Neutral Protection

Presentation

A long time overcurrent protection function is dedicated to the neutral protection.

Availability

Neutral protection is available on:

• MicroLogic 2.0 X, 5.0 X, 6.0 X, and 7.0 X control units for IEC standard

• MicroLogic 3.0 X, 5.0 X, and 6.0 X control units for UL standard

• 3-pole circuit breakers with the ENCT option (External Neutral Current Transformer) to measure the neutral current

• 4-pole circuit breakers

Description

Where the cross-sectional area of the neutral conductor is at least equivalent to that of the phase conductor, and the current in the neutral is expected not to exceed the value in the phase conductor, it is not necessary to provide overcurrent protection for the neutral conductor.

The neutral conductor must have protection against overcurrent if:

• The cross-sectional area of the neutral conductor is less than the cross-sectional area of the phase conductors

• Non-linear loads generating third order harmonics (or multiples thereof) are installed

It may be necessary to switch off the neutral for operational reasons (multiple source diagram) or safety reasons (working with power off).

To summarize, the neutral conductor can be:

• Non-distributed (3-pole circuit breaker)

• Distributed, not switched off and not protected (3-pole circuit breaker)

• Distributed, not switched off but protected (3-pole circuit breaker with ENCT option)

• Distributed, switched off and protected (4-pole circuit breaker)

MicroLogic X control units are suitable for all protection types. They incorporate the oversized neutral (OSN) function, which manages protection of the neutral conductor when third-order harmonic currents (and multiples thereof) are present.

Operating Principle

Neutral protection has the same characteristics as phase protection: Its threshold is proportional to the long-time protection threshold Ir. It has the same tr time delay values as long-time protection. Its short-time and instantaneous protections are identical.

Declaring the External Neutral Current Transformer (ENCT) on 3-Pole Circuit Breakers

On 3P circuit breakers the ENCT option must be declared in one of the following ways:

• On the MicroLogic X display screen, at Home > Configuration > Measures > System Type > ENCT

• With EcoStruxure Power Commission software

• By sending a setting command using the communication network (password-protected).

Setting the Neutral Protection for 3-Pole and 4-Pole Circuit Breakers

Set the type of neutral protection in one of the following ways:

• On the MicroLogic X display screen, at Home > Protection > Neutral

• With EcoStruxure Power Commission software (password-protected)

• By sending a setting command using the communication network (password-protected).

The following table shows the setting values of the neutral long-time protection and threshold for the type of neutral protection selected: