Classification, comparison and application scenarios of TN systems

Overview of TN System

The TN system is a power supply system. According to the definition of the International Electrotechnical Commission (IEC), it belongs to a type of low-voltage distribution system and is mainly used in the power supply systems of construction projects. The TN system can be further subdivided into TN-C, TN-S, and TN-C-S systems based on whether the protective earth wire (PE wire) is separated from the neutral wire (N wire).

Classification of TN System

1. TN-C System

The TN-C system is a three-phase four-wire system in which the protective wire and the neutral wire are combined into a PEN wire. The advantage of this system lies in its simplicity and economy. However, for lines with single-phase loads and three-phase unbalanced loads, current always flows through the PEN wire, which may have an adverse impact on sensitive electronic equipment.

2. TN-S System

The TN-S system is a three-phase five-wire system in which the working neutral wire N and the dedicated protective wire PE are separated. The advantage of this system is that it can effectively suppress the interference of the power supply system, and it is suitable for power supply to data processing and precision electronic instruments and equipment. It can also be used in explosive hazardous environments.

3. TN-C-S System

The TN-C-S system is a combination of the TN-C system and the TN-S system. In the TN-C-S system, the section from the power source adopts the TN-C system, and at a certain point near the electrical load, the PEN wire is separated to form separate N and PE wires. This system can reduce the voltage of the electrical equipment enclosure to the ground, but it cannot completely eliminate this voltage.

Applications of TN System

The TN system is a common form of grounding for low-voltage distribution systems. According to the different combination methods of the neutral wire (N wire) and the protective wire (PE wire), it can be divided into three types: TN-C, TN-S, and TN-C-S. Each type has its own characteristics and applicable scenarios. The following is a detailed analysis:

Types and Characteristics of TN System

Working Principle of TN System

In the TN system, the exposed conductive parts of all electrical equipment are connected to the protective wire and are connected to the grounding point of the power source. This grounding point is usually the neutral point of the distribution system. When a short circuit occurs due to contact with the enclosure, the short-circuit current forms a closed loop through the metal wires, resulting in a metallic single-phase short circuit, thereby generating a large enough short-circuit current so that the protection device can operate reliably and remove the fault.

Advantages and Disadvantages of TN System

Advantages

• It has relatively high safety and can effectively protect personal safety and equipment safety.
• The short-circuit current is large, and the protection device operates reliably.

Disadvantages

• The TN-C system has potential safety hazards. Especially when the three-phase load is unbalanced, the PEN wire may be electrified, increasing the risk of electric shock.
• It has a strong dependence on the protection device. If the protection device fails, it may lead to serious consequences.

Application Examples

1. Industrial Enterprises: Due to the large number of equipment in the industrial environment and high requirements for safety, the TN-S system is often adopted to ensure the safe operation of electrical equipment.
2. Large Civil Buildings: For example, in residences, shopping malls, etc., in order to ensure the safety of residents and customers, the TN-S system is also mostly adopted.
3. Data Centers and Communication Stations: These places have extremely high requirements for electrical safety and stability, and the TN-S system can meet their needs.

Differences in Grounding Methods between TN-C System and TN-S System

The TN-C system and the TN-S system are two common power supply systems, and they have obvious differences in grounding methods.

1. TN-C System

The TN-C system, also known as the PEN grounding system, where P stands for "Protective", E stands for "Earth", and N stands for "Neutral". In the TN-C system, the neutral wire (N) and the protective wire (PE) are combined together and share the same conductor. This means that in some electrical equipment, the neutral wire and the protective wire are connected together to form a common circuit. However, this grounding method has potential safety hazards in some cases. Because if the shared conductor breaks, it will cause the neutral wire and the protective wire to be disconnected at the same time, which may lead to grounding failure and potential safety hazards for electrical equipment.

2. TN-S System

In the TN-S system, the neutral wire (N) and the protective wire (PE) are independently separated and are respectively connected by different conductors. In the TN-S system, the independent connection of the neutral wire and the protective wire provides a more reliable grounding method and reduces the safety risks caused by the disconnection of the neutral wire. This makes the TN-S system superior to the TN-C system in terms of the safety of electrical equipment.

