I thought it was odd too, but the people insist that this thing originally came with a prewired plug, and that all they've done is replace the plug so that the flex could be passed through a worksurface to a socket.
5055W @240V
10A + 30% of remainder.
2400W + 30% of 2655W = 3196.5A = 13.32A @ 240V
They're pushing it a bit, eh?
They've been in the house 18 months and the hob is about 12 months old. The previous hob was wired in the same way and they have never had a fuse blow with either. In fact the current arrangement involves a 4-way fused "socket doubler" (IOW an extension lead which is screwed to the wall) and also feeds a TV and something else I've forgotten, and they've never had the fuse blow in that either.
Can't find anything in the regulations which prohibits this; nothing about cookers having to have their own dedicated circuits, just that the circuit must be appropriately rated.
The oven is a separate unit connected somewhere else altogether.
I've never seen (nor done) it before though, so any thoughts?
Assuming that you have a habit of turning all the hobs on full and then switching the whole unit on at the socket surely? I suspect that in a real life situation it will be the other side of 13A ~ 240V - close but not over IYSWIM.
Yes. Sounds like one of those bodges that seems to work in practice, even if it is somewhat naughty. Bear in mind 13A rated kit will survive significantly more than 13A indefinitely, and even more for limited periods.
Precisely. This couple admitted they rarely used more than two "burners", but I absolutely cannot see how this hob could *possibly* have been BS-whatever certified, BEAB approved or whatever given the rare, but not unlikely possibility of coming home from (say) church and turning 5kW of load on at pretty much the same time (we have gas, but if it weren't for the steamer we would do pretty much that for Sunday lunch).
I don't like it at all, but I can't offer a cast-iron argument to them to say that it absolutely shouldn't be done :-/
Martin Angove wrote: > I thought it was odd too, but the people insist that this thing > originally came with a prewired plug, and that all they've done is > replace the plug so that the flex could be passed through a worksurface > to a socket. >
Utterly, totally, and wildly inappropriately. Or near offer ;-)
It's entirely reasonable to apply the "10A + 30%-of-the-rest" guideline to say 'what realistic load will the cooking-appliance final circuit(s) present to the whole installation under domestic usage".
It's entirely INAPPROPRIATE to use that guideline to say 'how should I size the cables and overcurrent protection for the cooker final circuit'! Since the bleedin' thing's capable of drawing 5kW = 20A peak, it should freakin' well be connected through an arrangement which can safely provide 20 of the finest amperes in the land. A 13A plug-n-socket setup *can't* do that. No, the integral 13A fuse won't blow - it'll allow a mere 50% overload of nominal to pass for 30 minutes or more, and there's no way the hob will draw its 5kW for that long, as the individual simmerstats cut in and out. But that doesn't mean you have 'good practice' here - each time the hob does draw its full load, the flex and the pressure connections between plug and socket are overloaded, where overloaded means 'heating up above sensible temperature limits', the insulation's getting softer than it should and potentially flowing/creeping away from the conductors, and the effective life of your installation is being shortened.
You'd be a lot better off with a dedicated 20A radial and 20A DP switch close by the hob. As ding-dong discussions finally (?) settled here a year or more ago, it's also OK - disturbing at it may seem to the simplistic 'rate everything to the fuse in the fusebox' brigage - to have a 2.5mmsq feed (cable or flex) with a 20A switch at the end of a
40A 'cooker circuit' - the drop cable's adequately sized for the dedicated load, it can't overload, and the 40A MCB provides perfectly good short-circuit protection to the *short* length of 2.5mmsq, PROVIDED IT REALLY IS SHORT - archives would show the calcs, but 'a few' metres is all you can do. And you're better off doing any such final drop in
4mmsq if you can, since the 2.5mmsq is marginal if the ambient temp is over 30 degrees (which in the back of a kitchen unit near an oven or hob it might well be).
If that really is what is happening then the protection should have bloody well cut the supply, or are you saying that the whole frigging regulations and design of components are total crap in letting such a situation to happen were the protection is rated to the supposed maximum for the components...
