A long time overcurrent protection function is dedicated to the neutral protection.
Neutral protection is available on:
oMicroLogic 2.0 X, 5.0 X, 6.0 X, and 7.0 X control units for IEC standard
oMicroLogic 3.0 X, 5.0 X, and 6.0 X control units for UL standard
o3-pole circuit breakers with the ENCT option (External Neutral Current Transformer) to measure the neutral current
o4-pole circuit breakers
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:
oThe cross-sectional area of the neutral conductor is less than the cross-sectional area of the phase conductors
oNon-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:
oNon-distributed (3-pole circuit breaker)
oDistributed, not switched off and not protected (3-pole circuit breaker)
oDistributed, not switched off but protected (3-pole circuit breaker with ENCT option)
oDistributed, 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.
|
Circuit Breaker |
Possible Types |
Neutral Protection |
|---|---|---|
|
3-pole circuit breaker |
3P, 3D |
Off |
|
3-pole circuit breaker with ENCT option |
3P, 3D |
Off |
|
3P, 3D + N/2 |
Half neutral |
|
|
3P, 3D + N |
Full neutral |
|
|
3P, 3D + OSN |
Oversized neutral |
|
|
4-pole circuit breaker |
4P, 3D |
Off |
|
4P, 3D + N/2 |
Half neutral |
|
|
4P, 4D |
Full neutral |
|
|
4P, 4D + OSN |
Oversized neutral |
|
|
P: Pole, D: Control unit, N: Neutral protection |
||
|
Neutral protection has the same characteristics as phase protection: oIts threshold is proportional to the long-time protection threshold Ir. oIt has the same tr time delay values as long-time protection. oIts 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:
oOn the MicroLogic X display screen, at Home > Configuration > Measures > System Type > ENCT
oWith EcoStruxure Power Commission software
oBy 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:
oOn the MicroLogic X display screen, at Home > Protection > Neutral
oWith EcoStruxure Power Commission software (password-protected)
oBy 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:
|
Neutral protection type |
Neutral long-time threshold value |
|
|---|---|---|
|
OFF |
No long-time protection for neutral |
|
|
N/2 |
Ir/2 |
|
|
N |
Ir |
|
|
Oversized N |
3-pole (ENCT) |
1.6 x Ir |
|
4-pole |
1.6 x Ir limited to In |
|
