I participated in (but didn't initiate) the thread back in June - from which
I learned quite a lot - but have a few more questions.

My detached double garage is currently being built, and is about 1.1 metres
from the house. I intend to use a garage consumer unit - probably
http://tinyurl.com/zmmh4, which I intend to connect to a spare way in my
domestic CU. This
currently has a 6mm^2 T&E cable connected to it, which was originally
intended for an electric shower - but we used stored water and a shower pump
instead - so this way and cable (with a suitable MCB) can be re-deployed.

The 6mm^2 cable ends (with a couple of feet spare) just above the ceiling of
an en-suite bathroom - more or less vertically
above the point where the house end of the garage connection needs to be. I
intend to join some 6mm^2 SWA cable to this
cable, and take it through the wall and down the outside of the house, and
across a passage door frame into the garage.

Although the garage consumer unit can handle 40 amps, this is more than I
need (and the sum of the individual MCBs is a lot less than this anyway) so
I intend to use a 32 amp Type-B MCB at the house end of the connection -
which seems more
appropriate than a higher value if relying on the 2.5mm^2 earth within the
existing T&E cable.

My questions concern earthing arrangements, and physically joining the
cables together.

From my description of my incoming mains back in June, Andy Wade thought
that I have a TN-S system, and David Hansen thought it was more likely
TN-C-S. In case it helps, I've posted a photo of the main fuse and meter
connection at
http://www.mills37.plus.com/Incoming_mains.JPG Just to remind you, I
estimate that I have about 10 metres of T&E, and will need about 6 metres of
SWA.

The options for earthing appear to be as follows:

* Use 3-core SWA cable, and use one core (6 mm^2) as the earth - and join it
onto the end of the T&E's 2.5mm^2 protective earth conductor - thus
exporting the earth from the domestic CU

* Use 2-core SWA, and use the steel armour as an earth, again joined to the
T&E's earth conductor - again exporting the
earth. [There will be no external connections to be subjected to 'weather',
so corrosion shouldn't be a problem]

* Use 2-core SWA and an earth spike at the garage end - thus providing the
garage with its own earth - independent of the house supply

* Some combination of the above. [Is there a down side to exporting the
earth *and* having an earth spike?]

I would welcome constructive comments on the relative merits of each of the
above options - plus identification of any viable options I may have missed.

Now to joining the cables . .

The most convenient solution would be to have the joint inside a deep
(metal) socket box high on the wall of the en-suite
bathroom - sunk into the blockwork, and covered by a flush blanking plate.
The T&E would come down from above, and be chased into the plaster and the
SWA would go straight out through the wall from the back of the box - with
its gland fitted to a knock-out in the box. Does anyone see any problems
with this? How should I physically join the cables? Is a large chocolate
block connector (inside the box) ok - or is there anything purpose-made
which would be better? Do I need to earth the SWA's armour even if it isn't
being used as the protective earth?

Your expert advice will be much appreciated.

Oh, and in case anyone mentions Part P - I think that what I am doing is
probably covered by Part P. AIUI, extending an existing radial circuit
isn't, but when you start taking it outside, it is. The building itself is
exempt from building regs, being a detached garage of less than 30 M^2 floor
area, and meeting the other exemption conditions. My inclination therefore
is to forget about Part P and hope that it gets lost in the noise when the
property is eventually sold (hopefully far into the future). Nevertheless, I
want to ensure that I end up with a 'safe' electrical installation - hence
the questions.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored..
Messages sent to it may not be read for several weeks. PLEASE REPLY TO
NEWSGROUP!

Roger Mills wrote:

My detached double garage is currently being built, and is about 1.1 metres
from the house. I intend to use a garage consumer unit - probably
http://tinyurl.com/zmmh4, which I intend to connect to a spare way in my
domestic CU. This

It looks rather like the RCD covers both ways on that. Depending on how
you plan to use the garage you may wish to opt for some arrangement that
will not lose the lights at the same time you lop through a power cord
with a still spinning circular saw! (i.e. switched two way CU with a
RCBO for the sockets, or perhaps the one pictured with a non maintained
emergency light)

Although the garage consumer unit can handle 40 amps, this is more than I
need (and the sum of the individual MCBs is a lot less than this anyway) so
I intend to use a 32 amp Type-B MCB at the house end of the connection -
which seems more
appropriate than a higher value if relying on the 2.5mm^2 earth within the
existing T&E cable.

On the same theme as above you may need to bear in mind is
descrimination of the protective devices. You want to try and ensure
that a trip on the power circuit in the garrage only takes out the local
MCB and not the upstream one in the main CU as well hence losing your
lights.

My questions concern earthing arrangements, and physically joining the
cables together.

From my description of my incoming mains back in June, Andy Wade thought
that I have a TN-S system, and David Hansen thought it was more likely
TN-C-S. In case it helps, I've posted a photo of the main fuse and meter
connection at
http://www.mills37.plus.com/Incoming_mains.JPG Just to remind you, I
estimate that I have about 10 metres of T&E, and will need about 6 metres of
SWA.

That is a different cable head end from any I have seen in use in this
area, so can't give you a definitive answer. However the connection of
the (undersized!) earth connector to the side of the main cutout like
that would suggest TN-C-S.

Where does that other earth wire go that drops down from the CU? What is
that little box at the bottom of the frame?

The options for earthing appear to be as follows:

* Use 3-core SWA cable, and use one core (6 mm^2) as the earth - and join it
onto the end of the T&E's 2.5mm^2 protective earth conductor - thus
exporting the earth from the domestic CU

This is slightly more reliable that the next option in that there is
less chance of the earth becoming poor due to corrosian etc at the gland
- but as you later say these are internal connections and a new garage
is unlikly to be damp so there is not much in it. If doing this I would
tend to use one core *and* the screen.

* Use 2-core SWA, and use the steel armour as an earth, again joined to the
T&E's earth conductor - again exporting the
earth. [There will be no external connections to be subjected to 'weather',
so corrosion shouldn't be a problem]

One slight advantage with this one is the cable will be a little thinner
and easier to handle.

* Use 2-core SWA and an earth spike at the garage end - thus providing the
garage with its own earth - independent of the house supply

If the house end is not already TT then the arguments for a separate TT
supply in an outbuilding usually come down two one of two reasons -
either it is a long distance (obviuosly not the case here), or, when
there will be potential difficulty extending the house's equipotetial
zone into the outbuilding (of most concern with a PME house).

* Some combination of the above. [Is there a down side to exporting the
earth *and* having an earth spike?]

I would welcome constructive comments on the relative merits of each of the
above options - plus identification of any viable options I may have missed.

Now to joining the cables . .

The most convenient solution would be to have the joint inside a deep
(metal) socket box high on the wall of the en-suite
bathroom - sunk into the blockwork, and covered by a flush blanking plate.
The T&E would come down from above, and be chased into the plaster and the

If coming from above, could you not have an adaptable metal box in the
loft space instead?

SWA would go straight out through the wall from the back of the box - with
its gland fitted to a knock-out in the box. Does anyone see any problems
with this? How should I physically join the cables? Is a large chocolate
block connector (inside the box) ok - or is there anything purpose-made
which would be better?

