Electrical installation question for inset hobs and 2 ovens.....

[charles]

the 7th Edition cam eout a bit before your time, I suspect. It was issued in 1916 and replaced by the 8th in 1924. My first copy was the 13th. It's now the 17th. Just say BS7671.

[Tricky Dicky]

If it's a short you are worried about then changing the RCBO is the answer or a smaller MCB if you do not want to go to the expense of fitting a RCBO.

Richard

[John Rumm]

A "full on short" is what is known as a "fault current" or prospective short circuit current. The nominal rating of the cable is pretty much irrelevant since the current flow will be 100s if not 1000s of amps.

Hence what matters is can the cable survive long enough under adiabatic heating conditions to allow the protective device to operate, without melting, bursting into flames etc first. You also need to make the worst case assumption that its going to start at its maximum nominal conductor temperature of 70 deg C.

The way you would assess this if designing your own non standard circuit would be using the adiabatic equation:

s = sqrt( I^2 x t ) / k

Where s = the cross section of the cable in mm, I the current, t the time to clear the fault, and k a factor dependant on the makeup of the cable - typically 115 for a PVC insulated cable.

So say you have a fault current of 400A, that will open your RCBO on the magnetic response part of its trip. So allow a nominal 0.1 secs.

that means we get sqrt( 400^2 x 0.1 ) / 115 = 1.09mm

Which is smaller than any of the conductors in a 2.5mm T&E - hence it will survive.

This is a different situation from protecting it from an overload. e.g. drawing a sustained 40A from it.

(2.5mm^2 cable in its "clipped direct" or embedded in masonry installation method is good for 27A BTW)

No need - the single cable is adequately fault protected - this is why we allow spurs in 2.5mm2 cable from ring circuits and radials wired in larger diameters.

[Andy Wade]

Yes, if you retain the cooker control switch (CCS) as a master isolator for the group there'd be no need for individual switching for the hob & ovens. You could use three unswitched FCUs (with flex outlets) suitably placed, out of sight. They would be need to be accessible for fuse replacement, preferably without tooo much dismantling - but this is the same issue as when using plug & socket connections. This does assume that the CCS is not too far away, preferably no more than 2m from any of the cooking appliances. Label the CCS to indicate its function, e.g. "Hob & ovens (ign)"

The cooker hood/fan should, I suggest, have its own accessible switched FCU (whether or not it's fed via the CCS). This should be wall mounted at above worktop height, either in the 'run of sockets' or higher, near the appliance. This is to provide a readily recognisable means of 'switching off for mechanical maintenance' as required by the regs. An FCU feeding a separate flex outlet plate, linked via a run of 1.5 T&E buried in the wall, might be appropriate here.

[Stephen]

Thank you for your valuable comment... So are you saying its NOT regs compliant to use the CCS as a means of switching off the cooker hood for the purposes of mechanical maintenance? I would have thought this would be safer as this also ensures the cooker is off as well too....

The CCS will be in the run of sockets and would be in your words, "be wall mounted at above worktop height, in the 'run of sockets'in order to provide a readily recognisable means of 'switching off for mechanical maintenance'"

I am happy to stick a label on the CCS to make it clear that its hob, two grill/ovens and extractor fan if that makes any difference.

[Andy Wade]

No, I'm not necessarily saying that. I'd accept it if the CCS was the the only means of disconnection: you have provided means of isolation and switching off, as required by regs 537.2 and 537.3. The bloody-minded, though, might argue that the CCS is not continuously under the control of the maintenance-person, citing reg. 537.3.1.2, which requires suitable provision to ensure that the supply can't be inadvertently restored during maintenance. (Someone, not realising that the Hob & oven switch also controls the fan, might switch it back on, in order to pre-heat an oven, say...)

My point really is that a additional control switch would remove any doubt about such arguments, /and is just a much more conventional arrangement/, making the labelling simpler and clearer. One switch for the cooking appliances and one for the hood: simples.