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60A main Fuse, should I upgrade?
Trevor E
13 Posts
Answered

Supplier main Fuse is 60A and current consumer unit has 80A RCD (61008) as isolator with a notice saying 'Max load not to exceed 80A". Property is one storey with 5 rooms. Currently have 8 circuits, 2 lights with B10s, 2 ring finals with B32s, Cooker with B40, Shower with B40, Water Heater with B16 & Garage feed to a sub board with C40. Want to add a new 10.5kw shower so will need 50A protective device but not sure if should be approaching the DNO for an upgrade of the main fuse as running both showers together alone will draw a decent load

22 Replies
ebee
1336 Posts

You should be getting load approval if adding just one shower if one is not existing I think. Who does these days?  

Chris Pearson
3072 Posts

Don't ask us, ask the DNO!

Trevor E
13 Posts

ok, thanks

broadgage
878 Posts

I suspect that an upgrade would be needed if both showers are to be used at the same time.

AJJewsbury
2958 Posts

or if not, consider a shower priority unit… (especially if the DNO upgrade turns out to be more than just a fuse replacement - e.g. if it's a looped supply)

   - Andy. 

Sparkingchip
4625 Posts

I cannot imagine that the DNO will  entertain the idea.

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

Trevor E
13 Posts

Thanks David. I'm just starting out and whilst I understand the basis of diversity I was anticipating a high chance that both showers will be in operation concurrently. I take your point on the time it would take and should have thought about that. Would it then be safe to assume the main 60A fuse is likely to be a BS 3036 as this is the only table with a 60A rating?

No it will be basically a BS88 type, with similar characteristics. Use the 63A graph. BS3036 fuses are the rewirable type, and whilst still listed are not used for DNO fuses (not for at lease 60 years). The older cartridge types have been replaced with BS88 as the two are very similar, although the nearest rating is 63A. Diversity depends on TIME as well as how many things are in use, it is very badly understood, and I am afraid taught.

Is it likely both showers could be used at 5he same time? Apart from the electrical question I cannot see a normal water supply being able to provide enough pressure unless one of them (or both) aretank fed with a pump

Chris Pearson
3072 Posts

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?”

Zoomup
3856 Posts

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.

Trevor E
13 Posts

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.

AJJewsbury
2958 Posts

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.

Trevor E
13 Posts

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) ?

mapj1
4166 Posts

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.

 

Trevor E
13 Posts

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?

Zoomup
3856 Posts

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.

AJJewsbury
2958 Posts

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.

AJJewsbury
2958 Posts

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.

mapj1
4166 Posts

Sorry Mike, i'm relatively new to this and you lost me there.

No prob - we all start somewhere. Are you happy with I squared t being a measure of the energy needed to melt a fuse of given mass  in a short duration fast overload ?

If not come back and we can look at that in more detail. 

The pre-arc energy is that below which it is absolutely going to remain intact and not going to operate, and the other limit, the one at which it always operates is almost 3 times higher, is the one in the curves, and then extends south at a constant I2t - so divide the time by 9 if the current is 3 times higher. 

At the north end, the curve becomes more or less vertical - so never trips even after hours - 1ksec is about 20mins, and 10kseconds is nearly 3 hours.

 Now a 60A fuse must carry 60A all day, but is not guaranteed to carry much more but may not blow all day until we reach more like 90A.

6da80215825f2ecc000def8531d4a86d-origina
Read off the fault current at the bottom, and move your finger up to the curve for your fuse rating, and then find the maximum time it may take to blow at that current.

Note the ~ 2:1 ratio between the pre-arc (no blow) and the total for a 240v supply in the table. The all blow energy is higher for 415 V, as enough of the fuse has to melt to make enough opening  that the voltage could not sustain an an arc across the gap for the largest possible fault current.

0d08d214e1cba094e2e44e31ac4e3028-origina

Mike.

mapj1
4166 Posts

Consider the blow/no blow curves for the 13A plug fuse on the graph below.

Normally we only consider the right hand limit of the all-blow curve, but for any fuse it would have a form generally rather  like this. For the 13A fuse, note the vertical asymptote for the long term overload, could be between 20 an 25 amps for ever, while say a 100A fault may blow a 13A fuse as fast as 0.01 sec or as slow as 0,3 seconds.

Note that for these small fuses, even 10msec is not adiabaticlly fast, and the curve has not yet settled to an I2t form.

 

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