Cable calcs zs and ze values

[Simon-0116]

Cable calcs using cable calc app
Working out cable size for a workshop.
100m long, 100a supply, mccb 100a zs- 0.23 ohms, in a duct on its own, tt earth on 300ma rcd.- 20 ohms

App says 50mm swa which I expect.

But if I alter earthing to tncs 0.35 ohms it says max ze exceeded zs of device.

Am I missing something here / Friday afternoon brain.

Thanks

 

[Pretty Mouth]

For TT, the app is using the RCD for protection against earth faults, as you would - the OCPD almost certainly would not operate for fault currents on a TT system.

When changed to TN-C-S, it appears the app has decided to use the OCPD for protection against earth faults. However you can use an RCD for this protection on TN systems, it's perfectly acceptable (all other things being correct)

 

[Simon-0116]

For TT, the app is using the RCD for protection against earth faults, as you would - the OCPD almost certainly would not operate for fault currents on a TT system.

When changed to TN-C-S, it appears the app has decided to use the OCPD for protection against earth faults. However you can use an RCD for this protection on TN systems, it's perfectly acceptable (all other things being correct)

Thanks
Always wondered what people do when device operating zs is lower than ze. Never practically come across it till now.