Complete guide to earthing systems : types, components, calculations, measurements
Types of Earthing Systems
Types of Earthing Systems according to IEC :1) TT system
2) IT system
3) TN system :
• TN-C system
• TN-S system
• TN-C-S system
Neutral & exposed conductive part connections:
First letter : Relationship of the power system to earth:
T : Direct connection of neutral to earth;
I : Neutral isolated from earth, or one connected to earth through impedance.
Second letter : Relationship of the exposed-conductive-parts of the installation to earth :
T : Direct electrical connection of exposed-conductive-parts to earth.
N : Direct electrical connection of the exposed-conductive-parts to neutral.
Arrangement of N & PE conductors :
Subsequent letter(s) (if any) : Arrangement of neutral and protective conductors :
S : Neutral and protective conductor separate (N & PE).
C : Neutral and protective conductor combined (PEN conductor).
C-S : TN-C near the source, TN-S near the loads.
TT system
The source of energy (or the neutral point of the supply transformer) is directly connected to an earth electrode, the exposed conductive parts of the installation at the consumer side being connected to earth through a separate earth electrode which is electrically independent of the supply electrode .
– In the case of isolation fault, the potential of the exposed conductive parts will suddenly increase causing a dangerous situation of electric shock, this can be avoided with the use of RCD’s with the proper sensitivity in function of touch voltage.
– To ensure safety conditions in the installation, the earth values shall comply with :
RA x I∆n ≤50V
– RA = Earth resistance value of the installation.
– I∆n = Residual operating current value of the RCD.
Advantages of TT system
2) Fault currents are too week and can not activate over current protection devices and special devices (residual current devices RCD) can be used to detect the leakage current to ground and interrupt the circuit.
3) Safety of people is ensured by the use of RCD
5) The system can be easily extended without any need to calculate the lengths
7) Faults in LV network don't migrate into other LV customers.
8) In the event of a broken neutral, the TN system doesn't cause any potential rise of exposed conductive parts with the neutral conductor.
1) RCD's must be installed on each outgoing line to achieve horizental discrimination which increases the cost of the system.
2) In some cases, it is difficult to ensure vertical discrimination of RCD's.3) High over voltages may occur between all live parts and between live parts and PE conductor.
4) The level of safety is governed by the value of earth resistance of the system.Applications of TT system
IT system
In this system, the source of energy or the neutral point of the secondary winding of the supply transformer is either isolated from earth or connected to an earth electrode through a high current limiting impedance and the exposed conductive parts of the electrical installation is connected to a separate earth electrode .
– IT earthing system have no direct connection between live parts and earth.
– In case of insulation fault, the value of the fault current is not high enough to generate dangerous voltages.
– when first fault occurs, protection against indirect contact must be provided by an insulation monitoring device which shall provide visual and sonorous alarm.
in case of a second fault, the service shall be interrupted by means of breakers according to the following tripping conditions:
– To ensure safety conditions in the installation, it shall comply with:
RA x Id ≤ 50V
– RA = Earth resistance value of the installation.
– Id = Fault current value of the first fault.
Advantages of IT system
2) Optimal safety during first fault
4) when first fault occurs, no interruption of service is required and the insulation monitoring devices gives an alarm
Disadvantages of IT system
2) Dangerous touch voltage in the event of a double fault.
4) The network insulation shall be monitored permanently.
Applications of IT system
TN system
In this system, the source of energy (or the neutral point of the secondary winding of the supply transformer) is directly earthed at one or more points, the exposed conductive part of the electrical installation is connected to that point via protective conductor (PE) .
– There are three types of TN systems: TN-C, TN-S and TN-C-S
TN-C system
• In this system, a common conductor is used for both the neutral (N) and protective conductor (PE) all the way from the supply transformer to the consumer building, this common conductor is called (PEN) or "Protective earth and neutral conductor"
TN-S system
• In this system, the neutral (N) and protective earth conductor (PE) are separated throughout the system from the supply transformer to the consumer building and are not connected at any point within the building .
This separate protective earth conductor can take the form of the metalic sheath or armour of the underground supply cable .
TN-C-S system
• Part of the system uses a combined PEN conductor, which is at some point split up into separate protective earth PE and neutral N conductors,the combined PEN conductor is used between transformer or source of energy and entry point of the building while the separate conductors (PE & N) are used within the building .
