Hi,

I want to extend the ring mains in my (1930's) house to add two double
sockets to a room. I've got the floorboards up and know where the new
cable needs to go. However, the thing that's stopping me going ahead is
the fact that the existing cable appears to be ever so slightly thicker
than the 2.5mm^2 that I was going to use. It is also multi(6)-stranded
instead of single-stranded. Is this to be expected with older wiring (I
believe a rewiring was done in the 70's)? There is a 32A MCB at the fuse
box so I thought that 2.5mm^2 would do the job but having seen the
existing wire I now have doubts. I don't want to weaken the installation
by using wire that is too thin.

After a bit of Googling it seems that 4mm^2 cable might be used in a
radial installation but I disconnected the wire between two sockets in
the room and they both still worked - does this allow me to conclude
that I have a ring mains? Also, 4mm^2 cable doesn't seem to be available
from B&Q, etc. - the next size up is 6mm^2.

Maybe the fact that I'm having to ask these questions suggests that I
should get in an electrician?!

Cheers,
Grant

After a bit of Googling it seems that 4mm^2 cable might be used in a
radial installation but I disconnected the wire between two sockets in the
room and they both still worked - does this allow me to conclude that I
have a ring mains? Also, 4mm^2 cable doesn't seem to be available from
B&Q, etc. - the next size up is 6mm^2.

Maybe it was a radial at some point, then was taken back to the CU and made
into a ring?

Are there any markings on the cable?

Sparks...

Sparks wrote:
After a bit of Googling it seems that 4mm^2 cable might be used in a
radial installation but I disconnected the wire between two sockets in the
room and they both still worked - does this allow me to conclude that I
have a ring mains? Also, 4mm^2 cable doesn't seem to be available from
B&Q, etc. - the next size up is 6mm^2.


Maybe it was a radial at some point, then was taken back to the CU and made
into a ring?

Are there any markings on the cable?

Sparks...



No markings on the cable. What I have noticed is that some of the
existing cable doesn't appear to be copper - it is a silvery colour.

Cheers,
Grant

Grant wrote:
Sparks wrote:
After a bit of Googling it seems that 4mm^2 cable might be used in a
radial installation but I disconnected the wire between two sockets
in the room and they both still worked - does this allow me to
conclude that I have a ring mains? Also, 4mm^2 cable doesn't seem to
be available from B&Q, etc. - the next size up is 6mm^2.

Maybe it was a radial at some point, then was taken back to the CU and
made into a ring?

Are there any markings on the cable?

.... and can you see both ends of the ring coming back to the C.U.?


No markings on the cable. What I have noticed is that some of the
existing cable doesn't appear to be copper - it is a silvery colour.

It's tinned copper.

In an earlier contribution to this discussion,
Grant wrote:

Hi,

I want to extend the ring mains in my (1930's) house to add two double
sockets to a room. I've got the floorboards up and know where the new
cable needs to go. However, the thing that's stopping me going ahead
is the fact that the existing cable appears to be ever so slightly
thicker than the 2.5mm^2 that I was going to use. It is also
multi(6)-stranded instead of single-stranded. Is this to be expected
with older wiring (I believe a rewiring was done in the 70's)? There
is a 32A MCB at the fuse box so I thought that 2.5mm^2 would do the
job but having seen the existing wire I now have doubts. I don't want
to weaken the installation by using wire that is too thin.

After a bit of Googling it seems that 4mm^2 cable might be used in a
radial installation but I disconnected the wire between two sockets in
the room and they both still worked - does this allow me to conclude
that I have a ring mains? Also, 4mm^2 cable doesn't seem to be
available from B&Q, etc. - the next size up is 6mm^2.

Maybe the fact that I'm having to ask these questions suggests that I
should get in an electrician?!

Cheers,
Grant

2.5mm^2 is fine for a ring main as long as you *insert* it into the ring so
as to still have a continuous ring - rather than creating spurs. My house
has a mixture of older (stranded) cable and new solid stuff. Doesn't seem to
be a problem!

