On Tue, 11 Feb 2014 23:53:15 +0000, John Rumm
wrote:

On 11/02/2014 20:43, Johny B Good wrote:
On Tue, 11 Feb 2014 10:42:42 +0000, John Rumm
wrote:

On 11/02/2014 04:47, Bill Wright wrote:
John Rumm wrote:

No need for the 5A FCU. The 6A MCB or 5A fuse will take care of any
overload.

My reason for including it was not for protection of the wiring, but
for adding discrimination if possible.

Use a 1A or 3A fuse then.

Indeed... although it does depend on what the main circuit is protected
with. There are some 10A lighting circuits about which will offer more
scope.

I suspect such circuits would only qualify for the higher 10A rating
when completely wired up with Pyro cabling and appropriate fittings. a

10A is well within the capability of even 1.0mm T&E in some cases, and
1.5mm T&E in most.

There used to be a restriction on not using SBC and SES lamp fittings on
such circuits, but that was deleted with the issue of the 17th edition.

more likely scenarion in commercial premises rather than in premises
of the domestic persuasion (i.e. a house) where the limit has always
been 6A fusing at the CU panel afair.

10A is used domestically as well... I have certainly met it a few times.

Well, not being an electrician by trade, I've been unable to find any
references to the regulations, only anecdotes regarding 10A lighting
circuits (including the use of a ring lighting circuit) so I can't
quote 'chapter and verse' as to whether this is an acceptable
departure from the regulations or not (I know some leaway exists
within the regulations but this seems to be quite a large departure to
my mind).

All my knowledge comes from when, with the assistance of a retired
electrician (a family relative), I totally rewired this 3 story 6 bed
Victorian semi-detached house back around 1983.

This is where I learned about the CU fusing regulations with regard
to ring mains, lighting circuits, high power spur feeds to things like
cooker points and power showers as well as to the requirement to
provide a dedicated 15A fused feed to the immersion heater (classified
as a fixed connected load requiringa high temp flex connection between
the heater element terminals and the switched terminal box on the end
of said dedicated feed). Plus I also learned about the earth bonding
requirements of all exposed metalwork (pipework, stainless steel sinks
etc).

It isn't exactly rocket science for someone experienced with
electronics kit and a well founded knowledge of electrical theory.
Once you understand that the fundamental protection provided by the
fuse links (MCBs) in a CU is essentially to protect the permanent
cabling and fittings it's easy to see why 5A was chosen for lighting
and up to 30A for ring mains with 45A protecting 4mm FT&E cabling to a
cooker point and other similar heavy duty loads.

A small 3 bed semi can manage with just a single ring main circuit
and a single lighting circuit plus cooker point and immersion heater
feeds so could be nicely served by a 4 fuse CU with a set of fuses
comprising of 5A for the lighting, 15A for the immersion, 30A for the
ring main and a 45A for the cooker point.

In a bigger property, you'd be well advised to fit a 6 way CU so you
can split the lighting across two circuits, each with their own 5A
fuse rather than the ill advised use of a 10A fuse on a single circuit
(even when 1.5mm cabling is used - the ratings on the fittings are all
based on the protection of a 5A fused supply). Obviously, the second
extra fuse position will allow two seperate ring mains to be provided.

When we first moved into the current property, it didn't have a
cooker point so I was able to wire up three seperate ring mains
(ground floor, first floor and second floor mains socket supplies).

The basement was catered for by fitting 3 single outlet 13A sockets
onto the CU backboard each fed off the 3 ring main circuits so that
the freezer we kept in the basement could be readily powered from any
one of the ring main circuits as an insurance against any protracted
outage that might arise due to faults or planned changes in the ring
main wiring.

The freezer no longer resides in the basement but the 'diversity' of
this setup is still very handy although I have installed an extra twin
outlet wall socket away from the CU, connected most likely to the 1st
floor ring main circuit (but I'd have to take a gander at the fuse
cover labelling to confirm this - but it seems the most logical choice
of ring main so I'd be surprised if it were otherwise since the same
logic would have dictated my original choice).

When it came to replacing all the VIR cabling in conduit lighting
wiring, we knew it had to be split across two lighting circuits. As it
happened I bought a couple of 100m reels of 1.5mm FT&E for this job
and used nearly all of it.

The Mortgage company had held back £1000 of the loan on account of
the state of the lighting circuit wiring but when I delved a little
deeper into the state of the rest of the wiring, I discovered the
house had two ring circuits and a mix of spur fed outlets randomly
distributed about the property (one ring main fed half the ground
floor sockets and half the first floor sockets and the other ring main
fed all the 2nd floor sockets but with a length of heavy duty rubber
sheathed appliance cord being used instead of the regulation FT&E.

What had started out as just a lighting circuit 'rewire' developed
into a complete rewiring of the whole house. The job was essentially
an exercise in re-organising the randomly fed sockets into 3 distinct
ring mains.

Since the top floor was the only proper ring main in the whole house,
I replaced the rubber sheathed flex with 2.5mm FT&E to bring it up to
standard then dropped a very long mains extension down to the kitchen
with another extension lead into the basement to power the freezer
whilst I stripped out most of the 'ring main' wiring to the ground and
first floor sockets, sorting out the recovered lengths of FT&E so I
could rebuild the ring main circuits using shortest lengths first.

This minimised the need to add extra cable to the point where I was
able to beg the extra 20 or so metres from my dad thus reducing the
cable costs to nil with only the new dual gang 13A sockets as the main
parts cost.

It was a lot of work to sort it out but I felt it just had to be done
in the interest of safety (it's not good having sockets in the same
room powered from different fuses in the CU). needless to say (after
sorting out the earth bonding) I had no trouble getting the job
certified and the extra grand released by the Mortgage company.

I suspect anyone trying to get their house wiring certified whilst
they have a 10A fused lighting circuit in the mix will have a hard
time trying to convince an inspector that it is within regs and
standard wiring practice.

With many house owners now using CFLs and LED lamps in place of most
of the originally fitted 60 and 100 watt incandescent lamps, I think
any such 10A lighting circuits could now be 'downgraded' to 5A fuse or
6A mcb protected circuits without any problems.

Other than 'special cases' I think anyone with a 10A fuse protected
lighting circuit should downgrade to a 5A fuse for their own peace of
mind just on safety grounds alone. Using a 10A fuse, even in a larger
domestic property is probably unnecessary with modern lamps these
days.
--
Regards, J B Good