You could use crimps, but since it will remain accessable screw
connectors are ok. If you opt for using the screen as a CPC I would be
reluctant to have the gland connection burried in a wall behind a metal
box.

Do I need to earth the SWA's armour even if it isn't
being used as the protective earth?

It is good practice since it offers better protection to the cable
(especially since you don't currently appear to have a RCD at the head
end). If you were using TT in the garage then you can earth it at the
head end, but leave the other end disconnected (would require a
insualted CU in the garage)

Your expert advice will be much appreciated.

As usual it is worth what you paid! ;-)

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Thu, 24 Aug 2006 00:44:13 +0100 someone who may be John Rumm
wrote this:-

I've posted a photo of the main fuse and meter
connection at
http://www.mills37.plus.com/Incoming_mains.JPG

That is a different cable head end from any I have seen in use in this
area, so can't give you a definitive answer. However the connection of
the (undersized!) earth connector to the side of the main cutout like
that would suggest TN-C-S.

Agreed.

If you opt for using the screen as a CPC I would be
reluctant to have the gland connection burried in a wall behind a metal
box.

Agreed. An excellent way to introduce corrosion into the CPC. In
addition, although I don't share the panic about using steel as the
CPC I think it must be easily accessible for inspection. Provided
the extra lengths are acceptable I too would look at the loft as the
place to make the joint.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

John Rumm wrote:

It looks rather like the RCD covers both ways on that. Depending on how
you plan to use the garage you may wish to opt for some arrangement that
will not lose the lights at the same time you lop through a power cord
with a still spinning circular saw! (i.e. switched two way CU with a
RCBO for the sockets, or perhaps the one pictured with a non maintained
emergency light)

Yes, for TT a main switch and two RCBOs would be a good option, or main
switch with one MCB (lights) and one RCBO (power) for TN. I'd be
inclined to make the 'power' RCBO a 20 A one - RCBOs only come as Type B
and 16 A Type B might tend to trip on motor starting currents, or the
inrush current of a 230 - 115 V tool transformer. (Alternatively keep
the 16 A circuit for sockets only and provide separate motor circuits
for any machines.) You can buy all the MK consumer unit parts
separately from a wholesaler, including TLC if you want to buy on-line.

On the same theme as above you may need to bear in mind is
descrimination of the protective devices. You want to try and ensure
that a trip on the power circuit in the garrage only takes out the local
MCB and not the upstream one in the main CU as well hence losing your
lights.

That would probably require the feed to be via a separate switch-fuse
rather than an MCB in the CU - certainly a better arrangement, but does
involve teeing in to the meter tails with a service connector block.

That is a different cable head end from any I have seen in use in this
area, so can't give you a definitive answer. However the connection of
the (undersized!) earth connector to the side of the main cutout like
that would suggest TN-C-S.

Looks like TN-C-S to me, with old earthing and bonding conductors which
are undersized by current standards. These ought to be upgraded if you
intend to use the exported PME earth.

Where does that other earth wire go that drops down from the CU?

One of the main bonds?

What is that little box at the bottom of the frame?

Looks a bit like an old GPO telephone wiring junction box (a 'block
terminal' something-or-other)?

* Use 3-core SWA cable, and use one core (6 mm^2) as the earth - and
join it onto the end of the T&E's 2.5mm^2 protective earth conductor -
thus exporting the earth from the domestic CU

This is slightly more reliable that the next option in that there is
less chance of the earth becoming poor due to corrosian etc at the gland
- but as you later say these are internal connections and a new garage
is unlikly to be damp so there is not much in it. If doing this I would
tend to use one core *and* the screen.

* Use 2-core SWA, and use the steel armour as an earth, again joined
to the T&E's earth conductor - again exporting the
earth. [There will be no external connections to be subjected to
'weather', so corrosion shouldn't be a problem]

One slight advantage with this one is the cable will be a little thinner
and easier to handle.

Using a core as CPC gives you a 6 mm^2 conductor with a resistance of
about 3.7 milliohm/m. Using the armour gives you a CPC equivalent to
9.8 mm^2 copper, but with a resistance of around 7 milliohm/m. I'd use
the 2-core unless the CPC resistance (R2) is a problem in the design.

The weakest link in this design though is the 2.5 mm^2 CPC in the T&E
cable. Note that it would not be acceptable to use this if there are
any metal services rising in the garage that would need main bonding -
this because the CPC of the submain also acts as the main bonding
conductor, so would need to be a minimum of 10 mm^2 Cu equivalent.

* Use 2-core SWA and an earth spike at the garage end - thus providing
the garage with its own earth - independent of the house supply

If the house end is not already TT then the arguments for a separate TT
supply in an outbuilding usually come down two one of two reasons -
either it is a long distance (obviuosly not the case here), or, when
there will be potential difficulty extending the house's equipotetial
zone into the outbuilding (of most concern with a PME house).

As I usually say here, the TT/TN decision depends on the use of the
outbuilding and the practicality of creating an equipotential zone
there. At one extreme is a dry workshop building where there will be
little use of electrical equipment outdoors (TN OK here) and at the
other extreme is the metal greenhouse (TT essential). Any use extensive
of portable Class 1 (earthed) equipment outdoors again signals the need
for TT earthing.

* Some combination of the above. [Is there a down side to exporting
the earth *and* having an earth spike?]

No, and there is an advantage to doing that - but understand that it's
still TN-C-S with its attendant risks. You are adding another earth to
the M (multiple) in PME, so you'll help reduce the touch voltage in the
event of a broken supply neutral. You do need a damn good earth
electrode to do much good though, and it should be connected to the main
earth terminal in the house, not at the garage end via a weedy 2.5 mm^2 CPC.

[...]
SWA would go straight out through the wall from the back of the box -
with its gland fitted to a knock-out in the box. Does anyone see any
problems with this?

That's completely impractical because you've go no access to the back of
the box to tighten the gland nut. It would also be extremely fiddly to
do this with a single-gang accessory box. Better as John said to
connect the SWA in the loft, alternatively take the T&E through the wall
to a box on the outside, and run the SWA straight down from there.

[...]
Do I need to earth the SWA's armour even if it isn't being used as
the protective earth?

Yes, definitely, to provide protection in case the cable is damaged.

--
Andy

In an earlier contribution to this discussion,
John Rumm wrote:


On the same theme as above you may need to bear in mind is
descrimination of the protective devices. You want to try and ensure
that a trip on the power circuit in the garrage only takes out the
local MCB and not the upstream one in the main CU as well hence
losing your lights.

Many thanks for your response.

The local MCBs are 16A for power and 6A for lighting - so, under overload
conditions, should trip prior to the 32A one at the house end. Of course, if
I cut through a wire and connect live to earth before live to neutral, the
RCD will trip and I'll lose the lights as well. I guess I have to live with
that.



That is a different cable head end from any I have seen in use in this
area, so can't give you a definitive answer. However the connection of
the (undersized!) earth connector to the side of the main cutout like
that would suggest TN-C-S.

