Fuses don't really protect against overload.

[calumr135]

I know it sounds ridiculous but I don't think they really do. I have known fuses to take twice their rating capacity for quite some time before finally blowing. Best one is the very common main 60 amp cut out fuse in your normal household with the service cable only rated around 80 ish max, if the 60 amp main fuse can take 100 amps or slightly more for half an hour its not really protecting the cable against overload is it? Shorting the meter tails which I don't recommend would blow main fuse instantly however.

Another example which is very common in houses is when your dishwasher and washing machine are on the same 13 fused spur I have that set up myself, both dishwashers and washing machines are either 2kw or 3kw, add them together and they are in excess of 13 amps yet the fuse doesn't blow. There has been quite a lot of times over the years and still is where my dishwasher and washing were both on at the same time with both elements drawing heat and never once has the 13 amp fuse blown, for the record they both 3kw!. surely the 2.5mm cable would be getting pretty hot yet the fuse doesn't seem bothered.

Extension leads are also common where if you exceed 13 amps the fuse won't blow at least for a while or at all.

I believe fuses are more to protect against direct short circuits rather than overloads. Most fuses I have seen and tested can take over twice their rating.

Your opinions are welcome. Share your knowledge and experience.

 

[aptsys]

Thanks for the newsflash. This is what the fuse curves are for.

 

[ericmark]

I found MCB's were too slow to trip with a short circuit to protect solid state relays from short circuit, but semi-conductor fuses would rupture in time. There are so many fuse types it is impossible to be general, fast and slow blow all have their uses.

As to the BS1362 fuse it is there to protect the cable, both the fuse and the cable take time to warm up, and as long as the correctly selected fuse ruptures before the cable is damaged it has done its job. My dish washer takes around an hour and a half to do a wash, and in that time it uses 0.59 kWh so average use is around the 400 Watt mark it may take a peak watts of 1403 but the time it takes it over brings the average well down, and it is the sustained load over hours which would cause a fuse to blow.

Even the MCB is not designed to trip within 0.01 seconds until the load is between 3 and 5 times rated amperage (B type). The thermal tripping curve will as many have found allow showers which are a 1/3 again larger than the MCB rating to run for 15 minutes without tripping, since not use for long there is many a shower running on a MCB which is too small.

We have charts

and it is up to the designer to select a fuse by using the charts to suit what it is protecting. This is why we sign to say I being responsible for the design on the paperwork, we sign to say we have taken all that into consideration.

 

[EFLImpudence]

Think about it Calum - how could a piece of wire (say that in a 13A cartridge fuse) carry 13A "for ever" without damage and then get hot enough to melt at 13.1A?

The same goes for circuit breakers - run "for ever" at 16A, or whatever their rating, and then instantly heat up to trip (because that's how they do it) at 16.1.

All devices have a "fusing factor". This is a number "X" times their rated current which will cause them to trip in one hour.
For MCBs, this fusing factor is 1.45 - i.e. for a 16A x 1.45 = 23.2A
MCBs are designed not to trip up to 1.13 x their rating.

They have to have time to heat up.

Fuse wire has a fusing factor of 2.

The given current capacities of the cable obviously take this into account.

 

[John D v2.0]

Think about it Calum - how could a piece of wire (say that in a 13A cartridge fuse) carry 13A "for ever" without damage and then get hot enough to melt at 13.1A?

The same goes for circuit breakers - run "for ever" at 16A, or whatever their rating, and then instantly heat up to trip (because that's how they do it) at 16.1.

Totally right, but actually the logic goes the other way! It's intentionally designed to have a curve for good reason. In fact it would be quite easy to make an mcb that tripped straight away at the rated current, but since it's protecting copper conductors from overload it's desirable to follow the curve for damage too the conductors.
In fact mcbs actually have a magnetic trip that's designed for faults rather then overloads, and it does trip instantly as soon as you hit the threshold, as that's more desirable then a curve due to touch voltages on exposed conductive parts.

 

[EFLImpudence]

Totally right,

OK.

but actually the logic goes the other way! It's intentionally designed to have a curve for good reason. In fact it would be quite easy to make an mcb that tripped straight away at the rated current,

Not by heating. I presume they were designed to replicate fuses.

but since it's protecting copper conductors from overload it's desirable to follow the curve for damage to the conductors.

Not really. They may preempt the curve to avoid damage.
You would have to introduce time into circuit design otherwise.

In fact mcbs actually have a magnetic trip that's designed for faults rather then overloads,

Yes, I know. I did not mention fault current.
The OP did mention short-circuit but he was concerned with overload.

and it does trip instantly as soon as you hit the threshold,

Well, sort of - that threshold being a maximum when it must trip instantly. B, C or D.
They must have decided the threshold values before design but they are rather vague; 3-5, 5-10, 10-20.

as that's more desirable then a curve due to touch voltages on exposed conductive parts.

