TN system, that is, protective grounding system, refers to a protection method in which the non-charged metal parts of electrical equipment are closely connected to the neutral point of the power supply system through the protective neutral line (PEN line) during normal operation.
According to the different combinations of the neutral line (N line) and the protective line (PE line), it can be divided into the TN-C earthing system, the TN-S earthing system, and the TN-C-S earthing system.
| Earthing System | Advantages | Disadvantages |
|---|---|---|
| TN-C Earthing System | Cost-effective and simple wiring (combined PEN conductor) | Safety compromised if PEN fails; hazardous voltages on equipment enclosures |
| TN-S Earthing System | High safety and EMI performance; separate PE ensures clean protection | Higher installation cost; more complex wiring |
| TN-C-S Earthing System | Balances safety and cost; combines PEN up to distribution then splits | High risk if PEN conductor fails—dangerous voltages may appear on PE |
| TT Earthing System | Reduced interference and safer in case of broken neutral; independent earth | High earth impedance; requires RCDs due to slower fault clearing |
TN-C-S Earthing System
TN-C-S earthing system is a specific form of TN earthing system. TN-C-S earthing system consists of two parts: TN-C earthing system and TN-S earthing system.
In the first half of the system (such as from the transformer to the user’s distribution box), the neutral line (N line) and the protective line (PE line) are combined, called the protective neutral line (PEN line), while in the second half of the system (such as from the distribution box to the user), the neutral line and the protective line are separated.
In the TN-C-S system, the wiring of the surge protector should pay special attention to the treatment of the N line and the PE line. In the first half of the system (TN-C part), the N line and the PE line are combined (PEN line) and should be connected to the corresponding port of the surge protector.
In the second half of the system (TN-S part), the N line and the PE line are separated and should be connected to the N line port and the PE port of the surge protector, respectively.
TN-S Earthing System
In the TN-S earthing system, the neutral point (i.e., the neutral point of the secondary side of the transformer) is directly grounded to form a working ground. At the same time, the lead-out protection line (PE line) is used as a dedicated protection grounding line and is connected to the conductive parts, such as the equipment casing.
When the system is operating normally, an unbalanced current will pass through the working neutral line (N line), while no current will pass through the dedicated protection line (PE line). In this way, even if the equipment casing is charged for some reason, the leakage current can be quickly introduced into the earth through the PE line, thereby avoiding the occurrence of electric shock accidents.
In the TN-S system, the wiring of the surge protector should follow the principle of a three-phase, five-wire system. That is, the three L ports above are connected to three live wires, the N port is connected to the neutral line, and the PE port is connected to the ground wire. Note that the live wire, neutral wire, and ground wire cannot be connected incorrectly or reversely.
TN-C Earthing System
The TN-C earthing system is a power supply system that combines the neutral wire (N wire) and the protective ground wire (PE wire) into one wire. In this system, the working neutral wire also serves as the neutral protection wire, which can be represented by NPE (or PEN).
This system directly grounds the neutral point and connects the equipment casing to the neutral point of the power supply through the PEN wire to achieve a protective zero connection.
In the TN-C earthing system, when the equipment is operating normally, the current is transmitted through the working neutral wire. If the equipment casing is charged for some reason (such as the phase wire touching the shell or the equipment insulation is damaged), the leakage current will be quickly introduced into the earth through the PEN wire, thereby avoiding the occurrence of electric shock accidents.
However, since the N wire and the PE wire are shared, when the PEN wire fails or is cut off, the metal casing of the equipment may lose grounding protection, causing the casing to be charged, and there is a risk of electric shock.
According to the three-phase four-wire system of the TN-C system and the specific requirements of the system, usually for the TN-C system, a 3P surge protector can be selected because it is suitable for the three-phase TN-C system.
In the TN-C system, the wiring of the surge protector should follow the principle of a three-phase, four-wire system. That is, the three L ports above the surge protector should be connected to three live wires, respectively, and the N port below should be connected to the PEN line (that is, the combined line of the working neutral line and the protective line).
There is no separate PE line (protective grounding line) in the TN-C system, so there is no need to connect any port of the surge protector to the ground line. However, if there is an additional PE line in the system and it needs to be connected to the conductive part, such as the equipment casing, it should be connected in accordance with relevant regulations.
TT Earthing System
The TT earthing system, also known as the grounding system, is a power system with a specific grounding method.
The TT earthing system refers to a system in which the neutral point of the power supply is directly grounded, and the metal casing of the electrical equipment is also directly grounded.
In the TT earthing system, the grounding of the neutral point of the power supply is called the working grounding, and the grounding of the metal casing of the equipment is called the protective grounding.
The two groundings are independent of each other; that is, there is no electrical connection between the working grounding and the protective grounding.
When a single-phase shell-touching fault occurs in the equipment in the TT system, the grounding current will flow through the loop formed by the protective grounding device and the working grounding device of the power supply.
At this time, if someone touches the housing of the energized device, since the resistance of the protective grounding device is usually smaller than that of the human body, most of the grounding current will be shunted by the grounding device, thereby reducing or avoiding damage to the human body.
For a single-phase TT earthing system, you can choose a 1P or 2P surge protector; for a three-phase TT system, you may need to choose a 3P or 4P surge protector, depending on the specific needs and configuration of the system.
Single-phase TT earthing system: connect the L port of the surge protector to the live wire, the N port to the neutral wire, and the PE port to the ground wire (if the surge protector has an N port, but in the TT system, only the L and PE wires are usually involved).
Three-phase TT earthing system: The three L ports above are connected to the three live wires, respectively; the N port below (if any) is connected to the PEN wire (but in the TT system, it is more common to only connect the PE wire because the N wire and PE wire are separated); and the PE port or a separate PE wire interface is connected to the ground wire.
Earthing System SPD Selection & Standard References
Voltage Rating (Uc / MCOV): Choose an SPD whose continuous operating voltage exceeds your system voltage (often with a ~15% buffer); IEC 61643-11 defines Uc; in the U.S., UL 1449 uses MCOV terminology.
Key Parameters: Use IEC/EN 61643-11 or UL standards to guide SPD specifications; refer to:
- Uc – Maximum continuous operating voltage
- UT – Temporary overvoltage tolerance
- I n – Nominal discharge current
- I max / I imp – Maximum impulse current.
Design Considerations: SPDs with internal short-circuit protection, status indicators, thermal protection, and arc extinguishing should be selected to ensure a safe failure mode.
Common Wiring Mistakes & Safety Risks
A combined PEN should be used in the upstream section of a TN-C-S system, as using a TN-S rated SPD can cause improper operation and malfunction.
In a TT system, malfunctions in the downstream section of an RCD can be caused by a Type 1 or Type 2 SPD, which can trigger nuisance tripping and impair RCD functionality.
Convoluted and convoluted ground conductors longer than 0.5 meters reduce SPD effectiveness and increase let-through voltage, so wiring is crucial; keep cable runs as straight and short as possible.
One response to “TN-C, TN-S, TN-C-S and TT Earthing Systems”
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