Shower circuit design.

gkenyon: 
Sparkingchip, I think you have missed the point that Regulation 411.3.3 only applies where protection against overload is required. It's arguably not required for a shower (although see my earlier post, I would prefer it).

Slight typo, Graham, but we know what you mean.

433.3.1 deals with omission of protection against overload:

(ii) for a conductor which … is not likely to carry overload current …

The cable is not going to be overloaded because, if anything, it is over-sized, so 433.1.1(i) does not apply.

I cannot believe that the original electrician thought it out in these terms.

I don't have a regs book to hand to check the bit about omitting overload protection, I suspect that Graham is playing Devils Advocate. 

Last year I did an EICR for a flat that originally had a gas hob and an electric oven supplied by a B16 MCB and a 2.5 mm circuit. The landlord had the gas hob swapped for an electric hob which was connected to the existing circuit. 

I presume  @davezawadi (David Stone) will tell me that despite being unusable because the circuit breaker tripped if you turned more than one hot plate on along with the oven, it fully complied with the Wiring Regulations and I should not have coded it a a C2 in the EICR.

On that basis an installation that I condemned out of hand, and said a few rude words about when I found it, many years ago before folk started worrying about competent person schemes and so on, may also have been OK after all actually  !?!? 

In that case the cable to the shower came straight into the main switch on the CU and had no fuse at all, just the DNO one.  This was a (back then) fairly standard Wylex board, with not enough ways to add a shower, so the shower cable shared the switch terminals with the meter tails.?

Overload protection omitted, S/C fault covered by the DNO fuse. Compliant apart from no easy means of isolation. Maybe we do not need breakers at all for cookers and showers, and a straight link onto the bus bar would do ? Somehow though I think not really best practice.

Mike.

Sparkingchip: 
I presume  @davezawadi (David Stone) will tell me that despite being unusable because the circuit breaker tripped if you turned more than one hot plate on along with the oven, it fully complied with the Wiring Regulations and I should not have coded it a a C2 in the EICR.

I shall too!

I am not sure that BS 7671 says that you have to provide enough power. The circuit was perfectly safe, but the unwanted tripping would be a real nuisance. The circuit complied with 433.1.1 when it was erected. 133.2 may be relevant, but it certainly wouldn't merit more than C3. Overall, non-compliance is not obvious, so no code from me.

I think you may have missed the point, Andy, in some ways both Graham and I may have been encouraging you to think! Your C2 coding is simply wrong because the circuit is not in any way dangerous in your example. It might be a nuisance but I doubt that the breaker would operate with the normal use of both rings and oven unless one turned the whole lot on together. As I said above, no one would design these circuits this way, but they have ended up in a slightly curious state, where tripping could occur without a fault being present. However, this is still no different from a standard ring or radial circuit with sockets.

You should be aware that you would certainly fail your 2391 practical with that C2 and a couple of other mistakes, C1 or C2 codings are for danger, not nuisance problems.  A C3 suggests improvement, which is the outcome you would like to see, but you cannot force someone in this way, they may enjoy resetting the MCB.

As far as the suggestion that this breaker may get too hot in the OP, this is unlikely and even if it did it would trip at a lower current/shorter time from the data. I could show you many places with fairly hot switchgear, certainly too hot to touch for long, the dissipation in a large board can be a couple of hundred Watts, much more than a domestic CU, and the casing is not ventilated, and there may be a lot of bunched hot cables, whereas domestics rarely get even noticeably warm. These are the same BS 60898 breakers in many cases, and they generally do not burn up at all (which they should not from the spec!). It may be that they trip at a slightly lower current, but this rarely shows up.

I am NOT advocating using breakers in the way described in the op, by the way, just analysing the problem fully. It would be useful if the full range of currents were available easily, 6,10,16,20,25,32,40,50,63A, often there are inconvenient gaps in ranges even from manufacturers, particularly if one wants a C or D type.

Chris Pearson: 
 

gkenyon: 
Sparkingchip, I think you have missed the point that Regulation 411.3.3 only applies where protection against overload is required. It's arguably not required for a shower (although see my earlier post, I would prefer it).

Slight typo, Graham, but we know what you mean.

433.3.1 deals with omission of protection against overload:

(ii) for a conductor which … is not likely to carry overload current …

The cable is not going to be overloaded because, if anything, it is over-sized, so 433.1.1(i) does not apply.

I cannot believe that the original electrician thought it out in these terms.

Sorry yes, transcription of numbers 411.3.3 is also a common one to quote (RCDs required for socket-outlets up to 32 A). ?

Chris Pearson: 
 

Sparkingchip: 
I presume  @davezawadi (David Stone) will tell me that despite being unusable because the circuit breaker tripped if you turned more than one hot plate on along with the oven, it fully complied with the Wiring Regulations and I should not have coded it a a C2 in the EICR.

I shall too!

I am not sure that BS 7671 says that you have to provide enough power. The circuit was perfectly safe, but the unwanted tripping would be a real nuisance. The circuit complied with 433.1.1 when it was erected. 133.2 may be relevant, but it certainly wouldn't merit more than C3. Overall, non-compliance is not obvious, so no code from me.

So yes, 133.2 is where I'm going … but which piece of equipment is not selected and erected in accordance with this?

I'm going for TWO possible non-compliances  here … one of which warrants either C2 or FI (take your pick, I don't have enough information to choose without seeing the installation):

• Shower not selected and erected to prevent overheating of OCPD in use (severity dependent on adjacent OCPD loading factor). This is C2 or FI.
• Then there is the issue of OCPD not selected according to manufacturer's requirements for protection against overload current (given it's a concentric heating element device) - although because In of the OCPD is lower than that required by the shower manufacturer, this can only be a C3 - and I guess this is where David Z is coming from.

You cannot have it both ways. 

You're saying that if you don't need overload protection the MCB does not have to have a higher rating than the design current. 

But the 2.5 mm circuit supplying both an electric hob and an oven does need overload protection, therefore the rating of the MCB has to be higher than the design current, so the B16 is a clear breach of the regulations, despite the MCB being rated for the cable. 

Sparkingchip: 
 

You cannot have it both ways. 

You're saying that if you don't need overload protection the MCB does not have to have a higher rating than the design current. 

But the 2.5 mm circuit supplying both an electric hob and an oven does need overload protection, therefore the rating of the MCB has to be higher than the design current, so the B16 is a clear breach of the regulations, despite the MCB being rated for the cable. 

Yes from the cable's perspective that's correct.

From the OCPD perspective, it's a different story as it's being subjected to overload.

As I said earlier, despite it not being started specifically in BS 7671, I'd never select In to be less than Ib for that reason 

I know one thing, if there’s a couple of teenage girls in the house it will definitely trip.