There is a limit (can't remember what it is!) to the permitted number of
outlets on a single ring - but you probably won't exceed that unless the
ring has already been extended.

My understanding is that any such work now comes under the provisions of
Part P of the Building Regs - and that, legally, the work needs to be
certified by a qualified electrician - or by Building Control if they are
capable! Many competent DIY-ers choose to ignore this - particularly if
there's no obvious evidence as to *when* the work was carried out.

But if you're not confident *and* competent, get an electrician.
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.

No markings on the cable. What I have noticed is that some of the
existing cable doesn't appear to be copper - it is a silvery colour.

Probably tinned standed cable, as used yonks ago. You can probably use the
2.5mm cable as you wish. However, you should be doing a full circuit test
afterwards, which will ascertain that the circuit is a complete ring.

If it really is a radial (which is unlikely), your choices are to use bigger
cable, or drop the MCB to 20A, which is no hardship if the circuit just
covers a few reception or bedrooms. Don't even think about it for a kitchen,
though.

However, if you're unlucky the test may show that the old cable isn't up to
the earth loop impedence test. In that case, you probably must drop to 20A,
or even rewire.

You'll need an electrician to do the testing. It is probably well beyond
your experience, and the test equipment costs many hundreds of pounds.

Christian.

Set Square wrote:
My understanding is that any such work now comes under the provisions of
Part P of the Building Regs - and that, legally, the work needs to be
certified by a qualified electrician - or by Building Control if they are
capable! Many competent DIY-ers choose to ignore this - particularly if
there's no obvious evidence as to *when* the work was carried out.

Effectively, it doesn't matter when it was done, as long as there
isn't an exact record existing of what's there.

Chris Bacon wrote:
Set Square wrote:

My understanding is that any such work now comes under the provisions of
Part P of the Building Regs - and that, legally, the work needs to be
certified by a qualified electrician - or by Building Control if they are
capable! Many competent DIY-ers choose to ignore this - particularly if
there's no obvious evidence as to *when* the work was carried out.


Effectively, it doesn't matter when it was done, as long as there
isn't an exact record existing of what's there.

I'm in Scotland - I think the regs are different here aren't they? Even
stricter?

Cheers,
Grant

Christian McArdle wrote:
No markings on the cable. What I have noticed is that some of the
existing cable doesn't appear to be copper - it is a silvery colour.


Probably tinned standed cable, as used yonks ago. You can probably use the
2.5mm cable as you wish. However, you should be doing a full circuit test
afterwards, which will ascertain that the circuit is a complete ring.

If it really is a radial (which is unlikely), your choices are to use bigger
cable, or drop the MCB to 20A, which is no hardship if the circuit just
covers a few reception or bedrooms. Don't even think about it for a kitchen,
though.

However, if you're unlucky the test may show that the old cable isn't up to
the earth loop impedence test. In that case, you probably must drop to 20A,
or even rewire.

You'll need an electrician to do the testing. It is probably well beyond
your experience, and the test equipment costs many hundreds of pounds.

Christian.



Many thanks for your reply. I think it's time for a professional!

Cheers,
Grant

Chris Bacon wrote:
Grant wrote:

Sparks wrote:

After a bit of Googling it seems that 4mm^2 cable might be used in a
radial installation but I disconnected the wire between two sockets
in the room and they both still worked - does this allow me to
conclude that I have a ring mains? Also, 4mm^2 cable doesn't seem to
be available from B&Q, etc. - the next size up is 6mm^2.


Maybe it was a radial at some point, then was taken back to the CU
and made into a ring?

Are there any markings on the cable?


... and can you see both ends of the ring coming back to the C.U.?


I don't really know what I'm looking for at the CU so I think, based on
other advice in this thread, I should really be getting in a qualified
electrician.