This is usually referred to as protective multiple earthing (PME)
Advantages of TN system
1) In case of insulation fault,the fault or touch voltages in TN earthing system are smaller than in TT system ,this is due to the voltage drop in the phase conductor and the lower impedance of the PEN conductor compared with the TT earthing system.
Disadvantages of TN system
1) Faults in the MV network may migrate into the LV network grounding causing touch voltages at the consumers.
2) In the event of a broken neutral, the TN system causes potential rise of exposed conductive parts with the neutral conductor.
3) Any modification in the LV network causes increase in the fault loop impedance and therfore the protection devices should be fitted.
Applications of TN system
Methods of earthing
Pipe earthing
In this method, a galvanized iron pipe of approved length and diameter is used as an earth electrode and this pipe should have holes pierced at regular intervals along the length and is narrow at the bottom end .
The GI pipe is driven vertically and upright in a permanently wet soil and its size depends on the value of fault current to be carried and the type of soil.
The size of the pipe is usually 38.1 mm in diameter and 2 m in length for ordinary soils and the pipe length should be increased to 2.75 m in case of dry and rocky soils.
The depth at which the pipe should be buried depends upon the moisture condition of the soil.
The bottom of the pipe is surrounded by small pieces of coke or charcoal and salt for a minimum thickness of about 15 cm around the pipe and alternate layers of coke and salt are used till the top of the pipe as coke retains moisure and increases the effective area of the earth and salt decreases the earth resistance.
Another GI pipe which is 19.05 mm in diameter and 1.25 m in length (or longer) is then connected at the top of the GI pipe (the earth electrode) with the help of reducing socket .
A funnel is connected at the top of the 19.05 diameter pipe and the top of pipe with the funnel is placed inside an earth pit of size 30 cm × 30 cm × 30cm (1 ft × 1 ft × 1 ft) and the four sides of the pit have concrete or brick walls and is covered at the top with a cast iron plate .
During dry summer season, the earth resistance increases due to the decrease that happens to the moisure in the soil, so to increase conductivity and have an effective earth during summer , a suitable amount of water ( three or four buckets of water) is poured at regular periods into the funnel connected at the top of the 19 mm diameter pipe, which is further connected at the top of the GI pipe .
GI wire or a strip of GI earth wire is carried in a GI pipe (12 mm in diameter) at a depth of 60 cm below the ground level then this earth wire is fastened to the top of the GI pipe with the help of nuts and bolts .
The cross section area of the earth wire must be sufficient to carry the fault current safely .
Plate earthing
In this method, a plate is used as an earth electrode and the plate is made from one of the following two materials :1) a galvanized iron plate and its size should be not less than 60 cm × 60 cm × 6.35 mm (2 ft × 2 ft × 1/2 in) .
2) a copper plate and its size should be 60 cm × 60 cm × 3.18 mm (2 ft × 2 ft × 1/8 in) .
In the two cases the plate is buried vertically into the ground at a depth of not less than 3 m (10 ft) from the level of ground .
The plate is then surrounded by a small pieces of coke or charcoal and salt for a thickness of not less than 15 cm
A GI pipe of diameter 12.7 mm (1/2inch) is placed horizentally at a depth of 600 mm below the level of ground and then is bended to be in a vertical position till the top of the earthing plate and inside this pipe, an earth wire made of the same material as that of the earthing plate (copper earth wire for copper earthing plate and GI wire for GI earthing plate) is carried from the main earthing busbar till the top of the plate and is bolted tightly to the earthing plate with the help of bolts, nuts and washers .
Another GI pipe is placed at the top of the earthing plate and a funnel is connected at the top of this pipe ,the top of the pipe with the funnel is placed inside an earth pit (the same as above) and a three or four buckets of water is poured into the funnel to keep the earth resistance at minimum value as possible.
Strip or wire earthing
This method is used in rocky areas where excavation work is so difficult.
- Copper strips with minimum cross-section (25 mm × 1.6 mm)
- Galvanized iron strips with minimum cross-section (25 mm × 4 mm)
- Copper conductors with minimum cross-section (3 mm²)
- Galvanized iron conductors with minimum cross-section (6 mm²)
The length of conductors or strips buried horizentaly inside the ground shouldn't be less than 15 meters in order to get the required earth resistance.
earth plates
Earth plates are made of electrolytic solid copper sheet or steel sheet with electrolytic copper bondingEarth plates are also used in places where there is high resistivity soil
The use of earth plates as electrodes significally reduces the earth resistance in stony grounds as they increase the area of contact between the electrode and the ground.
