Is this correct ??

[Sav]

Hi guys,
Just a quickie for you to confirm if possible.

Father in law just finished his office/storage area brick built area at the end of the garden (approx 40 meters away from house).
The main supply to the house is a TN-S.
I run a 16mm swa from inside the house to the office ( clipped outside on the lower wall and then into the ground, covered etc as per regs).
Also, I run along with it another 16mm earth.
Inside the house the swa is connected to 60amp isolator, which in turn has 25mm tails into the henley blocks that also have 25mm tails for supplying the house c/unit.
The 16mm earth is connected to the MET along with other earths.
The office building in the garden has a dual rcd board with 63amp main switch.
Is this ok as far as you can "see", or have I missed anything else out ??

I used 16mm swa due to length of run, but Could I have used a smaller earth along with it (adiabatic equation) ??

Thanks again guys for all your help,
Sav

 

[Sav]

I think the design is right as 16mm swa is correct size for run and supply. The thing that threw me was the additional earth, which I should have insisted, was no reason to run alonmg with it.
Also earthing the sheath will be done tomorrow morning.
Just to clarify though, you would need an additional earth if services (water,gas etc) where to be installed. Thats correct ?

Cheers
Sav

 

[Des 56]

I posted this bit of comment on another thread
It may interest you for the future


Here is a quote from someone on the Iet forum that may be of interest to you

Whilst the primary function of the metallic armouring of an armoured cable is to provide overall strength, it may be used as a circuit protective conductor (cpc), main protective bonding conductor or, where practicable, an earthing conductor.
Whichever of these three functions is to be performed by the armouring of a cable, all the relevant requirements of BS 7671 need to be satisfied. The armouring of a cable may be used for more than one of the functions if all the appropriate requirements for each function concerned are met.

Like any conductor used as a cpc, armouring will experience thermal stress (heating effects) if it is subjected to earth fault current. To withstand this stress safely, the cross-sectional area (csa) of armouring used as a cpc must be not less than that determined by one of the two methods referred to in Regulation 543.1.1 of BS 7671, namely calculation or selection.

In an installation where Protective Multiple Earthing (PME) conditions apply (that is, where the means of earthing for the installation is a PME earthing facility), main protective bonding conductors have to carry sustained currents under certain conditions, which may result in heating of the bonding conductors. The armouring of a cable should not be used as a main protective bonding conductor where PME conditions apply.

 

[Sav]

Thanks Des,
Copied, pasted and printed into my notes for future reference.
Best regards,
Sav

 

[Davey101]

Usefull bit of information there Des, I had overlooked the PME/SWA situation

About the armour/sheath- It only needs bonding at one end, this should idealy (definatley?) be done at the supply end. If its bonded at the supply, you dont need to connect it at the new DB.

As for earthing/not earthing the SWA, I would consider it mandatory, rather than just good practice, as it is an exposed conductive part of the circuit. Can't see how you would get away with not earthing it.This question was the reason I joined this forum originaly, trying to find the reg number to back up this argument, but apparently there isnt one!

 

[Sparky Ninja]

.....as the armour of a 16mm 2 core PVC/XLPE SWA is not big enough by itself to serve as a CPC, which is why you would need to run a seperate one.

Table for 70 degree Thermoplastic PVC/SWA Cables

These figures are from prior 17th edition, but you get the idea.