Hob & oven circuit

[JohnW2]

Yes in a correctly designed and installed system, the fault impedance would be low enough to trip a 40A MCB. 230V/40A = 5.75ohms. This doesn't account for an over current condition that is between 24A and 40A ish.

BAS was talking about fault protection (since that was what you had asked about), not overload protection, and, essentially by definition, a fault current could not be as low as 40A.

Kind Regards, John

 

[EFLImpudence]

Yes I suppose you could argue that the fuse being downstream is sufficient,and the cable being in a fixed installation is sufficiently protected,

There is no argument.
It is what applies to every socket spur on a socket circuit in the country.

and of a short length but it would be ignoring 434.2.1 "shall be no branch circuits or socket outlets"

There is no branch.
It only applies after a reduction in cross-sectional area of the conductor which is the Cooker Connection Unit to Socket and the 3m. limitation (along with fire protection) only applies in the absence of fault protection.


Do you know the difference between overload and fault current?

Overload current is when too much current for the cable is drawn by the appliance or appliances.
An oven cannot do this so actually the 13A fuse is not required although, obviously, you cannot fit a plug without it.
The fuse does not have to be at the origin of the reduced csa cable.

Fault current is a short-circuit between L & N or a fault of negligible impedance between L & E.
This is protected against by the MCB at the origin of the circuit so exceptions in 433.3 or 434 are irrelevant.

 

[winston1]

Or should I just assume that in future no-one would be daft enough to plug a multi-way into the socket under the counter?

Multiways have 13 a fuses in them so it would not matter.

 

[EFLImpudence]

Yes in a correctly designed and installed system, the fault impedance would be low enough to trip a 40A MCB. 230V/40A = 5.75ohms.

As John above.

This doesn't account for an over current condition that is between 24A and 40A ish.

There can be no current between 24A and 40A with your oven.

Or should I just assume that in future no-one would be daft enough to plug a multi-way into the socket under the counter?

Yes, we do not have to account for stupidity.

Even so loads high enough for that in one adaptor are unlikely.
Multi-way extension leads have a 13A fuse in the plug.

 

[JohnW2]

I think this is not permitted by 434.2.1 but an FCU before the socket would be permitted.

Although I agree that it's difficult to conclude this from the actual wording of the regs, I really cannot see how 434.2 and 434.2.1 are in any way relevant if (as almost certainly will be the case in the situation we are discussing), the fault protection of the entire circuit (including both the larger and smaller CSA bits of cable) is already adequate - i.e. the Zs at the 'furthest point' in the circuit is low enough - regardless of any changes in cable CSA/CCC which may exist within the circuit (i.e. in the fault path).

Anyone got any thoughts about this?

Edit: Ah, I've just looked more widely - 434.2.2 says precisely what I was suggesting above - that if the device at the origin of the circuit provides adequate fault protection, then none of 434.2.1 applies

Kind Regards, John

 

[wrightsonm]

There is no argument.
It is what applies to every socket spur on a socket circuit in the country.

There is no branch.
It only applies after a reduction in cross-sectional area of the conductor which is the Cooker Connection Unit to Socket and the 3m. limitation (along with fire protection) only applies in the absence of fault protection.

I hadn't considered it to be the equivalent to a spur from a ring final circuit. Fair point.

Whilst it is not a branch, after the cable csa reduction it does say "no socket outlets."

 

[wrightsonm]

Multiways have 13 a fuses in them so it would not matter.

Yes, bad example. I was trying to think up a way of plugging in too many devices and overrating the cable.

 

[EFLImpudence]

I hadn't considered it to be the equivalent to a spur from a ring final circuit. Fair point.

Well, it's more than a fair point; it dismisses your doubt.

Whilst it is not a branch, after the cable csa reduction it does say "no socket outlets."

Except that it is in 434.2 which includes a lot of "except where ...". You cannot read just one sentence which apparently says what you want.

Also the MCB is at the origin of the circuit and is satisfactory therefore it does not apply.

 

[wrightsonm]

Well, it's more than a fair point; it dismisses your doubt.

Not quite. a ring final 2.5mm spur ref B is 24A 1.45x overcurrent is 34.8A which would trip the 32A MCB. The same cannot be said for 2.5mm spurred from a 40A circuit.

Except that it is in 434.2 which includes a lot of "except where ...". You cannot read just one sentence which apparently says what you want.

Yes, I am trying to justify the proposed solution back against the regulations, but there are so many if, but, except, when aspects that it is easy to miss something.

 

[EFLImpudence]

Not quite. a ring final 2.5mm spur ref B is 24A

Method C is 27A. Double socket with two maximum loads, which you aren't supposed to do - and is unlikely today - is two times 13A fuse, 26A.

1.45x overcurrent is 34.8A

That is allowed for in the given Current Carrying Capacity of the cables.

which would trip the 32A MCB.

No, it wouldn't 32 x 1.13 = 36.16A under which the MCB will never trip.

The same cannot be said for 2.5mm spurred from a 40A circuit.

Yes it can. It is protected by the two 13A fuses.

Yes, I am trying to justify the proposed solution back against the regulations, but there are so many if, but, except, when aspects that it is easy to miss something.

I assure you that everything I advised is correct.

 

[JohnW2]

Yes, I am trying to justify the proposed solution back against the regulations, but there are so many if, but, except, when aspects that it is easy to miss something.

I can understand that, but I believe you are attempting to seriously "overthink" the issues. You are being told what would be the standard (and regulation-compliant) practices of electricians in relation to the matters under discussion, and it would, in my opinion, probably make sense for you simply to accept that information/advice.

Kind Regards, John

 

[ban-all-sheds]

Not quite. a ring final 2.5mm spur ref B is 24A 1.45x overcurrent is 34.8A which would trip the 32A MCB. The same cannot be said for 2.5mm spurred from a 40A circuit.

You still seem unclear on the difference between overloads and faults.

 

[ban-all-sheds]

Yes in a correctly designed and installed system, the fault impedance would be low enough to trip a 40A MCB. 230V/40A = 5.75ohms.

For a 5s disconnection, the loop impedance required for a B40 is way less than that.

 

[JohnW2]

For a 5s disconnection, the loop impedance required for a B40 is way less than that.

Indeed - in fact, one fifth of the figure mentioned (before correcting for Cmin!) - and that because he doesn't seem to understand that the current required to produce an assured magnetic trip of B40 is 200A, not 40A.

 

[ban-all-sheds]

So i've extracted snippets of the regulations

Which indicates that you are neither clueless nor uninformed.

Can anybody here recommend a few decent guides to the regulations, basic theory etc?

Based on these extracts, it would appear:

• 2.5mm2 spurred socket is not permissible
• 2.5mm2 socket from CCU is not permissible
• socket in 2.5mm2 from 13A FCU wired from cooker isolator switch in 2.5mm2 or from CCU in 2.5mm2 is permissible

They are all permissible.