TT system: The N line of this system is only grounded at the neutral point of the transformer, and the N line and PE line are strictly separated for wiring. Therefore, surge protectors need to be installed between the phase line and the N line, and switch type surge protectors are often installed between the N line and the PE line.

TN-S system: The N and PE lines of this system are only connected to the low-voltage side of the transformer and to the ground. Before entering the building's main distribution box, the N and PE lines are independently wired, and surge protectors are installed between the phase and PE lines.