I currently have a 32A B curve RCBO in my house consumer unit.

6mm2 cable runs from this RCBO to the garage which is a run of less than 3m. Currently nothig is connected as a load to this 6mm2 cable.

I plan to connect the already buried 10mm2 SWA cable to this 6mm2 cable via a junction box. The SWA is 10mm2 due to the voltage drop across the garden.

I would like to then put in a shed CU at the other end of the SWA which is at the bottom of my garden.

This Shed CU would then have 3 x 6A RCBOs for a shed lighting circuit, a greenhouse lighting circuit, an external floodlighting circuit and a 20A RCBO for a double external socket.

We may want to add further circuits of up to 32A for a hot tub in the future.

The house has PME earthing (In old money) and there is a MET in the meter cupboard

One small fly is that the shed is a metal yardmaster so I plan to supplementary bond the shed metal work to the SWA's earth conductor (I propose to export the earth)

Now How do I sort out some form of discrimination between the house CU's 32A RCBO and all the RCBOs in the shed CU as I've never seen a time delayed 40A type C curve RCBO to replace the 32A RCBO with?

On 29/04/2019 23:17, wrote:
I currently have a 32A B curve RCBO in my house consumer unit.

6mm2 cable runs from this RCBO to the garage which is a run of less
than 3m. Currently nothig is connected as a load to this 6mm2 cable.

I plan to connect the already buried 10mm2 SWA cable to this 6mm2
cable via a junction box. The SWA is 10mm2 due to the voltage drop
across the garden.

I would like to then put in a shed CU at the other end of the SWA
which is at the bottom of my garden.

This Shed CU would then have 3 x 6A RCBOs for a shed lighting
circuit, a greenhouse lighting circuit, an external floodlighting
circuit and a 20A RCBO for a double external socket.

We may want to add further circuits of up to 32A for a hot tub in the
future.

The house has PME earthing (In old money) and there is a MET in the
meter cupboard

One small fly is that the shed is a metal yardmaster so I plan to
supplementary bond the shed metal work to the SWA's earth conductor
(I propose to export the earth)

Now How do I sort out some form of discrimination between the house
CU's 32A RCBO and all the RCBOs in the shed CU as I've never seen a
time delayed 40A type C curve RCBO to replace the 32A RCBO with?

At risk of saying you probably don't want to start from here, but...

Short answer is you can't. Even if you had a time delayed RCBO (not even
sure such a thing exists), then that would not offer protection for the
T&E at the head of the submain. This would need it unless protected in
some other way (say by burial = 50mm from the surface or by earthed
metallic shield).

You also have a second problem with the T&E in that its 4mm^2 earth is
inadequate to function as a main EQ bonding conductor for the PME supply
on its own.

You mention a greenhouse as well - and these are notoriously difficult
to extend a PME EQ potential zone into since there is such easy access
to alternative independent earth references.

I think I would be inclined to replace the short run of T&E with
earth-shield or similar, or protect it in earthed metal conduit. Replace
the head end RCBO with a normal MCB. Run the PME earth as far as the CU
in the shed, but then isolate that there - so all it is providing earth
fault protection for is the sub main itself and not the shed. Make the
shed a TT installation with its own earth spike. Now you have no need to
export the PME EQ zone to the metal shed or difficult greenhouse.

Discrimination will also be better since earth leakage at the shed end
won't be able to trip a RCD element at the head end.

Adam might be along shortly with a better idea?

--
Cheers,

John.

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\================================================= ================/

On 30/04/2019 02:49, John Rumm wrote:
On 29/04/2019 23:17, wrote:
I currently have a 32A B curve RCBO in my house consumer unit.

6mm2 cable runs from this RCBO to the garage which is a run of less
than 3m. Currently nothig is connected as a load to this 6mm2 cable.

I plan to connect the already buried 10mm2 SWA cable to this 6mm2
cable via a junction box. The SWA is 10mm2 due to the voltage drop
across the garden.

I would like to then put in a shed CU at the other end of the SWA
which is at the bottom of my garden.

This Shed CU would then have 3 x 6A RCBOs for a shed lighting
circuit, a greenhouse lighting circuit, an external floodlighting
circuit and a 20A RCBO for a double external socket.

We may want to add further circuits of up to 32A for a hot tub in the
future.

The house has PME earthing (In old money) and there is a MET in the
meter cupboard

One small fly is that the shed is a metal yardmaster so I plan to
supplementary bond the shed metal work to the SWA's earth conductor
(I propose to export the earth)

Now How do I sort out some form of discrimination between the house
CU's 32A RCBO and all the RCBOs in the shed CU as I've never seen a
time delayed 40A type C curve RCBO to replace the 32A RCBO with?

At risk of saying you probably don't want to start from here, but...

