I,m trying to design a submain for an outbuilding.The circuit comprises 50m of 3 core 10mm xlpe
on a bs88 40A fuse.Have worked out R1+R2 for the cores as 50x3.66x1.2/1000 =0.0216 ohms
Can the armour be included in the calcs to lower ZS and how would it be done?The supply is TNS 0.8 ohms (understand we have to work on that value)
Thanks for any help,
The measured ZE was 0.15 ohms,but I thought we had to work on the published value
of 0.8 in case there was a change in the network.
I don't think BS 7671 says we have to - it's certainly a good idea in the general case (although I think it unlikely that modern changes to the network would likely raise Ze much above 0.35 Ohms if it's below that already). In some (rare) situation that actual Ze can exceed the published figures - so you can't completely ignore the actual Ze. In others (e.g. sites with own transformers) the chances of significant increases in Ze are extremely unlikely.
We're about to take you to the IET registration website. Don't worry though, you'll be sent straight back to the community after completing the registration.Continue to the IET registration site