The most perfect, for today, tN-S grounding system (the type of electrical network) is highly recommended for the use of the PUE (Rules of the Electrical Installation Device). In Russia, up to now, a system similar to TN-C is used (the TN-C system is banned in new construction, in single-phase and direct-current circuits, this requirement does not apply to branches from a high voltage line up to 1 kV to single-phase electricity consumers - PUE 1.7.132).
TN-S grounding system - from the supply substations to the consumer there are two different zero wires: N - working zero and PE - protective zero, thereby providing the greatest electrical safety, both for human and for electric consumers.
In case of breakdown on the body, the leakage current goes along the zeroing (grounding) conductor to the protective zero - PE, which triggers the tripping RCD (currents through the differential transformer to the load and back are not equal). And with a large leakage current is triggered circuit breaker
In general, the grounding system "TN-S", was first developed in the 1930s and introduced in the territory of European countries, where the last 50 years is the main scheme for the protection of electricity consumers. Most likely, the same task is also faced by Russian enterprises of electrical networks, since when designing new lines of power supply development, it is recommended to use a five-core wiring for three-phase inputs and three-core - for single-phase connection, starting from the power source and ending with the outlet of a particular subscriber. As you know - recommendations very often turn into norms and provisions of standards, but for the time being one of the stages of such a transition is mandatory electrical installation in the system earthing TN-S-S, since the direct transition from TN-S to TN-S is associated with large capital investments and is comparable to the construction of a new HPP.
Fig. 1. TN-C System
Fig. 2. TN-S system
Fig. 3. TN-C-S System
What is so wonderful about it, if it is required, let it be a gradual but compulsory transition? To find this out, first of all, consider its electrical circuit. It is completely identical with the traditional power supply system, where a neutral conductor is included in addition to current-carrying lines, with the important difference that another zero conductor is added to the circuit, which does not require re-grounding either on the "N" line or on the "PE" line, which is carried out only on the initial power source. Thus, allowing to divide their working and protective functions on separate power rails. That is, the working conductor "N" performs only the functions of the EMF (electromotive force is a physical quantity characterizing the operation of external (non-potential) forces in direct or alternating current sources.) In a closed conducting circuit, the EMF is equal to the work of these forces by the displacement of a single positive charge along the contour) and the conductor "PE" - only the protection functions, while ensuring complete isolation from each other. This wiring diagram is especially relevant, in the cutting of problems, when there is absolutely no control over the condition of the protective grounded circuits, as can be seen, the need for this completely disappears.
Now, after the electrical circuit has been cleared up, it becomes obvious that such a TN-S grounding system maximizes the protection of electrical equipment and the person himself. Moreover, it eliminates high-frequency interference and other interference on consumer lines coming from some devices. A similar situation, for sure, was observed by each of us, when someone used an electric shaver, sometimes a drill or a welding machine in a nearby entrance, then jerky distortions appeared on the TV screen. Such a system, if not completely, then most of the interference, oscillatory and electromagnetic excitations, occasionally arising in electric networks, necessarily excludes. Therefore, the grounding system "TN-S" is very fond of employees who work with information, telecommunications, radar or locating equipment, since the maximum isolation from the housings and housings of other electrical devices, as well as interference through the "ground", in other words, from sources of interference.
Legend of grounding systems:
First letter - the state of the source neutral relative to the ground.
T - grounded neutral.
I - isolated neutral.
Second letter - the state of the open conductive parts relative to the ground.
T - the exposed conductive parts are grounded regardless of the attitude to the earth of the neutral of the power supply or any point of the supply network.
N - the open conductive parts are connected to the grounded neutral of the power source.
Letters after N - combination in one conductor or separation of functions of the zero working and zero protective conductors.
S - The zero working (N) and the zero protective (PE) conductors are separated.
FROM - the functions of the zero protective and zero working conductors are combined in one conductor (PEN conductor).
- Remove the tension that has arisen in a safety-threatening place, in a place where it does not harm anyone: this is the place-the land. Grounding connects all current-carrying parts, which in normal operation mode are not located under U, with ground.
Zeroing - is the connection of all parts of the appliance, which must not be under U, with a working zero. In this case, if there is a break in the phase for live parts under working zero, then a short circuit will occur and the circuit breaker will de-energize the appliance. This is of course less secure than grounding, a short circuit can cause subsequent malfunctions in the device. Unfortunately, zoning is the main type of protection in most residential areas.
