Understanding NCT Applications in 3-Pole and 4-Pole ACBs

Understanding NCT Applications in 3-Pole and 4-Pole ACBs

Understanding NCT Applications in 3-Pole and 4-Pole ACBs

Overview

Ground fault protection in Air Circuit Breakers (ACBs) can be implemented using several different sensing methods depending on the breaker configuration and system grounding arrangement.

This article explains the differences between Vector Sum, Source Return, and NCT-based ground fault protection, as well as the proper selection of Trip Units and Rating Plugs for 3-pole and 4-pole breakers.

Ground Fault Protection Methods

LS Electric ACBs support the following ground fault detection methods:

1. Vector Sum Method

The Trip Unit continuously measures the current flowing through all conductors passing through the breaker.

For a 3-pole breaker, the Trip Unit measures:

  • Phase A      (referred to as Phase R in LS Electric Applications)

  • Phase B      (referred to as Phase S in LS Electric Applications)

  • Phase C      (referred to as Phase R in LS Electric Applications)

Under normal conditions, the vector sum of these currents should equal zero.

Any imbalance is interpreted as leakage current or a ground fault condition.

This is the standard ground fault protection method available on 3-pole and 4-pole breakers.

2. Source Return Method (G EXT) 

Ground fault protection can also be achieved using an external Ground CT installed on the grounding conductor.

In this configuration:

  • A Ground CT is installed on the grounding conductor.

  • The Ground CT is connected to the Trip Unit's GEXT input.

  • Ground fault current is measured directly through the grounding path.

This method is commonly referred to as:

Source Return Type (G EXT + Ground CT)

3. NCT Method (Neutral CT)

When a 3-pole breaker is installed in a 3-phase, 4-wire system, the neutral conductor does not pass through the breaker and therefore cannot be measured by the breaker itself.

In these applications, an external Neutral Current Transformer (NCT) may be installed on the neutral conductor.

The NCT function provides the ability to detect and protect against ground fault current by monitoring the neutral conductor and incorporating that current into the Trip Unit's ground fault calculations.

This method is specifically intended for:

  • 3-pole breakers

  • 3-phase, 4-wire systems

  • Applications where neutral current must be included in the ground fault calculation

3-Pole Breakers (3P)

Available Ground Fault Protection Methods

A 3-pole breaker may utilize:

  • Vector Sum Method

  • Source Return Method (GEXT + Ground CT)

  • NCT Method (External Neutral CT)

NCT Applications

When using the NCT method, the following components are required:

  • NCT-capable Trip Unit ("O" Type)

  • NCT-compatible Rating Plug

  • External Neutral Current Transformer (NCT)

These components are designed to operate together and must be used as a complete system.

Important

A standard Rating Plug cannot be used with an NCT-type Trip Unit.

Likewise, an NCT Rating Plug cannot be used with a non-NCT Trip Unit.

The Trip Unit and Rating Plug must be selected as matching NCT-compatible components.

 

4-Pole Breakers (4P)

External NCT Not Required

In a 4-pole breaker, the neutral conductor passes through the fourth pole and is already measured by the breaker's internal current sensors.

Because neutral current is already included in the ground fault calculations, an external NCT is not required.

Trip Unit Selection

For 4-pole breakers:

  • Do not use an NCT-type ("O") Trip Unit

  • Do not use an NCT Rating Plug

  • Select a standard Trip Unit appropriate for the application

Available Ground Fault Protection Methods

A 4-pole breaker may utilize:

  • Vector Sum Method

  • Source Return Method (GEXT + Ground CT)

Because the neutral conductor is already monitored internally, the NCT method is not applicable.

Summary

  • LS Electric 3-pole ACBs can provide ground fault protection through Vector Sum, Source Return (GEXT), or NCT methods.

  • NCT protection is intended for 3-pole breakers installed in 3-phase, 4-wire systems where neutral current must be included in the ground fault calculation.

  • NCT applications require an NCT-type Trip Unit ("O"), NCT Rating Plug, and an external Neutral CT.

  • 4-pole breakers already measure neutral current internally and therefore cannot use NCT-type Trip Units or NCT Rating Plugs.

  • Ground fault protection on 4-pole breakers is achieved through Vector Sum or Source Return methods.