Lmax - Maximum Cable Length equation for TNS Vs IT system

Unless … the original formula makes an assumption that there's 10 % voltage drop in the supply to the origin of the circuit, and a further 10 % voltage drop to the point of utilization, in which case, depending on how you device Im (e.g. 5In for a Type B mcb) the formula sort of makes sense. However, it's not valid for use in the UK in general, because the total volt-drop would typically exceed the requirements of BS 7671 … but also certainly not take into account source impedance in the volt-drop assumptions.

For earth faults, however, the formula is not valid for cables with a metallic sheath or armour used as a protective conductor, or cables housed in containment used as a protective conductor - in those cases, for the UK at least, you need to follow the approach in the UK National Annex to IEC publication CLC/TR 50480 (published as PD CLC/TR 50480 in the UK).

Unless … the original formula makes an assumption that there's 10 % voltage drop in the supply to the origin of the circuit, and a further 10 % voltage drop to the point of utilization, in which case, depending on how you device Im (e.g. 5In for a Type B mcb) the formula sort of makes sense. However, it's not valid for use in the UK in general, because the total volt-drop would typically exceed the requirements of BS 7671 … but also certainly not take into account source impedance in the volt-drop assumptions.

For earth faults, however, the formula is not valid for cables with a metallic sheath or armour used as a protective conductor, or cables housed in containment used as a protective conductor - in those cases, for the UK at least, you need to follow the approach in the UK National Annex to IEC publication CLC/TR 50480 (published as PD CLC/TR 50480 in the UK).