For the TN-C grounding system, after the neutral line of the transformer is grounded, it is led out in the form of a protective center line PEN. This PEN line is the neutral line. The potential of the neutral line is zero, and it is equipped with multi-point grounding measures. Therefore, the shell of the electrical equipment is directly connected to the neutral line. We call it the protective zero connection.
If the zero line breaks, the potential of the zero line in front of the breaking point is still close to zero, and the zero line behind the breaking point will have potential drift due to the three-phase imbalance, and its voltage will rise, up to the phase voltage. It can be seen that for the protection of electrical equipment connected to the zero, if it is located behind the zero line breaking point, its shell will be charged and pose a great threat to personal safety.
Therefore, in the TN-C grounding system, a 3P (three-pole) switch must be used for three-phase power distribution, while a 1P (single pole) switch must be used for single-phase power distribution. fracture.
For the TN-S grounding system, after the neutral wire of the transformer is grounded, it is led out in the form of neutral wire N and ground wire PE, and there is no neutral wire in the system. The shell grounding wire PE of electrical equipment is called protective grounding. The neutral wire N of +TN-S can be broken, while the ground wire PE is not allowed to be broken, and the ground wire PE must be grounded repeatedly at multiple points to ensure that the enclosure of the electrical equipment maintains zero potential.
It is precisely because the neutral wire N in the TN-S system can be broken, so the neutral wire can enter the switch. Therefore, in the TN-S grounding system, the three-phase system can use 3P and 4P switches. The fourth pole of the 4P switch is the N line. It can be an ordinary contact that only plays a role in isolation, or it can have a line protection function. In the same way, single-phase systems can use 1P and 2P switches, and its second pole can be an ordinary isolated contact or a contact with line protection.
Let's imagine: two power transformers, numbered T1 and T2. The grounding form of the low-voltage power distribution network is TN-S. The low-voltage sides of T1 and T2 are connected to their respective incoming circuit breakers QF1 and QF2, and the outgoing ends of QF1 and QF2 are connected to the first section of the main bus and the second section of the main bus, and the two sections of bus are connected through the bus tie breaker QF3. The protection of QF1 and QF2 consists of four stages, namely the overload long delay L parameter, short circuit short delay S parameter, short circuit instantaneous I parameter and single-phase ground fault protection G parameter. +Note: The two incoming circuit breakers QF1 and QF2 and the bus tie circuit breaker QF3 are all 3P switches, so the ground PE in the system is integrated, and the neutral line N is also integrated.
When a single-phase grounding fault occurs at the exit of the feeder circuit of a section of the bus, the fault current returns to the power source along the ground wire PE. Since the ground wires PE of the two busbar systems are connected together, and the PE wire and the neutral wire N are also connected together at the neutral ground electrode of the power transformer, there are two paths for the fault current, one of which is along the The PE wire of a section of the busbar system returns to the T1 transformer, and the other is to follow the PE wire to the ground electrode of the 2-terminal transformer, and then return to the T1 transformer along the neutral line N. As a result, the single-phase ground fault G protection of the 2-stage incoming circuit breaker QF2 will be activated, and the load of the 2-stage bus system will therefore lose power supply, and the accident will be magnified.
If we use the 4P (four-pole switch) circuit breaker for the bus tie breaker QF3 and the two-stage incoming circuit breakers QF1 and QF2, because the N bus on the two sections of the bus is cut off by the bus tie breaker, the fault current is not It may flow along the N line again, and there will be no misoperation accidents. +This is one of the reasons why the 4P switch is used. +For the TN-CS grounding system for home power distribution, the neutral wire PEN at the outdoor watt-hour meter box is grounded again and again, and then separated into neutral wire N and ground wire PE, and enters the home together with the same phase wire L, so the home power distribution box The incoming line switch adopts 2P miniature circuit breaker MCB, that is, 2P air switch. +If we use a 1P switch and the N line is not cut off, it is possible that the N line will carry phase voltage when a fault occurs, causing equipment and personal safety failures.
Finally, make a summary: +1P switch is used for single-phase load power supply at the end of the distribution network, especially for lighting circuits and home power distribution. 1P switch is used a lot. The +2P switch is used for the incoming circuit of the single-phase power distribution system at the end of the distribution network. The +3P switch is used for the feed circuit of the three-phase system and the incoming circuit of the TN-C grounding system. The +4P switch is used for the incoming circuit, bus tie circuit, power switching circuit and feeder circuit of the TN-S grounding system.