Cheers,
Grant

Grant wrote:
Hi,

I want to extend the ring mains in my (1930's) house to add two double
sockets to a room. I've got the floorboards up and know where the new
cable needs to go. However, the thing that's stopping me going ahead is
the fact that the existing cable appears to be ever so slightly thicker
than the 2.5mm^2 that I was going to use. It is also multi(6)-stranded
instead of single-stranded. Is this to be expected with older wiring (I
believe a rewiring was done in the 70's)? There is a 32A MCB at the fuse
box so I thought that 2.5mm^2 would do the job but having seen the
existing wire I now have doubts. I don't want to weaken the installation
by using wire that is too thin.

After a bit of Googling it seems that 4mm^2 cable might be used in a
radial installation but I disconnected the wire between two sockets in
the room and they both still worked - does this allow me to conclude
that I have a ring mains? Also, 4mm^2 cable doesn't seem to be available
from B&Q, etc. - the next size up is 6mm^2.

Maybe the fact that I'm having to ask these questions suggests that I
should get in an electrician?!

Cheers,
Grant

Grant,
if it has 7 cores then it may be the old 7/029 standard. 7 cores,
0.29 something, and more tricky to work with than 2.5mm. 2.5mm came in
about the late 60s/ early 70s iirc. Yes it looks like a ring, yes I'd
just extend it with 2.5mm.
Phil

P.R.Brady wrote:

Grant,
if it has 7 cores then it may be the old 7/029 standard. 7 cores,
0.29 something, and more tricky to work with than 2.5mm. 2.5mm came in
about the late 60s/ early 70s iirc. Yes it looks like a ring, yes I'd
just extend it with 2.5mm.
Phil


I'm pretty sure that it has six cores, but I'll check when I get home
tonight. However, based on other advice in the thread regarding full
circuit & earth loop impedence tests I think I will hand this job over
to a professional.

Cheers,
Grant

Christian McArdle wrote:
If it really is a radial (which is unlikely), your choices are to use bigger
cable, or drop the MCB to 20A, which is no hardship if the circuit just
covers a few reception or bedrooms. Don't even think about it for a kitchen,
though.

Flipping the 32A MCB off shows that all sockets in the house (dining
room, sitting room, hall, 3 bedrooms & kitchen) are all on the same circuit.

Cheers,
Grant

Flipping the 32A MCB off shows that all sockets in the house (dining
room, sitting room, hall, 3 bedrooms & kitchen) are all on the same
circuit.

Ouch. You should really look into splitting the kitchen off it, unless it is
a tiny kitchenette.

Christian.

I'm pretty sure that it has six cores, but I'll check when I get home
tonight. However, based on other advice in the thread regarding full
circuit & earth loop impedence tests I think I will hand this job over
to a professional.

Note that there is no harm in running the cables before they come. That's
the long, expensive laborious bit, and I would say that there was an 80%
chance of the cabling being useful.

You only need the electrician for the skilled bit involving expensive
equipment.

Christian.

Christian McArdle wrote:
Flipping the 32A MCB off shows that all sockets in the house (dining
room, sitting room, hall, 3 bedrooms & kitchen) are all on the same

circuit.

Ouch. You should really look into splitting the kitchen off it, unless it is
a tiny kitchenette.

Christian.



No, it's a "proper" kitchen. :-(

Grant

Christian McArdle wrote:
I'm pretty sure that it has six cores, but I'll check when I get home
tonight. However, based on other advice in the thread regarding full
circuit & earth loop impedence tests I think I will hand this job over
to a professional.


Note that there is no harm in running the cables before they come. That's
the long, expensive laborious bit, and I would say that there was an 80%
chance of the cabling being useful.

You only need the electrician for the skilled bit involving expensive
equipment.

Christian.



Yes, I was planning on doing that. I am now having nighmares about my
house being ripped apart for a rewire (especially after having had the
floors done). Not enough forward planning I suppose :-(

Out of interest, maybe you could enlighten me on what a failed earth
loop impedence test would signify and what dangers does it put me at
risk from?

Cheers,
Grant

Out of interest, maybe you could enlighten me on what a failed earth
loop impedence test would signify and what dangers does it put me at
risk from?

When you have an earth to live fault, in the worst case, the current has to
flow from your incoming supply, through the consumer unit, along the circuit
cables to the furthest point. It then has to flow back down the earth
conductor back to your incoming supply. The incoming supply will also have
some resistance along its live and earth connection.

