cooker

[holmslaw]

..

 

[JohnW2]

If you think applying diversity to circuits and their equipment is inappropriate, then you have definitely missed something.

As I said, applying diversity in relation to cable and protective device sizing (and total installation current requirement) is fair enough.

Designing a circuit in which a switch rated at 45A could possibly be asked to switch a load of 50A is, IMO, a totally different matter.

Kind Regards,
John

 

[ELZ4742]

not really, from my understanding, the circuit is protected by a 32a mcb so will never pull 50amps anyway.

 

[JohnW2]

not really, from my understanding, the circuit is protected by a 32a mcb so will never pull 50amps anyway.

Well, for a start a 32A Type B MCB will (per generic spec - e.g. Fig 3.4 of the regs) will carry 50A for around 16 mins before tripping, and 46A/47A or so more-or-less indefinitely. That's why MCB sizing based on diversity calculations works, without lots of nusiance tripping.

Anyway, whatever the regs may require, I would personally be much more comfortable with a switch (or whatever) rated for the maximum current which could, under some circumstances, be drawn by the appliance, rather than the figure derived from applying diversity.

Kind Regards, John.

 

[riveralt]

Anyway, whatever the regs may require, I would personally be much more comfortable with a switch (or whatever) rated for the maximum current which could, under some circumstances, be drawn by the appliance, rather than the figure derived from applying diversity.

Thats a personal choice - I would apply diversity but also follow the manufacturers instructions. For example when fitting a double oven and hob together Hotpoint state they require a 45Amp dual pole cooker switch - thats good enough for me.

 

[Stoday]

30A plus 20A = 50A, which is 11.5 kW.

According to the On-Site Guide, "A 30 or 32 A circuit is usually appropriate for household or similar cookers of rating up to 15 kW.

That's good enough for me.

So is a 30 A isolating switch on a 30 A circuit.

 

[JohnW2]

Thats a personal choice - I would apply diversity but also follow the manufacturers instructions. For example when fitting a double oven and hob together Hotpoint state they require a 45Amp dual pole cooker switch - thats good enough for me.

I suppose I'm pretty conservative in relation to such matters - and,as you say, it's a matter of personal choice. I cannot deny that even 'bare' compliance with regs (if that is achieved) is still compliance. However, a few questions:

• 1...I cannot find any reference in the regs to the application of diversity in determining the required rating of switches etc. Am I missing something, or are the regs simply silent on this issue?
  
  2...In the example you give, what do the manufacturer's instructions for the cooker switch say? Do they indicate that it's acceptable to use it to switch a load which could, at the time of operation, be more than 45A?
  
  3...What, if anything, do the Hotpoint cooker instructions say about the maximum possible current draw and protective device requirements?

Kind Regards, John.

 

[JohnW2]

30A plus 20A = 50A, which is 11.5 kW.
According to the On-Site Guide, "A 30 or 32 A circuit is usually appropriate for household or similar cookers of rating up to 15 kW.
That's good enough for me.
So is a 30 A isolating switch on a 30 A circuit.

In analogy with what I've just asked riveralt about the 45A cooker switch, would the manufacturer's instructions for the 30A isolating switch you postulate indicate that it was OK to use it for a load which could be as high as 50A (and could certainly be considerably above 30A, indefinitely, without a 30/32A OPD operating)?

Kind Regards, John.

 

[riveralt]

However, a few questions:

• 1...I cannot find any reference in the regs to the application of diversity in determining the required rating of switches etc. Am I missing something, or are the regs simply silent on this issue?

• The OSG adds to reg 311 but makes no reference to switches - nor to cooker connection units. I'm certain BS 3676:2000 and BS4177:1992 will highlight the amp range that such switches/jbs must fall into.
  I'm also sure that, just like the MCB, a 45Amp cooker switch will not burst into flames at 46Amps.
  
  

  2...In the example you give, what do the manufacturer's instructions for the cooker switch say? Do they indicate that it's acceptable to use it to switch a load which could, at the time of operation, be more than 45A?

  This is an extract from the manual for the Hotpoint BD52P2 double oven. 'A double pole control switch having a minimum rating of 32 amps should be used to feed the cooker using a suitably rated cable. Where a hob is fitted adjacent to or over the cooker, a 45 amp Double Polce control switch should be used to feed both units via separate suitably rated cables'. So nothing about the load on the switch but then why would they - they are not making the switch.
  
  

  3...What, if anything, do the Hotpoint cooker instructions say about the maximum possible current draw and protective device requirements?

  Nothing. See answer to 2 above.
  
  As far as Hotpoint are concerned if I follow their instructions then they will honour their guarantee - they are happy to utilise a 45 Amp cooker switch to control both oven and hob - therefore so am I.

 

[JohnW2]

However, a few questions:

• 1...I cannot find any reference in the regs to the application of diversity in determining the required rating of switches etc. Am I missing something, or are the regs simply silent on this issue?

