How long do (plug) fuses last

Wyndham

Thanks for all the comments - I'm reassured. Though I am wondering what on earth I've started here  

Bendy_House

You've sparked off an extra conversation, that's all

Le_Kirk

Bendy_House said:

You've sparked off an extra conversation, that's all

Oh, you had no resistance to it among these current discussions!

RobM99

What on earth? It's a live conversation! 

Bendy_House

It certainly induced a few new lines.
Shame the conversations didn't stay parallel, tho', but became a series of somewhat choked points. The core of each should really have been terminated securely, instead of being daisy-chained onwards. We ended up running rings around each topic, the odd poster nearly blowing a fuse in the process. But, we didn't impede one another,  or cause unnecessary interference. The capacity of us to highly-charge a thread is quite impressive, whilst not leaving any residual or corrosive bad feeling. I'di-ode many on here a thanks for their huge pyl on insights, whilst re-coiling at some of the puns. Er...uh....umm.

Bendy_House

So, really, what makes an MCB trip when a filament blows? Do we KNOW for certain?

Section62

Bendy_House said:

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!

The arc has lower resistance than the filament. It will only stay on the two ends of the 'blow' if that represents the path of lowest resistance.

Otherwise it could move to the position where the total resistance is the lowest - i.e. a longer arc (of low resistance) plus shorter lengths of remaining filament (of higher resistance).

Bendy_House

Wow! So the arc could well travel down the filaments towards the power source, even if there was a narrower gap available to cross, because the overall resistance would be reduced by it doing this?

That is truly fascinating Thanks.