Cooker becomes Oven and Microwave....any ideas?

[timhoward]

I'm interested what people's gut reaction solution would be.

Kitchen design and customer said freestanding cooker. So I first fix 10mm, and boxes for cooker switch with socket, and cooker connection plate.
No one thought to tell me it became hob (elsewhere, that bit isn't a problem), and a vertical cupboard which will contain an Ikea Microwave combi thing (<13A which came with plug) and an Ikea oven (3.490 Kw so 15A max which came with bare wires). Instructions for oven do not specify an overcurrent device..

One has fan, one has motor, I can't really claim they are fixed resistive loads, chop plugs off and connect them!
I also don't really want to be even thinking about putting a socket on 10 sq mm cable.

Any creative ideas? (It would be really handy to know if the fan was fused internally in the oven....)
I can only think of tiny consumer unit at the moment. Any help welcome!

 

[timhoward]

Going back to the oven, it's not 100% clear to me whether the hob is still running from the same circuit. If not, it might well work on a B20 or certainly a B25, in which case the oven-maker's requirements might be met if one can find out what they are?

Yes, the hob is still upstream on the same circuit so as you say I don't want to reduce the OCPD.
Thanks for the other info - I was never serious about the fuses thing - but good to know some more theory.

Even a 3A fuse won't protect a fan motor from catching fire, so there is no relationship between the external OCPD for the oven and its current-using components. The manufacturer really ought to state what protection is required, which is quite likely determined by the need to protect the internal wiring against short-circuit.

The instructions are a mixture of a couple of helpful sentences on the website and a downloaded manual that appears to contain a copy/paste set of rules.

The website says:

"No plug is included. Installation to be carried out by a qualified installer."
"Minimum fuse = 16 amps"

Fine so far. The complete downloaded guide says:

-Make sure not to cause damage to the mains plug and to the mains cable
• The appliance must be earthed.
• Make sure that the parameters on the rating plate are compatible with the electrical ratings of the mains power supply.
• Always use a correctly installed shockproof socket.
-Connect the mains plug to the mains socket only at the end of the installation. Make sure that there is access to the mains plug after the installation.
• If the mains socket is loose, do not connect the mains plug.
• Do not pull the mains cable to disconnect the appliance. Always pull the mains plug.
• Use only correct isolation devices: line protecting cut-outs, fuses (screw type fuses removed from the holder), earth leakage trips and contactors.
• The electrical installation must have an isolation device which lets you disconnect the appliance from the mains at all poles. The isolation device must have a contact opening width of minimum 3 mm.

It looks like to fully follow the letter of the instructions a 16A socket would have to be fitted. I also believe a lot of these points are copied and pasted from other products. But no mention of a required maximum fuse or overcurrent device.

The circuit is RCD protected (the house happens to be TT). I know that thousands of these must be fitted without this degree of thought but my main concern is (still) a L-N short circuit as I don't trust the internal wiring to withstand the fault current allowed by a 32A breaker.

Ultimately the choice seems to be
a) dual plate, at least 2.5 sq mm T+E for single socket for microwave (433.2.2 allows this as @APE37 mentioned), connect oven in as no manufacturer maximum fuse is specified
b) Stick BG garage board in cupboard and run a 20A circuit for the oven and 16A for the microwave.

 

[timhoward]

The way I see it, is there is a oven, hob and combi oven/ microwave. I may be wrong here, but if you change the outlet plate to a dual plate that will deal with the oven and hob, I am sure I have read in the regs somewhere you can reduce the conductor size, as long as less than 3 meters?

Hob is half way along circuit.
Oven and Microwave at end.
Yes regarding that reg, the OCPD can be downstream in these circumstances, see reply above.
Thanks.

 

[Pretty Mouth]

my main concern is (still) a L-N short circuit as I don't trust the internal wiring to withstand the fault current allowed by a 32A breaker.

Would this be a problem though? If you've got a L-N short somewhere in the oven, it's dead anyway.

 

[timhoward]

Would this be a problem though? If you've got a L-N short somewhere in the oven, it's dead anyway.

Rightly or wrongly I'm more thinking about the 160A that flows before the instantaneous trip and whether that would start a fire.
As you know, I like to over-think and worry!

 

[Lucien Nunes]

Would this be a problem though? If you've got a L-N short somewhere in the oven, it's dead anyway.

You wouldn't want the short-circuit current to blow an internal CPC or its connector open-circuit and leave the faulty part live.

 

[Pretty Mouth]

Rightly or wrongly I'm more thinking about the 160A that flows before the instantaneous trip and whether that would start a fire.
As you know, I like to over-think and worry!

If you have an idea of the fault current at the outlet and brand of breaker, you could work out a min. CSA using the adiabatic?

 

[GBDamo]

Surely if the spec has changed then just quote for running in a new circuit.