Where does that other earth wire go that drops down from the CU? What
is that little box at the bottom of the frame?

The other earth wire goes into an adjoining cupboard which houses the gas
meter, and is clamped onto a gas pipe. The little box at the bottom is a
junction box for telephone wiring.




If coming from above, could you not have an adaptable metal box in the
loft space instead?

I *could* join it in the loft space - but that bit of loft is pretty
inaccessible, so I was trying to avoid it. What does an adaptable metal box
look like?


You could use crimps, but since it will remain accessable screw
connectors are ok. If you opt for using the screen as a CPC I would be
reluctant to have the gland connection burried in a wall behind a
metal box.

I guess I'd need a half-way decent ratchet crimping tool to use crimps -
which I don't have. I take your point about burying the gland in the wall.


Your expert advice will be much appreciated.

As usual it is worth what you paid! ;-)

If you're selective about what advice you accept, much of the advice given
here is actually worth a lot more than we pay for it!
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

Andy Wade wrote:
John Rumm wrote:

On the same theme as above you may need to bear in mind is
descrimination of the protective devices. You want to try and ensure
that a trip on the power circuit in the garrage only takes out the
local MCB and not the upstream one in the main CU as well hence losing
your lights.


That would probably require the feed to be via a separate switch-fuse
rather than an MCB in the CU - certainly a better arrangement, but does
involve teeing in to the meter tails with a service connector block.

Hager do a MCB sized HRC fuse carrier, which can solve that problem in
many cases.... although having just typed that, I remembered that
Roger's CU ain't a DIN rail jobbie anyway, so this is of little
practical use to him! ;-)


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Roger Mills wrote:

Many thanks for your response.

The local MCBs are 16A for power and 6A for lighting - so, under overload
conditions, should trip prior to the 32A one at the house end. Of course, if
I cut through a wire and connect live to earth before live to neutral, the
RCD will trip and I'll lose the lights as well. I guess I have to live with
that.

By a slight change to the choice of CU though that is easy enough to avoid.

Where does that other earth wire go that drops down from the CU? What
is that little box at the bottom of the frame?


The other earth wire goes into an adjoining cupboard which houses the gas
meter, and is clamped onto a gas pipe.

So no main bond to the water service then?

The little box at the bottom is a
junction box for telephone wiring.

Thought as much, but thought I had better check!

If coming from above, could you not have an adaptable metal box in the
loft space instead?


I *could* join it in the loft space - but that bit of loft is pretty
inaccessible, so I was trying to avoid it. What does an adaptable metal box
look like?

Box looks like:

http://www.tlc-direct.co.uk/Main_Ind...ble/index.html
or
http://www.tlc-direct.co.uk/Main_Ind...ded/index.html

You could use crimps, but since it will remain accessable screw
connectors are ok. If you opt for using the screen as a CPC I would be
reluctant to have the gland connection burried in a wall behind a
metal box.


I guess I'd need a half-way decent ratchet crimping tool to use crimps -
which I don't have. I take your point about burying the gland in the wall.

In which case why not chase down the bathroom wall as described, but
rather than installing a metal box in the bathroom, simply drill
throught the wall at that point (keeping within 6" of the bathroom
ceiling). You can stick some capping over the T&E to facilitate future
replacement if you wish.

Now install your cable and plaster over the chase leaving no visible
accessory in the bathroom. Now visit the other side of the wall where
hopefully you have a bit of T&E dangling, and poke it into the back of
one of the aformentiond boxes (a IP65 one would probably be a good
choice here). Screw it to the wall, and you can then stick a nice neat
SWA gland into the knockout at the base of the box. Choose the right box
ad you may even get one complete with junction terminals. That means
your joint is accessable and serviceable, no extra clutter to tile round
the bathroom, and no groveling in the loft!

Your expert advice will be much appreciated.

As usual it is worth what you paid! ;-)


If you're selective about what advice you accept, much of the advice given
here is actually worth a lot more than we pay for it!

Yup, second that many times over, thanks guys!

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:

Hager do a MCB sized HRC fuse carrier, which can solve that problem in
many cases....

Oops, quite right, I forgot that option.

although having just typed that, I remembered that
Roger's CU ain't a DIN rail jobbie anyway, so this is of little
practical use to him! ;-)

Doh! - his picture tells all, and I guess the new circuit is going in
the empty fuseway on the right (currently 15 A). In this case the
obvious thing to do is to use a 30 A BS 1361 fuse and not another
plug-in MCB.

--
Andy

Although the garage consumer unit can handle 40 amps, this is more than I
need (and the sum of the individual MCBs is a lot less than this anyway)
so I intend to use a 32 amp Type-B MCB at the house end of the
connection - which seems more appropriate than a higher value if relying
on the 2.5mm^2
earth within the existing T&E cable.

Don't be so sure. You want the house end to blow as slowly as possible to
get maximum possible discrimination with the garage CU. I would consider a
Type C MCB, or even an HRC fuse. If 40A is allowed as well, then use that,
too. I haven't done the calculations so can't tell you what you will get
away with. This way, any fault is more likely to be picked up at the local
MCB in the garage.

* Use 3-core SWA cable, and use one core (6 mm^2) as the earth - and join
it onto the end of the T&E's 2.5mm^2 protective earth conductor - thus
exporting the earth from the domestic CU

I'd just do this given you are only 1.1m from the house. This is only
possible if the garage has no main equipotential bonding requirements, or
the main equipotential bonding can actually be achieved.

Remember to use the armour as earth as well, both to get the ELI even lower
and because the armour must be earthed anyway. The reason to have the
additional copper conductor is that copper doesn't rust at the termination
and is more reliable over the decades.

The most convenient solution would be to have the joint inside a deep
(metal) socket box high on the wall of the en-suite
bathroom - sunk into the blockwork, and covered by a flush blanking plate.
The T&E would come down from above, and be chased into the plaster and the
SWA would go straight out through the wall from the back of the box - with
its gland fitted to a knock-out in the box. Does anyone see any problems
with this?

No problem.

How should I physically join the cables? Is a large chocolate block
connector (inside the box) ok - or is there anything purpose-made which
would be better?

Chocolate block or crimp is fine.

Do I need to earth the SWA's armour even if it isn't being used as the
protective earth?

Yes. Either earthed by using a metal box, or using a decent earth ring, not
the ****e one that you get with the gland.

Christian.

In an earlier contribution to this discussion,
Andy Wade wrote:

John Rumm wrote:

Hager do a MCB sized HRC fuse carrier, which can solve that problem
in many cases....

Oops, quite right, I forgot that option.

although having just typed that, I remembered that
Roger's CU ain't a DIN rail jobbie anyway, so this is of little
practical use to him! ;-)

Doh! - his picture tells all, and I guess the new circuit is going in
the empty fuseway on the right (currently 15 A). In this case the
obvious thing to do is to use a 30 A BS 1361 fuse and not another
plug-in MCB.

Yes, the circuit is going at the extreme right - which currently has a blue
base for a 16A MCB.

I may well still have a 30A fuse - not sure about the BS1361 bit though - is
that something special? [The CU started off with fuses, and was converted to
MCBs!]

