RCCB tripped this morning...

[razorback]

...upon having a good look at the CB there are left to right.. 1st RCCB 63A.. lights 1st floor 6A.. lights ground floor 6A.. central heating 6A... (hmmm).. 2nd RCCB63A then mcb for one socket i believe @ 16A although haven't checked... cooker @ 32A... sockets @ 32A then the main switch.
1) the dryer and central heating were on at the the time when the RCCB tripped
2) telly, dryer, cooker, white goods, upstairs and downstairs sockets all off when RCCB tripped
3) lights remained on
4) only the 2nd RCCB tripped nothing else
Finally, we have just had a new cooker installed. Last cooker was plugged this new one had to be "hard wired in". This new cooker needs to be installed to a suitable double pole control unit with a minimum rating of 40A. Am assuming that when it says control unit it is actually the consumer box. If so, then stating the obvious... 32A mcb needs to be changed to 40A... yes? I flipped the RCCB back on and everything ok so far. If it happens again will go through procedure, but an explaination perhaps as to why it tripped please?

 

[Chri5]

Suspect the drier, re set and don't use it.

Next time you do if the fault reoccurs you have a better idea, check for moisture via drier vent nears sockets and connections.

The control unit is the Ccu cooker control unit or switch in simple terms. So any cooker switch will cover the mi requirement.

As for loading what is the watts total load of the cooker, is it a all in one of oven and separate hob.

You can't just change the main fuse since the cable is ver likely to be designed for the existing / old load of 32A.

Physic laws are watt / 230v gives amps, so a 32A circuit can take a max load of 32 x 230 circa 7200w ( head rough calc).

 

[razorback]

Its a range, with two ovens a grill, five ceramic rings and a hotplate. Just looking at the spec... maximum wattage is 13.8 KW. Can't remember calcs so checked up internet 1000 x 13.8 / V (240 or 230 can't remember) = 57A & 60A respectively....

 

[Taylortwocities]

Am assuming that when it says control unit it is actually the consumer box. If so, then stating the obvious... 32A mcb needs to be changed to 40A... yes?

NO.

You will need the circuit to be checked and EFLI confirmed to confirm that your cable will carry 40amps

Needs knowledge and the right test equipment.

 

[razorback]

Taylortwocities... am just making sure that everything is safe, if after this little advisory I need to change a few things then my friend the sparky will come and do whatever needs to be done... but whilst we are here... on the central heating mcb it is a 6A breaker is that adequate?

 

[Chri5]

Not sure why you are asking if you have spark that will do the work. As for boiler circuit, mcb is ok and will likely need a fspur at 3A local to boiler.

The cooker is a big load lump, likely cable and total circuit will need upgrade.

 

[EFLImpudence]

There is nothing unusual or wrong about a 13.8kW cooker on a 32A MCB.

Razorback - look up 'diversity'.
13800/230 = 60A - 10 x 0.3 + 10 = 25A

Anyway, that's not the problem.
RCDs trip on Earth current faults.
It may be the cooker but not because of the above.

The 40A switch is just the current carrying capacity of it and so must be more than the MCB (fuse).


What was on when the RCCB tripped?

 

[JohnW2]

There is nothing unusual or wrong about a 13.8kW cooker on a 32A MCB. Razorback - look up 'diversity'.
13800/230 = 60A - 10 x 0.3 + 10 = 25A

Diversity is fine in terms of cable sizing, particularly given the oft-discussed 'considerable margins' seemingly built into CCC 'ratings'. However, do you feel that it's OK to apply the same diversity concept to the MCB rating? Indeed, is it compliant with the MIs of MCBs to have a design which involves up to almost double the MCB's In being drawn under occasional, even if fairly brief, 'normal operation' conditions (and more modest excursions beyond In much more common)? Quite apart from 'good design practice' and compliance with MIs, how often would you expect to get 'nuisance MCB trips' with a 13.8kW cooker on a 32A MCB (Christmas Day?!) - if such a cooker ever went 'full blast' (60A), one would expect a B32 to operate in around 2.5 mins; even at 45A, it should not last much longer than 15 mins. Such potential 'stressing' of an MCB beyond its In does not make me feel very comfortable, but maybe I'm just being silly or over-sensitive!

I'm not criticising or challenging - just genuinely interested in your answers!

Kind Regards, John

 

[EFLImpudence]

Well, them's the rules.

What else is there to say?

I have not come across this 'Christmas Day scenario'.
It could be a 'nice little earner' and probably more enjoyable than the usual fun.

Taking diversity to its limit - without integral socket - 19kW is acceptable on a 32A MCB. I do not really see any difference considering an MCB or the cable. If it complies then it must be satisfactory.

The OP's diversity of 25A still leaves over 25% tolerance for a 32A MCB or cable. I suspect the quoted wattage is stated at 240V and so would be even a little bit less.

Is it not like people saying "It's right on the limit. I'd better go up a size"?
If it's right on the limit then it is satisfactory.

