Neutral Protection

Presentation

Neutral protection on MicroLogic 5, 6, and 7 trip units protects all types of electrical distribution applications against overload and short-circuit currents.

It is available on:

MicroLogic 5, 6, and 7 trip units for 4-pole circuit breakers
MicroLogic 5 and 6 trip units with ENCT option for 3-pole circuit breakers

Description

Normally, the phase protection protects the neutral conductor (if it is distributed and identical to the phases in size, that is, full neutral).

The neutral must have specific protection if:

It is reduced in size compared to the phases
Nonlinear 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)

ComPact NSX circuit breaker trip units are suitable for all protection types.

Circuit breaker	Possible types	Neutral protection
3-pole circuit breaker	3P, 3D	None
3-pole circuit breaker with ENCT option	3P, 3D	None
	3P, 3D + N/2	Half neutral
	3P, 3D + N	Full neutral
	3P, 3D + OSN	Oversized neutral
4-Pole Circuit Breaker	4P, 3D	None
	4P, 3D + N/2	Half neutral
	4P, 4D	Full neutral
	4P, 4D + OSN	Oversized neutral
P: Pole; D: Trip unit; N: Neutral protection

Operating Principle

Neutral protection has the same characteristics as phase protection:

Its pickup is in proportion with the long-time Ir and short-time Isd protection pickups.
It has the same trip time delay values as the long-time Ir and short-time Isd protections.
Its instantaneous protection is identical.

The neutral protection settings are:

Ir: long-time protection pickup
tr: long-time protection time delay
IN: neutral protection pickup

Setting the Neutral Protection

4-pole trip unit

Set the IN pickup:

With the keypad on the MicroLogic trip unit
With EcoStruxure Power Commission software (password-protected)
By sending a setting command using the communication network (password-protected)

3-pole trip unit

Set the neutral declaration and the IN pickup:

With the keypad on the MicroLogic trip unit
With EcoStruxure Power Commission software (password-protected)
By sending a setting command using the communication network (password-protected)

Neutral Protection Setting Value

MicroLogic 5, 6, and 7 trip units incorporate the OSN (Oversized Neutral) function, which manages protection of the neutral conductor when third-order harmonic currents (and multiples thereof) are present.

The following table shows the setting values of the neutral long-time protection and neutral short-time protection pickups, according to the value of the IN/Ir function:

IN/Ir function	Long-time pickup value Ir(IN)	Short-time pickup value Isd(IN)
OFF	N/A	N/A
0.5 ⁽¹⁾	Ir/2	Isd/2
1	Ir	Isd
(1) For the MicroLogic 7 trip unit with 40 A rating, the IN/Ir = 0.5 setting is not available.

The setting values are identical for the phases, the neutral long-time, and short-time protection time delays.

The following table details the setting values of the neutral protection pickups (set to OSN) according to the phase protection pickup Ir setting and the In rating of the 4-pole trip unit.

Ir/In values		Long-time pickup value Ir(IN)	Short-time pickup value Isd(IN)
Ir/In < 0.63		1.6 x Ir	1.6 x Isd
0.63 < Ir/In < 1		In	In x Isd/Ir

Selection of the ENCT Option

The ENCT option is an external neutral CT for a 3-pole MicroLogic 5 or 6 trip unit.

The following table indicates the part number for the ENCT option installed according to the In rating of the MicroLogic trip unit or the need for OSN protection:

In rating (A)	Neutral protection limited to In	OSN neutral protection > In
40	LV429521	LV429521
100	LV429521	LV429521
160	LV430563	LV430563
250	LV430563	LV432575
400	LV432575	LV432575
630	LV432575	No ⁽¹⁾
(1) For the 630 A rating, the OSN function is limited to In (= 630 A).

Installing the ENCT Option

Step	Action
1	Connect the neutral conductor to the ENCT option primary (terminals H1, H2).
2	Remove (if existing) the jumper, between terminals T1 and T2 of the MicroLogic trip unit.
3	Connect the ENCT option secondary (terminals T1, T2) to terminals T1 and T2 of the MicroLogic trip unit.
4	Declare the ENCT option when setting the protection functions for the MicroLogic trip unit.

NOTE: If the ENCT option is declared before its installation, the MicroLogic trip unit trips and displays the ENCT screen. Either install the ENCT option or connect a jumper between terminals T1 and T2 on the MicroLogic trip unit. Clear the ENCT screen by pressing the OK key twice (enter and confirm).