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