# Sizing and protection of the neutral conductor (2)

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Continued from the first part – Sizing and Protection of the Neutral Conductor (1)

## Protection of the neutral conductor

If the neutral conductor is correctly sized (including harmonics), no specific protection of the neutral conductor is required because it is protected by the phase protection.

However, in practice, if the c.s.a. of the neutral conductor is lower than the phase c.s.a, a neutral overload protection must be installed.

### Protection against short-circuit

If the c.s.a. of the neutral conductor is lower than the c.s.a. of the phase conductor, the neutral conductor must be protected against short-circuit. If the c.s.a. of the neutral conductor is equal or greater than the c.s.a. of the phase conductor, no specific protection of the neutral conductor is required because it is protected by the phase protection.

## Breaking of the neutral conductor (Fig. 1)

The need to break or not the neutral conductor is related to the protection against indirect contact.

### In TN-C scheme

The neutral conductor must not be open-circuited under any circumstances since it constitutes a PE as well as a neutral conductor.

### In TT, TN-S and IT schemes

In the event of a fault, the circuit-breaker will open all poles, including the neutral pole, i.e. the circuit-breaker is omnipolar.

The action can only be achieved with fuses in an indirect way, in which the operation of one or more fuses triggers a mechanical trip-out of all poles of an associated series-connected load-break switch.

## Isolation of the neutral conductor (Fig. 1)

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

## Fig. 1 – Neutral conductor appearances

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.

Reference // Electrical Installation Guide 2010 – Schneider Electric

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#### Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

May 13, 2014

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2. SANJEEV SHARMA
Oct 04, 2012