The earth loop impedence is the total of all the resistances of the path
that the current has to flow down.

Now, a socket circuit must disconnect rapidly in the event of a fault. Using
MCBs, to trip quickly enough, you must be able to guarantee that the current
flowing will be 5 times the circuit rating. Therefore, a 32A circuit needs
160A to reliably flow. That requires a total earth loop impedence of 230/160
= 1.44 ohms, some of which will already come from the supply.

Now, the old cables you have found, I believe, had a smaller earth conductor
than modern cables. The horrendously large ring, including your entire house
has more of it, too. It is quite possible that the resistance of the cable
will be too much, either as is, or with your proposed extension.

The danger of not fixing this potential problem is that in the event of an
earth fault, it could take tens of seconds to disconnect the power, which is
a fire and electrocution risk.

In reality, such circuits are normally RCD protected, so provided the RCD is
present and working, then the danger is not real, although it is not
permitted to design circuits that rely on this, with the exception of TT
earthed (earth rod) systems, where it is simply not possible to comply.

Christian.

Christian McArdle wrote:
Out of interest, maybe you could enlighten me on what a failed earth
loop impedence test would signify and what dangers does it put me at
risk from?


When you have an earth to live fault, in the worst case, the current has to
flow from your incoming supply, through the consumer unit, along the circuit
cables to the furthest point. It then has to flow back down the earth
conductor back to your incoming supply. The incoming supply will also have
some resistance along its live and earth connection.

The earth loop impedence is the total of all the resistances of the path
that the current has to flow down.

Now, a socket circuit must disconnect rapidly in the event of a fault. Using
MCBs, to trip quickly enough, you must be able to guarantee that the current
flowing will be 5 times the circuit rating. Therefore, a 32A circuit needs
160A to reliably flow. That requires a total earth loop impedence of 230/160
= 1.44 ohms, some of which will already come from the supply.

Now, the old cables you have found, I believe, had a smaller earth conductor
than modern cables. The horrendously large ring, including your entire house
has more of it, too. It is quite possible that the resistance of the cable
will be too much, either as is, or with your proposed extension.

The danger of not fixing this potential problem is that in the event of an
earth fault, it could take tens of seconds to disconnect the power, which is
a fire and electrocution risk.

In reality, such circuits are normally RCD protected, so provided the RCD is
present and working, then the danger is not real, although it is not
permitted to design circuits that rely on this, with the exception of TT
earthed (earth rod) systems, where it is simply not possible to comply.

Christian.



Christian, thanks very much for that explanation. The 1.44 ohms figure
has jogged my memory - last year an engineer came to fix my washing
machine and he carried out a test on the circuit. He told me that the
desired figure for a sucessful test was 1.44 ohms but the figure he got
from my circuit was 3-point-something (I have a note of it at home). he
told me not to worry because some old houses have values above 30 ohms!
So it looks like that circuit is already dangerous and I need a rewire.

Thanks again,
Grant

Christian, thanks very much for that explanation. The 1.44 ohms figure
has jogged my memory - last year an engineer came to fix my washing
machine and he carried out a test on the circuit. He told me that the
desired figure for a sucessful test was 1.44 ohms but the figure he got
from my circuit was 3-point-something (I have a note of it at home). he
told me not to worry because some old houses have values above 30 ohms!
So it looks like that circuit is already dangerous and I need a rewire.

Make sure the sockets are RCD protected. Although the circuit still won't be
up to scratch and would still fail an inspection, this removes the main
potential danger.

Christian.

In an earlier contribution to this discussion,
Chris Bacon wrote:

Set Square wrote:
My understanding is that any such work now comes under the
provisions of Part P of the Building Regs - and that, legally, the
work needs to be certified by a qualified electrician - or by
Building Control if they are capable! Many competent DIY-ers choose
to ignore this - particularly if there's no obvious evidence as to
*when* the work was carried out.

Effectively, it doesn't matter when it was done, as long as there
isn't an exact record existing of what's there.

