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?
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
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."
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. 4220.127.116.11 applies (final ccts <= 32 A), but for a distribution
circuit (which this is) 418.104.22.168 allows 1 second disconnection time, so
you're OK - phew :-)
See also 531.2.9, and 531.2 generally. Happy reading. My copy arrived
last Friday and there is much to absorb...
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
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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