What is the rationale for using a fuse rather than an MCB? Is it that it
takes longer to blow, making it more likely that the one at the garage end
will trip instead? If so, it makes sense, but it flies in the face of the
advice given in the earlier thread which said that I needed a Type B (and
*not* Type C) MCB on account of only having a 2.5mm^2 earth wire for most of
the length.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

In an earlier contribution to this discussion,
Christian McArdle wrote:


Don't be so sure. You want the house end to blow as slowly as
possible to get maximum possible discrimination with the garage CU. I
would consider a Type C MCB, or even an HRC fuse. If 40A is allowed
as well, then use that, too. I haven't done the calculations so can't
tell you what you will get away with. This way, any fault is more
likely to be picked up at the local MCB in the garage.


Is there an idiot's guide somewhere, defining the calculations which I need
to do?

As mentioned in my post in answer to Andy Wade, I had previously been
advised (back in June) to use a Type B MCB, and to go for 32A rather than
40A in the light of my rather weedy 2.5mm^2 earth wire - so I'm getting a
bit confused!

* Use 3-core SWA cable, and use one core (6 mm^2) as the earth - and
join it onto the end of the T&E's 2.5mm^2 protective earth conductor
- thus exporting the earth from the domestic CU

I'd just do this given you are only 1.1m from the house. This is only
possible if the garage has no main equipotential bonding
requirements, or the main equipotential bonding can actually be
achieved.
It's 1.1 metres as the crow flies - but about 6 metres of SWA down the wall,
across the gap, and into the garage. Does this make any difference - or are
we only looking at the likely difference in earth potential between 2
adjacent buildings?
There will be no 'services' (water, gas, etc) in the garage, and no
equipotential bonding is required AIUI.

--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

In an earlier contribution to this discussion,
John Rumm wrote:


The local MCBs are 16A for power and 6A for lighting - so, under
overload conditions, should trip prior to the 32A one at the house
end. Of course, if I cut through a wire and connect live to earth
before live to neutral, the RCD will trip and I'll lose the lights
as well. I guess I have to live with that.

By a slight change to the choice of CU though that is easy enough to
avoid.
Could you elaborate on that please?


The other earth wire goes into an adjoining cupboard which houses
the gas meter, and is clamped onto a gas pipe.

So no main bond to the water service then?

Yes there *is* - but the water comes in elsewhere. The earth wire goes up
through the ceiling from the CU, and drops down into the kitchen where the
mains stop-tap is located.


Box looks like:

http://www.tlc-direct.co.uk/Main_Ind...ble/index.html
or
http://www.tlc-direct.co.uk/Main_Ind...ded/index.html

Thanks for the links. I now see what you mean.


I take your point about burying the gland in the wall.

In which case why not chase down the bathroom wall as described, but
rather than installing a metal box in the bathroom, simply drill
throught the wall at that point (keeping within 6" of the bathroom
ceiling). You can stick some capping over the T&E to facilitate future
replacement if you wish.

Now install your cable and plaster over the chase leaving no visible
accessory in the bathroom. Now visit the other side of the wall where
hopefully you have a bit of T&E dangling, and poke it into the back of
one of the aformentiond boxes (a IP65 one would probably be a good
choice here). Screw it to the wall, and you can then stick a nice neat
SWA gland into the knockout at the base of the box. Choose the right
box ad you may even get one complete with junction terminals. That
means your joint is accessable and serviceable, no extra clutter to tile
round the bathroom, and no groveling in the loft!


That's certainly a possibility. It means that I would be making the
connection while perched 16' up a ladder, but I could probably terminate the
SWA in the box before fixing the box to the wall. How would I seal round
where the T&E would come in?
--
Cheers,
Roger
______
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monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

Roger Mills wrote:

Yes, the circuit is going at the extreme right - which currently has a blue
base for a 16A MCB.

I may well still have a 30A fuse - not sure about the BS1361 bit though - is
that something special? [The CU started off with fuses, and was converted to
MCBs!]

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).

What is the rationale for using a fuse rather than an MCB? Is it that it
takes longer to blow, making it more likely that the one at the garage end
will trip instead?

In effect, yes. The problem with cascaded MCBs is that if the fault
current exceeds a certain figure both MCBs are on the 'instant' magnetic
tripping part of their characteristics, and they'll both trip together -
i.e. there's no discrimination.

If so, it makes sense, but it flies in the face of the advice given
in the earlier thread which said that I needed a Type B (and *not*
Type C) MCB on account of only having a 2.5mm^2 earth wire for most
of the length.

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.

- 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.

- 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.

--
Andy

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
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

Roger Mills wrote:

- Voltage drop: [...]

So what are we aiming at here - how high can it be?

4% (9.2 V) maximum between the meter and all points of utilisation.

- Disconnection time: [...]

Sounds like we're *well* in.

That's where you want to be.

- CPC sizing: [...]

What would be the effect on the calculations if it were a 40A or 45A BS1361
fuse rather than 30A?

The first question here is why would you want a 40+ A feed when your
stated load in the garage is only about 20 A? Gut feel says that for
40+ A the submain would need to be in at least 10 mm^2 and that 6 is
likely to be marginal at best.

Anyway, the current to blow the 45 A fuse in 5 s is 240 A. We
calculated 230 A earth fault current (at 70 deg. conductor temperature)
so calculations fail at the first hurdle in that case, unless you can
get positive confirmation that the supply is TN-C-S (the DNO should be
able to tell you). Even it did just meet 5 s disconnection, the
required CPC would be 6 mm^2, so that T&E cable would be at serious
risk. (BS 7671 doesn't give any data for a 40 A '1361 fuse, so I can't
comment on that.)

An option to overcome the Zs issue would be to fit a 100 mA Type S
(time-delayed) RCD at the house end of the feed.

To use 6 mm^2 T&E on a 45 A circuit the installation conditions would
have to be 'clipped direct' at all points - no grouping and no thermal
insulation allowable. This cable goes through the loft, so that might
not be practicable in any case.

By now you should see why (in June) I said stick to a 30/32 A circuit.

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?

You'll never seal it completely against porous brick. Drill a drain
hole in the bottom of the box.

Incidentally, there's a restriction on wiring in a bathroom that you
need to consider. Wiring which is in Zones 0, 1 or 2 and is not in
metal conduit or is not done in a metal-clad cable such as SWA is only
allowed to supply equipment in those bathroom Zones. This applies even
if the wiring is on the surface or in the normal safe zones - see Reg.
601-07-01. However it doesn't apply if the cable is buried 50 mm deep
in the wall. So if your old shower cable is over the bath (Zones 1 & 2)
you'll have to bury it more than 50 mm in the wall in order to be able
divert it for another purpose. The Zones finish at 3 m above the floor,
so there's a get-out if you have high ceilings. For the region
alongside a bath (where a shower pull-switch might previously have been
located) Zone 2 finishes at 2.25 m above the floor and 601-07-01 then
ceases to apply.

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.)?

