The CSN standard 33 2000-5-54 deals with this problem. The standard determines implementation of earth termination system, the resistance value of ground electrodes, voltage level of the ground electrodes, contact voltage and current-carrying capacity of particular ground electrodes. We will mention the grounding conductors in the following part.

Grounding conductors must suit by their cross-sections,which must not be smaller then cross-section designed for the following formula:

(This formula is only possible to be used during time of flowing I, which doesn´t overrun 5 seconds)
S – cross-section of grounding conductor in [mm2]
I – an effective value of alternating current in [A], traversing because of failure with irrelevant impedance by protection element
t – disconnection time of protection equipment in seconds[s]
k – coefficient depending on material of protection conductor, on isolation and on other parts, on temperature before and after short-circuit (values k for differently designed protection conductors are given in CSN standard 33 2000-5-54)