Max Zs multiple boards

[vwes59]

Had a bit of a brain freeze. Got to go and do an inspection on a property with 2 consumer units, the 2nd fed from the first on a 50A MCB.

Let's say theoretically I get 0.2 for the Ze on the main board, then 0.3 for the Zdb at the second board and 0.2 for R1+R2 on a ring final on a 32A MCB.

Is my max Zs for this circuit 0.7 ? Max Zs = Ze + Zdb + (R1+R2)

 

[timhoward]

The measured Zdb for the 2nd board incorporates the earth path of everything up to that point.
So the Max ZS for the ring final circuit on the 2nd board is the dead R1+R2 result added to the Zs at 2nd DB.

So in your example it would be 0.3 + 0.2 = 0.5

 

[timhoward]

To add, that will include parallel paths if bonding is connected at 1st DB, so if you are wanting to calculate Zs for a circuit on the 2nd DB it would be more accurate to add:
Ze + [(R1+R2) of distribution circuit to 2nd db] + [(R1+R2) of final circuit]

 

[vwes59]

Great, thanks for the explanation, I did wonder if I needed to treat the connection between the boards as essentially it's own radial circuit

 

[DefyG]

Is my max Zs for this circuit

The 'Max' Zs permitted will be determined by the OCD used for that final circuit.
The 'actual' Zs will be the sum of parts upto then eg Ze + R1+R2 dist'n cct etc etc.