Tn system

[gordy]

Ok. If my overcurrent protective device has a max zs of 1.20ohms and my measured zs is 1.30ohms in a tn installation. I know this is unsatisfactory as it will not meet disconection time permitted.

So if i replace my overcurrent protective device with and rcbo? Is this acceptable by bs7671?

Cheers

Gordy

 

[vambo]

yes, that is correct.

 

[ezzzekiel]

max disco for rcd is 1667 - think you would be hard pushed to reach that.
otherwise you would be looking at reducing R1 R2 not an easy task

 

[Guest123]

I think before just buging in an RCD/RCBO I'd check to see if there there was anything wrong first by checking the circuit constraints to see if they added up correctly with R1+R2 & Ze etc. If it is sound then fit the additional earth fault protection.

Obviously if it is a new circuit, a good design would have picked this up and a larger cable could have been selected, as the circuit may not neccessarily have needed RCD protection otherwise and could become a nuisance.