Well, on the basis of complying with the 11th commandment "Be thou not found
out" - it's as well to make sure that there's no incriminating evidence
which could prove that the work was done *after* the introduction of Part P.
This could be important if and when you want to sell the house.

For example, if the electrics are in an extension which was built this year,
they *couldn't* have been done before Part P.
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.

"Christian McArdle" wrote in message
. net...
Out of interest, maybe you could enlighten me on what a failed earth
loop impedence test would signify and what dangers does it put me at
risk from?

When you have an earth to live fault, in the worst case, the current has
to
flow from your incoming supply, through the consumer unit, along the
circuit
cables to the furthest point. It then has to flow back down the earth
conductor back to your incoming supply. The incoming supply will also have
some resistance along its live and earth connection.

The earth loop impedence is the total of all the resistances of the path
that the current has to flow down.

Now, a socket circuit must disconnect rapidly in the event of a fault.
Using
MCBs, to trip quickly enough, you must be able to guarantee that the
current
flowing will be 5 times the circuit rating. Therefore, a 32A circuit needs
160A to reliably flow. That requires a total earth loop impedence of
230/160
= 1.44 ohms, some of which will already come from the supply.

Now, the old cables you have found, I believe, had a smaller earth
conductor
than modern cables. The horrendously large ring, including your entire
house
has more of it, too. It is quite possible that the resistance of the cable
will be too much, either as is, or with your proposed extension.

The danger of not fixing this potential problem is that in the event of an
earth fault, it could take tens of seconds to disconnect the power, which
is
a fire and electrocution risk.

In reality, such circuits are normally RCD protected, so provided the RCD
is
present and working, then the danger is not real, although it is not
permitted to design circuits that rely on this, with the exception of TT
earthed (earth rod) systems, where it is simply not possible to comply.

Christian.

Thanks, Christian- that's probably the clearest explanation of a fairly
complicated (to a layman) subject I've ever seen.
If only my teachers at school could have been like you.........

For example, if the electrics are in an extension which was built this
year,
they *couldn't* have been done before Part P.

And date stamps on cable etc. are becoming more common.

Christian.

Set Square wrote:
Chris Bacon wrote:
Set Square wrote:
My understanding is that any such work now comes under the
provisions of Part P of the Building Regs - and that, legally, the
work needs to be certified by a qualified electrician - or by
Building Control if they are capable! Many competent DIY-ers choose
to ignore this - particularly if there's no obvious evidence as to
*when* the work was carried out.

Effectively, it doesn't matter when it was done, as long as there
isn't an exact record existing of what's there.

Well, on the basis of complying with the 11th commandment "Be thou not found
out" - it's as well to make sure that there's no incriminating evidence
which could prove that the work was done *after* the introduction of Part P.
This could be important if and when you want to sell the house.

For example, if the electrics are in an extension which was built this year,
they *couldn't* have been done before Part P.

There's the existing record, then - however, I shall be very interested
if anyone knows what information is collected - is it simply "electrics
passed", or is it "electrics passed, CU (type xxx), socket on wall X, Y,
Z; lighting to points a,b,c (type xxx) (etc., etc.)". What is recorded?

"Christian McArdle" wrote in message
. net...

Make sure the sockets are RCD protected. Although the circuit still won't
be
up to scratch and would still fail an inspection, this removes the main
potential danger.

But if it's RCD protected, don't you just need to meet ZsIn = 50V? (i.e.
1667ohm for 30mA)

To me, the regs are clear that it's *preferable* to design for TN systems to
disconnect on overcurrent rather than RCD, but seem to allow the use of
RCD's instead. (413-02-04 (ii) and 413-02-16).

What am I overlooking?

Will

There's the existing record, then - however, I shall be very interested
if anyone knows what information is collected - is it simply "electrics
passed", or is it "electrics passed, CU (type xxx), socket on wall X, Y,
Z; lighting to points a,b,c (type xxx) (etc., etc.)". What is recorded?

Bugger all. Your chance of being detected is between zero and nil.

