UK Swa as main earth ?!?

[DJP1997]

For main protective bond sizes on a TN-C-S system it is based on the DNO supply neutral not PFC.

For main protective bond sizes on a TN-C-S system it is based on the DNO supply neutral not PFC.

The supply is 25mm

 

[westward10]

What is 25.0, the tails from the head?
What size earthing conductor is installed from the head?

 

[timhoward]

I'm making a lot of assumptions, but if 100A is a sufficient max demand total for the 3 properties (eyebrow raised a bit), volt drop is ok, the CPC is sized by calculation for ADS of the switched fuse and you proceed as you suggest to make this distribution circuit TT the far end so each property is not connected to a PME supply....
..then my understanding is that you need to be above half the distribution circuit's calculated CPC size to support bonding within that building.
(I've got concerns that this hasn't been sufficiently planned, you've inherited a mess and someone else's attempts at a plan, along with a customer not wanting to spend money, and it sounds as though a 3ph supply should have been considered.)

 

[westward10]

If it becomes TT the distribution cpc is not relevant.

 

[DJP1997]

What is 25.0, the tails from the head?
What size earthing conductor is installed from the head?

This is what Ive got supply end straight into the swa. Earth is 16mm

 

[DJP1997]

If it becomes TT the distribution cpc is not relevant.

This was why I was thinking TT

This is what Ive got supply end straight into the swa. Earth is 16mm
View attachment 118019

 

[DJP1997]

And this

 

[westward10]

Judging by the supply cable I doubt your main protective bonds need to be above 10.0 and if that cable is 120.0 it's armour will support 10.0.

 

[timhoward]

If it becomes TT the distribution cpc is not relevant.

Obviously it's electrically irrelevant and I'm not trying to pick an argument that flies in the face of common sense....

Thing is, I've never found anything in the regs to specifically say how to size bonding in a separate building that becomes TT at point of entry.
541.1.1 seems to make no distinction for TT, just PME or non-PME, and the line about separate buildings.
Is there another section somewhere that is relevant ? Or is it just one for common sense?
Thanks.

 

[westward10]

You would treat it as a TT installation.

 

[BradTW]

Remote areas use TT, where the cost of an additional PE conductor outweighs that of a local earth connection.​

Installations on a construction site, a farm, or a swimming pool may be improper unless additional measures, like installing extra earth electrode is adopted.
For LV system earth, TT earthing helps in earthing such devices when low voltage arises in the system.
All metallic enclosures and extraneous parts are at equipotential.
Protection from lightning because of low fault current.
Protection from earth fault current because of low resistance pat
Advantages of TT Earthing:
-Simplicity (very few calculations when installing)
-Extension that does not necessitate the calculation of lengths
-Fault currents are low
-There is very minimal upkeep
-Operation with a low projected current source
-Ease of entry for conductors, smooth edges
-It is economical, safe and high graded in use
-Good withstanding capacity against harmonics supplied in the system

But you need to look out for two problems One maximum short circuit fault current (you may need a fuse as a breaker may fry, unlikely but you need to calculate) and the second is the neural developing a voltage if the ground becomes dry. Remember your earth rod or rods and neutral link. To achieve a safe installation you may also need to fit both fuse and RCD according to the UK electrical regulations (BS 7671), "Regulation 411.5.2 "One or More PROTECTIVE DEVICE.

411.5. 2 One or more of the following types of protective device shall be used, the former being preferred: (i) An RCD (ii) An overcurrent protective device. NOTE 1: An appropriate overcurrent protective device may be used for fault protection provided a suitably low value of Zs is permanently and reliably assured.

 

[buzzlightyear]

Basically what I’m trying to confirm is wether the steel armour on the 120mm has sufficient capacity to act as the main earth conductor from the supply to the 3 properties it feeds

more like three phase is in order .

 

[TerryB]

more like three phase is in order .

No need of the SWA as main earth use earthing rods in the out buildings Use SWA as protection from mech damage only connect to an earth point at its source.

Normally all exposed conductive parts of an installation have to be bonded to the main earth terminal. When feeding another building on a pme system then any services or exposed metal has to bonded back to the met with the correct size cable, depending on the original neutral.

That's why it's sometimes easier, as in your case, to make it a tt system, then you only have to bond back to the earth terminal of the out building remember protection devices including SPD's Also bond the barn if it is metal.

 

[buzzlightyear]

The op should show the out lay of the buildings has drawings, I understand
He is trying to gain if he is correct with his calculations,