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Zero Potential

The thin layer of material which covers our planet – rock, clay, chalk or whatever – is what we in the world of electricity refer to as earth. So, why do we need to connect anything to it?

After all, it is not as if earth is a good conductor.

What Is Earth And Why And How Do We Connect To It?
What Is Earth And Why And How Do We Connect To It? (photo credit:

It might be wise at this stage to investigate potential difference (PD). A PD is exactly what it says it is: a difference in potential (volts). In this way, two conductors having PDs of, say, 20 and 26 V have a PD between them of 26 and 20V. The original PDs (i.e. 20 and 26 V) are the PDs between 20 V and 0 V and 26 V and 0 V.

So where does this 0 V or zero potential come from?

The simple answer is, in our case – the earth. The definition of earth is, therefore, the conductive mass of earth, whose electric potential at any point is conventionally taken as zero.

Thus, if we connect a voltmeter between a live part (e.g. the line conductor of a socket outlet) and earth, we may read 230 V; the conductor is at 230 V and the earth at zero. The earth provides a path to complete the circuit. We would measure nothing at all if we connected our voltmeter between, say, the positive 12 V terminal of a car battery and earth, as in this case the earth plays no part in any circuit.

Figure 1 illustrates this difference.

Figure 1 - (a) Earth path, (b) No earth path
Figure 1 – (a) Earth path, (b) No earth path

So, a person in an installation touching a live part whilst standing on the earth would take the place of the voltmeter and could suffer a severe electric shock. Remember that the accepted lethal level of shock current passing through a person is only 50 mA or 1/20 A. The same situation would arise if the person were touching a faulty appliance and a gas or water pipe (Figure 2).

Figure 2 - Shock path
Figure 2 – Shock path

One method of providing some measure of protection against these effects is, as we have seen, to join together (bond) all metallic parts and connect them to earth. This ensures that all metalwork in a healthy installation is at or near 0 V and, under fault conditions, all metalwork will rise to a similar potential.

So, simultaneous contact with two such metal parts would not result in a dangerous shock, as there would be no significant PD between them. Unfortunately, as mentioned, earth itself is not a good conductor, unless it is very wet. Therefore, it presents a high resistance to the flow of fault current. This resistance is usually enough to restrict fault current to a level well below that of the rating of the protective device, leaving a faulty circuit uninterrupted.

Clearly this is an unhealthy situation.

In all but the most rural areas, consumers can connect to a metallic earth return conductor, which is ultimately connected to the earthed neutral of the supply. This, of course, presents a low resistance path for fault currents to operate the protection.

In summary, connecting metalwork to earth places that metal at or near zero potential and bonding between metallic parts puts such parts at a similar potential even under fault conditions. Add to this a low-resistance earth fault return path, which will enable the circuit protection to operate very fast, and we have significantly reduced the risk of electric shock.

Earth fault loop impedance

As we have just seen, circuit protection should operate in the event of a fault to earth. The speed of operation of the protective device is of extreme importance and will depend on the impedance of the earth fault loop path.

Figure 3 shows this path. Starting at the point of the fault, the path comprises:

  • The circuit protective conductor (cpc)
  • The consumer ’s earthing terminal and earthing conductor
  • The return path, either metallic or earth itself
  • The earthed neutral of the supply transformer
  • The transformer winding
  • The line conductor from the transformer to the fault.
Earth fault loop path
Figure 3 – Earth fault loop path

Figure 4 is a simplified version of the loop path.

Simplified earth fault loop path
Figure 4 – Simplified earth fault loop path

From Figure 4 , we can see that the total earth fault loop impedance (Zs) is made up of the impedance external to the installation (Ze), the resistance of the circuit line conductor (R1) and that of the circuit cpc (R2), i.e.

Zs = Ze + R1 + R2

we also have, from Ohm’s law, the value of the fault current that would flow from

I = U0 / Zs

where U0 is the nominal voltage to earth (230V).