Short answer is you can't. Even if you had a time delayed RCBO (not even
sure such a thing exists), then that would not offer protection for the
T&E at the head of the submain. This would need it unless protected in
some other way (say by burial = 50mm from the surface or by earthed
metallic shield).

You also have a second problem with the T&E in that its 4mm^2 earth is
inadequate to function as a main EQ bonding conductor for the PME supply
on its own.

You mention a greenhouse as well - and these are notoriously difficult
to extend a PME EQ potential zone into since there is such easy access
to alternative independent earth references.

I think I would be inclined to replace the short run of T&E with
earth-shield or similar, or protect it in earthed metal conduit. Replace
the head end RCBO with a normal MCB. Run the PME earth as far as the CU
in the shed, but then isolate that there - so all it is providing earth
fault protection for is the sub main itself and not the shed. Make the
shed a TT installation with its own earth spike. Now you have no need to
export the PME EQ zone to the metal shed or difficult greenhouse.

Discrimination will also be better since earth leakage at the shed end
won't be able to trip a RCD element at the head end.

Adam might be along shortly with a better idea?


A couple of things. The OP has not actually said that the 6mm requires
RCD protection. If it does then the last time I checked the 6mm earth
shield cable had still not pass the required tests.

You can always do a deviation from the regs and still have buried T&E
less than 50mm behind the plaster.

I agree about making the shed a TT supply.

--
Adam

The 6mm2 cable runs from the back of the consumer unit through a double skin wall straight into a ceiling-floor void for 2.5m and then straight through a double skin wall. The cable is then direct clipped on the wall at a height of 2.5m towards the garage door, past the garage door and then along the opposite side wall.

There will be a cooker switch style isolator which is how the 6mm2 cable will connect to the 10mm2 SWA.

I cannot see a situation where a rcbo is required in the house consumer unit....

The SWA is then buried under lawn before coming out into metal shed to feed the second consumer unit.

P.S....

The metal shed and metal framed greenhouse are both on a patio which in turn is sat on sharp sand followed by 100 mm mot1 hardcore.

The nearest bit of earth to hammer an earth rod into is the lawn and this rod would be at risk of lawnmower or strimmer damage.

Only alternative is to drill right through patio, sharp sand and hardcore and use an extra long earth rod...

I was going to use 20mm plastic conduit on all the cable runs between sub consumer unit and all the electrical accessories to improve both mechanical protection and implement "double insulation" between the conductors and the metal structures.

This is addition to using IP66 sockets and switches along with IP66 fluorescent lighting battens in both structures.

S.

wrote:
P.S....

The metal shed and metal framed greenhouse are both on a patio which in
turn is sat on sharp sand followed by 100 mm mot1 hardcore.

The nearest bit of earth to hammer an earth rod into is the lawn and this
rod would be at risk of lawnmower or strimmer damage.

Surround it with something like a bit of pipe/tubing.

Only alternative is to drill right through patio, sharp sand and hardcore
and use an extra long earth rod...

Decent drills and long enough bits can be hired.

Not a lot of point in asking for advice if you are concerned about the
difficulties of implementing it and are looking for excuses why you cant.


GH

On 01/05/2019 07:33, wrote:
The 6mm2 cable runs from the back of the consumer unit through a
double skin wall straight into a ceiling-floor void for 2.5m and then
straight through a double skin wall. The cable is then direct
clipped on the wall at a height of 2.5m towards the garage door, past
the garage door and then along the opposite side wall.

In which case you can probably argue that a deviation is acceptable, and
feed the T&E from a non RCD protected B32 MCB.

There will be a cooker switch style isolator which is how the 6mm2
cable will connect to the 10mm2 SWA.

ok

I cannot see a situation where a rcbo is required in the house
consumer unit....

Technically speaking, all cables should now have RCD protection unless
they are mechanically (or otherwise) protected from damage. Surface
mounting is in some cases an acceptable way of protecting it (i.e. you
are less likely to drill into it etc).

The SWA is then buried under lawn before coming out into metal shed
to feed the second consumer unit.


--
Cheers,

John.

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\================================================= ================/

On 01/05/2019 08:38, wrote:
P.S....

The metal shed and metal framed greenhouse are both on a patio which
in turn is sat on sharp sand followed by 100 mm mot1 hardcore.

The nearest bit of earth to hammer an earth rod into is the lawn and
this rod would be at risk of lawnmower or strimmer damage.

Then dig a small pit and put in a protective enclosure for the rod and
connection to it.

Only alternative is to drill right through patio, sharp sand and
hardcore and use an extra long earth rod...

Yup, that would probably be easy enough - in fact you can probably just
hammer drive it through the sand and MoT.

I would be inclined to start with a normal 4' rod, and then test that
once installed before deciding if you need to couple another rod to it.

http://wiki.diyfaq.org.uk/index.php/...ode_resistance

I was going to use 20mm plastic conduit on all the cable runs between
sub consumer unit and all the electrical accessories to improve both
mechanical protection and implement "double insulation" between the
conductors and the metal structures.

