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Home / Technical Articles / The basic understanding of an earthing protection system (diagram and definitions)

Protective conductors

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.

The basic understanding of an earthing protection system (diagram and definitions)
The basic understanding of an earthing protection system (diagram and definitions)

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.

The fuse(s) of the electrical device will operate and interrupt the power immediately. When the device is not fuse-protected the fuses, or miniature circuit breakers, mounted on the distribution panel after the Watt-hour meter will operate.

Let’s see now how earthing protection system looks like with all its parts, as presented in diagram below.


Earthing protection system diagram

Figure 1 – Earthing protection system diagram with definitions

Symbols

SymbolDescription
Earth symbolEarth, general symbol
Green/yellow dual colour protective conductorGreen/yellow dual colour protective conductor. Earth connection providing protection against electric shocks
Functional earth roleFunctional earth role, which does not necessarily include protection against electric shocks
Exposed conductive part, electrical connection of chassis, voltage reference pointExposed conductive part, electrical connection of chassis, voltage reference point
Equipotential linkEquipotential link
Exposed conductive part not linked to a protective conductorExposed 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 insulationDevice 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.


Definitions

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).

Earth electrode
Figure 2 – Earth electrode and earthing conductor

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.

Earth measurement bar used to measure the earth and make a break in the circuit
Figure 3 – Earth measurement bar used to measure the earth and make a break in the circuit

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.

Main earth bar
Figure 4 – Main earth bar

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.

General equipotential link
General equipotential link

6. General main equipotential link conductor

Connects the metal parts of the structure, the busbars and frames to the general equipotential link.

The crosssection must the same as that of the main protective conductor with a minimum of 6 mm2 (10 mm2 for aluminium) and a maximum of 25 mm2 (35 mm2 for aluminium).

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).

Main equipotential link conductors
Main equipotential link conductors

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.

Main protective conductor coming in distribution board
Main protective conductor coming in distribution board

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.

Circuit protective conductors
Circuit protective conductors

11. Additional equipotential links

These are used to ensure the continuity of the protective circuits:

  1. 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.
  2. 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.
In both cases, a minimum of 2.5 mm2 is necessary if the link is protected mechanically (in an enclosure,  ducting, sleeve, etc.) and 4 mm2 if it is not protected (flexible wire). These rules are applicable to the removable panels and doors of electrical panels and enclosures when no equipment is fixed in them.

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.

Earthing bolt on the switchboard roof
Earthing bolt on the switchboard roof

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.

HV/LV transformer protective conductor
HV/LV transformer 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.

Earthing of voltage surge protectors
Earthing of voltage surge protectors

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.

Non-earthed equipotential link
Non-earthed equipotential link

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.

Earthing conductor
Earthing conductor

19. Class II equipment

The exposed conductive parts of this equipment must not be connected to a protective conductor.

Sources:

  1. Electrical energy supply by Legrand

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

26 Comments


  1. Edward Grabczewski
    Mar 05, 2021

    I spend some time searching for the information in this article. I appreciate the detailed photos and can now proceed to create an earthing solution with confidence. Thanks!


  2. suresh c sahoo
    Sep 13, 2020

    Use full information on the Earthing system.,
    However, A lightning strike can be protected by 70 Sq mm Conductor @ 30 KA, @ 30 KV
    So comparing this we can use Max. 150 Sqmm for a 30 KA short circuit current for Main Grid.
    For interconnection of LV system fault level of 15 KA can be protected by 35 Sqmm conductor.


  3. Beverley Wood
    Jul 15, 2020

    Certainly food for thought – once you start to consider the consequences of doing things “off the cuff” With our 240 V system – if some misinformed person with a little knowledge decided to wire up a shower consuming 50 A – that means only 5 ohms resistance (roughly) is in the element – it would be less when cold – (I`m using 250 V here for simplicity`s sake). So, in that case even your naughty wire was only 5 ohms also it would still pass you 125 V, depending on where you In practice other things come into play – how could a wire have 125 volts across it – it would instantly melt. Precisely! Then you, if you were on the “wrong side” of the break, would get the full 250 V as your body`s resistances, being so sky high compared to the shower – or even a small bulb, wouldn`t stand a chance. I did hear of many amps being “lost” because of a leakage into a very damp wall. Seems so unlikely at first – but is perfectly logical, of course. Hope my example wasn`t too far fetched.


  4. Ali Abdulhadi
    Feb 17, 2020

    I have an existing “dirty earth”; how I can use this for the instrument panel (electronic equipment) earthing?


  5. Saleh Farman
    Feb 12, 2020

    RAMESH KUMAR KAPUR,

    Precisely……


  6. Che Wan Zainul Annuar
    Feb 12, 2020

    Dear Mr Edvard

    Details, informative and refreshing for me. Complete illustration and photo.
    Terimakasih.

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