I've been trawling through my new copy of BS7671 and noticed on page
196 a diagram of a suggested marina supply circuit where a three phase and neutral supply is fed (TT) via a TP&N RCD, then via a flexible cable to separate single phase socket outlets each with their own RCD. I can't see (maybe a mental block) but what gives discrimination between these RCDs = no mention of any time delay or use of type S at the TPN RCD Anyone else wondering about this?
Actually Fig. 709.2 shows a mixture of single- and 3-ph sockets.
Reg. 709.531.2 says that the sockets shall be individually RCD protected with RCDs as specified in 415.1.1, which is the normal (fast) 30 mA RCD. 709.533 also requires the overcurrent protection to be individual, but note that 709.531.2 stipulates all-pole disconnection by the RCD, which rules out using the many types of 'single-pole' RCBO with solid neutrals. DP RCBOs for single-phase do exist, but I'm not sure whether there are 4-pole ones available for 3-ph use.
Clearly to get discrimination the upstream device would have to be time delayed, but you still need to comply with the disconnection times in Chapter 41 - one area of major change from the 16th ed. Now, Type S RCDs won't meet the 70 ms disconnection required in Table 41.1 where reg. 411.3.2.2 applies (final ccts
I've posted bites about this before, based on the draft ed.
Regs 526.8.x refer to connection of "multiwire, fine wire and very fine wire conductors." These terms aren't included in the definitions, but I'm assuming that ordinary flex with 0.2, 0.25 or 0.3 mm strands is covered (BICBW).
526.8.1 says that the terminals must be suitable for these wire types, "or the conductor ends shall be suitably treated" - i.e. (IMO) fitted with bootlace ferrules, or crimped ends etc. as appropriate.
526.8.2 explicitly bans soldering or tinning wire ends if screw terminals are used.
526.8.3 bans soldered or tinned ends, regardless of the type of termination, where there is "relative movement between the soldered and the non-soldered part of the conductor."
When is the likely introduction of 17th Ed? I'm intending wiring our new extension (with building regs test and approval in this case) which is likely to be May/June this year - and am wondering which version will apply. Or is it the version in force when the buiding regs application goes through?
It was released on 1st January and you have the option of using it now. It comes into full effect on 1st July, or to quote: "installations designed after 30th June 2008 are to comply with BS 7671:2008."
You might want to talk to your building inspector about that. The actual Part P requirement doesn't mention BS 7671 at all, it merely requires that "reasonable provision shall be made in the design, installation, inspection and testing of electrical installations in order to protect persons from fire or injury," although the statutory instrument does mention BS 7671:2001 (i.e. 16th ed.) in connection with defining what is a special location for notification purposes. OTOH the guidance in Approved Document P mentions the 2001 edition many times.
Doubtless these things will get changed between now and July, and new versions of the IEE OSG and Guidance Notes will appear. I would be inclined to design to the new edition now though, as anything that complies with the 17th ed. will also comply with the 16th (I think).
The biggest single change [in reg. 522.6.7], if you're using 'normal' T&E house wiring practice, is that _all_ circuits will need 30 mA RCD protection. (Any non-RCD circuits will require steel conduit or special cable types, or burying more than 50 mm deep.)
There's also 522.6.8 which applies the same principle to cables in partitions or stud walls of metallic construction (increasingly popular in house building, I understand) _regardless_of_the_cable_depth_.
As the diagram is in the regs I "hoped" it would be more specific on my point. On the one hand the first outlet in the line (presumable "on terra firma while the next ones are on a floating dolphin) is a TPN socket and might be considered as requiring 30mA protection with the usual constraints of r30mA apid response for shock protection outside an equipotential zone. Hence my puzzlement about discrimination chances here. As I see it we have a need for official clarification but as things appear the chances are that a fault on one of the craft will likely take out the supply to the dolphin and all the others moored there as well as opening its own single outlet. Maybe making the shore connection via a fixed outlet rather than a socket would be a solution?
I can't recall my IET discussion group password (its at home) but maybe someone there can give a suitable answer
I don't see any great ambiguity; you just have to look elsewhere in the regulations - as ever. I agree that the annotation of the diagrams could be clearer though.
On the one hand the first outlet in the line (presumable "on
Ah, but bear in mind that the 30 mA fast RCD requirement only applies for sockets up to 20 A [411.3.3(i)]. As this socket is on a distribution circuit feeding a number of (presumably) 16 A sockets downstream (almost literally!) it's likely to be a 32 A or 63 A type and therefore an S-type RCD will be OK. It could also be a 100 or 300 mA RCD, if Zs is low enough.
Maybe - I read that from time to time and find the S/N ratio somewhat variable. I'd try the IEE enquiry service if you want a definitive reply.
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