Overcurrent protection does not, cannot economically, and is not designed to, provide protection against sustained "small" overloads.
Prevention of sustained overload is the job of final circuit DESIGN.
Fuses and MCBs aren't magic: an MCB rated at 20A will pass a 20A current forever without tripping. It will also pass a 21A, 22A, 23A, 24A, and
25A current effectively forever: these components just aren't precision devices. (Wylex tabulated graph for Type B MCB shows the trip time for a
1.2*nominal current to be between 400 seconds (about 7 minutes) and 1.4 hours.) For these 'small' overloads, the detection mechanism is thermal
- a strip heats up in direct proportion to the current being passed, bending as it heats, and when it's bent enough it trips the MCB mechanism. US trips, as I understand it, have only this thermal mechanism: all BS trips have a second, faster-acting mechanism - a solenoid which operates within a second at the low end of its rated sensitivity (3-5 times nominal for a Type B, higher multiples for Types C and D), and much faster than that for a gross overload, i.e. a L-N or L-E short. For fuses (rather than MCBs) the sensitivity to sustained small overloads is even worse - they'll let 1.5 times the rated current flow indefinitely.
So, no competent, Regs-compliant design EVER relies on the fuse/MCB to prevent small overloads: explicit design advice is that final circuit designs must NOT rely on the circuit protective element (MCB/fuse) to limit the current under normal operating conditions.
Instead, it's consideration of the LOAD which determines the size of cable (and ratings of associated control gear such as switches, naturally). You look at the sustained maximum current your known, fixed load could draw, choose a cable which can carry that current, check its performance under short-circuit conditions, and choose a protective device which will protect that cable. Where the load is capable of producing an overload - a motor is the classic example - your choice of protective device must be appropriate to that load and its overload characteristics.
Now, for 'general-purpose' power circuits - domestic/office rings and radials - you can't make nearly so precise an assessment of loads, as you don't have control over what gets plugged in. Therefore, the DESIGN (see, that word again!) guidelines for such circuits limit the LIKELY load, by suggesting limits on floor area served, by avoiding mixing fixed and variable loads (hence not putting immersion heaters, UFH, and similar on rings), and by telling the designer to take into account any known loads and their concentration (e.g. the long discussions here, and in IEE guidance notes, about rings which serve kitchens and avoiding situations where the bulk of the load is placed close to one end of the ring).
The take-away message is that fuses and MCBs do NOT protect against silliness, only against foreseeable gross faults.
Yes, I accept what you say, but surely the terminals etc. should be able to withstand such overloads? What is the point of a final protection device if it doesn't protect...
Yes well, kind of a moot point now. I've persuaded them to let me convert it back to a "cooker" circuit (someone had bunged a double socket on the end of the 32A/6mm2 radial in the first place).
Having had a day to think about it, they really aren't sure that the hob really did come with a fitted plug after all and erm, no, they can't find the instruction manual for it either (12 months old remember).
would they? It should all run no more than lukewarm at 13A, so in most cases it would be fine at 20C, well within limits. Whats being eroded is the safety margin more than the working ability.
I'm doubtful. I found it took 13A running thru thin 3A flex (IIRC) to reach that point. 13A flex would normally handle 20A no problem. Normally anyway...
yes, and the effective life of the user too. But not by much, even this bodge has only a very tiny chance of killing someone.
would they? It should all run no more than lukewarm at 13A, so in most cases it would be fine at 20C, well within limits. Whats being eroded is the safety margin more than the working ability.
I'm doubtful. I found it took 13A running thru thin 3A flex (IIRC) to reach that point. 13A flex would normally handle 20A no problem. Normally anyway...
yes, and the effective life of the user too. But not by much, even this bodge has only a very tiny chance of killing someone.
Why not just uprate the cable and fit a heatsink to the mains plug? :) (joke)
A ring circuit made from Flex and with each flex protected. Should be fine and at least as safe as a ring main provided you glue the plugs together so you have to remove them both. ;-)
You could always do the job properly and use a bigger connector that's designed to take 6kW.
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