Yes, it would be split load. This was covered earlier in the thread and
I made some suggestions then. Again:

Yes, for TT a main switch and two RCBOs would be a good option, or main
switch with one MCB (lights) and one RCBO (power) for TN. I'd be
inclined to make the 'power' RCBO a 20 A one - RCBOs only come as Type B
and 16 A Type B might tend to trip on motor starting currents, or the
inrush current of a 230 - 115 V tool transformer. (Alternatively keep
the 16 A circuit for sockets only and provide separate motor circuits
for any machines.) You can buy all the MK consumer unit parts
separately from a wholesaler, including TLC if you want to buy on-line.

--
Andy

Roger Mills wrote:

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?

There are a few options. I would be inclined to mount the box just above
the exit point of the T&E and come up into a hole in the bottom of the
box either with one of the connical grommets/blanking plugs that you get
with some of these, or, as you suggest silicone. The very short bit of
T&E ought to be either painted or covered by a short bit of mini
trunking to protect from the UV.

If you come throught the back of the box, then silicone is probably the
best option.

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.)?

You could use a split load unit - this has a switch on the incomer, and
then a RCD positioned mid way down the bus bar. Hence you can choose
which circuits are RCD protected by placing them on the protected "side"
of the CU. However for a garrage this might be overkill since a split
load CU will probably be a dozen ways wide.

If you are exporting the earth to the garage, then you could simply use
a small CU (say 4 way)[1] with a normal incomer switch, and then fit a
type B/C MCB for the lights, and a RCBO for the power.

[1] I say 4 way to allow for a little expansion, and also give you a
better choice of RCBOs - many of which take up two ways in width (there
are single width ones that are instead double height, but not all CUs
will take those)

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

In an earlier contribution to this discussion,
Andy Wade wrote:


Hi Andy,

Many thanks for your latest answers.


What would be the effect on the calculations if it were a 40A or 45A
BS1361 fuse rather than 30A?

The first question here is why would you want a 40+ A feed when your
stated load in the garage is only about 20 A? Gut feel says that for
40+ A the submain would need to be in at least 10 mm^2 and that 6 is
likely to be marginal at best.

I think someone else suggested 40A or 45A to provide greater discrimination
between the protection devices - but 30 A is adequate, and I've already got
some fuses, so I'll stick with that.


By now you should see why (in June) I said stick to a 30/32 A circuit.

Indeed!


Incidentally, there's a restriction on wiring in a bathroom that you
need to consider. Wiring which is in Zones 0, 1 or 2 and is not in
metal conduit or is not done in a metal-clad cable such as SWA is only
allowed to supply equipment in those bathroom Zones. This applies
even if the wiring is on the surface or in the normal safe zones -
see Reg. 601-07-01. However it doesn't apply if the cable is buried
50 mm deep in the wall. So if your old shower cable is over the
bath (Zones 1 & 2) you'll have to bury it more than 50 mm in the wall
in order to be able divert it for another purpose. The Zones finish
at 3 m above the floor, so there's a get-out if you have high
ceilings. For the region alongside a bath (where a shower
pull-switch might previously have been located) Zone 2 finishes at
2.25 m above the floor and 601-07-01 then ceases to apply.

It's not a bathroom per se in that it doesn't contain a bath. It's an
en-suite facility which has a toilet and bidet one end, and a wash basin and
shower cubicle the other end. Does that make any difference? The relevant
bit looks like http://www.mills37.plus.com/en-suite.JPG

The pull switch on the ceiling is for isolating the shower pump. It used to
be fed by the 6mm cable but is now fed by a fused spur off a ring main. You
can see the 6mm cable lying across the small hole I have made in the
ceiling. It would need to be chased into the wall, and then go out through
the wall just to the left of the fan. [It needs to come down to this level
to get below the fascia on the outside]. I don't know what zone that would
be. Please advise! What is the reason for not having any cables which are on
their way elsewhere - sounds a bit OTT?!

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.)?

Yes, it would be split load. This was covered earlier in the thread
and I made some suggestions then. Again:

Yes, for TT a main switch and two RCBOs would be a good option, or
main switch with one MCB (lights) and one RCBO (power) for TN. I'd be
inclined to make the 'power' RCBO a 20 A one - RCBOs only come as
Type B and 16 A Type B might tend to trip on motor starting currents,
or the inrush current of a 230 - 115 V tool transformer. (Alternatively
keep the 16 A circuit for sockets only and provide
separate motor circuits for any machines.) You can buy all the MK
consumer unit parts separately from a wholesaler, including TLC if
you want to buy on-line.

Thanks. John Rumm has also made some useful suggestions. Many thanks to you
both!
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

Roger Mills wrote:

It's not a bathroom per se in that it doesn't contain a bath. It's an
en-suite facility which has a toilet and bidet one end, and a wash basin and
shower cubicle the other end. Does that make any difference?

No, the same zoning applies.

The relevant bit looks like http://www.mills37.plus.com/en-suite.JPG

Ah, that's useful. The simplest thing would be to take the cable down
the wall in the RH corner, if poss, as that's more than 600 from the
shower cubicle, i.e. is in Zone 3. Otherwise, is the visible part of
the shower enclosure on the left fixed or movable, and how high is the
fan, above floor level?

The pull switch on the ceiling is for isolating the shower pump. It used to
be fed by the 6mm cable but is now fed by a fused spur off a ring main. You
can see the 6mm cable lying across the small hole I have made in the
ceiling. It would need to be chased into the wall, and then go out through
the wall just to the left of the fan. [It needs to come down to this level
to get below the fascia on the outside]. I don't know what zone that would
be. Please advise!

Zone 2, I suspect, up to a height of 2.25 m, then Zone 3 above that.

What is the reason for not having any cables which are on
their way elsewhere - sounds a bit OTT?!

I guess the thinking is that it's not obvious that it would be there, as
there's no electrical equipment in the room to give a clue to the
presence of a cable. The principle is to keep all non-essential cables
out of the the wet areas, thus avoiding danger when you decide to drill
through the wall while standing in the shower... It is a bit of a pain
in this case, although it looks as if chasing in to 50+ mm deep might
not be too difficult, if that is necessary.

I presume a new direct SWA run from the consumer unit to the garage is
out of the question.

--
Andy

In an earlier contribution to this discussion,
Andy Wade wrote:


The relevant bit looks like http://www.mills37.plus.com/en-suite.JPG

Ah, that's useful. The simplest thing would be to take the cable down
the wall in the RH corner, if poss, as that's more than 600 from the
shower cubicle, i.e. is in Zone 3. Otherwise, is the visible part of
the shower enclosure on the left fixed or movable, and how high is the
fan, above floor level?

There is a fixed panel alongside the vanity unit, which is 500 wide and
about 1860 high, then a folding door - which is open in the photo. Does the
fixed panel alter the zoning definitions?

The centre of the fan is about 2100 above the floor. The existing cable
isn't long enough to reach the RH corner - it's just long enough to come
straight down and through the wall, with enough to make the joint on the
outside. I am 6'2" tall (sorry for the mixed units!) and I can't reach to
where the cable would be, whilst standing in the shower cubicle - so would
have great difficulty drilling through the cable whilst having a shower!



