Isolation of the neutral conductor
From Electrical Installation Guide
(see Fig. G67)
It is considered to be the good practice that every circuit be provided with the means for its isolation.
| TT | TN-C | TN-S | IT | |
|---|---|---|---|---|
| Single-phase
(Phase-Neutral) |
or |
|
|
 [b]
or |
| Single-phase
(Phase-Phase) |
 [a]
or |
|
[a]
or |
|
| Three-phase four wires
Sn ≥ Sph |
|
|
[c]
|
 [b]
or |
| Three-phase four wires Sn < Sph |
|
|
[b]
or |
[a] 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
[b] 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.
[c] In some coutries the rules applied for TN-S are the same than the rules for TN-C
Fig. G67 – The various situations in which the neutral conductor may appear
