davezawadi (David Stone): 
Not again! Diversity, time of use etc. Ignore it, if the fuse fails then the DNO may increase it to 80A, otherwise, big bills may be expected if you make a fuss.

I am not sure that risking the fuse blowing is a good strategy because it would leave the household with no leccy.

There is a risk that there will be no headroom left. It all depends on the household's routine. Will they be showering whilst the dinner is in the oven and the washing machine is running, for example? Mention of a garage hints at an EVCP at some stage.

I would still advise the customer to approach the DNO with a carefully worded question. i.e. “What is the size of the main fuse; and what would be the cost of upgrading it if it turns out to be advisable?”

Many many main cutouts in houses have a main fuse made to B.S. 1361. The time current tables used to be printed in B.S. 7671. I have them in the red book. 17th edition.

Regarding the B.S. 88 part 2 cartridge fuses, they were required to carry an overcurrent of 20 per cent for up to one hour before opening if rated at 63 Amps or less. For fuses rated above 63 Amps, they must be able to carry 20 per cent more than their rated current for four hours of more without opening.

The latter are only required to operate within four hours if carrying 60 per cent more than their rated current.

Curves' chat

80A BS1361 Time Current Curve - IET Engineering Communities (theiet.org)

Z.

Thanks all. I already spoke to the DNO and apparently they don't keep records of what fuse is installed so would have to send someone out to confirm, at the same time they (UK Power Networks) can provide an assessment of whether they can upgrade the fuse to 80 or 100A and apparently there is no charge based on their website. Smart meter was installed last year so I am hoping the current supplier to the property has a record of what fuse is installed.

A 60A fuse will carry 80A for hours without failure, but you knew that didn't you? See fig 3A3 in the BBB.

You need to be a bit careful with the BS 7671 graphs - as they only show the slowest allowable disconnection times (useful for doing calculations for ADS, less so for discrimination). As all device standards allow quite a wide range of operating characteristics, if you want to be sure it won't open it's the fastest operating times you need to be looking for.

Suitable graphs are quite rare, but most cartridge fuses have a non-operating (or non-fusing) current (in conventional time) of around 1.25x In (compared with an operating current of 1.45x or 1.5x) - so it would be quite allowable for a 60A fuse to blow within an hour at 75A, or even shorter time at higher currents - although any individual fuse might last longer.

   - Andy.

Thanks Andy. Referring back to table 3A3(b) though isn't the data essentially saying that a 63A fuse will require roughly 100A of load for an hour before it begins to operate or am I misunderstanding the chart? Can't imagine a situation where both showers would be used for such a period at the same time so to my original query the existing main cutout fuse (if BS 1361 or BS 88-3) should be capable of dealing with the load should both showers be operated at the same time for a short period (say 10-15mins) ?

BS 1361 cartridge fuse curves for example (from TLC website)

Now, there curves are the ‘all fire’ lines, at this amp-time all fuses of that rating will always have blown clear, including slowest fuse off the production line, operating at coldest ambient . (at the fast end it becomes a near constant I2t, at the slow end a near infinite time, this curve is the interesting curved bit in between)

You need to consider pre-arc - the I2T below which we can be totally sure it will never open.

Between the two is ‘may operate’ and it is a wide band..

 datasheet shows similar graph to the above, but also tables a value of pre-arc for 60A to be 9,100 (so 300A for 0.1 seconds, or maybe a touch under100A for a second) and the let through to be 25,000 - compare that with the fast end of the 60A curve above, 0.1 second, and 900A - looks like I2t is more like 81,0000. This is why when you cascade fuses, to make sure the larger one does not blow at the same time as the small one it needs to be ~ 3 times the rating. In reality anything over the pre-arc level ‘might just’ fire the fuse if it is one of the friskier ones in the production spread.

Mike.

Sorry Mike, i'm relatively new to this and you lost me there. Are you saying that if it is BS 1361 rated at 60A it will open quicker if 100A being pulled through it?

Trevor E: 
 

Sorry Mike, i'm relatively new to this and you lost me there. Are you saying that if it is BS 1361 rated at 60A it will open quicker if 100A being pulled through it?

Yes Trevor, the more current that flows through a fuse the quicker it will heat up. If that current is excessive it will cause the fuse element inside to melt. The H.R.C. fuses contain a silver strip of metal that is sized to melt and open the circuit with a certain range of currents above its rated value. The 60 Amp fuse will carry 60 Amps all day forever. But if say, 200 Amps flows through it, it will open  in about perhaps 60 seconds. 300 Amps will open it even faster, and 500 Amps almost instantly.

Z.

Thanks Andy. Referring back to table 3A3(b) though isn't the data essentially saying that a 63A fuse will require roughly 100A of load for an hour before it begins to operate or am I misunderstanding the chart?

It's more saying that the fuse is guaranteed to open in that time for that current (the sort of reassurance you need for protecting conductors from thermal damage, or (for higher currents/shorter durations) that disconnection will be fast enough for ADS). Any given individual fuse is free to open quicker than that though - and the vast majority will be quicker to some extent (just due to manufacturing tolerances and need to stay below prescribed upper limits).  If the 1.25x non-fusing factor is correct, then all we can really say is that a 60A fuse will open in an hour at something between 75A and 100A. Lower currents (but above) 60A will take longer (perhaps infinitely long), higher currents will be quicker.

   - Andy.

Or for another persective - look at manufacturer's  graphs for MCBs - e.g. https://library.e.abb.com/public/114371fcc8e0456096db42d614bead67/2CDC400002D0201_view.pdf - the tripping characteristics are shown as a fairly large region (in orange in this example) rather than a single line. The BS 7671 graphs just show the top/right edge of those areas (i.e. the greatest time/largest required current) - i.e. the worst case for (most) BS 7671 purposes.

   - Andy.