Shower circuit design.

I have the IET Electrical Installation Design Guide 4th edition on my lap.

Mind your P’s and Q’s if commenting on my last post ?

Colin Haggett: 
 

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

Yes, I was called out to investigate a shower that had blown a 30A wylex fuse. The lady in question said that her niece had been enjoying the new water power spray for about ¾ of an hour when it stopped. It appeared the woman had had installed a new electric shower the day before.

Legh

Sparkingchip: 
 

Mind your P’s and Q’s if commenting on my last post ?

Sorry mate … the section you're looking at EIDG Section 4.2.2 it's entitled “overload currents”. That is, “protection against overload current”, or Section 433 (as posted in the margin of that Section on Page 44). See earlier posts - this can be omitted for some loads …

Legh Richardson: 
 

Colin Haggett: 
 

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

Yes, I was called out to investigate a shower that had blown a 30A wylex fuse. The lady in question said that her niece had been enjoying the new water power spray for about ¾ of an hour when it stopped. It appeared the woman had had installed a new electric shower the day before.

Legh

BUT … is that a failure of DESIGN (in relation to the characteristics of the fixed appliance and the OCPD) or OVERLOAD PROTECTION?

I really think it's NOT 433 …

… BUT as I've said all along in this discussion thread, I don't think that's a reason to say it's OK and complies with BS 7671! 

OBJECTS AND EFFECTS.  “….as to provide for safety and proper functioning for the intended use.” 120.1

Z.

gkenyon: 
 

Sparkingchip: 
 

Mind your P’s and Q’s if commenting on my last post ?

Sorry mate … the section you're looking at EIDG Section 4.2.2 it's entitled “overload currents”. That is, “protection against overload current”, or Section 433 (as posted in the margin of that Section on Page 44). See earlier posts - this can be omitted for some loads …

I will take that as an editoral commentary, reading from the start of 4.2.2 it does appear in context and the situation should never arise in the first place.

The circuit is the complete assembly including the circuit protective device, not just the wire (BS7671).

The start of 4.2.2 in the IET Design Guide 

We seem to have missed the point here. It is not a case of saying this is OK, it is a case of saying it is not dangerous. It is not a good design and may cause nuisance tripping, although this is simply because the load is more than the circuit rating. However, it is exactly the same situation as any circuit with sockets where too many appliances are operating together, this cannot be a non-compliance with BS7671. As a designer, one makes a guess at the possible socket circuit load and fits as many sockets per circuit as will cope with this load. If you like it is a similar problem to an office with too many mA of Earth leakage leading to RCD trips, it may or may not be a snag, but it is not a non-compliance with BS7671. It is entirely due to use that is not under the control of the regulations.

The only way that this can be “corrected” is to rate circuits by allowing the maximum current that could be taken, perhaps 20A for each 13A socket, leading to many circuits. This is however daft, because it is not preventing any problem, or more particularly danger, it is simply meeting some arbitrary idea with no reasonable basis in fact.

I am also very unhappy with the idea presented here that MCBs can overheat due to “overload”. This is again a silly idea, their exact job is to overheat and trip on small overloads. They have a negative temperature coefficient of trip current, so such overheat is impossible. As I said above, large groups of MCBs can get quite hot, it is not a problem as the only effect is that the trip current is reduced. Ambient temperature and self-generated temperature are two completely different things, although they obviously interact together.

It seems that some would like to make circuits impossible to “overload” by design, in other words, the CPD is only protecting against severe faults. This feeling is completely opposite to the understanding in BS7671, we protect against misuse of all kinds, as far as reasonably possible. I should point out that this all flies in the opposite direction of any kind of diversity, presumably in the desire to have 100% convenience.

Over heated M.C.B.s