Double oven and induction hob connections

[JohnW2]

I discussed this all further with him. He has been installing 2x6mm with seperate RCBOs in new builds for a couple of years contract work. Seems they are not applying the diversity rule for domestic kitchens of 10amps+30%+5amps for any socket.

One has to wonder "why?"!

I am surprised diversity makes such a big difference but understand that full load conditions for an oven/hob is nothing like (for example) an electric shower.

Yes, I think many of us are a little surprised when we first come across that calculation. I don't have an electric cooker, so cannot easily repeat the experiment, but a year or three ago I tried to emulate the infamous "Christmas Dinner Scenario" using the cooker of a family member - all hob plates on 'full blast' and two ovens running at high temperatures. The average current drawn over any appreciable period of time (I think I looked at 10 or 15 mins) was, over a period of 2 or 3 hours, actually always appreciably less than would be suggested by the 10A+30% formula.

Kind Regards, John

 

[securespark]

All I can say is that I have been called out on Christmas day to a blown 3036 on a cooker circuit.

 

[JohnW2]

All I can say is that I have been called out on Christmas day to a blown 3036 on a cooker circuit.

I can't argue! Can you recall what rating of 3036 and roughly what the maximum load of the cooker was? It ought to take about 1 hour of continuous ~55A to blow a 30A 3036.

I wonder if at least some of these stories may result from very old fuses blowing at currents far lower than they 'should'?

Kind Regards, John

 

[securespark]

Possibly. It was a 30A 3036, looked to be one of the original "Standard" boards - brown with wooden back, 60A main switch. I would have dated it to 1960. But there's no telling if the fuse wire was the same vintage.

 

[JohnW2]

Possibly. It was a 30A 3036, looked to be one of the original "Standard" boards - brown with wooden back, 60A main switch. I would have dated it to 1960. But there's no telling if the fuse wire was the same vintage.

I know them well! I do suspect that a aged and 'trigger happy' fuse may well have been the real cause (of this and similar stories), because, as I implied, I find it hard to believe that any sort of cooker could result in an average continuous current of ~55A for an hour (or anything remotely approaching that)! That would be worse than a 1-hour 11.5kW shower!

Kind Regards, John

 

[EFLImpudence]

Was it the correct fuse wire?

 

[securespark]

Yup. There wasn't a lot left, but the bits wrapped round the screws looked just like my roll of 30A wire.

 

[JohnW2]

Was it the correct fuse wire?

Even if it was (as secure has now confirmed), I would imagine that, particularly if it often carried high currents (even if briefly), hence got quite hot, over the years the surface would at least oxidise, if not worse, so it seems fairly likely to me that the fusing current might reduce significantly over a long period of time.

I certainly do remember fuses sometimes 'just blowing', for no apparent reason, even when they were the correct rating and not protecting a circuit for which diversity was relevant (so no 'peak' very high currents) - so maybe that was due to 'fuse senility'?

Kind Regards, John