It is a bit of a pain in this case, although it looks as if chasing in
to 50+
mm deep might not be too difficult, if that is necessary.

I *could* chase it in to a depth of 50mm, or I suppose I could join it to
SWA above the ceiling, and bring the SWA down and through the wall. I might
even be able to join the cables below the ceiling in a universal box, and
then poke the box up through the hole - but I doubt whether 6mm SWA is
flexible enough to allow that.

I presume a new direct SWA run from the consumer unit to the garage is
out of the question.

It's highly undesirable, involving interfering with the decorations in
several rooms, moving a lot of furniture and and taking up a lot of floor
coverings and floor-boards. To be avoided if at all possible!

--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

Roger Mills wrote:

There is a fixed panel alongside the vanity unit, which is 500 wide and
about 1860 high, then a folding door - which is open in the photo. Does the
fixed panel alter the zoning definitions?

It would if it was 600 wide and went up to the ceiling...

[...]

I *could* chase it in to a depth of 50mm, or I suppose I could join it to
SWA above the ceiling, and bring the SWA down and through the wall. I might
even be able to join the cables below the ceiling in a universal box, and
then poke the box up through the hole - but I doubt whether 6mm SWA is
flexible enough to allow that.

6 SWA isn't /that/ unflexible - buy your length and play around to get
the feel of it. Why not join the cables up in the loft in a metal
adaptable box fixed to the woodwork somewhere (not poked up thro' a hole
and left flapping around)? Then you can bring the SWA out through the
en-suite wall wherever you like, because it's a protected cable. Drill
through the wall at an angle, sloping downhill and round off the lower
outside edge of the hole, so the SWA can be dressed against the wall at
its exit point.

I presume a new direct SWA run from the consumer unit to the garage is
out of the question.

It's highly undesirable, involving interfering with the decorations in
several rooms, moving a lot of furniture and and taking up a lot of floor
coverings and floor-boards. To be avoided if at all possible!

That was a rhetorical question ;-)

--
Andy

In an earlier contribution to this discussion,
Andy Wade wrote:


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.)?

Yes, it would be split load. This was covered earlier in the thread
and I made some suggestions then. Again:

Yes, for TT a main switch and two RCBOs would be a good option, or
main switch with one MCB (lights) and one RCBO (power) for TN. I'd be
inclined to make the 'power' RCBO a 20 A one - RCBOs only come as
Type B and 16 A Type B might tend to trip on motor starting currents,
or the inrush current of a 230 - 115 V tool transformer. (Alternatively
keep the 16 A circuit for sockets only and provide
separate motor circuits for any machines.) You can buy all the MK
consumer unit parts separately from a wholesaler, including TLC if
you want to buy on-line.

Hi again Andy,

Thanks for the above information. I'm still a bit confused!

From earlier discussions, it looks likely that my main supply is TN-C-S or
*possibly* TN-S.

I've looked at the MK stuff on TLC's website, and can't quite work out what
I need - not for a reasonable price anyway! Their split load CUs are 70-odd
quid +VAT before putting any innards in. Can you suggest a shopping list of
bits which would give me MCB protection for lights and RCBO for power - and
be compatible with my mains supply - without costing an arm and a leg? [The
price reference is the Screwfix garage enclosure for £37 - so I'm hoping to
get something which *doesn't* take out the lights when the RCD trips for not
too much more than this].

Incidentally, my current thinking is to join the T&E to the SWA using an
adaptable box in the roofspace. This means that there won't be any joints
exposed to the weather, and the cable won't need chasing so deep into the
wall of the en-suite. Hopefully, I can strip the end of the SWA and fit the
gland in the en-suite, and then poke it up through the hole in the ceiling,
and then grovel in the attic to make the actual connection, which shouldn't
be too bad. Does this sound reasonable?
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

Roger Mills wrote:

Yes, for TT a main switch and two RCBOs would be a good option, or
main switch with one MCB (lights) and one RCBO (power) for TN. I'd be
inclined to make the 'power' RCBO a 20 A one - RCBOs only come as
Type B and 16 A Type B might tend to trip on motor starting currents,

Hi again Andy,

Thanks for the above information. I'm still a bit confused!

From earlier discussions, it looks likely that my main supply is TN-C-S or
*possibly* TN-S.

I've looked at the MK stuff on TLC's website, and can't quite work out what
I need - not for a reasonable price anyway! Their split load CUs are 70-odd
quid +VAT before putting any innards in. Can you suggest a shopping list of

If you go the split load route, then you don't need the RCBOs and can
use ordinary MCBs for all the circuits. However as you have found; split
load CUs tend to be big and expensive.

bits which would give me MCB protection for lights and RCBO for power - and
be compatible with my mains supply - without costing an arm and a leg? [The

Does it have to be MK? If not, how about (For a TN supply):

http://www.tlc-direct.co.uk/Products/CMM4N.html
http://www.tlc-direct.co.uk/Products/CM7106B.html
http://www.tlc-direct.co.uk/Products/CM92003SP.html

price reference is the Screwfix garage enclosure for ï½£37 - so I'm hoping to
get something which *doesn't* take out the lights when the RCD trips for not
too much more than this].

Your difficulty there is the price of RCBOs - one of them costs about that!

Cheapest (but not smallest) option may be:

http://www.tlc-direct.co.uk/Products...0slash80N.html
http://www.tlc-direct.co.uk/Products/CM7106B.html
http://www.tlc-direct.co.uk/Products/CM7120B.html

Although that still tops £50

Incidentally, my current thinking is to join the T&E to the SWA using an
adaptable box in the roofspace. This means that there won't be any joints
exposed to the weather, and the cable won't need chasing so deep into the
wall of the en-suite. Hopefully, I can strip the end of the SWA and fit the
gland in the en-suite, and then poke it up through the hole in the ceiling,
and then grovel in the attic to make the actual connection, which shouldn't
be too bad. Does this sound reasonable?

Goveling never sounds reasonable, bit if is what you have got to do.... ;-)


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

In an earlier contribution to this discussion,
John Rumm wrote:

Roger Mills wrote:


I've looked at the MK stuff on TLC's website, and can't quite work
out what I need - not for a reasonable price anyway! Their split
load CUs are 70-odd quid +VAT before putting any innards in. Can you
suggest a shopping list of bits which would give me MCB protection for
lights and RCBO for
power - and be compatible with my mains supply - without costing an
arm and a leg? [The price reference is the Screwfix garage enclosure
for ?37 - so I'm
hoping to get something which *doesn't* take out the lights when the
RCD trips for not too much more than this].


If you go the split load route, then you don't need the RCBOs and can
use ordinary MCBs for all the circuits. However as you have found;
split load CUs tend to be big and expensive.


Does it have to be MK?

Not necessarily. I quoted MK because someone said you could buy their loose
bits, and do a mix & match. I'm sure you can do that with other makes.

If not, how about (For a TN supply):

http://www.tlc-direct.co.uk/Products/CMM4N.html
http://www.tlc-direct.co.uk/Products/CM7106B.html
http://www.tlc-direct.co.uk/Products/CM92003SP.html

Your difficulty there is the price of RCBOs - one of them costs about
that!

