Topic on Talk:TN system - Earth-fault current calculation

Following the comment from Alberto, some clarifications were made on the values of conductor resistivity to be used in the calculation (resistivity at which temperature of the conductor material)

Hi: I think there is an error in the following sentence: "ρ = resistivity at normal working temperature in ohm-mm2/meter (= 23.7 10-3 for copper; = 37.6 10-3 for aluminium)". I think it should say: "ρ = resistivity at máximun working temperature in ohm-mm2/meter (= 23.7 10-3 for copper; = 37.6 10-3 for aluminium). The resistivity at the maximum temperature is used, because it is desired to calculate the lowest current produced in a ground fault."

Thanks.

Hello, I agree, The values of resistivity given (23.7 10-3) is valid for 115°C (Rho 20°C Cu = 1.72 10-8 Ohm.m Cu T° Factor / 20°C : 3.93 10-3 -> 23.7 10-3 @115°C.

I don't know why this value of 115°C has been selected ... but for sure it is not really normal working temperature. Your proposal for change is very valid.

@Mathieu GUILLOT I believe the values of ρ shown in this wiki page are for a temperature of 90°C. Taking as reference the IEC 60909 (values of ρ₀ at 20°C) completed with Cenelec CNC/TR 50480 (values at various temperatures, for example considering 90°C, it's ρ₁ = 1.28 ρ₀). In any case, we should clarify or re-phrase the text "normal working temperature", and probably also indicate that the values of ρ provided are example values for cables with maximum working temperature of 90°C. Let's connect to fine-tune how to update the page.

@Alberto Thanks for your useful comment! We will update the page as soon as we can, thanks in advance for your patience

@Alberto together with our expert @Mathieu GUILLOT, we have now prepared and implemented some changes to this page, for clarification on the values of ρ to use. You can check the page history to see which changes we made on this page. We have also taken the opportunity to check in other places accross the wiki, where we talk about conductors resistivity, and made some similar adjustments for clarification. You can see which updates were made using the recent changes