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Example of lightning current in TT system

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General rules of electrical installation design
Connection to the MV utility distribution network
Connection to the LV utility distribution network
MV and LV architecture selection guide for buildings
LV Distribution
Protection against electric shocks and electrical fires
Sizing and protection of conductors
LV switchgear: functions and selection
Overvoltage protection
Energy Efficiency in electrical distribution
Power Factor Correction
Power harmonics management
Characteristics of particular sources and loads
PhotoVoltaic (PV) installation
Residential premises and other special locations
ElectroMagnetic Compatibility (EMC)
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Common mode SPD between phase and PE or phase and PEN is installed whatever type of system earthing arrangement (see Fig. J61).

The neutral earthing resistor R1 used for the pylons has a lower resistance than the earthing resistor R2 used for the installation.

The lightning current will flow through circuit ABCD to earth via the easiest path. It will pass through varistors V1 and V2 in series, causing a differential voltage equal to twice the Up voltage of the SPD (Up1 + Up2) to appear at the terminals of A and C at the entrance to the installation in extreme cases.

Fig. J61Common protection only

To protect the loads between Ph and N effectively, the differential mode voltage (between A and C) must be reduced.

Another SPD architecture is therefore used (see Fig. J62)

The lightning current flows through circuit ABH which has a lower impedance than circuit ABCD, as the impedance of the component used between B and H is null (gas filled spark gap). In this case, the differential voltage is equal to the residual voltage of the SPD (Up2).

Fig. J62Common and differential protection