Ouch - that's expensive. I wasn't sure whether you could use an RCBO with
that board - 'cos it isn't listed among the compatible MCBs.

Cheapest (but not smallest) option may be:

http://www.tlc-direct.co.uk/Products...0slash80N.html
http://www.tlc-direct.co.uk/Products/CM7106B.html
http://www.tlc-direct.co.uk/Products/CM7120B.html

Although that still tops £50

According to my calculations, it's about £48 - including VAT and a load of
blanking bits. It would probably do - but it does seem a bit of an overkill
to have a bigger CU in the garage than in the house!

Just to stick another spanner in the works . . I was told today by one
supplier that a detached garage is considered to be 'outside', and that
everything in it has to be protected by an RCD - hence the design od the
standard garage CU. Is there any truth in this? If so, the above solutions
wouldn't comply anyway - so I'm back to losing the lights when I saw through
my power cable?!

Incidentally, my current thinking is to join the T&E to the SWA
using an adaptable box in the roofspace. This means that there won't
be any joints exposed to the weather, and the cable won't need
chasing so deep into the wall of the en-suite. Hopefully, I can
strip the end of the SWA and fit the gland in the en-suite, and then
poke it up through the hole in the ceiling, and then grovel in the
attic to make the actual connection, which shouldn't be too bad.
Does this sound reasonable?

Groveling never sounds reasonable, bit if is what you have got to
do.... ;-)

Actually, it's not *that* bad. It's a difficult bit of roofspace to get into
because it's part of an extension without proper access - so I have to
grovel under a purlin of the main house to get in. Once in, I can pull the
cables through to a place where there's a reasonable amount of headroom, and
make the joint there. I just have to make sure that I take everything I need
with me, to avoid multiple grovellings!
--
Cheers,
Roger
______
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monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

Roger Mills wrote:

Not necessarily. I quoted MK because someone said you could buy their loose
bits, and do a mix & match. I'm sure you can do that with other makes.

If not, how about (For a TN supply):

http://www.tlc-direct.co.uk/Products/CMM4N.html
http://www.tlc-direct.co.uk/Products/CM7106B.html
http://www.tlc-direct.co.uk/Products/CM92003SP.html

Your difficulty there is the price of RCBOs - one of them costs about
that!


Ouch - that's expensive. I wasn't sure whether you could use an RCBO with
that board - 'cos it isn't listed among the compatible MCBs.

The contactum ones are pretty tall[1], and ought to take the single
module RCBOs - but obviously TLC would be able to confirm if you askem
them (with a four way board you could afford the extra width of the
normal RCBOs).

[1] Here is one I dismantled befo

http://www.internode.co.uk/temp/contactum.jpg

Cheapest (but not smallest) option may be:

http://www.tlc-direct.co.uk/Products...0slash80N.html
http://www.tlc-direct.co.uk/Products/CM7106B.html
http://www.tlc-direct.co.uk/Products/CM7120B.html

Although that still tops ï½£50


According to my calculations, it's about ï½£48 - including VAT and a load of

Oh, bargain!

blanking bits. It would probably do - but it does seem a bit of an overkill
to have a bigger CU in the garage than in the house!

OK just thought of a cheaper scabbier solution[2]:

Two of: http://www.internode.co.uk/temp/contactum.jpg
http://www.tlc-direct.co.uk/Products/HGMT106.html
http://www.tlc-direct.co.uk/Products/HGMT120.html
http://www.tlc-direct.co.uk/Products/HGCD263U.html

Plus a plain incomer switch (which I am sure they must sell on its own,
but can't seem to construct the right search phrase to find it!)

Take your submain into one encosure and then to the second. Fit an
incomer switch and 6A MCB for the lights on one, and a 30mA trip RCD and
20A MCB for power on the other.

[2] Although to be totally correct you opught to use metal clad CUs for
a TN supply)

Just to stick another spanner in the works . . I was told today by one
supplier that a detached garage is considered to be 'outside', and that
everything in it has to be protected by an RCD - hence the design od the
standard garage CU. Is there any truth in this? If so, the above solutions
wouldn't comply anyway - so I'm back to losing the lights when I saw through
my power cable?!

Depends on your garrage I guess. If it has a bare soil floor, then to
all intents it is outside - you would be hard pushed to extend an
equipotential zone into it. A nice concrete floored building on the
other hand is probably as "inside" as your front room!

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Wed, 30 Aug 2006 22:50:55 +0100 someone who may be "Roger Mills"
wrote this:-

Just to stick another spanner in the works . . I was told today by one
supplier that a detached garage is considered to be 'outside', and that
everything in it has to be protected by an RCD

Of course a supplier will say this, they want to sell you things.

However, there are garages and garages. Some garages are essentially
built to much the same standard as houses, some are a few sheets of
rusty corrugated iron or asbestos. It is up to the designer to
assess the possible risks and proceed from that.

Actually, it's not *that* bad. It's a difficult bit of roofspace to get into
because it's part of an extension without proper access - so I have to
grovel under a purlin of the main house to get in. Once in, I can pull the
cables through to a place where there's a reasonable amount of headroom, and
make the joint there. I just have to make sure that I take everything I need
with me, to avoid multiple grovellings!

That will give you an incentive to triple check you have everything
before going in the first time:-)


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

In an earlier contribution to this discussion,
John Rumm wrote:


Cheapest (but not smallest) option may be:

http://www.tlc-direct.co.uk/Products...0slash80N.html
http://www.tlc-direct.co.uk/Products/CM7106B.html
http://www.tlc-direct.co.uk/Products/CM7120B.html


OK, after a lot of deliberation, this is the one I've gone with - and the
bits have just arrived.

A couple of questions - surprise, surprise!

The live busbar is supplied in one piece. I'm sure I've read somewhere -
though it ain't in the instructions supplied! - that I need to cut this into
two in a place appropriate to how many ways I want protected by the RCD. Is
this correct.

There is already a live cross connection between the output of the main
switch and the input of the RCD. I assume that the part of the busbar
serving the non-RCD-protected ways needs to connect into the same output
from the main switch?

As supplied, neither the main switch nor the RCD will turn on. Is this
because they need a mains supply before they will operate, or are they duff?

Finally, bearing in mind that this is an insulated rather than metalclad CU,
what's the best way of terminating the incoming 6mm SWA cable? There's no
obvious place on the casing to fit an SWA gland. Do I need a junction box
next to the CU for this purpose?

Thanks.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

Roger Mills wrote:
ice will be much appreciated.

Oh, and in case anyone mentions Part P - I think that what I am doing is
probably covered by Part P. AIUI, extending an existing radial circuit
isn't, but when you start taking it outside, it is. The building itself is
exempt from building regs, being a detached garage of less than 30 M^2 floor
area, and meeting the other exemption conditions. My inclination therefore
is to forget about Part P and hope that it gets lost in the noise when the
property is eventually sold (hopefully far into the future). Nevertheless, I
want to ensure that I end up with a 'safe' electrical installation - hence
the questions.


a) It's covered by part P
b) I'd google anyone I was buying a house from, and find this thread.
c) It probably won't get lost in the noise, because it's the noise that
is precisely the problem when buying a house. Well, that and solicitors
covering their backs.
d) But, if I found it by google, I'd then chat to you, and provided I
was convinced that you knew what you were doing and did it properly, I'd
tell you to forget about disclosing anything about it, and the
solicitors would never be the wiser. Much the best way.

