What are different AC Power Systems (TN, TT & IT earthing) and which one should you choose?

Anyone who works with electric vehicle charging stations hardware and installation might have come across different power system terminologies such as TN-S, TN-C, TN-C-S, TT & IT.

What are they? How are they different from each other? Why can’t we have one standard earthing scheme? What reasons make the electrical installers & manufacturers to choose these different schemes?

This article may give an quick (and hope a simplified) explanation for all the above.

Different AC power distribution systems

Electrical installers across the world may call the distribution systems in different names: like three phase- three wire system, three phase – four wire system, single phase- one wire, single phase = two wire system…etc.

But to bring an uniform definition, International Electrotechnical Commission (IEC), under the standard IEC 60364-3 classified the AC power distribution systems, according to the different grounding methods as: TN, TT & IT systems; and the TN system is further separated into TN-C, TN-S, TN-C-S.

First letter: denotes the relationship of the power distribution system to earth

➤ T – means direct connection of one pole to earth
➤ I – means system isolated from earth, or one point connected to earth through an impedance.

Second letter: denotes grounding of the equipment
➤ T - means direct electrical connection of the equipment to earth, independently of the grounding of any point of the power distribution system.
➤ N means direct electrical connection of the equipment to the earthed point of the power distribution system ( in ac systems, the earthed point of the power distribution system is normally the neutral point or, if a neutral point is not available, a phase conductor).

Third letter: denotes the arrangement of neutral and protective conductors
➤ S - means the protective function is provided by a conductor separate from the neutral or from the earthed line (or in ac systems, earthed phase) conductor.
➤ C - means the neutral and protective functions are combined in a single conductor (PEN conductor). 

 

Characteristics of different power / earthing systems

TN-C earthing:

The TN-C mode power supply system uses the working neutral line as the zero-crossing protection line, which can be called the protection neutral line and can be represented by PEN.

TN-C-S earthing:

For the temporary power supply of the TN-C-S system, if the front part is powered by the TN-C method, and the construction code specifies that the construction site must use the TN-S power supply system, the total distribution box can be divided at the rear part of the system.

TN-S earthing

The TN-S mode power supply system is a power supply system that strictly separates the working neutral N from the dedicated protection line PE. It is called the TN-S power supply system.

TT power supply system

The TT method refers to a protective system that directly grounds the metal housing of an electrical device, which is called a protective earthing system, also called a TT system. The first symbol T indicates that the neutral point of the power system is directly grounded; the second symbol T indicates that the conductive part of the load device that is not exposed to the live body is directly connected to the ground, regardless of how the system is grounded. All grounding of the load in the TT system is called protective grounding.

The characteristics of this power supply system are as follows.

1) When the metal shell of the electrical equipment is charged (the phase line touches the shell or the equipment insulation is damaged and leaks), the grounding protection can greatly reduce the risk of electric shock. However, low-voltage circuit breakers (automatic switches) do not necessarily trip, causing the earth-leakage voltage of the leakage device to be higher than the safe voltage, which is a dangerous voltage.

2) When the leakage current is relatively small, even a fuse may not be able to blow. Therefore, a leakage protector is also required for protection. Therefore, the TT system is difficult to popularize.

3) The grounding device of the TT system consumes a lot of steel, and it is difficult to recycle, time, and materials.

At present, some construction units use the TT system. When the construction unit borrows its power supply for temporary use of electricity, a special protection line is used to reduce the amount of steel used for the grounding device.

TN power supply system

In the TN system, that is, the three-phase five-wire system, the N-line and the PE-line are separately laid and insulated from each other, and the PE line is connected to the housing of the electrical device instead of the N-line.

Therefore, the most important thing we care about is the potential of the PE wire, not the potential of the N wire, so repeated grounding in a TN-S system is not a repeated grounding of the N wire. If the PE line and N line are grounded together, because the PE line and N line are connected at the repeated grounding point, the line between the repeated grounding point and the working ground point of the distribution transformer has no difference between the PE line and the N line.

The original line is the N line. The neutral current that is assumed is shared by the N line and the PE line, and part of the current is shunted through the repeated grounding point. Because it can be considered that there is no PE line on the front side of the repeated grounding point, only the PEN line consisting of the original PE line and N line in parallel, the advantages of the original TN-S system will be lost, so the PE line and N line cannot be Common grounding.

Due to the above reasons, it is clearly stated in the relevant regulations that the neutral line (ie. N line) should not be grounded repeatedly except for the neutral point of the power supply.

IT system

IT mode power supply system “I” indicates that the power supply side has no working ground, or is grounded at high impedance. The second letter T indicates that the load side electrical equipment is grounded.

The IT mode power supply system has high reliability and good security when the power supply distance is not long. It is generally used in places where no blackouts are permitted, or places where strict continuous power supply is required, such as electric power steelmaking, operating rooms in large hospitals, and underground mines.

The power supply conditions in underground mines are relatively poor and the cables are susceptible to moisture. Using the IT-powered system, even if the neutral point of the power supply is not grounded, once the device is leaking, the relative ground leakage current is still small and will not damage the balance of the power supply voltage. Therefore, it is safer than the neutral grounding system of the power supply. However, if the power supply is used for a long distance, the distributed capacitance of the power supply line to the earth cannot be ignored.

When a short-circuit fault or leakage of the load causes the device case to become live, the leakage current will form a path through the earth and the protection device will not necessarily act. This is dangerous. Only when the power supply distance is not too long is it safer. This type of power supply is rare on the construction site.

Reasons for different earthing systems

Why do we have different earthing systems like TN, TN-C, TN-S, TT & IT ?. Why can’t we have one standard earthing scheme? What reasons make the electrical installers & manufacturers to choose these different schemes?

Choosing an earthing scheme is not that direct; It is all about saving money vs provide a sufficient coverage against electric shock.

For instance,

➤ TT- is mainly for consumer power supplies. The owner must install the earthing protection by their own connection to the earth. The advantage is the reduction noise of high or low frequency, no risk of failure and suitable for premises where all AC power circuits are residual current device (RCD) protected.

➤ IT-This system is similar with TT system but different in earthing supply. The distributor system has only a high impedance connection. This type is not ideal for consumer power supply and used for power distributor such as substation or generator area.

➤ TN-S system The consumer’s earthing terminal is usually connected to the metallic part of the distributor’s cable .This is used for underground power supply to the premise or factory from the distributor substation to customer substation.

➤ TN-C-S system-This system has the supply neutral conductor of a distribution main connected with earth at source as protective multiple earthing.

➤ TN-C-This system is a combined PEN conductor fulfils the functions of both a PE(protective conductors) and an N (neutral) conductor.

Above reflects only general scenarios; but one must always adhere to the local regulations, if there are any. As mentioned there’s no standard solution, there are different types of earthing are needed to suit the particular consumers such as domestic, industrial, HT/LT…etc.