Power supply system grounding

The TT system refers to a protection system in which the metal casing of electrical equipment is directly connected to ground, known as a protective grounding system. The characteristics of this power supply system are as follows. 1. When the metal casing of electrical equipment is charged, grounding protection can **reduce the risk of electric shock**. However, the low-voltage circuit breaker may not trip, resulting in the voltage of the enclosure of the leaking device relative to ground being higher than the safe voltage, which constitutes a hazardous voltage. 2. When the leakage current is low, a fuse may not blow even in the presence of one, so a leakage protector is also needed for protection; therefore, the TT system is difficult to implement on a wide scale. 3. The grounding systems in TT systems consume a large amount of steel, and they are difficult to recycle, requiring much labor and materials. The TN-C-S system can reduce the voltage of the motor casing with respect to ground, but it cannot completely eliminate this voltage; it requires that the current resulting from load imbalance be low, and repeated grounding should be provided on the PE wire. The TN-C-S power supply system is a temporary adaptation of the TN-C system. When a three-phase power transformer has proper working grounding and the three-phase loads are relatively balanced, the TN-C-S system remains a viable option for construction electrical applications. However, in the case of unbalanced three-phase loads or when there is a dedicated power transformer at construction sites, a TN-S power supply system must be used.

　　The TT system is a system in which the low-voltage distribution network is directly grounded, and the metal enclosures of electrical equipment are also grounded. The first capital letter “T” indicates that the distribution network is directly grounded, while the second capital letter “T” indicates that the metal casing of the electrical equipment is grounded. 　　The TT system can significantly reduce the voltage of the casing of leaky equipment with respect to ground, but it generally cannot reduce it to a safe level. Therefore, when using a TT system, an automatic safety device capable of cutting off the power supply within the specified fault duration should be installed. The TT system is mainly used for low-voltage shared users, that is, small users who do not have distribution transformers and draw low-voltage power from an external source. 　　In a grounded distribution network, if there are no safety measures on equipment with leakage current, the voltage across it with respect to ground is equal to the phase voltage. In the TT system, when a device leaks electricity, its voltage with respect to ground and the voltage of the neutral wire with respect to ground are respectively: where RN is the grounding resistance of the working ground. Since RE and RN are on the same order of magnitude, the fault voltage in devices with leakage current is significantly reduced, but it is almost impossible to keep it within safe limits. On the other hand, fault current is not short-circuit current; for conventional overcurrent protection, the power supply cannot be disconnected quickly, so the fault will persist for a long time. 　　For this very reason, the TT system cannot be used under normal circumstances. If it is truly difficult to avoid using the TT system, measures must be taken to prevent the risk of the neutral wire becoming live, and protective devices that can automatically cut off the power supply must be installed to keep the duration of faults within acceptable limits. A residual current protection device or an overcurrent protection device can be installed in the TT system, with the former being preferred. 　　The TT system is mainly used for low-voltage customers who do not have distribution transformers and draw low-voltage power directly from outside.