Rating of main fuse

Hi,
I am currently having a 10.8KW shower installed. I have noticed that
the main fuse on my supply is in a box marked "60A". This would seem
to suggest that if, for instance, someone turned the kettle on while I
was showering the main fuse might blow...
Obviously only the supplier can change the main fuse, for, say, a 100A
one, but presumably they would need to ensure the cables running to my
main fuse could take that... any thoughts?
Cheers,
Ben
Reply to
Ben
In article , Ben writes:
Yes. They'll also check that your consumer unit and tails are suitable, and that the main earthing and service bonding is up to spec (correct size cables, etc).
I last did this >10 years ago, and it was free back then. This might not be true now, and there might be more checks required of your own wiring.
Reply to
Andrew Gabriel
I can't see any writing on either the meter tails or the supply from the other side of the fuse. However, slightly worrying is that the consumer unit is rated at 100A, and the meter tails are definitely larger diameter than those supplying the fuse in the first place. I hope they can cope with uprating the fuse... However, on a possible plus, just below my cutout I have another box with a screwed on lid with two conduits coming in, and two coming out. One goes to my cutout, the other through the wall (presumably to the cutout next door), each carrying L,N and earth.
If the remaining two conduits coming in carry live and neutral (as seems likely), then they would be rated much higher since they supply at least two flats. However, replacing the cable from there to the cutout would involve working live, which might cause issues. Suffice to say I'm not going anywhere near it myself! I'll see what the electrician says on Monday.
Cheers,
Ben
Reply to
Ben
A fuse does not blow at + a tiny bit. The rating is what it will carry pretty much indefinately. At 120A it'll be getting warm if not rather hot after few minutes but could take considerably longer to blow. To make a fuse blow in short order requires an overload several times it's rating.
Reply to
Dave Liquorice
Don't panic.
A 10.8kW shower + 3kW kettle = 13.8kW = 60A at 230v
Now, a 60A fuse takes a long time to blow with 60A going through it (there are curves which predict how long they take to blow at varying currents). Typically, with 59.5A it will never blow, with 60A it *may* take an hour or, more likely, never blow, with 65A it may take 45mins, with 80A it may take 3mins etc. (values made up on the spur of the moment!). The incoming fuse is intended to protect the wiring against short circuits in the fuse box so it is a slow type. Your own internal fuses or circuit breakers are faster, so that they blow before the main fuse.
Unless you are in the habit of taking *very* long showers you aren't going to blow that fuse. Even if you add a toaster it won't go as the kettle will boil dry and the toast will burn first!
Reply to
mick
Thanks - I realise that. But presumably it is also bad practice to design a system where it is known the current may occasionally exceed the rated value of the protection, while still functioning correctly?
Reply to
Ben
The rule for diversity of a consumer unit is 100% of the biggest break and 40% of all the others.
So 10.8 Kw (Assumed at @240V) is 45 but ... small print on unit will say 9.9kW at 230Vac. Anyway breaker will be 45A.
Given that you only have 60A supply already installed you probably have a flat so the other circuits might just be 32A sockets and 6A.
So (40% x 38A) = 15.2A so together with the shower that makes 60.2A so it's just about OK to leave the main fuse alone.
If you have more circuits you'll need to contact the supplier (after a big run around) they might come and up the fuse to 80A if their supply is ok.
Reply to
Ed Sirett
I don't think the rated value of the protection comes into it. It's bad practice to design a system where there will be an expected long term moderate overload, as that's the sort of overload that can overheat cables but not trip protection.
Provided the protection is adequately protecting the cables, then any load the protection permits should be safe.
Owain
Reply to
Owain
Agreed!
In fact fuses are *far* more forgiving than that even. To blow a 60A fuse "instantly" you would be looking for an overload of something north of 700A. 60A or even 80 or 90A it will most likely carry indefinitely. At 100A you could be looking at 45 mins. 200A would probably do it in 30 secs or so.
Reply to
John Rumm
indefinitely.
So with a conventional 100 amp mains fuse how long do you reckon it'd sustain 150 amps? I'm having a new 3 phase supply run into my new workshop, and for the vast majority of the time the load will be less than 10 KW, but for very occassional use (once a month or so) I need 150 amps per phase for maybe ten minutes to run an induction furnace. A 100 amp 3 phase supply can be provide for £1100, but a 200 amp per phase becomes 'a project;' and the cost escalates hugely but they won't quote specifics.
