Zs for a CU with sub main and fused isolator

[JosephandAmber]

Setup:
Consumer unit fed via sub main from a 60A fused isolator switch. This is connected to the service head MET via a 2m earth connection.

How would one go about both calculating and documenting the Zs?
Would you be treating the fused isolator switch as it’s own db and therefore be documenting Zdb1 (at point of isolator switch) and Zdb2 (at consumer unit). Zdb2 would be be the reading minus Zdb1? Or would you do Zdb1 and then the R1 + R2 of the sub main?

I’m getting myself confused so if anyone would be kind enough to talk through how you’d test this setup it would be hugely appreciated?

 

[GBDamo]

In this situation I record the distribution circuit and the switch fuse, as a DB with the one circuit, on is own test results sheet.

So you'd have;

1 = Ze - obvious.
2 = Zs at switch fuse.
3 = R1+R2 of distribution circuit.
4 = Zs at Db = 1+2+3.

 

[DefyG]

Zs at 2 already includes [Ze + meter tails (R1+R2)] though - so 4 = 2+3

 

[GBDamo]

Zs at 2 already includes [Ze + meter tails (R1+R2)] though - so 4 = 2+3

Good, you're paying attention.?

 

[JosephandAmber]

When you say distribution circuit are you referring to the sub-main?

 

[dodger421]

When you say distribution circuit are you referring to the sub-main?

Is it a sub-main or is it just that the meter is >3m from the CU?

 

[JosephandAmber]

So we have meter, 3m run to fused isolator, 15m run to CU

 

[dodger421]

So we have meter, 3m run to fused isolator, 15m run to CU

So it’s not a sub-main, it’s the consumers tails. In that case the 15m length of (hopefully) SWA is the distribution circuit in GBDamo’s post.

You can also measure the Ze at the CU the same way you would for a more traditional setup, but you need to remove everything other than the suppliers earth and the earth to the CU from the MET before you do.