UK diy (uk.d-i-y) For the discussion of all topics related to diy (do-it-yourself) in the UK. All levels of experience and proficency are welcome to join in to ask questions or offer solutions.

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 148
Default Electrical question....

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?



  #2   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 25,191
Default Electrical question....

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.

/================================================== ===============\
| Internode Ltd -
http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
  #3   Report Post  
Posted to uk.d-i-y
ARW ARW is offline
external usenet poster
 
Posts: 10,161
Default Electrical question....

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
  #4   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 148
Default Electrical question....

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.
  #5   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 148
Default Electrical question....

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.


  #6   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 601
Default Electrical question....

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

  #8   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 25,191
Default Electrical question....

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 |
\================================================= ================/
  #11   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 25,191
Default Electrical question....

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 |
\================================================= ================/
  #12   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 3,237
Default Electrical question....

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
  #13   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 148
Default Electrical question....

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.
  #16   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 148
Default Electrical question....

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.
  #17   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 25,191
Default Electrical question....

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.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
  #18   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 148
Default Electrical question....

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
  #20   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 148
Default Electrical question....

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?


  #22   Report Post  
Posted to uk.d-i-y
external usenet poster
 
Posts: 148
Default Electrical question....

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?
Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Electrical putty vs. silicone for sealing exterior electrical holes? blueman Home Repair 3 April 1st 05 11:06 PM
Quick electrical question Sneezy UK diy 15 March 5th 04 09:07 PM
Basic electrical question MattP UK diy 32 February 6th 04 09:42 PM
Electrical question - vac with power take-off mike UK diy 8 January 31st 04 09:16 PM
another electrical question alan UK diy 8 September 29th 03 01:11 AM


All times are GMT +1. The time now is 07:01 AM.

Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 DIYbanter.
The comments are property of their posters.
 

About Us

"It's about DIY & home improvement"