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# What Is Earth And Why And How Do We Connect To It?

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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.

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.

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).

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.

Figure 4 is a simplified version of the 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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### 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.

1. Anthony
Sep 02, 2018

How do I subscribe to your magazine

2. GAponte
Jun 22, 2018

Something more about the “why” part. A voltage source has two terminals, power flows from one to the other and that’s it. Power does not go from one terminal to anywhere else. That means that to get electrocuted you need to close the circuit between the terminals.

Ideally, a transmission line does not need to be connected to anything else but the load. If you were to touch any of the terminals, you would be fine even if you stood barefooted on the ground. You’ll get hurt only if you were to touch both terminals at once.

Now, let’s say that by some accident (or fault) one of the lines touches the ground (a fallen branch laying on one line, a bad electrical installation that makes one line touch the earth, some other guy miles away also trying to make the point that he won’t be electrocuted by touching one line while barefooted, etc.). You would not see anything wrong and still believe both lines are isolated, but if you happen to touch the other line, the circuit will close and you’ll be in for a shock.

To make the system safer, connect one terminal to ground, ground being the earth itself or the metal casing of the equipment. Mark the grounded terminal or line. Now you know it has “0” voltage to ground (ideally). A tree branch falling on this terminal will have no effect, but if it were to fall on the other, there will be a short circuit that will knock the system out. You are now dealing with only one live line, with the other at 0 volts, making the danger predictable and the system better to handle.

3. 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?

4. Ahmed Fawzi
Apr 09, 2017

Simple and fantastic article.
Thanks a lot

5. 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
steve

6. Affan
Dec 16, 2016

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

7. 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.

• Justin Guest
Feb 06, 2018

Read about step voltage – the current will create a step voltage but the resistance across the length of an earth worm will be in the order of milli or micro ohms (depending on the soil resistivity). If the current was flowing near the surface of the earth creatures such as snakes or cows could be electrocuted during fault clearance. However, earth rods and tapes are buried at least 600mm below ground surface, partly to reduce the magnitude of the step voltage.

8. mohammed boawainah
Aug 10, 2015

9. Pedro Perez
Jul 20, 2015

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

10. 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.

11. 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.

12. john
May 09, 2015