• The OSG adds to reg 311 but makes no reference to switches - nor to cooker connection units.

• 
  Indeed. So I wasn't missing anything. This seems to indicate that the regs (including the OSG) do not provide any basis for applying diversity to the rating of switches etc.?
  
  

  [quoteI'm also sure that, just like the MCB, a 45Amp cooker switch will not burst into flames at 46Amps.

  So am I, but proper design practice should not rely on built-in 'safety margins' over and above an accessory's rating. I'm less sure about the 30A switch suggested by Stoday, even though that is still within the load calculated using diversity - has anyone tried breaking a 50A current with a 30A-rated switch? Still possibly OK, but I imagine that it goes way beyond normal 'built-in safety margin' of the rating.
  
  I think there is a lot of misunderstanding about the concept of diversity. It is a statistical concept which is fine in terms of cable and OPD loading. However, when it comes to switches (and probably some other accessories), that statsitical concept is irrelevant. What matters when a switch attempts to break a current is the level of the current at that moment. The fact that the current may usually, perhaps nearly always, be far less than the present value is irrelevant to the switching job it is being asked to do at that moment in time.
  
  

  2...In the example you give, what do the manufacturer's instructions for the cooker switch say? Do they indicate that it's acceptable to use it to switch a load which could, at the time of operation, be more than 45A?

  This is an extract from the manual for the Hotpoint BD52P2 double oven.....

  That's no use - Hotpoint don't make the switch! I was asking what the instructions provided by the manudacturer of a 45A switch would say about it's suitability for switching a load potentially as high as 50A.
  
  

  3...What, if anything, do the Hotpoint cooker instructions say about the maximum possible current draw and protective device requirements?

  Nothing. See answer to 2 above.

  They don't say what maximum current their appliance draws?
  
  

  As far as Hotpoint are concerned if I follow their instructions then they will honour their guarantee - they are happy to utilise a 45 Amp cooker switch to control both oven and hob - therefore so am I.

  I don't really think Hotpoint's guarantee comes into this. Their guarantee only covers their appliance, and I really can't see how the rating of a switch in the circuit supplying it could possibly affect the appliance!
  
  Rather, we're talking about whether it's both safe and compliant to have a switch rated for less than the total current it may have to break - and that compliance depends, amongst other things, on what the instructions provided by the manufacturer of the switch say about its maximum current-breaking rating. It was, I remind you, you who introduced the matter of complying with MIs - and that obviously would apply just as much to the switch you specify or fit as to the appliance itself.
  
  Kind Regards, John

 

[riveralt]

I'm also sure that, just like the MCB, a 45Amp cooker switch will not burst into flames at 46Amps.

So am I, but proper design practice should not rely on built-in 'safety margins' over and above an accessory's rating.

What a load of codswallop - The whole of BS7671 is about built in saftey margins and as a practitioner I will continue to follow its guidelines for diversity.

If you wish to challenge the diversity view with respect to switches then by all means read BS 3676:2000 and BS4177:1992 and if still not satisfied take it up with the IET, I'm sure they will listen to your concerns.
But before doing so have another look at reg 311.1 which seeks to balance economics with reliable design - and does this by giving you the option to incorporate diversity into the installation.

 

[JohnW2]

I'm also sure that, just like the MCB, a 45Amp cooker switch will not burst into flames at 46Amps.

So am I, but proper design practice should not rely on built-in 'safety margins' over and above an accessory's rating.

What a load of codswallop - The whole of BS7671 is about built in saftey margins and as a practitioner I will continue to follow its guidelines for diversity.

That's your prerogative. BS7671 itself doesn't actually give any guidelines (but the OSG gives some); 311.1 merely states that diversity may be taken into account in assessing maximum demand and as we've discussed, there is certainly no guidance (in either BS7671 or OSG) in relation to the ratings of items such as switches.

As I said, I believe that there is a lot of misunderstanding about the concept of diversity.

Kind Regards, John.

 

[ban-all-sheds]

Is diversity to be used for designing individual circuits?

Or is its purpose to provide an assumed load to be used in assessing the maximum demand of the whole installation?

 

[riveralt]

Is diversity to be used for designing individual circuits?

Or is its purpose to provide an assumed load to be used in assessing the maximum demand of the whole installation?

Both I think.
For full rewires in particular, I always produce a circuit design sheet/table with the full calculations for each individual element of the equation for each individual circuit - then a cumultive one for the whole installation.

 

[DetlefSchmitz]

Is diversity to be used for designing individual circuits?

Or is its purpose to provide an assumed load to be used in assessing the maximum demand of the whole installation?

If you assume that the OSG is authoritative, i.e. intended as a companion to BS7671, then the wording of appendix 1 in the latest edition has been watered down in an apparent move towards your second possibility.

However the second paragraph of 8.4 is fairly unequivocal, pointing at the first possibility.