AWEM
Reply to
Andrew Mawson
If look at the graph for BS 88 style fuses (fig 3.3B in BS 7671) (not necessarily the correct fuse type - but it does have curves for the ratings under discussion), it suggests a 150A fuse would carry that load indefinitely (or at least for several hours).
I don't know whether one is allowed to apply diversity to a induction furnace, but I presume it is thermostatically controlled(?), so the average loading when on should be significantly below the maximum.
Have you asked them what supply they recommend for your usage? (one could couch the question in terms of "if I were to get a furnace that required...." etc to not give the game away!)
Reply to
John Rumm
furnace.
induction
I like the approach in your last point. In practice the furnace runs flat out when on, and though I can control the power level simply by tweaking a front panel knob (power in KW is digitally displayed) the advantage of rapid melting is lost. Is there an online source of relavant fuse charateristics ?
AWEM
Reply to
Andrew Mawson
On 7 Oct, 09:34, "Andrew Mawson" wrote:
To the original poster
Response curves for the BS types of fuses are tabulated in the appendices of BS7671 (16th edition regs). Cheap copies are always for sale on ebay if you can't run to a brand new one. also the 17th edition is just around the corner when no doubt there will be 16th editions in the clearance book stores BS1361 fuses are the "norm" for electricity board main fuses. From my copy you would expect the fuse to operate in 5 seconds at a current of 330A, within 0.4 seconds at 600A and 0.1 seconds at 880 A. The curve only goes up to 3500 seconds which would require 100A sustained current (about an hour to open the circuit) You don't know what the conductors of the incoming main are but I'd hazard a guess at least 25mm or equivalent depending on age of the units. As others have said ask the supplier if their supply is or would be suitable for the additional load. Be prepared for a run-around by call centre staff!
Regarding the furnace, diversity might taken into account if the 10 minutes are a true description of the situation but it falls outside the readily available tablulated situations and would need examining by a competent engineer. The makers might be willing to advise/ negotiate with the REC
Reply to
cynic
In our area (Manweb) we'll typically replace a 60A fuse for an 80A fuse where possible - most of the cable that was put in for a 60A supply is good for that.
An upgrade to 100A would be a chargeable job, and we'd usually take it all the way back to the main in the street.
All this depends, of course, on the type of network supplying your house - if you're on an overhead, particularly if you're in a small cluster of houses fed by a small pole mounted transformer, it might be quite costly...
Reply to
Colin Wilson
The last guidance note I saw for our electricians (a couple of years ago) is that you don't check the customers' installation at all. The RECs' responsibility ends at the double pole switch - the customer has complete liability for their own side of the installation.
Absolutely no checks are made on earthing, and if the service cable is replaced / refurbished, the existing earthing arrangements are kept "as they are" even if the new service is capable of being PMEd.
Reply to
Colin Wilson
In our new (to us) house we have perhaps a similar problem - but the shower is already fitted.
I have noticed that when the shower is turned on the lights dim slightly, suggesting that not enough electrons are moving up/down/side to side/whatever which in turn suggests that the mains supply may not be quite up to the current load.
I haven't had a chance to check all the ratings thoroughly (new house is somewhere that I am not) but would overloading the main fuse produce these symptoms, or am I looking potentially at an expensive upgrade to my mains supply?
Just throwing this in as it seems possibly related.
Cheers
Dave R
Reply to
David W.E. Roberts
It is not an indication that you are overloading the fuse as such, more demonstrating that the impedance of your supply is perhaps a little higher than you would like. Being realistic, its not usually a problem unless the impedance is so high that you drop significant voltage on high loads.
For example we have an overhead supply with 60A main fuse, and a supply impedance of about 0.25 ohms. So a 40A load would drop the normally 240V supply to a bit under 230V - certainly enough to see a brightness change, but not enough to drop the supply out of spec or cause any real problem.
Reply to
John Rumm
thanks for that - reassuring.
Presumably it might be an issue if I wished to extend the house and add significantly to the existing load?
Cheers
Dave R
Reply to
David W.E. Roberts
In article , John Rumm writes:
Also, a tiny change in the voltage causes a much larger change in the light from filament lamps.
Reply to
Andrew Gabriel

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