UK: main earth at board

[bobbyroberts]

need some info please, a customer requested a new fuse board to replace an old bord with discontinued mcb s ect. it is fed via a 35mm 2 core swa sub main and is using the armour as the main earth. Will i need to run a new 16mm earth to the new board or is the swa armour ok as the main earth. I ask this as the main electrical intake is 30 meters from the board and running in a new earth would increase the price of the job. p.s is there a calculation to show if the armour is o.k to use incase of fault.

[sparkie]

http://www.theiet.org/publishing/wir...s.cfm?type=pdf
without looking in regs book
regs 547-02-01

For both PVC and XLPE cables its is not until cable sizes in the order of 90mm are reached that the copper equivalent of steel armouring complies with the lowest permissible conductor size.


Due to the nature of glanded armoured joints the installer cannot guarantee integrity for life. Corrosion between different metal types.

run a separate earth

[Docara]

Hi Bobobyroberts

You dont necessarily NEED to run a separate earth (but i't good practice to do so as it solves many problems eg RCD tripping times). It depends on your test readings!!!

BUT When you walk away from that intallation can you assume during it's life that the armoured cable isn't going to corrode etc and always always carry the fault currrent - No

As per your request for a formula ?????? - Test the installation before quoting. Find out Ze and Zs (don't rely on the lecky boards quoted figures) Ze=Zs+(R1+R2)

If you are about to change a concumer unit and your next question is what is Ze or Zs - you will not get an answer!!!!

Doc

[ohm-my-god]

Originally Posted by Docara

If you are about to change a concumer unit and your next question is what is Ze or Zs - you will not get an answer!!!!

lol :P i'm presuming by this setup you mean it has a TN-S supply where the earth must be connected into a MET then 16mm to the consumer unit (not forgetting your 10mm bonds for services of course)