There is a famous quote that goes like
- The safe way is the best way
Electricity is a fascinating form of energy, but could be dangerous if not handled properly. Utilizing its potential while ensuring man and machine safety is equally important. One of the best ways to keep a check on electrical hazards is by Earthing every electricity operated equipment.
Earthing not only ensures the safety of man and equipment from fatal damage but is also important for limiting the magnitude and duration of the ground-fault current. A well-designed system earthing ensures safe operations and really helps to check even fire or explosive hazards, hence ensures better performance of the electrical system.
In this article we will learn about types of Low Voltage (LV) earthing systems, that are implemented to ensure electrical safety and the significance of terminologies used to express them.
As per the International Electrotechnical Committee standard IEC 60364, LV system earthing for both, source-side and the load side can be broadly designed in three ways:
1) TT System
2) TN (C/S) System
3) IT System
The first question that might come to our mind is what do these terms T, N, I, C and S mean?
T- Solid/effectively earthed
N- Load side earthing is connected to source-side earthing only
I- Isolated or No earthing connected or Un-earthed
C- Combined
S- Separate
PE- Potential Earth or directly connected to earth
Okay, let us now look at these terms from a broader perspective.
1) TT system
The first letter signifies the source side of the earthing arrangement and similarly the second letter signifies the load side arrangement. First T means that the source neutral is solidly earthed. While, the second T means that the load side is also solid earthed but to a separate earthing, might be to a local earth pit (as illustrated below). As both source and load have their separate earthing arrangements, Earth fault loop impedance is very high. With an RCD (residual-current device) on the load side it makes the TT system safe and reliable.
2) TN System
Here the first letter signifies the source side is solidly earthed. N signifies that the neutral is connected to the source side earthing. This is again of three types,
I) TN-S
II) TN-C
III) TN-C-S
I) TN-S
In TN-S, S stands for separate while, neutral and earth are connected on the source site, but are also available at the load side (as illustrated below). Since the earth and neutral at the source side are combined, it has very low earth fault impedance and a separate conductor provided at the source side neutral that makes the system safe.
II) TN-C
In TN-C, C stands for combined i.e. earth and neutral are combined all the way from the source to the load (as illustrated below). Since the earth and the neutral at the source side are combined, so it has very low earth fault impedance and there is no separate conductor provided for earthing from the source side. This makes the system a little unsafe. The conductor from source is used for the earth as well as the neutral this saves the cost of another conductor hence, this is economic to install.
III) TN-C-S
In the TN-C-S system, the source-side has combined earth and neutral conductors. At the load side, a separate earthing conductor is provided by tapping from the neutral conductor connected from the source to the load (as illustrated below). This system counters the disadvantages of the TN-C system by, providing earth conductor tapped between the source and the load. It is also a cost-effective system, as the installation requires only one neutral conductor from the source to the load and at the load side earthing is given by tapping the neutral from the source.
3) IT System
The first letter I signifies that the source is un-earthed or the neutral point of the source is completely isolated. The second term T signifies that the load side is solidly earthed. Since the source is isolated and the Load side is separately Earthed locally, this makes this system earth fault impedance very high. IT system is moderately safe but has certain advantages such as, (i) continuity in operation (ii) less downtime (iii) least signal interference.
Summary
In India, Most of Industries as well as our electricity distribution system is designed on the TN-S system of earthing. The purpose of earthing is to provide a safe and continuous operation of electrically operated equipments. So far in this article we have learned the types of system earthing and terminologies used to express them. For various applications like Hospitals, households, Laboratories, construction sites, etc we can use these types of systems based on their priority or needs.
- Anshu Prakash
Electrical engineer
Yash Pakka Pvt Ltd
E-mail: send2anshup@gmail.com
- The safe way is the best way
Electricity is a fascinating form of energy, but could be dangerous if not handled properly. Utilizing its potential while ensuring man and machine safety is equally important. One of the best ways to keep a check on electrical hazards is by Earthing every electricity operated equipment.
Earthing not only ensures the safety of man and equipment from fatal damage but is also important for limiting the magnitude and duration of the ground-fault current. A well-designed system earthing ensures safe operations and really helps to check even fire or explosive hazards, hence ensures better performance of the electrical system.
In this article we will learn about types of Low Voltage (LV) earthing systems, that are implemented to ensure electrical safety and the significance of terminologies used to express them.
As per the International Electrotechnical Committee standard IEC 60364, LV system earthing for both, source-side and the load side can be broadly designed in three ways:
1) TT System
2) TN (C/S) System
3) IT System
The first question that might come to our mind is what do these terms T, N, I, C and S mean?
T- Solid/effectively earthed
N- Load side earthing is connected to source-side earthing only
I- Isolated or No earthing connected or Un-earthed
C- Combined
S- Separate
PE- Potential Earth or directly connected to earth
Okay, let us now look at these terms from a broader perspective.
1) TT system
The first letter signifies the source side of the earthing arrangement and similarly the second letter signifies the load side arrangement. First T means that the source neutral is solidly earthed. While, the second T means that the load side is also solid earthed but to a separate earthing, might be to a local earth pit (as illustrated below). As both source and load have their separate earthing arrangements, Earth fault loop impedance is very high. With an RCD (residual-current device) on the load side it makes the TT system safe and reliable.
 |
| TT system of earthing |
2) TN System
Here the first letter signifies the source side is solidly earthed. N signifies that the neutral is connected to the source side earthing. This is again of three types,
I) TN-S
II) TN-C
III) TN-C-S
I) TN-S
In TN-S, S stands for separate while, neutral and earth are connected on the source site, but are also available at the load side (as illustrated below). Since the earth and neutral at the source side are combined, it has very low earth fault impedance and a separate conductor provided at the source side neutral that makes the system safe.
 |
| TN-S system of earthing |
II) TN-C
In TN-C, C stands for combined i.e. earth and neutral are combined all the way from the source to the load (as illustrated below). Since the earth and the neutral at the source side are combined, so it has very low earth fault impedance and there is no separate conductor provided for earthing from the source side. This makes the system a little unsafe. The conductor from source is used for the earth as well as the neutral this saves the cost of another conductor hence, this is economic to install.
 |
| TN-C system of earthing |
III) TN-C-S
In the TN-C-S system, the source-side has combined earth and neutral conductors. At the load side, a separate earthing conductor is provided by tapping from the neutral conductor connected from the source to the load (as illustrated below). This system counters the disadvantages of the TN-C system by, providing earth conductor tapped between the source and the load. It is also a cost-effective system, as the installation requires only one neutral conductor from the source to the load and at the load side earthing is given by tapping the neutral from the source.
 |
| TN-C-S system of earthing |
3) IT System
The first letter I signifies that the source is un-earthed or the neutral point of the source is completely isolated. The second term T signifies that the load side is solidly earthed. Since the source is isolated and the Load side is separately Earthed locally, this makes this system earth fault impedance very high. IT system is moderately safe but has certain advantages such as, (i) continuity in operation (ii) less downtime (iii) least signal interference.
 |
| IT system of earthing |
Summary
In India, Most of Industries as well as our electricity distribution system is designed on the TN-S system of earthing. The purpose of earthing is to provide a safe and continuous operation of electrically operated equipments. So far in this article we have learned the types of system earthing and terminologies used to express them. For various applications like Hospitals, households, Laboratories, construction sites, etc we can use these types of systems based on their priority or needs.
- Anshu Prakash
Electrical engineer
Yash Pakka Pvt Ltd
E-mail: send2anshup@gmail.com