Circuits with high protective conductor currents

[RMS]

Hi,

I have a job for the company i work for to install a power pole for sockets supplying computers.

I have been told to fit a new OPD at the consumer unit and run a 6mm twin and earth cable to a 32amp switch mounted in the power pole. Then to run a ring final circuit within the pole for sockets.

I understand that the protective conductors within the ring should be terminated in separate earthing terminals in the sockets due to high protective conductor currents but would i have to run a separate protective conductor with the 6mm radial?

Also how many computers would need to be connected to the circuit for the total protective conductor current to exceed 10mA?

any answers appreciated thanks.

 

[Lectrician]

You would.

I would be more inclined to run a 10mm earth to the switch in supplement to the CPC in the cable - you do not need to form a ring of the earths if using cable in excess of 10mm (hence why you don't on submains etc!)

 

[ColJack]

i never understood that... it's only up to 30mA leakage.. so why the 10mm?

i get the reason for high integrity earthing by putting earths in separate terminals at the sockets and the board.. that's so that if one comes loose there is still an earth path..

but surely if the single 10mm earth comes loose then there is no earth path..?


( please excuse any incoherencies or spelling mistakes, i'm half way down a bottle of wine... )

 

[Adam_151]

Rightly or wrongly, the 10mm² is considered big enough to withstand mechanical stresses which may cause it to go open circuit, below that you have to provide two paths so its 'double fault to danger'

a random train of thought - Afaik, the regulation just says 10mm² not 10mm² copper or calculated equivelent to 10mm (correct me if I'm wrong!), which means you could use the armour on just about any SWA without supplimenting it, which I suppose could be a bit iffy if the swa runs through an area with vehicles or forktrucks in it, which could pull the armourings out of the gland and leave the conductors connected, I suppose its left upto the designer to recognise that and design accordingly