touch something it shouldn't, resulting in this.