Using SWA Armour & Core in PME TNCS?

[Robinson]

Hi guys , was wondering if any could advise. Got a job when client is moving his shed containing Swin pool pump filter etc etc a few metres. I have disconnected for him and made safe.
Unfortunately though he thought he could just bish bosh in himself but advised that would be a notifiable and would require extra work to bring it in line with Regs.

To the point basically Its PME TNCS at house with 4mm SWA 20m run feeding a small 4 way RCD DB in Shed. I need to put a isolator with OCP in in house as it was just slapped into a henley.
What i was wondered was as the 3rd core at present is being used for earth is undersized. Is ok to use SWA armour combined with the core ? Looking at chart for 4mm 3Core SWA armor i can see the Steel would be 22mmsq which is
equiv in copper to 9.75mmsq.

Is the use of the Steel permit-able on PME?

Was considering the Earth rod but kinda pointless if other means poss. especially as armoured is already run in and no chance really of running a 10/16mm out to Pool Shed.

Oh and there is no extranious parts, metal work or copper pipes at all in shed.

Any thoughts


kind regards to you all !

 

[trev]

Tons of threads about exporting PME mate, have a dig

 

[Robinson]

Yeah had a look around a got loads on the exporting side etc etc, but find nothing on actually using the steel armoured in a PME situation, and indeed the possibility of combining a core and steel to meet the 10mmsq Min for a PME earth conductor.

 

[johnboy6083]

Why is the 3rd core undersized, if there is no bonding required in the shed? If its the same size as the line conductor, then it will comply with no calculations required. Adiabatic will probably allow an even smaller conductor to be used.

 

[Robinson]

perhaps i need keep digging. i know the export thing is a biggy on here and i read some great threads inc the one with the best practice guide.

 

[Guitarist]

So all the pipework entering the shed is plastic ? If no extraneous conductive parts entering the shed then you don't need a 10mm earth bond, just the core of the swa as a cpc.

Amazed that the 4mm is henley'd off the main fuse at present tho.

 

[Robinson]

So all the pipework entering the shed is plastic ? If no extraneous conductive parts entering the shed then you don't need a 10mm earth bond, just the core of the swa as a cpc.

Amazed that the 4mm is henley'd off the main fuse at present tho.

Yeah straight off the old block. 100amp. tag cut bosh. So YES ALL pipes in plastic, NO metal work (extraneous). Great News. making life hard for myself werent i. air of caution at least.

 

[Guitarist]

Yeah straight off the old block. 100amp. tag cut bosh. So YES ALL pipes in plastic, NO metal work (extraneous). Great News. making life hard for myself werent i. air of caution at least.

Just make sure you abide by regs section 702 mate.

 

[Robinson]

Just make sure you abide by regs section 702 mate.

Sure thing. On inspection the installation in shed is fantastic. very good workman ship which leads me to believe was a different person to the numpty who just
stuck it in the henley other end.
But yes 702 will be most certainly abided by. thanks mate.

 

[Robinson]

Why is the 3rd core undersized, if there is no bonding required in the shed? If its the same size as the line conductor, then it will comply with no calculations required. Adiabatic will probably allow an even smaller conductor to be used.

Yeah same size as line conductor, and adiabatic would be less. why did i think PME export required minimum 10mm earth conductor regardless of Bonding. hmmmmm reading time.

thanks johnboy

 

[IQ Electrical]

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.

 

[Guitarist]

Excellent quote mate, especially the very end part. On this occasion tho, the OP isn't using any form of bonding conductor, just using his cpc.
Interesting that many people use 10mm swa to supply outbuildings with extraneous conductive parts in order to use the earth core as the 10mm bonding. This excerpt is basically saying "run a separate earth cable".

 

[malcolmsanford]

Just an aside to this, the amendment in regulation 521.5.1 as for the 1st recognized the use of a parallel conductor being use as a protective conductor with SWA

 

[Robinson]

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.

Bang on ! this exactly the info i was wondering BUT As Guitarist kindly said by redirected me with regards me not even needing to worry, as i have no exteraneous parts in shed. So there is no bonding ALL plastic..

Still thanks IQ top quote. very interesting to note the additional core must be sized as if it were alone.

REALLY HELPFUL. Again morrrrre reading for me GN3 stylee........man i just need to not ever take my head out of a book..:0)