The most basic problem with DIY work

[securespark]

What 'harm' resulted, eventually?

Kentucky Fried Cable!

 

[JohnW2]

... Apart from going round in circles It would seem one of us is confused.
An appliance which cannot cause an overload (an overcurrent occurring in a circuit which is electrically sound) can be installed on an OPD whose rating is greater than the CCC of the cable because it (the cable) will not have to deal with loads greater than that of the appliance's stated rating for which the cable has been selected.
Should a fault occur the OPD will operate as this will have been determined. That was what I thought wrong with Simon's hypothesis because, I don't think, an immersion can cause a fault current which is (as you mention) only double the cable's CCC.

The situation, particularly the vocabulary, is, indeed confusing - and I think the reason is that BS7671 is very non-exhaustive in its definitions. AFAICS, there are, in fact, three main categories of situations in which 'overcurrents' (currents greater than the design current of the circuit which, if high and persistent enough, could damage wiring or other parts of the installation) can flow:

• 1...A 'fault', per BS7671 definition - a connection of negligible impedance betwen L & N or L & E/CPC. Such currents will be very high and, in a properly designed and functioning installation, will, as you say, result in operation of an OPD.
  
  2...'Something wrong with' a load (which I confusingly refered to as a 'fault', in the everyday, not BS7671, sense) resulting in a current (flowing L-N or L-E/CPC) which is above the design current, but less (usually far less) than the fault current seen with (1).
  
  3..."An overcurrent occurring in a circuit which is electrically sound" - which, as you quote above, is the BS7671 definition of an "overload". The simplest example would be plugging too high a total load into a sockets circuit. In fact, it's difficult to think of any other situations in which a properly-designed installation could experience a 'BS7671 overload' - since proper design should preclude the installation of any fixed-wired load which resulted in an overcurrent when 'electrically sound'. Can you think of any examples?

AFAICS, the problem is that, whilst extensively addressing (1) and (3), BS7671 'ignores' (2) - yet our discussion has, implicitly, been nearly all about (2) - Simon's hypothetical immersion example (if it occurs) and a jambed motor (without protection of its own) would be two examples.

I don't know what you'd want to call it (since it doesn't qualify as either 'fault' or 'overload' per BS7671 definitions) but, if you accept that an overcurrent of type (2) may sometimes occur, do you not feel that it is something that wiring ought to be protected against?

The fact that BS7671 has 'hijacked' the everyday word "fault" makes the vocabulary difficult. As I did yesterday, it's only too easy to make the mistake of using the everyday meaning and refering to 'something going wrong' which results in a abnormal L-N or L-E path of non-negligible impedance (which resukts in an 'overcurrent') as a "fault" - which is when confusion arises.

In what sort of circumstances would you personally consider it 'useful or necessary' to invoke 433.3.1(ii)?

The first that springs to mind is connecting a small oven to the cooker circuit by the supplied flex (MIs notwithstanding).

Fair enough, but in any situation like that, I would say that it's only reasonable if one feels that a 'type (2)' situation (as well as a 'BS7671 overload') cannot be created within the appliance. Maybe that's what's confused you, and maybe others - I (and Simon, and maybe others) have been talking about 'overload' when we have usually meant 'type (2)', not a 'BS7671 overload'.

Kind Regards, John

 

[JohnW2]

What 'harm' resulted, eventually?

Kentucky Fried Cable!

I'm still pretty surprised, unless there was a very adverse installation method. Per the CCCs we work with and the known characteristics of MCBs, we know that 2.5mm² cable clipped direct is considered able to cope with about 39A for 1 hour without suffering any harm. That's almost as much as a 10.8 kW shower, which will rarely be on for anything like as long as an hour at a time (unless, I suppose, teenagers are in the equation ).

Kind Regards, John

 

[securespark]

To be fair, I didn't go into the nitty gritty of the installation: I just condemned it on the basis of the csa alone.

But there may well have been loft insulation involved (there often is!) and given that is seemed to be an immersion conversion (poetic) and therefore DIY, there could have been poor connections too.

But the cable was badly melted for quite some distance.

 

[JohnW2]

To be fair, I didn't go into the nitty gritty of the installation: I just condemned it on the basis of the csa alone. But there may well have been loft insulation involved (there often is!) and given that is seemed to be an immersion conversion (poetic) and therefore DIY, there could have been poor connections too. But the cable was badly melted for quite some distance.

Fair enough. 'Condemning it on the basis of the CSA alone' is obvioulsy what you had to do, since that's clearly non-compliant. I was merely pointing out that the reason why these 'frightening' situations often persist (without any melting or other incidents) is that, in practice, one can often go quite far into non-compliance before anything actually 'happens'! - the regs seem to have some pretty large safety margins built in.

I guess we need RF to run 43A through some 2.5mm² for a while and see what happens (with and without insulation)!

Kind Regards, John

 

[securespark]

Yeah, where is he and his trusty welder when you need him, eh?

 

[ringi]

I know this is an old thread, but….

Remember in most of world there are no fuses in plugs. The freezer has therefore to be designed so it is safe with an unfused plug.

For it to be legal to sell the freezer in the UK, it must have a CE mark, so must keep to the EU standards, this includes having any protection it needs for its cable inside of its case.

 

[ban-all-sheds]

Don't worry, Ukip have a plan to fix that, and make our electrical appliances less safe.