Range Cooker Connection Refusal.

davezawadi (David Stone): 
 

I find that manufacturers' instructions are less and less satisfactory by the week. They are often written for multiple countries, whose supplies differ, without any account taken of this. They are often “get out of jail” kinds of nonsense. … They often require RCD protection which BS7671 may not. This is all very bad indeed.

I can't disagree with that, but this is nothing new.

davezawadi (David Stone): 
 

The idea that a cable can be “too large” is simply daft, and fundamentally untrue. 

Except for the suitability of connecting terminals … but if there is a connecting point or CCU, surely the breaker could be reduced to a 40 A, and lower CSA cable connected between connection point/CCU and appliance.

BS 7671 says that B22 etc lighting accessories must be protected by no greater than a 16A OCPD, so I see no reason why appliance manufacturers can't make a similar stipulation.

Ideally, manufacturers should be specifying a range, e.g. 32A-50A, to cover maximum demand (low end), and the capacity of the terminals + the internal wiring to those terminals (high end).

The annoying thing is when you have an induction hob which insists on exactly 25A breaker.

wallywombat: 
 

BS 7671 says that B22 etc lighting accessories must be protected by no greater than a 16A OCPD, so I see no reason why appliance manufacturers can't make a similar stipulation.

That's rubbish. GLS (general lighting service) is the lighting equivalent to a socket-outlet circuit, for general purpose lamps in that case, rather than general purpose appliances.

We are talking about specific fixed appliances.
 

Ideally, manufacturers should be specifying a range, e.g. 32A-50A, to cover maximum demand (low end), and the capacity of the terminals + the internal wiring to those terminals (high end).

Why? We are talking about fixed appliances that have a dedicated circuit. It could well be that you move up from an oven requiring 25 A to a new appliance requiring 32 A … that may need new cables run? What's the issue here? Whatever range you choose, the next appliance may well not fit that.

The annoying thing is when you have an induction hob which insists on exactly 25A breaker.
 

The only reason this one's annoying, is that B25 are not (currently) used as frequently in the UK as they are in other EU countries. It's nonsense that this could well lead to a new CU requirement because not all ranges of CU have a B25 available, but that's again a UK thing going on here, that we let happen. BS EN 60898 and BS EN 61009 have B25 as a preferred value. Why should a manufacturer have to trawl through the catalogues of all providers of CUs in the UK to see whether the CU manufacturer provides a rating that's preferred in the standard. Surely the requirement here is for manufacturers to offer the range of rated mcb's and RCBO's, not cherry-pick  … UK get on board with the British Standards please.

I have sat in meetings where it's been debated whether structured cabling should be permitted because it's de-skilling the trade.

If it's so hard to provide a dedicated circuit for a fixed appliance, particularly where there are installation instructions or detailed protection requirements from the manufacturer, I am left considering wondering whether structured cabling is simply a response to a “plug and play” type of approach that some posting here seem to be working towards.

I mean, we're already at the point, for single-phase installations up to 100 A maximum demand, where the majority (perhaps all) common circuit configurations are covered, and there's no need to do any calculation whatsoever. Just select the correct OCPD/cable combination and don't exceed the circuit lengths quoted in Table 7.1(i) of the OSG …

Or am I missing something?

Oh … yes … in this particular case we're discussing from the OP, it's probably not a “Part P competent person" organisation that's doing the job, and because it might need a new circuit protective device, they will need to “call an electrician in”.

But at this point, the customer has already been given a quote for a few (perhaps countable on one hand) brown beer tokens for the “connection” of their shiny new product and isn't best-pleased.

gkenyon: 
 

Ideally, manufacturers should be specifying a range, e.g. 32A-50A, to cover maximum demand (low end), and the capacity of the terminals + the internal wiring to those terminals (high end).

Why? We are talking about fixed appliances that have a dedicated circuit. It could well be that you move up from an oven requiring 25 A to a new appliance requiring 32 A … that may need new cables run? What's the issue here? Whatever range you choose, the next appliance may well not fit that.

