Chapter G

Sizing and protection of conductors


Isolation of the neutral conductor

From Electrical Installation Guide
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DB422336.png

(see Fig. G68)

It is considered to be the good practice that every circuit be provided with the means for its isolation.

Fig. G68 – The various situations in which the neutral conductor may appear
TT TN-C TN-S IT
Single-phase

(Phase-Neutral)

DB422337.png

or
DB422340.png

DB422338.png DB422337.png DB422339.png[a]

or
DB422337.png

Single-phase

(Phase-Phase)

DB422344.png[b]

or
DB422345.png

DB422345.png DB422344.png[b]

or
DB422345.png

DB422345.png
Three-phase four wires

Sn ≥ Sph

DB422341.png DB422342.png DB422341.png[c] DB422343.png[a]

or
DB422341.png

Three-phase four wires Sn < Sph DB422343.png DB422343.png DB422343.png[a]

or
DB422341.png

  1. ^ 1 2 3 The neutral overcurrent protection is not necessary:
         - If the neutral conductor is protected against short-circuits by a device placed upstream, or,
         - If the circuit is protected by a RCD which sensitivity is less than 15% of the neutral admissible current.
  2. ^ 1 2 Authorized for TT or TN-S systems if a RCD is installed at the origin of the circuit or upstream of it, and if no artificial neutral is distributed downstream of its location
  3. ^ In some coutries the rules applied for TN-S are the same as the rules for TN-C
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