Reference: Electric Wiring: Domestic – Brian Scaddan IEng, MIET

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About Author


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 fascilities. Professional in AutoCAD programming. Present on


  1. abdellah kamli
    Jul 29, 2017

    Bonjour Edvard
    Tes lessons sont précieux, et je voudrai bien poser les deux questions suivantes
    Pourquoi la terre se trouve au potentiel zéro ? ce potentiel est il le même avec différents types du sol par exemple le sable et si le sol est humide a t il un effet sur le potentiel zéro de la terre?
    Avec le timer 555 et certains composants on peut obtenir un potentiel négatif soit disant (-70)V si on alimente le circuit avec un chargeur 12V et l’on mesure l’écart entre le phase 230V et (-70)V est ce que l’afficheur donnera 300V?

  2. Ahmed Fawzi
    Apr 09, 2017

    Simple and fantastic article.
    Thanks a lot

  3. stephen camilleri
    Apr 08, 2017

    Dearsir, can you tell me where i can buy a book or something how to troubleshoot motor controls and how to arrive to find a fault in a control circuit? i ve been looking for a long time but i cant find

    thanks a lot

  4. Affan
    Dec 16, 2016

    very understandable and easy to digest knowledge of earthing i have read so far. thanks Edward.

  5. Ag
    May 22, 2016

    Hello Edward,

    I really love this portal. I have a question. When the current returns through the earth, do the creatures (like earthworms) in the path of the current get electrocuted?
    I have heard that in HVDC sea links are used as return paths. So the fishes in the water get electrocuted?
    I know these are silly questions.. but you seem to be the perfect person to answer them.

  6. mohammed boawainah
    Aug 10, 2015

    could you please add me in your network

  7. Pedro Perez
    Jul 20, 2015

    As always, highly informative. I really enjoy this site and it’s wealth of information.

  8. Antro P.
    Jul 18, 2015

    Dear Edvard, please clarify functioning of electrical apparatus at current zero of AC supply. Also please explain how AC sinusoidal wave travel through a conductor.

  9. Rahul
    Jul 17, 2015

    Hello Edvard, I had a question. If earth is a bad conductor of electricity then how is the current flowing from the live end to the transformer via the ground? After all they are not connected via any conducting media.

    • Nicholas Jones
      Aug 10, 2015

      Although the earth itself is considered to not be a good conductor. What it does offer is multiple parallel paths back to the source, infact because there are so many parallel paths the overal resistance of the earth is negligble.

  10. john
    May 09, 2015

    Am happy about that. I request your newsletter

  11. barclinton
    Apr 29, 2015

    EEP..your articles are very educating, it has put our young Engineer on their toes.

    please Keep it coming

  12. Cristiano Ferreira
    Apr 07, 2015

    Another amazing article!

  13. ganesh
    Jan 29, 2015

    It’s really useful and interesting too thank u,

  14. chista
    Jan 21, 2015

    hi. if possible please tell some more about smart home technology

  15. Kevin
    Jan 15, 2015

    Interesting article.

  16. Mahesh
    Jan 03, 2015

    Thanks for this all articles for everyone.

    • Edvard
      Jan 03, 2015

      You’re welcome Mahesh. Glad you like EEP’s articles!

    Oct 22, 2014


  18. ROBIN
    Oct 17, 2014

    Very informative and good learning

  19. Nduaguba Chinedu
    Oct 12, 2014

    Please I’m a graduate of electrical engineering seeking employment. Please would you be kind to assist me?

  20. Yusuf Muhammad Gwani
    Oct 10, 2014

    Nice piece ,thanks for providing the information.

  21. Wasi Ahmed Siddiqi
    Oct 10, 2014

    It is my absolute pleasure to read about a least understood yet very important topic. It enhanced my knowledge on earthing. I will use the knowledge in practical work.

  22. dothan
    Oct 07, 2014


    thanks for the article..very interesting…
    quick fig 3..the link between N and E is absent for the TN-S and TT system right?

  23. Abdullah Habib Zia
    Oct 07, 2014

    Awesome Man. Thanks for sharing.

    Oct 07, 2014

    Excellent narration of Earth

  25. mj
    Oct 06, 2014

    Thanks for such a good articles

  26. amin
    Oct 06, 2014

    thank you for informiantion
    why Get PDF IS inactive

    • Edvard
      Oct 06, 2014

      It works just fine, I created PDF a minute ago. Maybe it was a temporary glitch on their side.

      • mohammed nizamuddin
        Oct 07, 2014

        Mr. Edvard i kindly request you for pdf of this and other articles…

      • FARHAN
        Jan 07, 2016


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