How long do (plug) fuses last

Eldi_Dos

Wyndham said:

However, the replacement fuse has been in the box for some time - I'm talking years and maybe about 20. Do they have a 'shelf life'? Would I be better off buying a new one?

Depends where and how they have been stored,if there was a powdery coating(oxide) around the metal bands that would indicate to me they had been stored in damp conditions.In cupboard above kettle I would consider a damp environment. In some fuses there is a powder around wire and I suppose over decades unused it could become damp and affect how fuse functions.

For the sake of a few pence and if you are concerned it is only a few pence to replace.

As others have said buy from reputable source for peace of mind.

Section62

Bendy_House said:

Anyone got a definitive explanation why blowing filament bulbs often trip MCBs (or even fuses, as per the OP)?!

Read a few, but they don't seem quite convincing.

Most likely due to an arc forming between the broken ends of the filament - once initiated, the current flowing in an arc can get to silly numbers.  Hence the effort made by electrical engineers to design arc supression into equipment and protection devices where arcs could form.

Also why humble (and genuine) BS1362 fuses should be filled with sand to quench any arc before things get nasty.

daveyjp

Whenever I throw out an electrical item I remove the fuse and keep it.  I have a box of fuses some of which will be 40-50 years old.  I have no idea how old they are once in the box, but no problem using them.

However with LED bulbs and more modern manufacturing processes Of electrical equipment I find blown fuses are now very rare.  The last one I had blow was on a steam generator iron with a pump or element which blew up so that needed far more than a new fuse and was skipped.

Ebe_Scrooge

daveyjp said:

Whenever I throw out an electrical item I remove the fuse and keep it.  I have a box of fuses some of which will be 40-50 years old.  I have no idea how old they are once in the box, but no problem using them.

However with LED bulbs and more modern manufacturing processes Of electrical equipment I find blown fuses are now very rare.  The last one I had blow was on a steam generator iron with a pump or element which blew up so that needed far more than a new fuse and was skipped.

I'm exactly the same - I've got two jam-jars full of various fuses in the garage that I never use.  In the olden days when we had actual fuses in the "fuse-box" (and a stock of different thickness fuse-wires to re-wire them when they blew), fuses in plugs would quite often blow.

These days it always seems to be the trip-switch in the consumer unit that trips first - I'm guessing they're a lot more sensitive than a fuse?  And yes, as others have said, they'll even trip when a light-bulb blows.  I suppose it's a good thing that they are so sensitive, shows that they're offering a decent level of protection?

Bendy_House

Section62 said:

Bendy_House said:

Anyone got a definitive explanation why blowing filament bulbs often trip MCBs (or even fuses, as per the OP)?!

Read a few, but they don't seem quite convincing.

Most likely due to an arc forming between the broken ends of the filament - once initiated, the current flowing in an arc can get to silly numbers.  Hence the effort made by electrical engineers to design arc supression into equipment and protection devices where arcs could form.

Also why humble (and genuine) BS1362 fuses should be filled with sand to quench any arc before things get nasty.

That's along the lines of what I read, but I still don't understand it. The arc will be created between the two ends of an otherwise intact filament, so that filament should still be providing the original resistance, surely, and the arc theoretically add to this if anything? What's the resistance of an arc?!

Unless, if the remaining filament now remains 'cold', it will have far less resistance, and that's wot gives the larger current?

From looking at the broken filaments, I don't think it's due to a broken filament end touching a different part - a partial short - but I'm not certain.

Section62

Bendy_House said:

That's along the lines of what I read, but I still don't understand it. The arc will be created between the two ends of an otherwise intact filament, so that filament should still be providing the original resistance, surely, and the arc theoretically add to this if anything? What's the resistance of an arc?!

Unless, if the remaining filament now remains 'cold', it will have far less resistance, and that's wot gives the larger current?

From looking at the broken filaments, I don't think it's due to a broken filament end touching a different part - a partial short - but I'm not certain.

