Three-Phase Four-Wire Meter Wiring Diagram (2)

Three-phase four-wire system (TN-C system)

The characteristics of the three-phase four-wire system are: the neutral point of the power transformer is grounded, and the protective neutral line (PE) and the working neutral line (N) are shared (referred to as PEN).

It is suitable for situations where the three-phase load is basically balanced. If the three-phase loads are unbalanced, there is unbalanced current in the PEN line. In addition, the harmonic current caused by some load equipment will also be injected into PEN, so that the neutral line N is charged, and it is very likely to be higher than 50V. It not only electrifies the equipment casing, which is unsafe to the human body, but also cannot obtain a stable reference potential.

The PEN line should be grounded repeatedly, and its function is to effectively reduce the neutral-to-ground voltage when the zero-connected equipment contacts the housing.

Defect:

(1) When the three-phase load is unbalanced, an unbalanced current appears on the neutral line, and the neutral line presents a voltage to the ground. Touching the neutral wire may result in electric shock.

(2) The zero line passing through the leakage protection switch can only be used as the working zero line, and cannot be used as the protection zero line of electrical equipment, which is determined by the working principle of the leakage switch.

(3) Single-phase electrical equipment connected with a two-pole leakage protection switch, such as the protective neutral line used in the metal shell of the TN-C system is strictly prohibited to be connected to the working neutral line of the circuit, and it is also not allowed to be connected to the PEN line in front of the leakage protection switch.

(4) It is strictly forbidden to connect the connecting line of the repeated grounding device with the working zero line passing through the leakage switch. The TN-S power supply system completely separates the working neutral line from the protection neutral line, thus overcoming the defects of the TN-C power supply system. Therefore, the TN-C system is no longer used at the construction site, and the TN-S system is recommended.