In terms of RCD Zs then your cable hot/cold change is going to be negligible to the overall acceptable value. Less than measurement accuracy for sure, unless you have a huge cable R2 and then you have something more fundamentally wrong.
The Ra value limit of 200 ohms is an attempt to make sure you have a stable earth rod behaviour. It should be lower than that under practically all cases in the UK. Some regions with very dry soil or rock substrate might have difficulties unless you drill down far, but mostly for the UK using typical incomers of 100mA (500 ohm, but 200 ohm for stability) or 300mA (166 ohm or less) you ought to meet it under all but drought conditions if you have a 2.4m (8') deep rod so it finds some moisture.
As for the Zs values to use, it should be based on the max working temperature. So:
The Ra value limit of 200 ohms is an attempt to make sure you have a stable earth rod behaviour. It should be lower than that under practically all cases in the UK. Some regions with very dry soil or rock substrate might have difficulties unless you drill down far, but mostly for the UK using typical incomers of 100mA (500 ohm, but 200 ohm for stability) or 300mA (166 ohm or less) you ought to meet it under all but drought conditions if you have a 2.4m (8') deep rod so it finds some moisture.
As for the Zs values to use, it should be based on the max working temperature. So:
- If you cable is CCC limited then it will run at max (say 70C) so you should be using 80% when measured cold prior to use.
- If you have a cable that is VD limited, so CCC ample, it won't run as hot so a de-rating of less might be appropriate if it is proving a close call.
- If you measured it just after normal operations (say EICR) then you could may not need derating at all as it is measured hot.