Earth are available in 600mm x 600m and 900mm x 900mm dimensions and 1.5mm or 3mm thick copper.
Earth lattice
Earth lattice are often used for potential grading and are a preferred option on telecommunican towers and other applications to reduce the danger of high step and touch voltages that can cause problems to operators in situations such as high voltage switching.
Earth lattice are used to provide a long-lasting, durable and high reliable earthing solution in high fault current applications as they provide a good earthing contact with a wide surface area of the surrounding soil.Earth lattice are available in many sizes such as (500 × 500, 600 × 600, 900 × 900, 1000 × 1000) and thickness of 2 mm or 3 mm.
Earth lattice provide high corrosion resistance, abrasion resistance and high electrical conductivity.
Rod earthing
It is suitable for sandy areas and it is used for temporary earthing purpose
In this method, an earth rod of 12.7 mm (1/2 inch) diameter or 16 mm (5/8 in) or 19 mm (3/4 in) diameter made of galvanized steel or solid copper or copper clad steel or stainless Steel is driven straight down into the ground manually to a depth not less than 2.44 m (8 ft) (as per nec code)The rod is hammered manually or with the help of pneumatic hammer
The upper end of the rod should be kept below the ground level inside an earth pit the four sides of which have concrete or brick walls and is covered by cast iron plate.
Types of earth rods
Solid copper earth rods
It is made from high conductivity hard drawn copper
Advantages
It is used for soil with high moisure and salt content and is very corrosive
It is used where high fault currents are expected .
Disadvantages
The solid copper is a ductile metal that will often bend when driven into soil so it is not suitable for use when a deep driving into the ground is needed
Copper bonded steel earth rods
Advantages
It is considered the most efficient and cost effective type of earth rods.
It has high mechanical tensile strength and resistance to corrosion at lower cost as compared to other types of earth rods (solid copper or stainless Steel)
The rod is usually treated to avoid oxidation of the copper bonding.
Galvanized Steel earth rods
The zink coating will delay the corrosion of the steel bar by providing a sacrificial barrier.
Advantages
It has good corrosion resistance and conductivity.
Disadvantages
It has the lowest resistance to corrosion
It has the lowest electrical conductivity and poor current carrying capacity.
Applications
Stainless steel earth rods
It is made of stainless steel bar
Advantages
It is used when extremely corrosion resistance and high mechanical strength is needed .
The stainless Steel earth rod is treated with an oxide layer that makes it more resistant to corrosion than copper.
It is used to overcome many of the problems caused by the galvanic corrosion that can take place between buried dissimilar metals being in close proximity to each other .
Stainless Steel earth rod provides high mechanical strength so it is unlikely to break or bend when driven into the ground even in rocky soil.
Disadvantages
It is the most expensive of all types of earth rods .
The current carrying capacity of steel earth rods is poor compared with copper earth rods
Application
The earthing resistance of the grounding electrode
1 the resistance of the electrode itself, which depends on the type of the material it is made of and the contact resistance between the electrode and the connections to it (earh wire and clamp) so to keep this resistance as low as possible, the electrode shall be made of a high conductive material and incorrect terminations and corrosion should be avoided .
2 the contact resistance between the electrode and the surrounding ground it is driven in , so to reduce this resistance to a negligible value ,the electrode should be free of grease and paint and the soil surrounding the electrode shall be packed firmely .
3 the resistance of the surrounding soil which depends on the soil's composition, temperature of soil and moisure content .NEC grounding electrodes Requirements
Pipe electrodes : Electrodes of pipe or conduit shall not be smaller than (3/4 in) and requires corrosion protection where made of iron or steel
steel or iron plate electrodes shall be at least 6.4 mm (¼ in) in thickness , but nonferrous plate electrodes shall be at least 1.5 mm (0.06 in.) in thickness.
Rod electrodes : Iron or steel rods must not be smaller than 15.87 mm (5/8 in) in diameter and shall be galvanized for corrosion protection.
Rods electrodes shall fulfill the following requirements :
1) shall not be less than 1.5 m (5 ft) in length.
2) shall be driven, where practicable, below permanent moisure level.
3) shall be separated at least 1.8 m (6 ft) from any other electrode electrodes including those used for signal circuits, lightning system, or any other purpose.
5) If rock bottom is encountered before the earth rod is 8 ft (2.44 m) into the earth, in this case it should be driven into the ground at an angle not over 45° from the vertical to have at least 8 ft (2.44 m) of its length in the ground.
