Chapter G

Sizing and protection of conductors


Isolation of the neutral conductor

From Electrical Installation Guide

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

  •      - 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.
  1. ^ 1 2 3 The neutral overcurrent protection is not necessary:
  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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