UK Swa as main earth ?!?

[DJP1997]

So I have turned up on a job… someone previously has installed a 120mm 2 core swa from the meter to a barn building 160m away and them split off for the house and barn building in 25mm tails and have used the armour on the swa as the main earth.

Both ends of the swa have been terminated badly, the ze reading is 0.80 and the armour csa isn’t large enough to be use as the main earth.

I have calculated the for that distance a 70mm swa should have been used and then in that case the csa of the existing swa would be sufficient.

I’m conflicted now if I should make off both ends of the existing swa and if I get a low enough reading continue to use the armour as the main earth or to convert all 3 building that are fed from it to TT systems.

It’s a mess and the customer refused to put in the new correct cable… so what are your thoughts ??

 

[westward10]

What is the earthing system?

 

[DJP1997]

So I have turned up on a job… someone previously has installed a 120mm 2 core swa from the meter to a barn building 160m away and them split off for the house and barn building in 25mm tails and have used the armour on the sea as the main earth.

Both ends of the swa have been terminated badly, the ze reading is 0.80 and the armour csa isn’t large enough to be use as the main earth.

I have calculated the for that distance a 70mm saw should have been used and then in that case the csa of the existing swa would be sufficient.

I’m conflicted now if I should make off both ends of the existing swa and if I get a low enough reading continue to use the armour as the main earth or to convert all 3 building that are fed from it to TT systems.

It’s a mess and the customer refused to put in the new correct cable… so what are your thoughts ??

View attachment 117993

What is the earthing system?

Currently TNCS

 

[westward10]

What size of main protective bonding is required?

 

[DJP1997]

What size of main protective bonding is required?

35mm I believe

 

[westward10]

The armour of a 120.0 two core will support a maximum of 10.0 main protective bonding from a TN-C-S system.
The size of main protective bond is not dictated by the size of this submain so I don't understand why you say 70.0 is fine but 120.0 is not.

 

[UNG]

So I have turned up on a job… someone previously has installed a 120mm 2 core swa from the meter to a barn building 160m away and them split off for the house and barn building in 25mm tails and have used the armour on the swa as the main earth.

How much further are you going from the split to house and barn distribution

Both ends of the swa have been terminated badly, the ze reading is 0.80 and the armour csa isn’t large enough to be use as the main earth.

That does need sorting the 951 clamp is not the way to earth an SWA cable

I have calculated the for that distance a 70mm swa should have been used and then in that case the csa of the existing swa would be sufficient.

What numbers are you using to get to this solution

I’m conflicted now if I should make off both ends of the existing swa and if I get a low enough reading continue to use the armour as the main earth or to convert all 3 building that are fed from it to TT systems.

It will tidy up the cable termination and at least one end of the SWA will need earthing

It’s a mess and the customer refused to put in the new correct cable… so what are your thoughts ??

If the correct cable is the 70mm you say then I think you may need to revisit your calcs and take into account the whole installation as I don't believe you are looking at the total volt drop through to the final circuits

Is the SWA in a duct, if it is you could just pull in a suitably sized earth cable

 

[DJP1997]

The armour of a 120.0 two core will support a maximum of 10.0 main protective bonding from a TN-C-S system.
The size of main protective bond is not dictated by the size of this submain so I

The armour of a 120.0 two core will support a maximum of 10.0 main protective bonding from a TN-C-S system.
The size of main protective bond is not dictated by the size of this submain so I don't understand why you say 70.0 is fine but 120.0 is not.

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…

 

[DJP1997]

How much further are you going from the split to house and barn distribution

That does need sorting the 951 clamp is not the way to earth an SWA cable

What numbers are you using to get to this solution

It will tidy up the cable termination and at least one end of the SWA will need earthing

If the correct cable is the 70mm you say then I think you may need to revisit your calcs and take into account the whole installation as I don't believe you are looking at the total volt drop through to the final circuits

Is the SWA in a duct, if it is you could just pull in a suitably sized earth cable

 

[DJP1997]

How much further are you going from the split to house and barn distribution

That does need sorting the 951 clamp is not the way to earth an SWA cable

What numbers are you using to get to this solution

It will tidy up the cable termination and at least one end of the SWA will need earthing

If the correct cable is the 70mm you say then I think you may need to revisit your calcs and take into account the whole installation as I don't believe you are looking at the total volt drop through to the final circuits

Is the SWA in a duct, if it is you could just pull in a suitably sized earth cable

Yes the plan is to make both ends of the swa of properly.. And I’m not bothered the fact that it’s 120mm but the bit I’m struggling to get a definitive answer on is wether the armour on the 120mm is sufficient to be used as the main earth…

 

[westward10]

It isn't the main earth it will be a cpc. If your main protective bonds need to be 35.0 then no it isn't adequate so you need to establish the bonding csa.

 

[timhoward]

It isn't the main earth it will be a cpc. If your main protective bonds need to be 35.0 then no it isn't adequate so you need to establish the bonding csa.

It should be a cpc, but reading his post it sounds as though the SWA heads off before the consumer unit or any protective device and he essentially has 160m meter tails!

I think that is why it’s being called the ‘main earth’.

A diagram would be worth 1000 words in this thread. I suspect it needs tidying up at the origin to include an OCPD, some adiabatic calcs to determine CPC requirements, then considerations of bonding requirements the far end.

 

[DefyG]

It would help to know the supply details ...... if 25mm 100A supply TN-C-S then 16mm earthing conductor would be required with 10mm bonding conductors thereafter, irrespective of the 120mm circuit conductors as they are probably only sized for volt drop and will never exceed 100A. An adiabatic calc would determine if the swa (10mm) is adequate.

 

[UNG]

It should be a cpc, but reading his post it sounds as though the SWA heads off before the consumer unit or any protective device and he essentially has 160mm meter tails!

I assume you have added an m here and it was meant to read 160m

A diagram would be worth 1000 words in this thread. I suspect it needs tidying up at the origin to include an OCPD, some adiabatic calcs to determine CPC requirements, then considerations of bonding requirements the far end.

A diagram would be a great help as well a little more information on the configuration of rest of the installation

The 120mm cable has clearly been used to cover off the volt drop and sized to leave some overhead for the volt drop in the final circuits but there is a lot of information missing to be able to offer any informed advice with regard to the options on the earthing

 

[DJP1997]

It should be a cpc, but reading his post it sounds as though the SWA heads off before the consumer unit or any protective device and he essentially has 160m meter tails!

I think that is why it’s being called the ‘main earth’.

A diagram would be worth 1000 words in this thread. I suspect it needs tidying up at the origin to include an OCPD, some adiabatic calcs to determine CPC requirements, then considerations of bonding requirements the far end.

Hi, yes this is essentially what I have… 25mm tails connected strait to swa that’s run 160m then connected back to 25mm tails that’s then sent of in 3 different buildings..

 

[westward10]

Is there no circuit protection?

 

[DJP1997]

Is there no circuit protection?

Not at the moment… do the solution I have come up with as the customer doesn’t want to run new cable is… 100amp fused switch then run the armoured . Make it off the other end properly and convert all 3 buildings to tt

 

[westward10]

What size main protective bonding is required?

 

[DJP1997]

R

What size main protective bonding is required?

Re done my calves and 16mm I believe… based on 100amp pfc of 658a

 

[westward10]

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

 

[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,