6) if rock bottom is so shallow that it is not possible to get 8 ft (2.44 m) of the rod in the earth at a 45° angle, then it is necessary to lay the rod horizentaly in a 2½ ft (750 mm) deep trench.
How to reduce earth resistance ?
1) increasing the diameter of the earth rod
2) increasing the length of the earth rod
3) Using multiple rods in parallel
4) Chemical treatment of the soil
increasing the diameter of the earth rod
Doubling the diameter of the earth rod will result in reduction in the earth resistance by approximately 10%
increasing the length of the earth rod
Doubling the length of the earth rod will reduce the earth resistance by around 40%
Using multiple rods in parallel
The NEC requires that a single electrode with a resistance to ground greater than 25 Ω should be augmented by an additional electrode.
- If two well-spaced earth rods (of the same size and depth) are driven into the ground,this will result in reduction in the overall earth resistance by about 40%
- If three rods are used,the reduction in the earth resistance will be 60%
- If four rods are used,the reduction will be 66%
The following curve shows this effect :
Each two earth rods should be separated from each other by a distance not less than the depth to which they are driven or preferably twice the depth and the NEC requires that the minimum spacing between two parallel rods driven into the ground is 6 ft .
Measurement of Soil resistivity (Wenner method)
Procedures
1) A four terminal instrument like megger earth tester is used to measure the earth resistivity
3) Use a separate four lead wires to connect the four electrodes to the four terminals on the instrument as follows:-
- The two outer electrodes (1 & 4) are connected to the two terminals labelled C1 and C2 (current terminals) on instrument
- The two inner electrodes (2 & 3) are connected to the two terminals labelled P1 and P2 (potential terminals) on instrument
V1 and V2 is referred to a very distant point.
Using ohm's law (R=V/I), the earth tester automatically calculates the earth resistance and its value will appear on the screen of tester.
If the soil resistivity measured by the megger earth tester is 20 Ω and the distance between electrodes is 3 m . Calculate the soil resistivity ??
The earthing resistance can be calculated using the following equation :
The following equation can be used to calculate the overall earth resistance for multiple rods:
Calculate the earthing resistance of an earthing electrode of length 3m and its diameter is 16 mm driven in a soil of 50 Ω.m resistivity.
Hence the equivalent earthing resistance will be :
Chemical treatment of the soil
Chemical treatment of the soil is a good way to reduce the earthing resistance of the grounding electrode in situations when we can not drive the electrode into a deeper depths because of hard underlying rock and when using multiple rods is not practical.
Magnesium sulfate is the most widely used chemical compound because of its low cost and and it has high electrical conductivity and the least corrosive effect .
Installation of chemical compounds
2) Supply a little water into the hole for good absorption of salts (chemicals) periodically.
One of the good advantages of chemical treatment is that it reduces the seasonable variation on resistance that results from wetting and drying out of the soil.
Bentonite is an off-white sodium montmorillonite clay formed from altered volcanic ash.
It is a moisure retaining clay that is used for reducing the contact earth resistance and increase the effective size of grounding electrodes as a backfill for grounding rods installed in buried holes or as a layer encapsulating earth conductors buried horizentaly in trench.
Advantages of Bentonite as a backfill material
1 Bentonite is a super absorbant material as when it comes into contact with water, it will absorp up to five times its weight and expand up to thirteen times its original dry volume so it increases the serface area contact between the grounding electrode and the surrounding soil which improves the total earthing resistance.2 Bentonite is strongly hydrated in water so it has a great water absorption capability
3 Bentonite has the ability to absorb moisure from the surrounding Soil and to retain water or its moisure content for a considerable period of time at atmospheric pressure, and this will reduce the earth resistance.
4 Bentonite is a non-corrosive material even for a long period of time so it protects the grounding electrode.
5 Bentonite is a stable material because it does not change its characteristics as the time elapses.
6 Bentonite is cost-effective as we can get the required earth resistance by using a little amount of it when compared with other cement based solutions.7 The resistivity of Bentonite depends on its moisure content as its resistivity varies from 3 ohm.m in wet condition and upwards to 18 ohm.m in dry condition .