Anyway, just pointing out that posting plans to ignore buildings control
isn't a good idea when done under your real name!


Cheers,

Ben

Roger Mills wrote:

In an earlier contribution to this discussion,
John Rumm wrote:


Cheapest (but not smallest) option may be:

http://www.tlc-direct.co.uk/Products...0slash80N.html
http://www.tlc-direct.co.uk/Products/CM7106B.html
http://www.tlc-direct.co.uk/Products/CM7120B.html



OK, after a lot of deliberation, this is the one I've gone with - and the
bits have just arrived.

A couple of questions - surprise, surprise!

The live busbar is supplied in one piece. I'm sure I've read somewhere -
though it ain't in the instructions supplied! - that I need to cut this into
two in a place appropriate to how many ways I want protected by the RCD. Is
this correct.

This is often the case. IIRC it was with the last contactum split load
CU I fitted... (thinking about it I remember it because I forgot to take
a hacksaw!)

There is already a live cross connection between the output of the main
switch and the input of the RCD. I assume that the part of the busbar
serving the non-RCD-protected ways needs to connect into the same output
from the main switch?

Yup, check the wiring carefully though, making sure it is correct and
tight! (I had a contactum CU where the wiring as supplied was seriously
wrong!)

[Next bit I am doing this from memory - so if it does not make sense
when you look at your one, ignore me!]

Assuming you are going with the main switch on the right, and the RCD in
the centre, then the phase and neutral from your SWA run into the top
terminals of the main switch (phase on left). The bottom right of the
main switch ought to have a neutral connection to the non RCD neutral
busbar - there may also be a connection to the top neutral on the RCD
(or that will more likely come from a wire on the end of the first
nutral bus bar). The phase (bottom left) of the main switch ought to
connect to the main phase busbar AND the flylead to the (top left) phase
input of the RCD. The phase output of the RCD (bottom left) should then
feed the other bit of phase busbar, while the bottom right of the RCD
should have a lead to the second neutral busbar.

Any of that seem plausable?

As supplied, neither the main switch nor the RCD will turn on. Is this
because they need a mains supply before they will operate, or are they duff?

It is normal for a RCD, but not a switch (assuming it is just a bog
standard two module wide switch). The Switch should snap from one side
to the other, with a bias toward "off".

Finally, bearing in mind that this is an insulated rather than metalclad CU,
what's the best way of terminating the incoming 6mm SWA cable? There's no
obvious place on the casing to fit an SWA gland. Do I need a junction box
next to the CU for this purpose?

Personally I would drill a suitable sized hole to take the SWA gland to
make it neat and tidy. Then use a ring terminal on the gland to pick up
the earth from the shield, and take that to the earth busbar (along with
the third core if you have one!).


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Ben Blaukopf wrote:

Roger Mills wrote:

Oh, and in case anyone mentions Part P - I think that what I am doing
is probably covered by Part P. AIUI, extending an existing radial
circuit isn't, but when you start taking it outside, it is. The
building itself is exempt from building regs, being a detached garage
of less than 30 M^2 floor area, and meeting the other exemption
conditions. My inclination therefore is to forget about Part P and
hope that it gets lost in the noise when the property is eventually
sold (hopefully far into the future). Nevertheless, I want to ensure
that I end up with a 'safe' electrical installation - hence the
questions.



a) It's covered by part P

Yup, agreed.

b) I'd google anyone I was buying a house from, and find this thread.

I bet few would though.... still its certainly possible.

(I hope you like reading - there are getting on for 10,000 posts that
mention Rogers's name!)

c) It probably won't get lost in the noise, because it's the noise that
is precisely the problem when buying a house. Well, that and solicitors
covering their backs.

I expect a few years into the future they wil be showing as much
interest as they do in FENSA certs now (i.e. getting progressively less)

d) But, if I found it by google, I'd then chat to you, and provided I
was convinced that you knew what you were doing and did it properly, I'd
tell you to forget about disclosing anything about it, and the
solicitors would never be the wiser. Much the best way.

Yup, I would much rather find evidence of someone asking sensible
questions before doing a job!

Anyway, just pointing out that posting plans to ignore buildings control
isn't a good idea when done under your real name!

Hmmm, doubt there are many on this group who haven't wrt part P!

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

In an earlier contribution to this discussion,
John Rumm wrote:


There is already a live cross connection between the output of the
main switch and the input of the RCD. I assume that the part of the
busbar serving the non-RCD-protected ways needs to connect into the
same output from the main switch?

Yup, check the wiring carefully though, making sure it is correct and
tight! (I had a contactum CU where the wiring as supplied was
seriously wrong!)

Yes, the way it's pre-wired corresponds with the leaflet which came with it.

[Next bit I am doing this from memory - so if it does not make sense
when you look at your one, ignore me!]

Assuming you are going with the main switch on the right, and the RCD
in the centre, then the phase and neutral from your SWA run into the
top terminals of the main switch (phase on left). The bottom right of
the main switch ought to have a neutral connection to the non RCD
neutral busbar - there may also be a connection to the top neutral on
the RCD (or that will more likely come from a wire on the end of the
first nutral bus bar). The phase (bottom left) of the main switch
ought to connect to the main phase busbar AND the flylead to the (top
left) phase input of the RCD. The phase output of the RCD (bottom
left) should then feed the other bit of phase busbar, while the
bottom right of the RCD should have a lead to the second neutral
busbar.
Any of that seem plausable?

Yes, that's exactly right.

As supplied, neither the main switch nor the RCD will turn on. Is
this because they need a mains supply before they will operate, or
are they duff?

It is normal for a RCD, but not a switch (assuming it is just a bog
standard two module wide switch). The Switch should snap from one side
to the other, with a bias toward "off".

On trying again, they *do* in fact turn on. It's just that they take more
force than I was expecting - and I didn't want to break anything!


Finally, bearing in mind that this is an insulated rather than
metalclad CU, what's the best way of terminating the incoming 6mm
SWA cable? There's no obvious place on the casing to fit an SWA
gland. Do I need a junction box next to the CU for this purpose?

Personally I would drill a suitable sized hole to take the SWA gland
to make it neat and tidy. Then use a ring terminal on the gland to
pick up the earth from the shield, and take that to the earth busbar
(along with the third core if you have one!).


There isn't really anywhere suitable in the body of the thing to drill
holes - apart from some knockouts in the back. But I'm not planning in
coming straight through the wall into the back for several reasons. My
current thoughts are that I will drill a 20mm hole in a piece of aluminium
angle to take the gland, and screw the angle to the wall near the top RH
corner of the CU, and just take the conductors in through a cutout in the
cover. If I fix the angle and gland *first*, and then position the CU next
to it, there won't be any exposed cables.

Thanks again for your help and support.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!