Cuprager wrote:

Dumb question time!

I know that you are not supposed to take more than one socket off a ring
as a spur but have never figured out why! Can anyone enlighten me?

Several reasons....

Firstly there is the issue of the cable. On a ring, each socket will be
fed by two cables, either of which will carry the full load current of
the socket. Hence if you spur from the ring with one similar cable there
is little danger of causing damage to the cable with an overload. If
however you add several sockets to that spur cable then you run the risk
of exceeding its capacity. That is why the regs require the introduction
of a fuse into the spur if you are going to do this.

The second issue is that of load distribution. Rings work well as a
result of both diversity (i.e. the statistical likelihood that only a
small proportion of the actual maximum load will ever be required at any
one time), and the fact the loads will be spread around the ring. A ring
with all the load concentrated at one end, could for example, overload
the short cable run back to the CU since it would have a proportionately
lower resistance than the longer run, and hence carry more than its
"fair share" of the current. So again adding lots of sockets to a single
spur (even if you wire it in heavier cable so as to not allow the spur
to overload), you will still be concentrating a large load on to a
single point on the ring.

Other issues: fault loop impedance; a ring socket will have two earth
wires connecting to each socket, while a large spur will have ultimately
half the total earth conductor size/area. Hence the resistance to earth
from a spur socket is likely to be a bit higher than for a ring one. The
longer you make it and the more sockets you add the worse this gets.
Hence you potentially lower safety since the fault current that can flow
to earth is reduced, and the time to blow/trip the protective fuse or
breaker gets longer.

Maintenance: there is the danger than someone will mistake a socket on a
multiple spur as a part of a ring (since it will have a cable in and out
and hence look like a part of the ring). Adding to the possibility that
later modifications will increase the risk of overload.

Increased testing time: testing a ring is relatively simple using a low
ohms meter at various points with the conductors cross connected (and
disconnected from the CU!). Each spur adds complexity to the process.



--
Cheers,

John.

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