Common mode SPD between phase and PE or
phase and PEN is installed whatever type of system earthing arrangement (see
Fig. 1).
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. 1 – Common 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.2)
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.2 – Common and differential protection
