As you already know, protective conductors are the main part of every earthing protection system, but the complexity of the system will increase with the requirements of information technology, voltage surge protection, local area networks, etc. with the risk of muddling the terminology somewhat.
Earthing of the supply in a house or building serves as a protection for the users. It protects them from electrical shocks when a piece of electric equipment has an insulation failure to ground.
When such an insulation failure occurs, a short-circuit current, which is many times higher than the normal operating current, flows through the safety ground wire and via the earth back to the star point of the distribution transformer.
Let’s see now how earthing protection system looks like with all its parts, as presented in diagram below.
Earthing protection system diagram
|Earth, general symbol|
|Green/yellow dual colour protective conductor. Earth connection providing protection against electric shocks|
|Functional earth role, which does not necessarily include protection against electric shocks|
|Exposed conductive part, electrical connection of chassis, voltage reference point|
|Exposed conductive part not linked to a protective conductor. If a functional link is necessary (for example linking exposed conductive parts), use the symbol|
|Device with double insulation achieved by construction, or assembly with double insulation (referred to as fully insulated), achieved by installation.|
Ok, now after we have a complete picture of a earthing structure, let’s say a word about each part.
1. Earth electrode
Set of conductive elements in contact with the earth. The earth connection is established according to local conditions (type of ground) and the required resistance value (Figure 1).
2. Earthing conductor
Conductor providing the link with the earth electrode. It is generally not insulated, and has a minimum crosssection of 25 mm2 (copper) or 50 mm2 (galvanised steel).
See Figure 1 above.
3. Isolating device
This is inserted in the earthing conductor. The device is opened in order to measure the earth connection.
4. Main earth terminal
Electrical link between the earth circuit and the general equipotential link. Can be an integral part of the general equipotential link or the isolating device.
5. General equipotential link
Located at the origin of the installation and/or at the point of entry in each building. It links all the earthing conductors, the main equipotential link and the various protective conductors.
6. General main equipotential link conductor
Connects the metal parts of the structure, the busbars and frames to the general equipotential link.
7. Main equipotential link conductors
Connect the conductive parts near the main LV distribution board to the protective conductor terminals.
Same as above, the cross-section must be the same as that of the protective conductor with a minimum of 6 mm2 (10 mm2 for aluminium) and a maximum of 25 mm2 (35 mm2 for aluminium).
8. Main protective conductor
Conductor linking the main earth terminal to the main protective conductor terminal. Its cross-section is determined according to the rules given in this technical article.
9. Protective conductors main terminal or collector
This is located in the main LV distribution board.
10. Circuit protective conductors
These are determined in accordance with the current of each load circuit.
11. Additional equipotential links
These are used to ensure the continuity of the protective circuits:
- Between exposed conductive parts: the cross-section is at least that of the smaller protective conductor of the two exposed conductive parts to be linked.
- Between exposed conductive parts and conductive parts: the cross-section must be at least half that of the protective conductor of the exposed conductive part to be linked.
When equipment is fixed in them or there are specific risks of indirect contact with these exposed conductive parts (feed-throughs for controls, no faceplate, etc.), flexible braids provides an ideal solution for all installation requirements.
12. Local equipotential link
If in a TN or IT neutral earthing system, the lengths of the circuits upstream of the terminal circuits are not known or they are too long, a local equipotential link is created in each distribution board supplying the terminal circuits.
Its cross-section must be at least half that of the protective conductor supplying the board, with a minimum of 6 mm2 (10 mm2 for aluminium), and a maximum of 25 mm2 (35 mm2 for aluminium).
13. HV/LV transformer protective conductor
The cross-section is determined according to the type of conductor, the power of the transformer and the reaction time of the HV protection.
In practice, its cross-section is almost always identical to that of th main protective conductor.
14. HV exposed conductive parts conductor
If the installation is supplied via a delivery substation, the cross-section used is 25 mm2 (35 mm2 for aluminium). For other types of supply, the cross-section must be calculated.
15. Earthing of voltage surge protectors
This is designed to discharge the fault currents resulting from the elimination of overvoltages. These conductors must be as short as possible and only used for this purpose.
The minimum cross-section is chosen according the manufacturers’ instructions: generally 4 to 16 mm2.
16. Earthing conductor with no safety function
This provides the earth connection, for functional reasons or due to the level of disturbance. Only use the green/yellow dual colour if the conductor also performs the protective function.
The terms “noiseless earth” or “clean earth” must not be used.
17. Non-earthed equipotential link
Link specific to certain restricted applications in non-conducting environments (test platform, etc.). All the exposed conductive parts and parts that are accessible simultaneously must therefore be linked.
The cross-sections are taken as being identical to those of the additional equipotential links.
18. Earthing conductor
Regarding conductor for functional use only: voltage referencing (electronic exposed conductive parts), its cross-section is then chosen according to the actual current.
Regarding electromagnetic compatibility: the conductors must be chosen to be as short and wide as possible to reduce their impedance at high frequencies.
19. Class II equipment
The exposed conductive parts of this equipment must not be connected to a protective conductor.
- Electrical energy supply by Legrand