Design Query

[Spikenaylor]

Just a query to clarify.

Client wants a socket, inside light and external security lights installed.
this is running off 3 phase cabinet. Inside light already connected fed from L1 BS88 HBC 6A Fuse, TT system ZE 14 Ohms

To give client what they want, will run small RCD CU from replacement 20A BS88 HBC (replacing 6A BS88) but do the ZS calcs apply for the now installed MCB's or the BS88 Fuse.

 

[telectrix]

for both, really. your Zs for the circuits fed from your new CU should comply with the max. for the MCBs. the Zdb at your new CU should relate to the BS88 fuse feeding it.

 

[Spikenaylor]

Thanks
that makes sense