8 Bentonite can be in the form of granular or powder, the granular form is preferred to be used for filling trenches while powder form is preferred for pouring into boreholes.9 As there is a moisure in the soil, bentonite will retain enclosing the buried earthing rod or electrode and will not get washed away.hence we will not need to replace the bentonite
10 Bentonite shouldn't be used in very dry locations and free draining locations.Bentonite installation
Mixing bentonite
As a rule of thumb assume an expansion ratio of 2:1
1×25 kg bag = 1 ft³ or 0.0283 m³ (dry)
1×25 kg bag = 2 ft³ or 0.0566 m³ (wet)
Example
If we have 4 earth rods inserted in boreholes (10 cm wide , 3 m deep), calculate the quantity of bentonite required for backfilling these boreholes??
Volume of boreholes = 2 × ( r² × h) = 2 × (3.1416 × (0.1)² × 3)
Number of bags = / 0.0566 = bag
1) At the desired electrode location, dig a hole of 75 - 100 cm (3 - 4 in) wide at a depth that is determined by the earthing system's designer.
2) Insert the electrode or rod vertically in the center of the borehole with its top at the correct level for wire connections.
4) remove excess standing water from the trench
1) At the desired rod location, dig a trench of 200 - 300 mm (8 - 12 in) wide at a depth of 600 mm below the ground level or the depth determined by the designer of the earthing system.
3) Apply another layer of bentonite 25 - 50 cm (1 -2 in) thick and ensure that the earth strip is fully covered .
Advantages of GEM as a backfill material
1 Ground Enhancement Material (GEM) is a superior conductive material that improves grounding effectiveness especially in areas of poor conductivity and solves the toughest grounding problems.
2 GEM is a low-resistance, non-corrosive, carbon dust-based material that improves grounding effectiveness, regardless of soil conditions. It is the ideal material to use in areas of poor conductivity, such as sandy soil, rocky ground and mountain tops.
3 GEM contains portland cement, which sets in 3 days and fully cures within 28 days , to become a conductive concrete that is permanent, maintenance-free and will never leach or wash away.
4 GEM is the best material that you can use for reducing the grounding resistance .
5 GEM maintains a permanent low earth resistance and provides high conductivity for the life of the grounding system once in its set form.
6 GEM does not adversely affect soil as it doesn't contain any hazardous chemicals so it will not pollute the soil or the ground water.
7 GEM does not depend on the continuous presence of water to maintain its conductivity.
8 GEM is a permanent material that doesn't decompose, dissolve or leach out as time elapses.
9 GEM doesn't require periodic charging treatment nor replacement.
10 GEM performs in all soil conditions irrespective of the presence of water.
11 GEM is easy to install as it doesn't need any special tools and requires only one man to install and also requires no maintenance.
12 GEM can be easily installed in both dry or slurry form.
13 GEM is little affected by freezing as freezing increases the resistivity by only 10 - 15%.
14 GEM is non corrosive as it contains a corrosion inhibitor that forms a film on the rod surface creating a barrier against corrosion.
15 GEM reduces theft and vandalism as grounding rods and conductors are hard to remove when set in concrete.
16 GEM has a very low resistivity (less than 0.02 ohm.m) which is only 1% the resistivity of bentonite clay.
Applications
Grounding enhancement material is the ideal material to use in areas of poor conductivity, such as sandy soil, rocky ground and mountain tops.
GEM installation
2) Mix GEM into a slurry form by using a cement mixer or mix in a mixing box or bucket. Use 1.5 to 2 gallons (5.7 to 7.6 liters) of clean water per bag of GEM.
Do not mix GEM with salt water.
3) Cover the bottom of the trench with a layer of GEM 2 cm (5 inch) thick.
5) Apply another layer of GEM on top of conductor and make sure that the conductor is fully covered – about 2 in (5 cm) deep.
Notes :-
- You must apply 10 cm (4 inches) of insulating material to the conductors and ground rods exiting the GEM, starting 2 inches (5 cm) inside the GEM.
- Excess standing water must be removed from trench.
1) Auger a hole of 3-inch (7.5 cm) or larger diameter to a depth of 6 inches (15 cm) shorter than the length of the earth rod.
The top of the earth rod will be approximately 6 inches (15 cm) below ground level.
3) Mix GEM into a slurry form. Use 1.5 to 2 gallons (5.7 to 7.6 liters) of clean water per bag of GEM.
The installation of GEM in a dry state is acceptable for vertical earth rod applications.
5) Wait 30 minutes to 1 hour then fill the hole with soil backfill.
- You must apply 10 cm (4 inches) of insulating material to the conductors and ground rods exiting the GEM, starting 2 inches (5 cm) inside the GEM.
- Excess standing water must be removed from trench
