Regardless of TN system or TT system, the connection wires at both ends of the SPD connected to the live conductor and PE line should be short and straight. The IEC standard stipulates that the total length of the SPD connection wire should not exceed 0.5m. This is because the lightning impulse current has high-frequency characteristics, and the lightning impulse overvoltage applied to the protected electrical equipment is the residual voltage on the SPD plus the Ldi/dt high-frequency voltage drop on the connection wire (L is the inductance of the wiring at both ends of the SPD, which is proportional to the length of the connection wire, and di/dt is the steepness of the change of the lightning impulse current). The residual voltage of the SPD is determined by the product performance and cannot be changed, while the Ldi/dt on the connection wire can be reduced in the design of the electrical device by reducing the length of the connection wire, that is, reducing the inductance. Therefore, it is best to install the SPD directly between the live conductor busbar and the PE busbar of the distribution box to minimize the length of the connection wire. Now some SPD products have been made according to the modulus of the miniature circuit breaker in the distribution box so that they can be directly installed in the distribution box, which is very beneficial to shorten the length of the SPD connection wire.

  If this is difficult to do for some reason, the SPD can be installed elsewhere. In this case, it is best to use a "V" connection method for the SPD, as shown in Figure 1. The length of the SPD connection line in the figure is almost zero, but this connection method will encounter some difficulties and inconveniences in actual construction.

Figure 1 V-shaped connection in SPD connection