The only real chance of problems is if you sell the house and lie on the
form, with would be regarded as fraud and could end you in prison. Your new
house owner will be far more motivated at picking up infringements and
evidence of new work than a BCO who is more interested in his next job where
he gets to say how deep the foundations go.

Christian.

Christian McArdle wrote:
For example, if the electrics are in an extension which was built
this year, they *couldn't* have been done before Part P.

And date stamps on cable etc. are becoming more common.

Readers will have to refer to previous messages in this thread
for context. Why not quote a bit more?

Anyway:

Even if date stamps are on *everything*, then what does it
matter if there's no previous record of what was installed?

But if it's RCD protected, don't you just need to meet ZsIn = 50V? (i.e.
1667ohm for 30mA)

Technically. Although they also specify a 200 ohm maximum, even this is a
recommendation, I believe.

To me, the regs are clear that it's *preferable* to design for TN systems
to
disconnect on overcurrent rather than RCD, but seem to allow the use of
RCD's instead. (413-02-04 (ii) and 413-02-16).

What am I overlooking?

It comes under bad practice. You could certainly attempt to argue the toss,
though, if it came to court or something. As I suggested, just adding an RCD
to the existing system I would regard as sufficiently safe, although much
over 3 ohms and you're starting to look at lighting and fixed circuits, too.
However, I would not expect to see such shoddiness in a new or rewired
installation and would regard such an installation as a cowboy bodge.

Best practice is to always design main equipotential bonding and circuit
cable sizing/resistance for TN-C-S, even on a TT or TN-S system. That way,
the system can just be transferred to TN-C-S when it becomes available,
which is increasingly common, even on overhead lines.

Christian.

For example, if the electrics are in an extension which was built
this year, they *couldn't* have been done before Part P.

And date stamps on cable etc. are becoming more common.

Readers will have to refer to previous messages in this thread
for context. Why not quote a bit more?

There was enough context in what I quoted to get the gist. I don't like to
quote more than necessary. It wastes the reader's time.

Even if date stamps are on *everything*, then what does it
matter if there's no previous record of what was installed?

The fact that if the date stamps are post Part-P, there should be
documentation proving that it was tested. You don't need prior documentation
to see what was there before if your cables are stamped 2006, so must have
been installed after the rule change.

Christian.

Christian McArdle wrote:
The only real chance of problems is if you sell the house and lie on the
form

Or potentially if somebody got injured or worse as a result of the work
and an investigation highlighted that the work was done illegally.

Christian McArdle wrote:
Christian, thanks very much for that explanation. The 1.44 ohms figure
has jogged my memory - last year an engineer came to fix my washing
machine and he carried out a test on the circuit. He told me that the
desired figure for a sucessful test was 1.44 ohms but the figure he got
from my circuit was 3-point-something (I have a note of it at home). he
told me not to worry because some old houses have values above 30 ohms!
So it looks like that circuit is already dangerous and I need a rewire.


Make sure the sockets are RCD protected. Although the circuit still won't be
up to scratch and would still fail an inspection, this removes the main
potential danger.

Christian.



Lots of valuable advice so far - thanks everyone.

So, I will protect the sockets with an RCD until I get the rewire done.
Currently there is a 32A MCB for the sockets. Do I simply need to
replace this MCB with an appropriate RCD or is the RCD in addition to
the MCB? Do RCDs have the same kinds of ratings? In short, do I buy a
32A RCD and put it where the MCB currently is? Excuse my ignorance!

Cheers,
Grant

Grant wrote:
Christian McArdle wrote:

Christian, thanks very much for that explanation. The 1.44 ohms figure
has jogged my memory - last year an engineer came to fix my washing
machine and he carried out a test on the circuit. He told me that the
desired figure for a sucessful test was 1.44 ohms but the figure he got
from my circuit was 3-point-something (I have a note of it at home). he
told me not to worry because some old houses have values above 30 ohms!
So it looks like that circuit is already dangerous and I need a rewire.



Make sure the sockets are RCD protected. Although the circuit still
won't be
up to scratch and would still fail an inspection, this removes the main
potential danger.

