On Monday, February 18, 2013 6:18:39 PM UTC, Stephen H wrote:

I did actually sit down and think this through from a good design point
of view...
As stated, there is a 2.5mm2 ring main with a 32A RCBO.
I was not worried about L-E or N-E shorts as there is the 30mA RCD part
in the RCBO that takes care of that.
However, I had two scenarios to think about regarding my outside sockets.
I wanted to put a single external socket in the front porch.
So that meant a single 2.5mm2 spur T&E cable from the 32A ring main in
the garage to a single socket in the front porch via a bedroom.
Given that the max current available on the ring main would be 32A
before the consumer unit's 32A RCBO opens and that the maximum

32A is what it'll pass indefinitely. Real life trip current is far higher

in-service non-fault conditions continuous current would be 13A at the
porch socket, (limited by the 13A or less fuse in the plug of whatever
appliance plugged in)

fuses don't limit current at all. A 13A fuse can pass around 20A for a long time


I had thought about what could happen if there was a dead L-N short on
the single 2.5mm T&E cable running between the ring main and the single
gang socket.

PSCC is anywhere upto 6,000A. Whatever PSCC actually is it'll normally trip the 32A mcb first. If its north of the mcb's breaking capacity it'll take out the incomer fuse instead.

Obviously this single 2.5mm cable is capable of taking a 13A current
continuously between the ring main and the single socket via the spur
cable, I felt uncomfortable with it potentially carrying up to 32A
before RCBO opens in case of a L-N short on the spur cable.

It would carry anything upto 6kA. Its not a problem for the very short time involved.

Thus, I put a 13A fused switched neon faceplate at the point where the
spur is connected to the ring main in the garage to protect the spur.

no point

The other scenario was the double gang socket on the front driveway.
Now I could plug in two appliances. Lets say both draw up to 13A before
their individual plug fuses pops. Thats a total load of 26A which is on
a 32A rated 2.5mm ring main.
Now doing 26A continuously on a single 2.5mm2 spur cable between the
ring main and double socket I was not comfortable with, let alone a

its within the 27A cable rating. But what real life loads would draw 26A continuous?

potential up to 32A flowing in the case of a dead L-N short.

more like thousands of amps

Switch isolators come rated as 20A or 45A. So 20A is less than 26A that
the double gang socket could be going up to. So I went to 45A rating for
the switch neon isolator.

no need. 20A's enough for a double socket.

So I put a 45A rated neon switch on the 2.5mm2 ring main and reasoned
that as current flows both ways round the ring, that 5.0mm2 is the same
as 2 x 2.5mm2. So I opted to run the spur cable as 4.0mm2 from the 45
amp isolator to the double gang socket.

no need

Now I am then comfortable with
up to 26A flowing continuously on this 4.0mm cable from isolator to
double gang socket.
I fully appreciate the above scenario is highly improbable and that I
have probably over-engineered my circuit but hopefully at least a sparky
can't find fault with my reasoning!
Stephen.




NT