Ze

[malcolmsanford]

The 21 ohms is the DNO earth at their supply transformer. No matter what system of earthing you have in your house the supply transformer will have an earth electrode and that is the DNO responsibilty

If you have TNS or a TNC_S then the DNO have the responsibility to maintain that earth.

If you have a TT then it's the consumers responsibilty to maintain that not the DNO so they don't have the responsibilty of your Ze at a TT. All they have to ensure is that the Electrode at the transformer is there at that 21 ohms. and it is your responsibilty to get the earth path back to it.

 

[martysparky]

The Ze in a TT changes with the weather and is unreliable so we don't really know it; isn't that why its RCD'd

 

[trebor]

The 21 ohms is the DNO earth at their supply transformer. No matter what system of earthing you have in your house the supply transformer will have an earth electrode and that is the DNO responsibilty

If you have TNS or a TNC_S then the DNO have the responsibility to maintain that earth.

If you have a TT then it's the consumers responsibilty to maintain that not the DNO so they don't have the responsibilty of your Ze at a TT. All they have to ensure is that the Electrode at the transformer is there at that 21 ohms. and it is your responsibilty to get the earth path back to it.

I would agree that it is the consumers responsibility to maintain a low resistance electrode 200ohms or less at the installation, however if the DNO did not maintain a source electrode of 20 or 21 ohms max then the earth fault path would be incomplete for TT, I know that in addition the source electrode is there for referencing the neutral of the transformer for the TN systems and voltage purposes, but this does not preclude it from a TT system, by enquiry to the DNO their standard quote for TT will be that the source electrode is 20 or 21 ohms which is an external impedance ie Ze which in turn is a requirement of the ESQCR

 

[martysparky]

The Ze in a TT changes with the weather and is unreliable so we don't really know it; isn't that why its RCD'd

To overcome this we should install and earth electrode 2.4m deep opposing a 1.2m deep electrode. as this will be more stable than the 1.2m deep one even with a higher resistance.