Christian.



Lots of valuable advice so far - thanks everyone.

So, I will protect the sockets with an RCD until I get the rewire done.
Currently there is a 32A MCB for the sockets. Do I simply need to
replace this MCB with an appropriate RCD or is the RCD in addition to
the MCB? Do RCDs have the same kinds of ratings? In short, do I buy a
32A RCD and put it where the MCB currently is? Excuse my ignorance!

Cheers,
Grant
Its not quite as simple as that as MCBs are single pole on the live side
of the circuit whereas RCDs are double pole so work on both the live and
the neutral side. It may be possible, depending on your consumer unit
to replace the MCB with an RCBO - which is a combined RCD and MCB in one
unit that comes with the appropriate wire/teminals to connect in the
neutral as well.

Christian McArdle wrote:
Even if date stamps are on *everything*, then what does it
matter if there's no previous record of what was installed?

The fact that if the date stamps are post Part-P, there should be
documentation proving that it was tested. You don't need prior documentation
to see what was there before if your cables are stamped 2006, so must have
been installed after the rule change.

Not so - nothing is recorded, as you say - so even if the cable
and fittings are date marked, it doesn't matter, as replacement
of fittings and damaged wiring is allowed without notification.

So, I will protect the sockets with an RCD until I get the rewire done.
Currently there is a 32A MCB for the sockets. Do I simply need to
replace this MCB with an appropriate RCD or is the RCD in addition to
the MCB?

If it is an RCD, then it is in addition to the MCB. You could put it in a
separate box after the MCB to just protect the socket circuit.

However, you can get an RCD that replaces an MCB. It is called an RCBO.
However, you may find it difficult to find a suitable device for some older
boards. If you have a DIN rail system, then you should be able to find a
suitable single width RCBO. These are a direct replacement.

Note, that when you get the circuit sorted, you should probably install an
extra few circuits in the kitchen. It is usual these days to have a separate
kitchen ring that is RCD protected and to have one or more circuits for
fixed appliances (washing machines/freezers/fridges/tumble
dryers/dishwashers) that is not RCD protected, so that your frozen sausages
are still frozen when you get back from holiday.

Christian.

"Christian McArdle" wrote in message
. net...

It comes under bad practice. You could certainly attempt to argue the
toss,
though, if it came to court or something. As I suggested, just adding an
RCD
to the existing system I would regard as sufficiently safe, although much
over 3 ohms and you're starting to look at lighting and fixed circuits,
too.
However, I would not expect to see such shoddiness in a new or rewired
installation and would regard such an installation as a cowboy bodge.

But that implies that 413-02-16 is just there for the use of cowboys.

I absolutely agree that one wouldn't design a new circuit like this, but I
can't see how it would fail a periodic inspection. It's in compliance with
the regs and the GN3 guidance.

Of course, it might be a symptom of more serious problems, which *would* be
failures.

Best practice is to always design main equipotential bonding and circuit
cable sizing/resistance for TN-C-S, even on a TT or TN-S system. That way,
the system can just be transferred to TN-C-S when it becomes available,
which is increasingly common, even on overhead lines.

Well, it could be anyway, as long as the RCD wasn't removed.

I'm really not arguing with you that it would be a good idea to look at
what's really happening here, rather than sticking an RCD in and forgetting
about it - I'm just always interested in what really is compliant and what
isn't.

Will

Not so - nothing is recorded, as you say - so even if the cable
and fittings are date marked, it doesn't matter, as replacement
of fittings and damaged wiring is allowed without notification.

It would be laughed out of court if you attempted to suggest that your
entire new installation had needed every single fitting and cable replaced
one at a time.

Christian.

I absolutely agree that one wouldn't design a new circuit like this, but I
can't see how it would fail a periodic inspection. It's in compliance
with
the regs and the GN3 guidance.

I'd expect to see it as an advisory, provided there was appropriate RCD
support and a plastic consumer unit.

But that implies that 413-02-16 is just there for the use of cowboys.

