How to distinguish: TN-C system, TN-S system, TN-C-S system, TT system

TNC, also known as a four-wire system, means that in this system the neutral wire and the protective ground wire are combined at a single grounding point, with the exposed conductive parts of electrical equipment connected to PE-N. The disadvantage is that current flows through the PE-N wire in the case of an unbalanced three-phase circuit or when only one phase is present. When the transformer in a substation is struck by lightning or experiences a short circuit, large currents flow through the PE-N wire to the equipment’s enclosure, and people who come into contact with the equipment can suffer electric shock. TNS, also known as the five-wire system, means that 5 wires emerge from the power grid, with N and PE being separate. The advantage is that PE has no current flow, resulting in good electromagnetic compatibility ； The downside is the high cost. TNC.S, also known as the four-and-a-half-wire system, separates the neutral wire from the PE wire inside a building (without combining them). The advantages are relatively high safety and good electromagnetic compatibility. In the TT system, the neutral wire and the exposed metal parts of the equipment are grounded separately. The disadvantage is that when there is a short circuit at the high-voltage side of the transformer, or when the lightning rod comes into action, the large current in the ground grid causes the ground potential to rise. This results in a voltage difference between the equipment’s casing and its interior that exceeds the insulation rating of the equipment, leading to breakdown or short circuits in the equipment.