Voltage (Uo) for calculating permissible (Zs) earth loop impedance and disconnect times

That's not the case. As a simple example, Tables 41.2 to 41.5 in BS 7671 are based on 230 V supplies with C_min in accordance with ESQCR. They can't be used for a 240 V system unless the C_min of that system just happens to be 0.91. With fixed tap changer, the real value of is likely to exceed 0.95, meaning you would need to as the OP says, those Tables lead to an under-estimate of the possible earth fault loop impedance - not necessarily a safety issue though.


HOWEVER, it's not always the case that dropping to 230 V puts you o the "safe side" - for example, the calculations in A722.3 - use of 230 V in that case, as my previous post indicates, would mean you calculate:


(a) for a 230 V single-phase supply, with a MD of 100 A: R_{A ev} ≤ 70 × 230 × 1.1 ÷ (100 × (230 × 1.1 - 70 )) = 0.97 Ω


(b) for a 240 V single-phase supply, with a MD of 100 A: R_{A ev} ≤ 70 × 240 × 1.1 ÷ (100 × (240 × 1.1 - 70 )) = 0.95 Ω


So in that particular case, you would be calculating 2 % above the safety margin using the wrong nominal voltage ... Granted in this particular case it's a moot point because the value is too low to be practicable, but if you were doing the calculation for three-phase it would be a different story. It's quite important for that calculation, as things change regarding the impact of a shock only a few V above 70 V, so there's not a lot of margin for error or indeed assumption.

That's not the case. As a simple example, Tables 41.2 to 41.5 in BS 7671 are based on 230 V supplies with C_min in accordance with ESQCR. They can't be used for a 240 V system unless the C_min of that system just happens to be 0.91. With fixed tap changer, the real value of is likely to exceed 0.95, meaning you would need to as the OP says, those Tables lead to an under-estimate of the possible earth fault loop impedance - not necessarily a safety issue though.


HOWEVER, it's not always the case that dropping to 230 V puts you o the "safe side" - for example, the calculations in A722.3 - use of 230 V in that case, as my previous post indicates, would mean you calculate:


(a) for a 230 V single-phase supply, with a MD of 100 A: R_{A ev} ≤ 70 × 230 × 1.1 ÷ (100 × (230 × 1.1 - 70 )) = 0.97 Ω


(b) for a 240 V single-phase supply, with a MD of 100 A: R_{A ev} ≤ 70 × 240 × 1.1 ÷ (100 × (240 × 1.1 - 70 )) = 0.95 Ω


So in that particular case, you would be calculating 2 % above the safety margin using the wrong nominal voltage ... Granted in this particular case it's a moot point because the value is too low to be practicable, but if you were doing the calculation for three-phase it would be a different story. It's quite important for that calculation, as things change regarding the impact of a shock only a few V above 70 V, so there's not a lot of margin for error or indeed assumption.