Cabling rules of Surge Protection Device

Rule 1

The first rule to comply with is that the length of the SPD connections between the network (via the external SCPD) and the earthing terminal block should not exceed 50 cm.

Figure J40 shows the two possibilities for connection of a SPD.

Fig. J40 – SPD with separate or integrated external SCPD

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Rule 2

The conductors of protected outgoing feeders:

• should be connected to the terminals of the external SCPD or the SPD;
• should be separated physically from the polluted incoming conductors.

They are located to the right of the terminals of the SPD and the SCPD (see Figure J41 ).

Fig. J41 – The connections of protected outgoing feeders are to the right of the SPD terminals

Rule 3

The incoming feeder phase, neutral and protection (PE) conductors should run one beside another in order to reduce the loop surface (see Fig. J42).

Rule 4

The incoming conductors of the SPD should be remote from the protected outgoing conductors to avoid polluting them by coupling (see Fig. J42).

Rule 5

The cables should be pinned against the metallic parts of the enclosure (if any) in order to minimize the surface of the frame loop and hence benefit from a shielding effect against EM disturbances.

In all cases, it must be checked that the frames of switchboards and enclosures are earthed via very short connections.

Finally, if shielded cables are used, big lengths should be avoided, because they reduce the efficiency of shielding (see Fig. J42).

Fig. J42 – Example of improvement of EMC by a reduction in the loop surfaces and common impedance in an electric enclosure