Earthing Advantages

U need a residual current to earth network to operate an RCD dude.

But the RCD trips if there is unbalance between line and neutral currents. what is the function of the earthing here?

RCD will not act on unbalance between line and neutral. If properly installed, RCD is used to encompass all 4 wires in a 3phase/4wire system. The summation CT will take the vector sum of all four currents, and will act only when their resultant is not zero. So, an earth is a must.

Thanks for your reply, but if i touch an unearthed refrigerator body or water pipe " not connected or bonded to the earthing terminal " and a live conductor in the same time, i think that the RCD will trip.

Theoretically speakig, if the refrigerator is NOT earthed, and if its insulation is good, you will not get any shock. RCD will not see any current and will not trip.

Please download and read this paper, it is very informative:

ect114.pdf

sry but i meant with unearthed that there is no connection with the earth via the protective bonding conductor, look at the picture, the pipe water is already connected to the earth so why i need to connect it via the protective bonding conductor?
there is a fault current path through my body, water pipe, earth to the transformer earthing rod. if i use a 30 mA RCD so the min. current needed in the live conductor is 30mA in the case of there is no load connected, hence the max. resistance= 220 v / 30 mA = 7.3 kohm, if the transformer is adjacent to my house so the total resistance will not reaches this value so why i need to earthing my house?
i hope u understand me.

Well explained. i have a doubt here though...as per the IEC designations which are given in the Schneider ECT114, the system you show as TT seems to be actually TN-C.

Anyhow, if you touch that refrigerator, AND if there is no insulation failure, you will not get a shock. If there is an insulation failure, the RCD will trip anyway(provided the leakage is =>30mA), because the vector sum of the currents is no longer zero. If you touch a live wire and an earthed pipe, the RCD should trip, again if the overall resistance of the path results in a current of =>30mA.

However, if your body resistance+earth resistance to the transformer neutral is >8kΩ, then the RCD may not trip, but you will suffer from various ill-effects as shown in this chart.

It is better to minimise this risk by having a specific earth conductor and not depend on the conductivity of the soil only.

i suppose you are asking from an academic angle since earthing is mandatory as per most Codes.

So the main function of earthing is to reduce the fault loop resistance to a specified value so the fault current is high enough to detected.
if the fault happened between line and neutral conductors via my body, the RCD or ELCB will never trip and the fault current is not high enough to trip the MCB. what is the solution for this problem?

i am perplexed... how do you propose to create a fault through your body between line and neutral ? Difficult i think ? You need to stand on an insulated stool and touch both line and neutral.

On the other hand, if you stand on earth and touch line conductor (God, my electrical engineer's body shivers!) the current through you will not flow through the neutral, so the RCD will trip. No issues, i hope?

The maximum resistance should be calculated using the touch voltage of 50V and not 230V. For an adult allowable leakage is 30mA,but for kids it is much low(may be 5mA) Refer reputed manufacturers like ABB,Siemens,Schneider etc

• The earthing system is there to operate the circuit protective device(s) in the event of a fault.
• The circuit breaker is there to protect the wiring against overheating and fire, and will operate if the current exceeds its setpoint value.
• The RCD is there to prevent death by electric shock, and will operate when the current imbalance between the phase conductor(s) and any neutral conductor exceeds its set value.
• The value of the human body impedance is usually low enough to operate an RCD that trips at 30mA, irrespective of the supply voltage.

If in doubt, consult a qualified electrician.

If the metal work is not earthed,the voltage due to leakage might rise to a value closer or higher than the allowable touch voltage and endanger persons/children touching it. There are RCD with low tripping currents(about 5mA) used in children wards. This voltage will depend upon the resistance of the contact made by metal to live wire. If the metal work is earthed there will be a leakage current flowing through the RCD causing it to trip and ensure safety of persons.

Some advantages already mentioned to you and let me also add some information for you:

the main advantage to earthing is that the whole electrical system is tied to the potential of the general mass of the earth and can not float at another potential. By connecting earth to metal work a path is provided for fault current which can be detected by a protective conductor.

the disadvantage of earthing is mainly the cost to provide proctive conductors, earth electrodes and etc.

The four basic systems available for earthing system. Each one has its own detail and I am not going to detail.

The place where the consumer system has a separate earthing arrangement from the substation earthing system it is known as a TT installation. As such it may have a very high earth return impedance through the ground and must accordingly be protected by RCDs. A 30 mA RCD for example will operate with an earth loop impedance Z of up to

Z = 230 volt/ 0.03 = 7667 ohm.

Taking 50 volt as the maximum allowable safe value, it is necessary to ensure that the RCD will trip before the product of the earth leakage current and the earth loop impedance Z exceed 50 volt. The RCD rated sensitivity In must such that

In x Z < 50.

A TT system may also be used where it is not possible to creat an equipotential zone. TT system depends upon sensitive RCD protection but requires some determination of the earth fault loop impedance in order to specify the RCD sensitivity.

Maximum permissible earth loop impedance Z for a TT system protected by a 30 mA RCD will be

0.03 X Z = 50
Z = 1670 ohm.

its clear. but i still confused, the walls, ceiling and floor of the building act as an earthing electrode so why i need to install an earthing rod?

could u tell me the concrete resistivity plz?

I dont know the concrete resistivity and you can find in google but I will give you some information regarding to grounding and ground rode.

The main pupose of ground rode is safety and ground rods are installed to the electrical system of a building to prevent electrical shock of equipment or individuals. inserting the ground rod into the ground will assure adequate contact with moist dirt and allow for conduction.

There are seven common grounding methods and I will just give you a brief explaination about each of this.

1-Connecting to building frame, the metal frame of the building has to be grounded to the earth. This connection should have a sufficiently low impedance and enough current carrying capacity to prevent an accumulation of voltage within the frame.

2-connecting to underground water supply is a common type of grounding system. The underground metallic water pipe must be in direct contact with the earth for a distance of at least 10 feet.

3- the Ufer ground system is installed in a building foundation when the foundation is poured the Ufer electrode has to be surrounded by at least 2" of concrete poured near the bottom of a foundation.

4- connecting to a bare copper ring that encircles the building in direct contact with the earth at a depth of not less than 2.5 feet. the bare copper conductor has to be at least 20 feet long and not smaller than 2 AWG.

5- A driven ground rod or electrode has to be installed so that at least 8 feet of electrode is in contact with the earth. The electrode has to be made of steel or iron and must be at least 5/8" in diameter.

6- Connecting to pltate electrode, plate electrode are acceptable for grounding if the electrode has at least 2 square feet of sureface exposed to the earth.

7- other grounding requirements of electrical equipement need a separate grounding system. Metallic raceway serving the equipment and the metallic enclosure may have to be grounded. electrically connected equipment housings must also be ground.