I did not devise the diversity calculation but should we not accept, after so long, that it has been done correctly?

Although we do not know what the MIs demand for the MCB rating, as the cooker has already been installed I would suppose that that has been followed.


I would just restate for the OP that this has nothing to do with the RCCB tripping.

 

[JohnW2]

Firstly, to reiterate what you say, razorback should understand that this discussion has nothing to do with his tripping problem, and he therefore should probably ignore it.

Well, them's the rules. What else is there to say? ... I do not really see any difference considering an MCB or the cable. If it complies then it must be satisfactory. ... I did not devise the diversity calculation but should we not accept, after so long, that it has been done correctly?

I did try to avoid this by the final comment in my previous post, but it seems to have happened despite that . As I said, I am not 'challenging' - I was merely genuinely asking ('for my education') whether (it is your view that) the concept of diversity can be applied to MCB ratings in the same way as we apply it to cable sizing.

You talk about 'compliance' but, as you are aware, BS7671 is pretty silent about all this - and virtually everything we 'know' about diversity (particularly 'the diversity calculation' to which you refer) comes from the OSG. As we know, BS7671 requires that the In of the OPD must be at least as high as the 'design current' of the circuit, and the only definition of 'design current' appears to be that in Part 2, which says that it is 'the current 'in normal service', without any reference to diversity. About the only reference to diversity I can find is the very general single sentence in 311.1.

So, to re-state my question slightly differently (for my education, not because I am arguing about it!) do you believe that everyone is happy that, although BS7671 itself does not appear to say this explicitly, the value for 'design current' used to confirm compliance with 433.1.1(i) may take into account diversity, using 'the calculations' indicated in the OSG? If everyone is happy, and that's how it's always done, then I'm not going to argue.

As for your not seeing a difference between the MCB and the cable, I think I see a potential one. As we have discussed before, the maximum CCC figures for cables we work with appear to have very generous built-in margins, which recognise that fairly short periods of current well above the tabulated 'maxima' will do no harm to the cable. Whether or not MCBs are similarly happy with (maybe even fairly frequent) currents above their 'rated' value, I don't know - and I have so far not found anything useful about this is any manufacturer's literature on MCBs. However, I'm mindful of our recent discovery that at least Wylex aren't happy about adjacent MCBs even running 'close to' their rated current.

One can, as you say, look at this the other way around, and see what the cooker manufacturers are asking for - since that may well largely over-ride diversity considerations. I've had a quick look and found a cooker fairly similar to the OP's (Belling DB4 90E, 2 ovens + 5 ceramic hobs). This 'totals' about 12.7 kW (~55A at 230V) and the manufacturer specifies a 45A MCB.

Kind Regards, John

 

[JohnW2]

Duplicated deleted

 

[JohnW2]

Another duplicate deleted

 

[EFLImpudence]

I did try to avoid this by the final comment in my previous post, but it seems to have happened despite that . As I said, I am not 'challenging' - I was merely genuinely asking ('for my education') whether (it is your view that) the concept of diversity can be applied to MCB ratings in the same way as we apply it to cable sizing.

I realise that. I said I see no difference.
Diversity does work. It has been around for a very long time.

To answer the question - If diversity allows an appliance with a large total load to be connected to a 32A CCC cable (say 4mm²) we obviously cannot then fit a larger MCB .

You talk about 'compliance' but, as you are aware, BS7671 is pretty silent about all this - and virtually everything we 'know' about diversity (particularly 'the diversity calculation' to which you refer) comes from the OSG.

I'm not a fan of the OSG but I think diversity is quoted in all similar publications and is generally accepted.

As we know, BS7671 requires that the In of the OPD must be at least as high as the 'design current' of the circuit, and the only definition of 'design current' appears to be that in Part 2, which says that it is 'the current 'in normal service', without any reference to diversity. About the only reference to diversity I can find is the very general single sentence in 311.1.

Do you not think 'in normal service' would be the diversity calculation result?
No different than three 32A ring/radial finals being over the rating of the main fuse in most properties but in 'normal service' this will not be used.

So, to re-state my question slightly differently (for my education, not because I am arguing about it!) do you believe that everyone is happy that, although BS7671 itself does not appear to say this explicitly, the value for 'design current' used to confirm compliance with 433.1.1(i) may take into account diversity, using 'the calculations' indicated in the OSG? If everyone is happy, and that's how it's always done, then I'm not going to argue.

Yes but 'that's how it's always done' sounds derogatory.
How about 'has been shown to work'.

As for your not seeing a difference between the MCB and the cable, I think I see a potential one. As we have discussed before, the maximum CCC figures for cables we work with appear to have very generous built-in margins, which recognise that fairly short periods of current well above the tabulated 'maxima' will do no harm to the cable. Whether or not MCBs are similarly happy with (maybe even fairly frequent) currents above their 'rated' value, I don't know - and I have so far not found anything useful about this is any manufacturer's literature on MCBs. However, I'm mindful of our recent discovery that at least Wylex aren't happy about adjacent MCBs even running 'close to' their rated current.

