We have a 3ph&n(415v) installation protected by a bs88 160A fuse. Bs7671 states that Zs maximum is 0.25 ohms for a 5s disconnection time. The swa is being used as the protective conductor.
To estimate our value of Zs I have assumed Ze is 0.35 ohms as we have no other information.
I am struggling to find formula for estimating r1+r2 again as we have no other information
So I can check the actual Zs based on Zs=Ze+(r1 +r2)
Can anyone please point me in the direction of estimating r1+r2 using the tables in bs7671.