Supplying outbuilding, bonding and earthing

[kropaske]

Hi Guys,

Just join this great forum so lets get straight to the point...

Soon we will be working on property (TNS) with 1 outbuilding on the back of the garden which have kitchen, bathroom and 1 extra room. Also there is gas central heating with combi boiler. At the moment they feed this outbuilding from kitchen ring on a 30amp fuse. They spur off to the outside socket with 2.5mm then 6mm swa from that socket to the CU in the outbuilding(cable not even burried). CU have 2x20amp and 1x6amp circuits as per image (bit of a mess). House CU is approx 20mtrs away from outbuilding CU.
We going to rewire kitchen and outside bit and also have to reconnect outbuilding back all this will go into 3 or 4 way CU for now. So the questions are:

1. 6mm swa gland at house CU from 32 amp mcb without rcd to the outside plastic waterproof junction box to reconnect to existing 6mm swa. Will that be ok?
2. Would you gland both swa cables at the outside junction box and reconnect armour with banjo or is there better solution?
3. At the moment armour is used as the earth, should i connect 3rd wire back to earth as well?
4. There is no bonding provided in the outbuilding, mains water is in plastic externally but all inside is copper. Wouldit be acceptable to bond gas and water back to outbuilding CU? I read it somewhere that it may be different scenario with PME supply, but TNS should be fine.
5. And finally do i need to retest whole outbuilding or just the sub main cable?

Lucas

 

[westward10]

I seem to remember that you cannot combine swa & core, as form of earthing or bonding, but I might be wrong. What was the exact reading you obtained for suspect extraneous conductive parts?

No you can't and if he did the csa is still inadequate.

 

[kropaske]

It was 1837 ohms.
I believe should be over 23k to omit bonding.

 

[Midwest]

It was 1837 ohms.
I believe should be over 23k to omit bonding.

Yep what are your plans now?

PS Anything below 0.02Moh and above 1667oh (rcd protected) needs bonding (citation Mr Kitcher vid)

 

[tony mc]

I seem to remember that you cannot combine swa & core, as form of earthing or bonding, but I might be wrong. What was the exact reading you obtained for suspect extraneous conductive parts?

GN8 mentions this but it refers to PME


Just a quick excerpt from Guidance Note 8 on this:

For installations forming part of a TN-C-S system in which the supply is earthed at
multiple points (PME conditions applying), and it is intended to utilise the armouring of
a cable or a core of an armoured cable as a protective bonding conductor the designer
needs to consider the effects of the currents that may flow in the conductor due to
network conditions. The armouring or a core of such a cable is normally not used as
a protective equipotential bonding conductor where PME conditions apply unless the
electrical installation designer determines that the heat produced in the armouring or
core due to its use will not cause overheating of the live conductors of the cable when
on full load.

Then there is another passage on combined armour/cores:

9.3.4 Armouring inadequate for a CPC
There will be occasions where the actual CSA of the armouring of a cable is less than
the value of S calculated by use of Equation (9.1). To remedy this, the designer has
three options to consider:
1 selecting a cable with an extra internal core, and using it as the CPC (note that all
the cores are required to be suitably identified – Regulation 514.3.1 refers)
2 selecting a larger cable with a corresponding larger CSA and using the armour as
the CPC
3 a separate green-and-yellow covered copper conductor.
For options 1 and 3, it cannot be accurately predicted how the current will divide
between what is effectively two parallel conductors (i.e. the core or separate conductor
and the armouring) due to the magnetic effect of the armouring. It is therefore
important that the additional core or separate CPC is sized as if it alone were to take
the earth fault current. In other words, it is not permissible to add the CSAs of the two
conductors together.

 

[Midwest]

Well of course, I knew I was right.

 

['mr2'gd4u]

Hi Guys,

Just join this great forum so lets get straight to the point...

Soon we will be working on property (TNS) with 1 outbuilding on the back of the garden which have kitchen, bathroom and 1 extra room. Also there is gas central heating with combi boiler. At the moment they feed this outbuilding from kitchen ring on a 30amp fuse. They spur off to the outside socket with 2.5mm then 6mm swa from that socket to the CU in the outbuilding(cable not even burried). CU have 2x20amp and 1x6amp circuits as per image (bit of a mess). House CU is approx 20mtrs away from outbuilding CU.
We going to rewire kitchen and outside bit and also have to reconnect outbuilding back all this will go into 3 or 4 way CU for now. So the questions are:

1. 6mm swa gland at house CU from 32 amp mcb without rcd to the outside plastic waterproof junction box to reconnect to existing 6mm swa. Will that be ok?
2. Would you gland both swa cables at the outside junction box and reconnect armour with banjo or is there better solution?
3. At the moment armour is used as the earth, should i connect 3rd wire back to earth as well?
4. There is no bonding provided in the outbuilding, mains water is in plastic externally but all inside is copper. Wouldit be acceptable to bond gas and water back to outbuilding CU? I read it somewhere that it may be different scenario with PME supply, but TNS should be fine.
5. And finally do i need to retest whole outbuilding or just the sub main cable?

Lucas

View attachment 34422

View attachment 34423

What a bodge

 

['mr2'gd4u]

What a bodge

How dare the moderators edit my post!! What I wrote was perfectly valid!! anyway... If I was you, I would get a competent spark In to do the job for you, then charge the client making a small profit :S

 

['mr2'gd4u]

OMG look at the main earthing conductor wrapped in the green n yellow tape looks like its actually some of the steel wire armorings no gland there then!

I prefer the live coming from switch to rcd which is actually a neutral cable with red tape... looks like a quality job.

 

[Pete999]

I think bodge is the correct analysis