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)
Minimal cross-section of grounding conductors must be minimally 16mm2 Cu. They are placed in earth and protected against corrosion but not protected against mechanical damage. If these are not protected against corrosion (doesn´t mater if they are protected against mechanical damage or not), minimal cross-section of grounding conductor must be 25mm2 Cu (exceptionally Al, which is however not recommended for use in earth).
The grounding conductor must be laid in such a way to resist all external influences, which can be expectedduring operating. At the same time it must not causefire danger, eventually it should not influence operating of other equipment. It is laid to be as short as possible, without sharp curves, unnecessary arcs and loops.
Overground parts of grounding conductors must be placed so they can be controllable. Outside part of grounding conductor must be suitably protected by panelling or by placing into tubes in places where danger of damage can occur (for example while going through a wall, going into earth).
Conductive construction elements of metal constructions can be used as random grounding conductors. They create continually connected complex,as for example cable trays, cable frames, pillars, rails of crane, steel poles, reinforcement of columns made of flow spinning concrete and metal conduits. Connectionsof grounding conductors and ground electrodes must be correctly carried out and must be desirably dimensioned. While using clamps principle governs that the used clamp must not mechanically damage neither ground electrode (for example conduit) nor grounding conductor.