I don't follow your logic.

What would be the point of the device taking longer to trip?
Tripping more slowly would not be tripping instantly - the desired criterion - at a maximum allowed current.


Edited - thank you Risteard.

 

[stillp]

Let's not get into manufacturing tolerances...

 

[John D v2.0]

I think you missed my point, I was saying there is no fundamental reason why they would use heating. If it were more appropriate to protect from overload in another way they would have done that instead.
By way of example, I pointed out that they changed the way of dealing with short circuits from heating a fuse to a magnetic trip (because it's better) so they would have used a different method for overload too if it were better.

 

[EFLImpudence]

I think you missed my point, I was saying there is no fundamental reason why they would use heating.

There is if that's what they wanted.

If it were more appropriate to protect from overload in another way they would have done that instead.

Shall we deduce it wasn't - because...

By way of example, I pointed out that they changed the way of dealing with short circuits from heating a fuse to a magnetic trip (because it's better)

...maybe the only way of getting a (cheap) reusable instant trip...

so they would have used a different method for overload too if it were better.

...and, presumably they didn't want a specific cut-off point for all circumstances - overload is gradual.

Obviously you could design a current/time monitor to do the job.
Heating does the same and, I would say, more reliably - like a fuse.

 

[John D v2.0]

I think we're basically agreeing but with different wording, we could keep this debate going on longer than the general election campaign if we keep going this way

 

[EFLImpudence]

 

[ban-all-sheds]

I know it sounds ridiculous but I don't think they really do.

Of course they do. They protect against overcurrent in the way they are supposed to.

if the 60 amp main fuse can take 100 amps or slightly more for half an hour its not really protecting the cable against overload is it?

Assuming that when you use the word "protecting" you actually mean "protecting against damage", then yes, it is.


Shorting the meter tails which I don't recommend would blow main fuse instantly however.

both dishwashers and washing machines are either 2kw or 3kw, add them together and they are in excess of 13 amps yet the fuse doesn't blow.

How often, and by how much, are they in excess of 13A together?

There has been quite a lot of times over the years and still is where my dishwasher and washing were both on at the same time with both elements drawing heat and never once has the 13 amp fuse blown, for the record they both 3kw!.

But for how long was the total load 6kW?

surely the 2.5mm cable would be getting pretty hot

Why?

Extension leads are also common where if you exceed 13 amps the fuse won't blow at least for a while or at all.

Do you think that the fuse in an extension lead is different to one in a plug not on an extension lead?

I believe fuses are more to protect against direct short circuits rather than overloads.

They are there to protect the cable from overcurrent.

Most fuses I have seen and tested can take over twice their rating.

What testing have you done? Using what equipment and in what environments?

Your opinions are welcome.

Good oh.

Please welcome this:

What is your point?

 

[calumr135]

I just found an interesting video on youtube made by john ward demonstrating a normal 13 amp fuse taking up to 30 amps before finally blowing,my point is how is that fuse going to protect cables such as an extension lead which are 1.5mm and only rated at 16 amps at best? surely the cable or fcu/plug would melt before the fuse the blows. His video is called "fake 15A BS1362 fuses tested" check it out if you don't believe me. I am just convinced fuses are there just for short circuit protection only, they do not provide sufficient overload protection and something would already catch fire before the fuse operates.Just sharing my experience that's all. cheers

 

[EFLImpudence]

I just found an interesting video on youtube made by john ward demonstrating a normal 13 amp fuse taking up to 30 amps before finally blowing,

Yes, I explained why.

my point is how is that fuse going to protect cables such as an extension lead which are 1.5mm and only rated at 16 amps at best?

The cross-sectional area of 15A fuse wire is 0.2 square millimetres.

surely the cable or fcu/plug would melt before the fuse the blows.

Have you ever had a fuse blow?
Have you ever had a cable melt?

His video is called "fake 15A BS1362 fuses tested" check it out if you don't believe me. I am just convinced fuses are there just for short circuit protection only, they do not provide sufficient overload protection and something would already catch fire before the fuse operates.Just sharing my experience

Obviously if they are fake then that is a different problem.

 

[bernardgreen]

they do not provide sufficient overload protection and something would already catch fire before the fuse operates.Just sharing my experience that's all. cheers

A fuse, if correctly selected, will prevent the cable from becoming dangerous overheated. They are not intended to and cannot prevent fire in a damaged or mis used appliance. That protection has to be internal to the appliance.,