Plastic gives some mechanical protection and can make a surface wired
job look neater. Trunking is a bit easier to install.

This is addition to using IP66 sockets and switches along with IP66
fluorescent lighting battens in both structures.

Pick whatever is appropriate for the location. Inside a shed you would
be ok with a lower IP rating normally so long as it doesn't leak.

--
Cheers,

John.

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

In article , John Rumm
scribeth thus
On 01/05/2019 08:38, wrote:
P.S....

The metal shed and metal framed greenhouse are both on a patio which
in turn is sat on sharp sand followed by 100 mm mot1 hardcore.

The nearest bit of earth to hammer an earth rod into is the lawn and
this rod would be at risk of lawnmower or strimmer damage.

Then dig a small pit and put in a protective enclosure for the rod and
connection to it.


Plenty of decent earthing stuff here


https://www.tlc-direct.co.uk/Main_In...ing/index.html
--
Tony Sayer


Man is least himself when he talks in his own person.

Give him a keyboard, and he will reveal himself.

On 01/05/2019 08:38, wrote:
P.S....

The metal shed and metal framed greenhouse are both on a patio which in turn is sat on sharp sand followed by 100 mm mot1 hardcore.

The nearest bit of earth to hammer an earth rod into is the lawn and this rod would be at risk of lawnmower or strimmer damage.

Only alternative is to drill right through patio, sharp sand and hardcore and use an extra long earth rod...

I was going to use 20mm plastic conduit on all the cable runs between sub consumer unit and all the electrical accessories to improve both mechanical protection and implement "double insulation" between the conductors and the metal structures.

This is addition to using IP66 sockets and switches along with IP66 fluorescent lighting battens in both structures.

S.


Is it 3 core SWA and how long is the run?



--
Adam

On 30/04/2019 02:49, John Rumm wrote:

You also have a second problem with the T&E in that its 4mm^2 earth is
inadequate to function as a main EQ bonding conductor for the PME supply
on its own.

Quick brain fart correction, the CPC on 6mm^2 T&E is only 2.5mm^2 (its
10mm^2 that has the 4mm^2 CPC)


--
Cheers,

John.

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

wrote:

P.S....

The metal shed and metal framed greenhouse are both on a patio which in
turn is sat on sharp sand followed by 100 mm mot1 hardcore.

The nearest bit of earth to hammer an earth rod into is the lawn and this
rod would be at risk of lawnmower or strimmer damage.

Only alternative is to drill right through patio, sharp sand and hardcore
and use an extra long earth rod...

I was going to use 20mm plastic conduit on all the cable runs between sub
consumer unit and all the electrical accessories to improve both
mechanical protection and implement "double insulation" between the
conductors and the metal structures.

This is addition to using IP66 sockets and switches along with IP66
fluorescent lighting battens in both structures.

S.

I don't think you want to double insulate the circuits you are using
outside.. At least, it might be ok if the only things connected to it
were double-insulated light fittings with cord operated ceiling
switches, but if you are having any other wired appliances, let alone
sockets, then you want the metal building bonded to the earth you are
using for the local sockets. As discussed, their are theoretical
dangers if this is the exported house PME earth, which is why you are
being recommended to have a local TT earth stake for the outside
circuits.

--

Roger Hayter

Yes it is three core SWA in the new three phase colours so I will have to use coloured tape on both ends....

The cable is 20 m long from end to end but about 5m is above ground and 1.2m below down to the trench so the physical lateral distance between garage and shed/greenhouse is only about 13.8 metres.

On 01/05/2019 21:07, wrote:
Yes it is three core SWA in the new three phase colours so I will have to use coloured tape on both ends....

The cable is 20 m long from end to end but about 5m is above ground and 1.2m below down to the trench so the physical lateral distance between garage and shed/greenhouse is only about 13.8 metres.


Go TT.



--
Adam

On 01/05/2019 21:07, wrote:
Yes it is three core SWA in the new three phase colours so I will have to use coloured tape on both ends....

The cable is 20 m long from end to end but about 5m is above ground and 1.2m below down to the trench so the physical lateral distance between garage and shed/greenhouse is only about 13.8 metres.


OK so you could actually export the earth for main equipotential bonding
purposes by adding a 10mm earth between the CU and the SWA. However IMHO
both the shed and the green house should be supplied from a TT supply.

--
Adam

Ok so this now leads to a question of:

What are the pros and cons of exporting the main house earth Vs going TT with an earth rod?

S.

On 03/05/2019 20:28, wrote:
Ok so this now leads to a question of:

What are the pros and cons of exporting the main house earth Vs going TT with an earth rod?


http://wiki.diyfaq.org.uk/index.php/...rting_an_earth

(see the subsequent section on exporting an equipotential zone as well)


--
Cheers,

John.

