TN system neutral point directly grounded, the exposed accessible conductor of the electrical device through the protective grounding wire connected to the grounding point, that is, the equipment is not individually grounded, only the system grounding of the low-voltage power distribution system.
In the TN system is divided into TN-C, TN-S and TN-C-S three kinds of systems:
1) TN-C system - in the whole system N wire and PE wire is united (the legendary four-wire system)
2) TN-S system - in the whole system N wire and PE wire are Separate (the legendary five-wire system)
3) TN-C-S system - in the whole system, usually only in the low-voltage electrical installations, power supply into the point before the N wire and PE wire is united, power supply into the point that is divided into two wires (power supply to the box is a four-wire box to the equipment is a five wire)