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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


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.


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.


  1. Electrical energy supply by Legrand

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More Information

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.


    Feb 11, 2020

    I have one suggestion or a correction to recommendation of size of Earthing Conductor – which is suggested as 6 sq mm or 25 sq mm Cu. This is risky. The earthing conductor has to be sized according to expected maximum fault level at the equipment to be earthed, which will flow through the earth conductor.
    The size of main earth Bus or Earth Conductor shall be greater than the designed fault level or expected fault level, example if it is 16kA for 1 Sec for a Single phase to earth fault for solidly grounded System Neutral -Then the Main Earthing PVC insulated Copper Conductor shall be 150 sq mm which has a Short Circuit Rating of 17.30kA for 1 Sec.

    Similarly, second example, of secondary earth conductor, if Incomer/main Circuit Breaker of a switchboard is rated at 630A and Short Circuit Protection is set at 3.5 x Current rating (which is minimum for Distribution type MCCB), that is, 630 x 3.5 = 2205A, the minimum size of Conductor used for earthing body of Switchboard or its earthing bar shall be 25 sq mm which has Short Circuit rating of 2880A for 1 Sec.

    Had we grounding the panel using 6 sq mm Cu cable, the earth conductor must have melted off as its fault rating is 690A for 1 Sec. AND RESULTING IN LOOSING BASIC SAFETY AGAINST SHOCK.

    If the equipment, in above example, is earthed using 25 sq mm conductor, then in case of an earth fault, we avoid risk of Fuseing off of Earthing Conductor.

    Hence in my opinion (the practice I follow, I always size the earth conductor based on maximum fault current expected to flow through it). Yes I did learn in the begining of my Engineering Carrier in 1972 with Tata Electric, noticed on drawing of EBASCO (I hope I remember the spellings correctly – can be wrong) that minimum size of earth conductor shall be 1/4″ from mechanical strength point. EBASO were the project consultant of thermal power plant constructed somewhere in 1955.

  2. Vinodkumar prasad
    Feb 11, 2020

    Very useful notes.

  3. alfred odhiambo audi
    Apr 19, 2019

    Paying the premium membership tomorrow. Have got 30% discount but forget the code. Can I move on and pay 130 usd

  4. Randy Yatco
    Mar 14, 2019

    Very useful information and reference.

    Thanks a lot for all the topics posted.

  5. Brijendra P Singh
    Feb 13, 2019

    Very useful information on basic electrical engineering related to earthing and protection.

  6. Eduart
    Feb 11, 2019

    I have a question about a specific case, in the case of oil deposits (underground or above ground) or similar should be the grounding or Lightning-rod system is connected?; maybe it is wrong?..

  7. James Wascisin
    Feb 07, 2019

    I am concerned when I hear a statement such as “current will always find the shortest way to the ground”, or “electricity always follows the path of least resistance”. Both statements are not true. They also imply that no current flows on any other paths. This is a very dangerous thought pattern to get into, and can get you killed. It indicates a lack of understanding of basic electricity and Ohms law. Ohms law states quite plainly that “electricity follows any and all paths, the current determined by each circuit impedance”. A 1 ohm circuit will conduct a larger current (per applied voltage) than a 500 Ohm or a 1000 Ohm circuit, but if those three paths are available, current will flow on each! If you are part of a path, and and more that 6 mA flows through you, the potential for it to be fatal is there. Please remember the basics in these discussions, for those that are not as experienced and/or knowledgeable.

    • Manivannan
      Sep 19, 2019

      Not the shortest way but current always found low resistance path…R=pL/A the area of conductor increased meant to say it’s low resistive path

  8. Mohammed Abuu
    Feb 05, 2019

    I like this subject, because is very useful in our domestic and industrial

  9. Abdullatif Abdulkarim
    Feb 02, 2019

    Dear Mr Edvard

    How many safely max ohm of earthing please.


    • Mitul
      Jan 16, 2020

      Earthing value should not exceed 10 ohms in any case. If you have a neutral point and Earthing point ( both proper operational) then you can measure the voltage between those 2 points. Under normal condition that voltage should not measure more then 5 volts ( in case you are using 230 V AC ). If you are getting voltage more then this , you need to check your wiring and earthing.

  10. Atlano Fortes
    Jan 18, 2019

    Bom dia,
    Queria saber qual e o valor da terra que podemos encontrar quanto uma aterramento e considerado bom.

  11. Muhammad salman
    Jan 17, 2019

    sir please tell me why electric motor external ground,please tell standard

    • Mitul
      Jan 16, 2020

      It is done to prevent over-voltage of phase winding of generator/motor during earth fault condition.

  12. J.uma
    Jan 17, 2019

    How to new earth connecting

  13. Rajendra Itagi
    Jan 17, 2019

    It’s very useful notes for.. Industrial and home application..

  14. David Renshaw
    Jan 16, 2019

    The conductor sizes in sections 6, 7, and 12 specify a MAXIMUM size or 25 or 35 sq mm. as applicable, and are worded in a way that implies these are mandatory limits – which some readers will find puzzling.

    The intention of the regulation was that there’s no requirement for these sizes (25 or 35) to be exceeded (but you are not prohibited from doing so).

  15. Anil Paul Jacob
    Jan 16, 2019

    Dear Mr Edvard
    Your given details on ground system were informative and refreshing for me. I have the following doubt. Please reply as soon as you get free to answer.
    1. in Figure 1 above, “Earthing protection system diagram”, I have seen the LV/ HV earthing had been linked with General equipotential link (5), and in same link there is Lightning conductor is also connected, such as Link 5 is used as a common link bar. My doubt is if there is grounding of charged cloud is happening, is there any chance of getting reverse potential to the connected equipment through the link 5 back? Or any device to be connected in this system? Please reply

    • Edvard
      Jan 16, 2019

      Well, current will always find the shortest way to the ground, which means that cannot go through other sides/links other than directly to the ground either through surge arrester and lightning rod or directly through main ground rod.

      • Vivek Jagtap
        Jan 18, 2019

        Dear Mr.Edvard,
        Very true .Current will always find the shortest way to the ground .But in many cases a seperate /dedicated earth electrode is preferred for earthing of lightning conductor .This ensures that in event of a loose connection or deterioration in connection over a period of time/neglected maintenance ,reverse current does not flow .By having a common connection , the extra safety aspect gets compromised / is susceptible to an undesired fault .Your views please!

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