The point I was trying to make is that every appliance, by the nature of its design, will have a range of CB ratings that it can comfortably and safely work with. If the manufacturer tells us this info, it gives us maximum flexibility. For example, if an appliance is specified as needing a 32-50A CB, then we can wire it to an existing 40A circuit without changes. If the manufacturer just says “must use a 25A breaker” then the circuit has to be modified, or an an extra circuit added, before the appliance can be added. Which is annoying, as one might suspect that it would actually be ok with a 40A breaker too, if only the manufacturer would tell us!

Look on the bright side, it does not yet impose a Zs limit, or that the supply is TT. 

What this really tells you is that the cooker shop installers are just fitters, and have been told not to touch the set-up if it does not fit in with the pre-pack instructions. That is annoying but probably safer than if they were told they could  pick up a supply on any old red and black cable they could find. 

It also frees up proper electricians to do something less soul destroying instead.

I presume the OP just connected it up and it all works.

Mike.

wallywombat: 
The point I was trying to make is that every appliance, by the nature of its design, will have a range of CB ratings that it can comfortably and safely work with. If the manufacturer tells us this info, it gives us maximum flexibility. For example, if an appliance is specified as needing a 32-50A CB, then we can wire it to an existing 40A circuit without changes. If the manufacturer just says “must use a 25A breaker” then the circuit has to be modified, or an an extra circuit added, before the appliance can be added. Which is annoying, as one might suspect that it would actually be ok with a 40A breaker too, if only the manufacturer would tell us!

I think that we can all see why a minimum size of MCB should be specified, but I am struggling to see why the maximum should be for a dedicated one appliance circuit. If the cooker specifies, say 40 A and 6 sqmm cable, but 50 A and 10 sqmm are used, the circuit cannot be overloaded. You could even use 6 sqmm cable on the 50 A MCB subject to the higher let-through energy of the larger MCB not being excessive - 433.3.1(ii).

wallywombat: 
The point I was trying to make is that every appliance, by the nature of its design, will have a range of CB ratings that it can comfortably and safely work with. If the manufacturer tells us this info, it gives us maximum flexibility. For example, if an appliance is specified as needing a 32-50A CB, then we can wire it to an existing 40A circuit without changes. If the manufacturer just says “must use a 25A breaker” then the circuit has to be modified, or an an extra circuit added, before the appliance can be added. Which is annoying, as one might suspect that it would actually be ok with a 40A breaker too, if only the manufacturer would tell us!

Agreed … some manufacturers could be a little more open about this. Having said that, some manufacturers have bad experiences of horror installations, and having to prove it's not their appliance that caused the fire … it works both ways.

There are also cultural issues here. For example, installers in some countries feel the manufacturer should specify the OCPD, whereas I get the impression certainly many competent electricians in the UK feel they are being undermined, or, as you say, restricted, by the manufacturer being so specific … especially when it's printed alongside other installation instructions that are clearly nonsense or not applicable in the UK !

I cannot agree with you above Graham, although the “paying for 2 MCBs” comment might suit the manufacturer to make £1 more profit is possible. 

Looking at the technical points, this appliance has a rating (the maximum possible draw) of 11.75kW. this is a consumption of 51A at 230V. There can be no reason why this should not be the final circuit rating, although it will probably never be reached due to diversity. If I fit a 40A breaker it will probably never trip, but even so, it is directed as a maximum by the instructions. These may have been written to minimise the chance of a fault damaging the appliance wiring, but at either 40 or 50A, this is unlikely unless it is the power to the clock. In any case, if there is an Earth fault that does damage something other than the fault, does it matter? I suggest not, except to the manufacturer who hopes to limit guarantee costs. I expect that an additional £1 for an MCB would be cheap insurance. It is worth noticing that most appliances contain some kind of CPD, be it a fuse or whatever. Even wall warts tend to have a thermal switch or fuse, although made down to a very low cost. The size of the appliance terminals is a weak excuse, any competent electrician will easily cope with this, by using crimp tags or the methods given above.

edit : I will add one extra point. It is often difficult to add another circuit to many modern properties because of the construction, typically concrete floors both up and down. Mostly it should not be necessary, and it is simply needlessly expensive.

I am glad that you too have noticed how poor the instructions often are.