Why do you say that? Are you 'quibbling' about the fact that the connection between existing cable and extension would (presumably) be in a kitchen? Apart from that, I can't see why extending an existing circuit to provide a socket (which is not part of an 'outside electrical installation') would be notifiable.
We've been through that before, and I thought the general consensus was that, although it's surprising that running an outdoor, maybe underground, cable should be non-notifiable, the wording of Schedule 4 certainly seems to indicate that it is not, per se, provided that it is terminated 'indoors' (house or outbuilding) at both ends.
Indeed, and that would obviously move the goalposts. However, I was responding to a question about running SWA from the circuit in a kitchen to a socket in an outhouse.
Not notifiable.
Quite - as I said/implied ... provided, of course, that one does not quibble (and I know/presume that you won't!!) about the fact that one would (presumably) be installing an accessory (JB or whatever) within a kitchen to effect the join.
It's not the existance of JB which is the issue - and I disagree with what you say, since neither is there any mention of cable in relation to (non-notifiable) extensions to add sockets, FCUs, lights and switches. I will therefore happily interpret the Schedule as meaning that things like cable and JBs are implicitly allowed as part of these works that can be done without notification.
Why does that mean that you have to disagree?
Indeed. So we are agreed, then.
For the purposes of Schedule 4 its location has to be that of the item of which it is an implicit part. If it inherits that part's notifiability status it inherits its location.
You don't have to break it.
We know your view/interpretation - and you may be right - in which case, as you say, I wouldn't need to break the law. However, if you're wrong I would, in this case, break the law.
Ive done some reading on discrimination.
In your case, the simplest solution is not to 'add in discrimination' but, rather to reduce the number of overcurrent protective devices (OPDs - MCB/fuse) to one - so there are not two things to 'discriminate' between. An OPD is only there to protect the cable. Provided all cable is fat enough to be adequately protected by the OPD at the house CU, there is no need for any MCB/fuse anywhere else. If you want to run a 'sub-circuit' in smaller cable (e.g. for lighting), then you can take that off the main supply via a fused connection unit (FCU), with a fuse in that small enough to protect the size of cable concerned (say 3A or 5A). That should discriminate satisfactorily with, say, a 20A or 32A MCB in the CU.
I see, so I would just have the MCB in the consumer unit, which would protect the cable and everything beyond it, with then a 5amp FCU to the lighting.
Exactly. That's the usual 'simple' supply to a garage.
Sure, but that's unavoidable unless you either run a totally separate circuit from house to garage for the lighting (with its own MCB in house), or run a fat cable (hence allowing a high rated MCB at CU) all the way from house CU to a CU in the garage (which could then give you discrimination between MCB in house and power circuit MCB in garage). As regards the latter, the 4mm² cable in your existing cooker circuit would limit you to a 32A MCB, which is not hih enough to give you discrimination from a 20A (and probably not 16A) power circuit MCB in the garage. If you could run 6mm² or 10mm² cable all the way from house CU to a garage CU, things would be different, because you could then have a higher-rated MCB in house.
In the face of high fault currents, I don't think one could ever guarantee discrimination, no matter what the ratio of MCB ratings. However, to stand a reasonable chance of getting discrimination, you would need at least a 2:1, probably 3:1 ratio - e.g. you might get discrimination between, say, a B6 and a B16. In other words, I don't think you would, in practice, find an MCB to put upstream of a B32 which would offer much hope of discrimination (and, even if you could, you'd have to use hefty cable). If the upstream device is a fuse, then you're much more likley to get discrimination but, even then, I think you'd still need at least 2:1.
Indeed. Of course, in the case of two 30mA RCDs, there's no reason to even expect any discrimination. The only 'upside' of have two RCDs is that their reliability is uncertain (one often hears it quoted that about 7% are found to be faulty when tested), so there's some 'belt and braces' reassurance to be derived by having two!
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