Applicable Scenarios of TN-C-S System

The TN-C-S system is a hybrid power supply system that combines the characteristics of the TN-C system and the TN-S system. In this system, usually only before the incoming line point of the low-voltage electrical installation, the N wire and the PE wire are combined. After the incoming line point of the power source, they are divided into two wires, that is, the N wire and the PE wire are separated.

The TN-C-S system may be more applicable than the TN-C or TN-S system in the following situations:

• External Power Supply of Substations: When the power supply source is located outside the building and the building interior needs to adopt the TN-S system, the TN-C-S system can be used as a transitional solution. In this case, the cable from the substation to the building adopts the PEN wire, and after entering the building, it is separated into the PE wire and the N wire, so that the building interior can still adopt the TN-S system.
• Anti-interference of Information Technology Equipment: For information technology equipment, the TN-C-S system can reduce common-mode voltage interference. This is because in the TN-C-S system, the neutral wire and the PE wire are separated at the incoming line point of the low-voltage power source, rather than at the outgoing line point of the substation. Therefore, the potential difference or common-mode voltage between the neutral wire and the PE wire in the TN-C-S system within the low-voltage user building is smaller than that in the TN-S system.
• Three-phase Unbalanced Loads: The TN-C-S system has better ability to resist three-phase unbalanced loads in its TN-C part. When serious three-phase unbalance occurs in the system, the TN-C-S system can maintain a lower voltage of the equipment enclosure to the ground, while the TN-S system may have a larger unbalanced voltage.
• Temporary Power Supply during Construction: In the temporary power supply during building construction, if the front part is powered by the TN-C method and the construction specifications stipulate that the TN-S power supply system must be adopted on the construction site, the PE wire can be separated at the on-site main distribution box in the rear part of the system, and the TN-C-S system can be adopted.

In summary, the TN-C-S system may have advantages over the TN-C or TN-S system in specific application scenarios such as external power supply of substations, anti-interference of information technology equipment, handling of three-phase unbalanced loads, and temporary power supply during construction.

Why TN-S System is More Suitable for Power Supply to Data Processing and Precision Electronic Instrument Equipment

1. Characteristics of TN-S System

The TN-S system, also known as the three-phase five-wire system, is a power supply system characterized by the separation of the protective wire (PE wire) and the neutral wire (N wire). In this system, during normal operation, the PE wire does not carry load current. Therefore, the metal enclosures of electrical equipment connected to the PE wire are not electrified during normal electrical operation. This characteristic makes the TN-S system very suitable for power supply to data processing and precision electronic instrument equipment, because these equipment are very sensitive to electromagnetic interference, and the isolation of the PE wire can effectively reduce electromagnetic interference.

2. Applicability Analysis

The TN-S system is not only suitable for power supply to data processing and precision electronic instrument equipment, but also suitable for explosive hazardous environments. This is because in the TN-S system, since the PE wire does not carry load current, in the event of a fault, the fault current can be conducted through the PE wire, thereby quickly removing the fault and ensuring the safe operation of the equipment.

3. Economic Considerations

Although the TN-S system performs excellently in terms of safety, its initial investment cost is relatively high because an additional PE wire is required. However, for application scenarios with high requirements for safety and electromagnetic compatibility, such as data centers and laboratories, the long-term operating cost of the TN-S system may be lower, because it can provide a more stable and reliable power supply, reducing equipment damage and downtime.

In summary, the reason why the TN-S system is suitable for power supply to data processing and precision electronic instrument equipment is mainly due to its unique electrical characteristics, which can effectively reduce electromagnetic interference and improve system safety. At the same time, although the initial investment is high, its long-term operating cost and stability advantages make it the preferred power supply system for many critical application scenarios.

Summary

The main difference between the TN-C system and the TN-S system lies in their grounding methods. In the TN-C system, the neutral wire and the protective wire are combined, while in the TN-S system, the neutral wire and the protective wire are separated. This difference in design leads to different performances in terms of safety and reliability. The TN-S system is generally considered to be safer because it reduces the safety risks caused by the disconnection of the neutral wire.