We do not use these 'built-in margins' in the calculations but you still seem to be assuming that the cooker WILL overload the MCB rather than it will work as diversity allows and only - say - half the total load will be drawn at any one time.
After all, why is it 10A + 30%? I don't actually know but it works.

One can, as you say, look at this the other way around, and see what the cooker manufacturers are asking for - since that may well largely over-ride diversity considerations. I've had a quick look and found a cooker fairly similar to the OP's (Belling DB4 90E, 2 ovens + 5 ceramic hobs). This 'totals' about 12.7 kW (~55A at 230V) and the manufacturer specifies a 45A MCB.

In that case the instructions shoulb be followed but whether the manufacturer has measured the maximum current drawn or noted the maximum CCC of 6mm² cable we don't know. Either way it is less than 60A.
If that is the case with the OP's then if not perfect installation method 10mm² may be required (it may be there already).

What would we do without the diversity calculation?
Would we have to fit 60A circuits for these cookers or actually measure the maximum current drawn when working?

I may be giving officialdom more credit than I usually do but I can but assume that is what someone did when introducing diversity.

 

[JohnW2]

To answer the question - If diversity allows an appliance with a large total load to be connected to a 32A CCC cable (say 4mm²) we obviously cannot then fit a larger MCB .

Obviously true. If one 'needed' a larger MCB, then the cable would have to be sized approriately. However, that's not inconsistent with the concept of 433.1.1 which separately cites the requirements for the relationships between In, Ib and Iz and requires all to be satisfied. There is no suggestion that a design which satisfies the In ≤ Iz requirement automatically satisfies the In ≥ Ib one, since that obviously is not necessarily true.

I'm not a fan of the OSG but I think diversity is quoted in all similar publications and is generally accepted.

It's a bit odd that the regs themselves are so silent about this. However, as I said, my interest was is knowing if it is 'generally accepted' in relation to MCB rating, as well as cable sizing. You appear to be saying 'yes'.

Do you not think 'in normal service' would be the diversity calculation result?

It might well be (and you obviously think it does) - but, again, I'm surprised that the regs are not more explicit about such a crucial definition as "design current". They might not feel that it's appropriate to talk actual 'diversity figures' in the regs (because situations vary), but they could at least mention the acceptability in applying the concept of diversity in their definition of design 'current'.

No different than three 32A ring/radial finals being over the rating of the main fuse in most properties but in 'normal service' this will not be used.

Agreed, but at least the OSG explicitly addresses the issue of diversity in that situation.

If everyone is happy, and that's how it's always done, then I'm not going to argue.

Yes but 'that's how it's always done' sounds derogatory. How about 'has been shown to work'.

I think you are being over-sensitive. "How it's always done" is simply what I've been trying to ascertain, and I certainly didn't intend it to be derogatory.

... you still seem to be assuming that the cooker WILL overload the MCB rather than it will work as diversity allows and only - say - half the total load will be drawn at any one time. After all, why is it 10A + 30%? I don't actually know but it works.

Except for those appliance which deliberately limit the maximum load (in which case this discussion becomes moot), diversity is a probabilistic concept - the implication of which is that the MCB (or cable) will sometimes be technically 'overloaded'.

... (Belling DB4 90E, 2 ovens + 5 ceramic hobs). This 'totals' about 12.7 kW (~55A at 230V) and the manufacturer specifies a 45A MCB.

In that case the instructions shoulb be followed but whether the manufacturer has measured the maximum current drawn or noted the maximum CCC of 6mm² cable we don't know. Either way it is less than 60A.

Slightly. Those Belling figures suggest that they would call for a 50A MCB (well, 49A!) for the OP's 13.8kW cooker. Not quite 60A, I agree, but it suggests that they are far less comfortable with applying OSG diversity calculations to a cooker than you are ...

If that is the case with the OP's then if not perfect installation method 10mm² may be required (it may be there already).

Indeed, but 6/10mm² cable on a 50A MCB is very different from the 2.5mm² (clipped direct) on a 25A MCB which would theoretically be acceptable per your diversity calculation. As above, if Belling are typical, the cooker manufacturers seem to be much more conservative about applying diversity.

What would we do without the diversity calculation? Would we have to fit 60A circuits for these cookers or actually measure the maximum current drawn when working?

As above, the implication is that Belling would probably feel that a 13.8kW cooker (60A max) requires a 50A MCB, and appropriate cable. I'm not 'taking sides' - merely presenting them!

I may be giving officialdom more credit than I usually do but I can but assume that is what someone did when introducing diversity.

The concept of diversity has been around for a long time, and may even have predated the appearance of modern MCBs as we know them. The applicability of 'standard diversity calculations' obviously depends upon how well MCBs tolerate sub-tripping 'overloads'

Kind Regards, John