Earthing systems
Consider the systems used in household premises:
TN-C
The first letter T means that the neutral of the power source is connected to the ground, which means that the conductor of the working zero at the substation goes into the ground. The second letter - N - means the connection of the open conductive parts of the electrical installation of the building with the ground point of the power source. The third letter is C-means that the protective and working zero are on the same common PEN, that is, the working zero and is protective. In fact, this system is the same "zanuleniem". The most insecure of systems. All current-carrying parts that must not be under U are under working zero. The defense is built upon the action of the machine after a short circuit. The protective and working zero is in one conductor to the switchboard. 
Figure 1 TN-C system
1 - open conductive parts.
2- power supply
3- switchboard to the apartment.
TN-S
The first two letters also, as in the previous system, mean that the neutral of the power source is connected to the ground (which is located at the power source) and the open conductive parts of the electrical installation of the building are connected to the ground point of the power source. The third letter-S- means that the zero and protective PE and the working N are on different conductors (grounding). This means that two separate wires are sent from the power station to the working zero and ground. This system is the safest for multi-storey buildings.
Figure 2 TN-S system
2-power supply
The diagram shows that two separate wires are sent from the power source to the working zero and to the ground, then the conductors are not met.
TN-C-S
It is a modernized TN-C system. The functions of the zero working and zero protective conductors are combined in one conductor in the part of the network that comes from the power source. Then a grounded conductor is added to a certain area. For multi-storey houses, a normally grounded conductor is added to the ASP (input switchgear at home). This system also provides sufficient security. 
Figure 3 TN-C-S system
1-open conductive parts
2-power supply
3-Distribution board for an apartment
The diagram shows the network before modernization - TN-C system and after modernization - TN-C-S system.
TT system
Usually used in the construction of private houses. The second letter T means that grounding and working zero are not connected anywhere. The first letter has already been mentioned above. The house comes in the same way as in the TN-S system, three wires: a working zero, a phase wire and a ground wire. Only here the grounding wire does not go from the power source (as in the TN-S system), but near the private house its own ground loop is mounted according to all the rules of the electrical installation (rules for the installation of electrical installations), it is from the ground loop and the ground wire is going.
Figure 4 TT system
1-open conductive parts
2-power supply
3-earth ground loop at the private house and the conductor leaving it.
The power supply systems are classified by the International Electrotechnical Commission (IEC), depending on the method of grounding of the distribution network and the measures of protection against electric shock applied. Distribution networks are divided into networks with earthed neutral and networks with isolated neutral. The IEC-364 standard divides distribution networks depending on the configuration of current-carrying conductors, including the zero working (neutral) conductor, and the types of grounding systems. The following notation is used. The first letter, I or T, characterizes the connection to the earth of current-carrying conductors (network grounding). The second letter, T or N, characterizes the connection to the ground of open conductive parts (HRE) and third-party conductive parts (HF) (equipment grounding).
The first letter (I or T) The first letter I means that all current-carrying parts are isolated from the ground, or that one point of the network is connected to the ground through a resistance, or through an arrester or an air gap. Networks with isolated neutral (I) can be: (1) very small networks, such as ultra-low voltage safety networks (SELV or SELV) with electrical separation by means of separating transformers, or (2) medium-size networks such as those used for power separate workshops, or (3) distribution networks for powering entire areas of the city, such as three-phase networks with a voltage of 230 V (IT system). In the past, in Europe the IT system was usually used, but then it was replaced almost everywhere with systems with grounded neutral.
There are several reasons for this replacement. One of these reasons is overvoltage protection. Only in Norway, the IT system is still widely used. The system with an isolated neutral is gradually replaced by a three-phase 230/400 V system with a grounded neutral. Everywhere in the world, the use of the IT system is limited to a special application in those industries where a break in power supply can be dangerous. For example, for the supply of explosive industries.
The first letter T indicates a direct connection of at least one point of the network to the ground (terra). For example, a three-phase distribution network powered by a secondary winding connected to a star, with a neutral conductor, voltage 127/220 V or 220/380 V with a neutral connected to ground via a grounding device.
Special requirements for grounding devices, depending on the type of network, will be discussed in later chapters.
Second letter (T or N) The second letter denotes the type of connection between the HRE, the protective earth conductor (equipment earthing) of the electrical installation and the ground. The second letter T means a direct connection between HRE and the HRO and ground (terra), independent of the system ground, which may or may not contain live parts of the system. The second letter N means a direct connection between HRE and HH with a grounded point (s) of the network through a PEN or PE conductor. Network grounding and measures of protection against electric shock are subject, each, to independent consideration.
