In electrical design, what should be done if the TT system becomes a TN system?


Release time:

2023-04-19

According to the current national standard "Code for the Design of Low-voltage Distribution" (GB50054), the low-voltage distribution system has three grounding forms, namely IT system, TT system, and TN system. So how to avoid TT systems becoming TN systems in building electrical design?Derun ElectricShare with you:

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According to the current national standard "Code for the Design of Low-voltage Distribution" (GB50054), the low-voltage distribution system has three grounding forms, namely IT system, TT system, and TN system. So how to avoid TT systems becoming TN systems in building electrical design?Derun ElectricShare with you:

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First, the characteristics of TT system

The TT system has only one point directly grounded, and the exposed conductive part of the device should be connected to the ground electrode that is electrically independent of the power supply system. As shown in Figure 1

Advantages of TT system: it can avoid the high voltage failure on the system side (such as when the high voltage line is connected to the low voltage line or the insulation breakdown between the high and low voltage windings is distributed), the high voltage is transmitted to the metal shell of the electrical equipment through the PE line, and an electric shock accident occurs.

The main disadvantages of the 1.TT system:

(1) When low and high voltage lines are lightning striked, the configuration may change the overvoltage in positive and reverse directions

(2) The protection effect of the grounding of the low-voltage electrical shell is not as good as that of the IT system.

(3) 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 risk of electric shock can be greatly reduced due to grounding protection. However, the low-voltage circuit breaker (automatic switch) may not necessarily trip, causing the housing voltage of the leakage equipment to ground to be higher than the safe voltage, which is a dangerous voltage.

(4) When the leakage current is relatively small, even if there is a fuse, it may not be able to fuse, so it also needs a leakage protector for protection, so the TT system is difficult to promote.

(5) The TT system grounding device consumes a lot of steel, and it is difficult to recycle, time-consuming and material-consuming.

Applications of 2.TT systems:

TT systemSince the grounding device is near the equipment, the probability of PE wire disconnection is small and easy to detect.

The TT system equipment is not charged during normal operation, and the high potential of the shell will not be transmitted to the whole system along the PE line in the event of a fault. Therefore, TT systems are suitable for powering voltage-sensitive data processing equipment and precision electronic equipment, and are advantageous in hazardous locations such as explosion and fire hazards.

TT systems can greatly reduce the fault voltage on leakage equipment, but generally cannot be reduced to a safe range. Therefore, the TT system must be equipped with leakage protection device or overcurrent protection device, and the former is preferred.

The TT system is mainly used for low-voltage users, that is, for small users who are not equipped with distribution transformers and introduce low-voltage power from the outside.

Second, the analysis of the reasons why the TT system changes TN

TT systems are also commonly used in civil buildings, such as street lamps, pumps and other electrical equipment located outside the building. Usually, when the TT system is used to supply power to outdoor electrical equipment, the distribution box is set in two positions: indoor and outdoor (as shown in Figure 2 and Figure 3).

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The key to the TT system is to ensure that the metal shell of the outdoor electrical equipment is grounded separately, and the metal shell or grounding wire (PE) of the equipment cannot be connected with the PE wire of the indoor power supply system. Figure 2 and Figure 3 appear to be TT systems, but they are actually TN systems. Here's why:

The PE line of the indoor power distribution system forms a path through the metal shell of the distribution box - metal wire pipe - the metal shell of the electrical equipment, resulting in the PE line of the power supply system being connected with the metal shell of the outdoor electrical equipment, which actually becomes the TN system.

3. Measures to ensure the TT system

Obviously, to ensure that the TT system does not become a TN system, just cut off the connection path between the power PE line and the metal shell of the electrical equipment. The usual practice is:

(1) Change the wire pipe in the outdoor part of Fig. 2 and Figure 3 to PVC pipe.

(2) If the cable adopts metal armored cable, the metal armored steel strip should not be connected with the metal shell of the electrical equipment and its grounding system.

Fourth, TT system indirect contact protection measures

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The protection action time is shown in the following table:

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Anhui high and low voltage distribution cabinet manufacturer Derun Electric 400-128-7988