I suspect it is written like that so that the OP can continue to use his
system (with an RCD) without having the power disconnected by overzealous
contractors/maintenance people. It really isn't that unsafe, provided that
the circuit conductors passed their continuity tests with the expected
numbers.

Christian.

Christian McArdle wrote:
Not so - nothing is recorded, as you say - so even if the cable
and fittings are date marked, it doesn't matter, as replacement
of fittings and damaged wiring is allowed without notification.

It would be laughed out of court if you attempted to suggest that your
entire new installation had needed every single fitting and cable replaced
one at a time.

But in context we're talking about adding a socket to a ring,
which may be exempted anyway!

Christian McArdle wrote:
So, I will protect the sockets with an RCD until I get the rewire done.
Currently there is a 32A MCB for the sockets. Do I simply need to
replace this MCB with an appropriate RCD or is the RCD in addition to
the MCB?


If it is an RCD, then it is in addition to the MCB. You could put it in a
separate box after the MCB to just protect the socket circuit.

However, you can get an RCD that replaces an MCB. It is called an RCBO.
However, you may find it difficult to find a suitable device for some older
boards. If you have a DIN rail system, then you should be able to find a
suitable single width RCBO. These are a direct replacement.

I don't know if I have a DIN rail system or not. I think I'll call an
electrician ASAP and get things sorted once and for all. I am not
willing to take any risks whatsoever!

Note, that when you get the circuit sorted, you should probably install an
extra few circuits in the kitchen. It is usual these days to have a separate
kitchen ring that is RCD protected and to have one or more circuits for
fixed appliances (washing machines/freezers/fridges/tumble
dryers/dishwashers) that is not RCD protected, so that your frozen sausages
are still frozen when you get back from holiday.

Christian.



Christian, I can't thank you enough for your excellent advice. I'll
certainly bear this in mind when discussing my requirements with the
electrician.

Incidentally, how disruptive is a rewire likely to be? The two public
rooms and hallway have nicely finished floors and I would prefer not to
have them damaged. I know it will depend on my installation but will an
electrician be able to pull through most of the new wiring using the
existing stuff? None of the socket cabling is plastered in - the sockets
are either behind skirting or fed via metal conduits.

What about cost? £1000-ish?

Cheers,
Grant

P.S. Anyone know a good electrician in Edinburgh?

"Grant" wrote in message
...
Hi,

I want to extend the ring mains in my (1930's) house to add two double
sockets to a room. I've got the floorboards up and know where the new
cable needs to go. However, the thing that's stopping me going ahead is
the fact that the existing cable appears to be ever so slightly thicker
than the 2.5mm^2 that I was going to use. It is also multi(6)-stranded
instead of single-stranded. Is this to be expected with older wiring (I
believe a rewiring was done in the 70's)? There is a 32A MCB at the fuse
box so I thought that 2.5mm^2 would do the job but having seen the
existing wire I now have doubts. I don't want to weaken the installation
by using wire that is too thin.

After a bit of Googling it seems that 4mm^2 cable might be used in a
radial installation but I disconnected the wire between two sockets in the
room and they both still worked - does this allow me to conclude that I
have a ring mains? Also, 4mm^2 cable doesn't seem to be available from
B&Q, etc. - the next size up is 6mm^2.

Maybe the fact that I'm having to ask these questions suggests that I
should get in an electrician?!


It sounds like 7/.029 cable which was the forerunner of 2.5 solid conductor
and ws usually tinned copper conductors.
A simple solution of just using 2.5mm T&E to extend it is possible but
ignores the ramifications of cicuit protective conductor sizing (earth
conductor) and fault currents. However I suspect most of the run of the mill
sparkys might do just that.
4mm cable is readily available from half decent suppliers and some
wholesalers might even be willing to cut lengths for you.
For a fuller answer we would need to know what earthing arrangements apply
to your installation and an idea of possible cable lengths involved. You
have already advised us that you have a 32A MCB but can you tell us if it a
type B or otherwise? If the protection is problematic it might be possible
to use a 32A RCBO but we need answers first