Network (working) and protective earthing
| System designation | Mains ground |
Protective earthing of conductive parts |
There is no direct connection to the ground. It is allowed to connect to the ground through a resistance, an air gap, an arrester, and so on. |
||
Connection to ground in one or several points of the distribution network outside the consumer's network |
Direct ground connection, independent of mains ground |
|
Connection to the ground at one or several points of the distribution network and at one or more points in the customer's network |
Connection to the "network ground" using a PE or PEN conductor |
|
Ground connection at one or more points in the distribution network |
There are no connections to the ground and to the network ground |
Current-carrying parts of the network are connected to ground to limit the voltage that may appear on them as a result of a direct lightning strike (pdm) or secondary manifestations of lightning (induced overvoltage waves), or as a result of unintended contact with higher voltage lines, or as a result of breakdown of insulation of current-carrying parts of the distribution network.
The reasons why the live parts of the distribution network are not connected to the ground are as follows: in order to avoid interruption of the consumer's supply with a single damage (breakdown of the insulation on the ground of the current-carrying suits of the distribution network); to avoid sparking in explosive and fire hazardous areas with a single damage to the insulation of live parts of the network. The grounding of electrical equipment, or more precisely the grounding of exposed conductive parts (HRE), is one of many measures that can be used to protect against electric shock. Grounding HRE involves creating an equipotential medium, which reduces the likelihood of stress on the human body. In the TN system, the grounding of the HRE provides the creation for a short-circuit current with a low resistance. This facilitates the operation of overcurrent protection devices.
The designations TN, TT and IT refer only to the distribution network configuration. These designations are of limited relevance to various methods that can be used to provide protection against electric shock, including grounding of HRE. Although each system is provided by connecting the HRE to the ground, the effective method used in the installation for protection against electric shock may include other protection measures, such as double insulation.
The configuration of the distribution network and the measures used to protect against electric shock are, each, the subject of independent consideration.
In Fig. 1. - 5. systems of three-phase networks are given. The symbols used in the figures have the following meaning:
T - direct connection of one point of current-carrying parts of the power source to the ground,
I - all current-carrying parts are isolated from the ground, or one point is grounded through the resistance.
The second letter is the nature of the grounding of the open conductive parts (HRE) of the electrical installation:
T - direct connection of HRE to the ground, regardless of the nature of the connection of the source of litany to the ground,
N - direct connection of the HRE to the ground point of the power supply (in systems of alternating current it is usually grounded by neutral).
Subsequent letters (if any) are the device of the zero working and zero protective conductor.
S - the function of the zero protective and zero working conductor is provided by separate conductors.
C - the functions of the zero protective and zero working conductors are combined in one conductor (PEN-conductor).
TN System
The supply networks of the TN system have a point directly connected to the ground. Open conductive parts of the electrical installation are connected to this point by means of zero protective conductors.
Depending on the device of the neutral working and neutral protective conductors, the following three types of the TN system are distinguished:
System TN-S - zero working and zero protective conductors operate separately throughout the system.
Fig. 1. System TN-S (zero working and zero protective conductors operate separately) 1 - grounding of the power supply; 2 - open conductive parts
The TN-C-S system - the functions of the zero working and zero protective conductors are combined in one conductor in a part of the network. 
Fig. 2. TN-C-S system (in the network part, the zero working and zero protective conductors are combined) 1 - grounding of the power supply; 2 - open conductive parts
The TN-C system - the functions of the zero working and zero protective conductors are combined in one conductor throughout the network. 
Fig. 3. The TN-C system (zero working and zero protective conductors are connected all over the network) 1 - grounding of the power supply; 2 - open conductive parts
TT system
The supply network of the TT system has a point directly connected to the ground, and the open conductive parts of the electrical installation are connected to the earth electrode, which is electrically independent from the neutral of the power supply neutral. 
Fig. 4. The TT system
1 - grounding of the power source; 2 - open conductive parts; 3 - grounding of the equipment casings
IT system
The feeding network of the IT system does not have a direct connection between current-carrying parts and the ground, and the open conductive parts of the electrical installation are grounded. 
Fig. 5. IT system
1 - resistance; 2 - grounding of the power source; 3 - open conductive parts; 4 - grounding of the equipment casings