 

[timhoward]

Surely if the spec has changed then just quote for running in a new circuit.

Fair point and would love to. Sufficient to say this job hasn't been easy and relations already a little strained. I've already had to play the "nobody told me" card once which led to a rewire of 1st floor lighting as no CPC and smokes needed.

If you have an idea of the fault current at the outlet and brand of breaker, you could work out a min. CSA using the adiabatic?

Thanks for this very helpful thought.
It seems I haven't yet measured PSCC at the board and only have an R1+R2 for this circuit, not an R1+RN

Estimating things a bit....
R1+R2=0.26 ohms. Circuit is therefore 20.6m long. Using figures for 2 x 10 sq mm singles, I get rough R1+RN of 0.09 ohms
It's an overhead supply and I can't imagine the impedance to transformer being less than 0.2 ohms.
So if I go with 0.29 ohms, I get 2.1 sq mm out of the adiabatic which is likely bigger than the real values.
I can check with some real data when next there, but this does give me some confidence that the flex supplied and likely the internal wiring can withstand this scenario.
Thanks again.

 

[timhoward]

Just realised I should probably be using the other formula for a live conductor t=(S²k²)/I²
Using this one with a fault current of 703 amps, k=115 to disconnect in 0.1s needs a live conductor size of 2.18mm.
I've a feeling the 2.5mm flex is going to be fine.

 

[LastManOnline]

Rightly or wrongly I'm more thinking about the 160A that flows before the instantaneous trip and whether that would start a fire.
As you know, I like to over-think and worry!

Your conscientious is to be admired.But surely, is,nt it down to the appliance manufacture to display the same sense of care by ensuring his product is properly engineered to avoid been a potential danger ?.Appliance manufacturers know exactly what kind of scenario to be expected?.All domestic installations are wired to exactly the same standards.

 

[timhoward]

I've just realised the 'normal' adiabatic formula and the one in 543.1.2 are exactly the same, just rearranged...!

 

[LastManOnline]

Even a 3A fuse won't protect a fan motor from catching fire, so there is no relationship between the external OCPD for the oven and its current-using components.

Thank you for this comment.Would love to have written it but my technical knowhow is not quite there yet.All fan manufacturers please take note.

The manufacturer really ought to state what protection is required, which is quite likely determined by the need to protect the internal wiring against

Look at a "stand alone " cooker 40 years old or more.?.You will see control wiring of 0.75 mm or less.Yet the manufacturer was quite comfortable with a 32amp mcb/fuse as circuit protection.?

 

[Pretty Mouth]

I've just realised the 'normal' adiabatic formula and the one in 543.1.2 are exactly the same, just rearranged...!

There's the nice n easy table B7 in the OSG, giving a minimum CSA of 1.5mm for faults of up to 3kA, and 2.5mm for up to 6kA.

Using manufacturers data will allow a much smaller CSA than that, eg Hager B32 = 1.5mm at 6kA fault.

Gotta agree with @LastManOnline though, it should be the manufacturer worrying about this.

 

[Lucien Nunes]

Appliance manufacturers know exactly what kind of scenario to be expected?.

In an ideal world, yes.

All domestic installations are wired to exactly the same standards.

No, this is where it goes wrong. What is the normal way of wiring an oven in Germany? Would you find a 40A cooker circuit in Sweden? Manufacturers should take regional differences into account in preparing instructions, but we know from experience this sometimes doesn't work out. For example, we've discussed on this forum appliances from German manufacturers supplied with the German instructions translated into English. They were written on the basis that all domestic plugs are 16A and that the oven will come with a 16A plug attached. They assume that a 16A socket will be provided on a 16A or 20A circuit specifically for the oven and take no account of larger OCPD ever being used or that the oven would ever be hard-wired. The designers might have allowed for the UK scenario where the 13A plug isn't man enough and hard-wiring is a must, but that information has not filtered through to the published instructions so we are left wondering.

 

[timhoward]

To be fair the the manufacturer, normally I'd read the plate, say "Oh it needs a 20 amp circuit", read the manual, see the min 16A, and crack on.
It's more that when things backfire and you end up in abnormal circumstances it's nice if the manufacturer can document the parameters of the design and not leave me guessing about it being probably ok.
It's also an unfortunate reality of the UK that convenient compact fusing stops at 13A.

There's the nice n easy table B7 in the OSG, giving a minimum CSA of 1.5mm for faults of up to 3kA, and 2.5mm for up to 6kA.

This has been most enlightening. So as the RCBO in question (like almost all) has an energy limiting class 3 rating, providing the PSCC is less than 3ka, 1.5 sq mm csa live conductors will always disconnect before melting under fault conditions.

Right, check PSCC, dual plate, single socket, and run!
Thanks all.