In an earlier contribution to this discussion,
Andy Wade wrote:
Hi Andy,
Many thanks for your very detailed reply - complete with calculations. A few
more questions, I'm afraid . . .
The BS 1361 fuse is a cartridge fuse, like this
http://www.tlc-direct.co.uk/Products/WYC30.html
as opposed to the rewireable one that you probably still have (which
is to BS 3036).
I've looked at my old ones, and they *are* BS 1361 cartridges - so I already
a couple.
OK - I've probably confused you there. We're looking at different
aspects of the design in isolation - discrimination only in this
sub-thread - and have lost sight of the whole picture. Let's look at
some other aspects. I've found the June thread and assume that 10 m
of T&E plus 5 m of SWA still stands. I'll also assume that the 30 A
BS 1361 cartridge fuse is to be used.
- Voltage drop: 15 m (total) of 6 mm^2 will have a voltage drop of
under 3.3 V at 30 A; that's 1.4% of Uo, so OK there.
So what are we aiming at here - how high can it be?
- Disconnection time: 10 m of 6/2.5 T&E will contribute about 0.13 ohm
to the earth fault loop impedance (Zs) and 5 m of 6 mm^2 2-core SWA
will contribute a further 0.06 ohm; taking the cautious assumption
that the supply is in fact TN-S we add 0.8 ohm for the external
(supplier's side) part of Zs, so Zs (total) is 0.8 + 0.13 + 0.06,
which is 1 ohm, as near as damn. The earth fault current it thus 230
A which will take out the 30 A fuse in somewhere between 0.2 and 0.3
seconds - so the 5 s disconnection time requirement is met.
Sounds like we're *well* in.
- CPC sizing: assuming 0.3 s disconnection then for the T&E section
S(min) = sqrt(230^2 * 0.3) / k, with k = 115 (from Table 54C). This
gives a min CPC size of just over 1 mm^2, so the available 2.5 mm^2
CPC is OK. It's hardly necessary to do this calculation for the SWA
section, but for completeness the relevant k value is 51 (Table 54D,
assuming 70 deg.) so S(min) is just under 2.5 mm^2 (steel). The
armour CSA in 2-core 6 mm^2 cable to BS 5467 is 22 mm^2, so
definitely no problems there.
Unless I've missed anything, we can conclude that the submain design
complies with BS 7671.
What would be the effect on the calculations if it were a 40A or 45A BS1361
fuse rather than 30A?
Others have suggested going out through the wall with the T&E, and joining
it to the SWA using an IP65 box mounted on the outside of the wall. That
might be easier in some ways, but I'm not sure how best to seal the T&E
cable entry into the box. Any suggestions? Do I drill the back of the box,
and align the hole with where the cable comes through the wall, and seal
round the cable with silicone sealant? Or what?
Someone suggested using a different type of CU in the garage - so that if I
were to do something with a power tool which caused the RCD to trip, it
wouldn't also take out the lights. Is that what is meant by a 'split load'
CU? Any suggestions as to exactly what I would need (make, model, etc.)?
Many thanks.
--
Cheers,
Roger
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