Checking Zdb at Garage Sub-board

[ringer]

Have just installed a garage CU and want to check Ze/Zdb is ok
Main board in house, Ze is 0.19ohm
Garage board, Zdb is 0.44 ohm
Zs on all circuits in the garage is well within limits given in table 41.3 in the BRB

I know max Ze for the main board is 0.35ohm (as per the OSG for a TN-C-S supply), so Ze is well within limits, but how do I check the sub-board complies? Am unsure what maximum value of Zdb would be acceptable. Is there a calculation? Or do I use table 41.3 to check the BSEN61008 RCCB in the Garage unit complies? It is a 63A RCCB so max Zs for that is 0.73ohm.

Also, the 0.35ohm figure comes from the OSG. Is there a particular regulation that covers this, because I cannot find it in the BRB/BGB?

 

[spark 68]

Hi the maximum Zdb value would be whatever complies with the OCPD/cable combination supplying the second DB, not the suppliers cutout.

Is there a calculation? Or do I use table 41.3 to check the BSEN61008 RCCB in the Garage unit complies? It is a 63A RCCB so max Zs for that is 0.73ohm.

You would treat Zdb as a Zs as this is what it is in reality

 

[Deleted member 9648]

It is a 63A RCCB so max Zs for that is 0.73ohm[/B].

As already stated the Zdb is really the Zs for the distribution circuit supplying the db, hence the max value will be the max value for the overcurrent device protecting it at source.
An RCCB is not an OCPD,,,so I'm confused as th how you are applying a max Zs of 0.73 ohms to that??