/================================================== ===============\
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| John Rumm - john(at)internode(dot)co(dot)uk |
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Incidentally I can buy RCBOs that have a short flying white wire which is a functional earth.

As I understand it, if there was ever a loss of earth, the RCBO then trips.

So can such an RCBO be used in a TT install as the electrode resistance will be much higher than a electricity board supplied earth via what I know as PME in old money

On 04/05/2019 13:35, wrote:
Incidentally I can buy RCBOs that have a short flying white wire which is a functional earth.

As I understand it, if there was ever a loss of earth, the RCBO then trips.

So can such an RCBO be used in a TT install as the electrode resistance will be much higher than a electricity board supplied earth via what I know as PME in old money


The fly lead is there so that the RCD part of the RCBO will still
function if the neutral is lost.

They are fine to use on TT supplies.

And that reminds me of an NICEIC inspections I had. The inspector chose
a house where we had added circuits to the CU using RCBOs and there were
already existing RCBOs. We did not over sleeve the white fly lead with
green/yellow sleeving but the RCBOs that were already installed were
oversleeved.

The inspector asked "Why did you not over sleeve the fly lead?" and I
replied "Because it not an earth it's a functional earth and it's colour
should be white". That was the correct answer.

--
Adam

When you say neutral lost, do you mean neutral before the RCBO or after the RCBO?

In any case, if there was loss of neutral, the load would not work as there would be no return path for the current from the live, i.e. an open circuit surely?

wrote:

When you say neutral lost, do you mean neutral before the RCBO or after the RCBO?

somewhere in the street usually

In any case, if there was loss of neutral, the load would not work as there would be no return path for the current from the live, i.e. an open circuit surely?

except with a broken neutral the neutral conductor is likely to rise to
about half mains voltage as all houses on the street try to find neutral
paths via each others' appliances ... can get quite nasty if it lasts
long, motors overheating trying to run on low voltage, but many bits of
kit with switch mode PSUs carry on regardless.

And also if there is a broken neutral, the RCBO would still work if there was more than 30mA flowing in the live as no current would be flowing in the broken neutral?

wrote:

And also if there is a broken neutral, the RCBO would still work if
there was more than 30mA flowing in the live as no current would be
flowing in the broken neutral?

it might be flowing in the neutral conductor out of your house, to
"somewhere" other than the star point in the substation, so no tripping.

In article , ARW adamwadsworth@blueyo
nder.co.uk scribeth thus
On 04/05/2019 13:35, wrote:
Incidentally I can buy RCBOs that have a short flying white wire which is a
functional earth.

As I understand it, if there was ever a loss of earth, the RCBO then trips.

So can such an RCBO be used in a TT install as the electrode resistance will
be much higher than a electricity board supplied earth via what I know as PME in
old money


The fly lead is there so that the RCD part of the RCBO will still
function if the neutral is lost.

They are fine to use on TT supplies.

And that reminds me of an NICEIC inspections I had. The inspector chose
a house where we had added circuits to the CU using RCBOs and there were
already existing RCBOs. We did not over sleeve the white fly lead with
green/yellow sleeving but the RCBOs that were already installed were
oversleeved.

The inspector asked "Why did you not over sleeve the fly lead?" and I
replied "Because it not an earth it's a functional earth and it's colour
should be white". That was the correct answer.


Where do they get the inspectors from, Adam?...
--
Tony Sayer


Man is least himself when he talks in his own person.

Give him a keyboard, and he will reveal himself.

On 05/05/2019 19:50, tony sayer wrote:
In article , ARW adamwadsworth@blueyo
nder.co.uk scribeth thus
On 04/05/2019 13:35, wrote:
Incidentally I can buy RCBOs that have a short flying white wire which is a
functional earth.

As I understand it, if there was ever a loss of earth, the RCBO then trips.

So can such an RCBO be used in a TT install as the electrode resistance will
be much higher than a electricity board supplied earth via what I know as PME in
old money


The fly lead is there so that the RCD part of the RCBO will still
function if the neutral is lost.

They are fine to use on TT supplies.

And that reminds me of an NICEIC inspections I had. The inspector chose
a house where we had added circuits to the CU using RCBOs and there were
already existing RCBOs. We did not over sleeve the white fly lead with
green/yellow sleeving but the RCBOs that were already installed were
oversleeved.

The inspector asked "Why did you not over sleeve the fly lead?" and I
replied "Because it not an earth it's a functional earth and it's colour
should be white". That was the correct answer.


Where do they get the inspectors from, Adam?...


This one was an ex electrician. I have not had a bad one yet. The way he
asked the question was I think was to try and catch me out. They are
however actually very helpful blokes.

--
Adam