The filament doesn't usually stay intact though. It is suspended by wire loops and once broken, gravity will cause the two ends to move apart (whatever the orientation of the lamp). The arc will grow in length as the two ends of the filament move away from each other.

Once formed, an arc effectively acts as a short (lower resistance than the filament). If the arc can move along the two remaining parts of the filament towards the supply ends then the effective overall resistance (of the lamp) decreases - giving a higher current for a given voltage. i.e. up to the point there is virtually a dead short across the terminals of the lamp.

The randomness of the position the filament breaks and whether/how an arc forms, is the reason for the randomness in whether the resulting current is sufficient to blow/trip a fuse and/or breaker.

GDB2222

Tungsten filaments are incredibly long. They are wound as a double helix, so the wire you see is actually a tiny spiral, made up of an even tinier spiral. When it fails, there’s masses of wire there, and a small bit could cause a short. 

onejrmmrj

Section62 said:

RobM99 said:

I wouldn't think they'd decay much -they're metal, after all. As for new ones, £2.10 for 10 at Screwfix. Use the correct ampage,  1A for a light. Fuses protect equipment, not people!

Fuses protect cables, equipment AND people.

You won't get a 1A BS1362 fuse from Screwfix.

3A is commonly the correct rating for portable lamps/lights. If there is no label on the lamp saying what the correct rating is, and the fitted fuse is anything other than 3A, then further investigation is required.

The purpose of a fuse is only to protect the cable from overheating. There are plenty of possible fault conditions that wouldn't cause an excessive current to flow and would therefore not blow a fuse, a short circuit via a person being the obvious one. That's why the regs now demand RCDs in electrical installations.

 Provided the lamp manufacturer fitted a cable rated at more than 3A then a 3A fuse is fine.

Section62

onejrmmrj said:

Section62 said:

RobM99 said:

I wouldn't think they'd decay much -they're metal, after all. As for new ones, £2.10 for 10 at Screwfix. Use the correct ampage,  1A for a light. Fuses protect equipment, not people!

Fuses protect cables, equipment AND people.

You won't get a 1A BS1362 fuse from Screwfix.

3A is commonly the correct rating for portable lamps/lights. If there is no label on the lamp saying what the correct rating is, and the fitted fuse is anything other than 3A, then further investigation is required.

The purpose of a fuse is only to protect the cable from overheating.

If you take a very narrow view, perhaps.  In practice, a plugtop fuse can do considerably more than just protect the cable from overheating.

onejrmmrj said:

There are plenty of possible fault conditions that wouldn't cause an excessive current to flow and would therefore not blow a fuse, a short circuit via a person being the obvious one. That's why the regs now demand RCDs in electrical installations.

There are also plenty of possibly fault conditions in which an excessive current could flow that would be sufficient to cause a plugtop fuse to blow.  Some of those could pose a danger to life and property. The protection afforded by fuses (and other protective devices) extends beyond the narrow view (even if not designed for).

Bendy_House

Section62 said:

The filament doesn't usually stay intact though. It is suspended by wire loops and once broken, gravity will cause the two ends to move apart (whatever the orientation of the lamp). The arc will grow in length as the two ends of the filament move away from each other.

Once formed, an arc effectively acts as a short (lower resistance than the filament). If the arc can move along the two remaining parts of the filament towards the supply ends then the effective overall resistance (of the lamp) decreases - giving a higher current for a given voltage. i.e. up to the point there is virtually a dead short across the terminals of the lamp.

The randomness of the position the filament breaks and whether/how an arc forms, is the reason for the randomness in whether the resulting current is sufficient to blow/trip a fuse and/or breaker.

Is that actually what happens when it trips an MCB, or an educated guess?

It's plausible, certainly, but also seems unlikely to me. I'd imagine the two ends of the 'blow' would spring/fall apart, and the arc wouldn't carry on further down each line as they'd surely be further apart?

Ooh - time to check YouTube for a slo-mo!