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Home / Technical Articles / Application of 4-pole Switchgear Devices
Application of 4-pole Switchgear
Application of 4-pole Switchgear Devices (on photo Electrical panel board motors control by C&P Engineering @ Flickr)


The majority of low-voltage switchgear is equipped with three contacts in the main circuit, which switch three-phase loads in all poles. In some applications switchgear with four main poles is required, either for safety reasons or for an optimum solution of the application.

This may require various device configurations.

Applications of switchgear with 4 NO contacts

Four NO contacts are required or at least very advantageous for the below applications:

Application no.1

Applications which require the interruption of the neutral line for switching off or disconnecting loads. This can be the case in supplies with adverse grounding conditions, in TT supplies, for protective disconnection in IT or impedance-grounded networks. Attention has to be paid that the neutral pole closes before or at the same time as the other poles and opens after them or at the same time.

When switching non-linear consumers, specific attention has to be paid regarding the current loading of the neutral line.

The effect of higher frequencies on the performance of low-voltage devices should be considered both in networks with higher basic frequencies (for example 400 Hz) and also in cases where current-harmonics occur. Such current-harmonics occur if the supply voltage contains harmonics or if non-linear consumers are connected.

Such consumers may for example be compensation devices for luminescent lamps that operate in the range of saturation or devices with phase angle control. With consumers with phase angle control and with frequency converters harmonics with frequencies up to several kHz may arise in the supply. The harmonic content can be increased by capacitors connected to the supply, whose current consumption increases with increasing frequency.

Special attention should be paid to this factor in individually compensated motors and a correction of the current settings of the protective relay may be required.

In applications in which current-harmonics arise, the effect of the harmonics (for example additional heating effects) is added to that of the basic frequency. This can be especially critical in devices that contain coils or ferromagnetic materials (bimetal heating coils, magnetic releases etc.).

In the case of loads with connection to the neutral conductor (e.g. single-phase loads such as luminescent lamps, small power adapters etc.), a high harmonic content can result because of the formation of a zero-sequence-system that may lead to thermal overloading. This should also be taken into account in the use of 4-pole switchgear.

Application no.2

Switching-over of supply systems (for example for standby power supplies), for which complete separation of the two supply systems is required.

Application no.3

Switching of several single-phase loads (heaters, lamps) with one switchgear unit.

Application no.4

Switching direct current loads with higher rated voltage that requires the series connection of four contacts.

Switchgear designed for alternating current can carry at least the same rated continuous operational DC current. With direct current the skin effect in the circuits disappears and none of the specific effects associated with alternating currents such as hysteresis or eddy current losses occur.

DC devices that are operated at low voltage can be switched by AC switchgear without difficulty, as their direct current switching capacity at low voltages is practically the same as for alternating current.

With voltages in excess of around 60 V, the direct current switching capacity of AC switchgear with double-breaking contacts (for example contactors) decreases strongly. By connecting two or three circuits in series (Figure 1) this limit can be raised to twice or three times the voltage.

Examples of diagrams for poles connected in series
Figure 1 – Examples of diagrams for poles connected in series. Where grounded power supplies are used (top graph) with loads switched on both sides, it should be noted that ground faults can lead to bridging of contacts and hence to a reduction in the breakable voltage.

The reason for the reduced switching capacity with DC compared with AC is the absence of the current zero crossover that with AC supports the quenching of the electric arc. The electric arc in the contact system can continue to burn under larger direct voltages and thus destroy the switchgear.

With direct voltages, the contact erosion and hence also the contact life span differ from those at alternating voltage. The attainable values for direct current are specifically tested and documented. With direct current, the load affects the switching capacity more strongly than with alternating current. The energy stored in the inductance of the load must largely be dissipated in the form of an electric arc.

Hence with a strongly inductive load (large time constant L/R) the permissible switching capacity for the same electrical life span is smaller than with an ohmic load due to the much longer breaking times.

Applications of switchgear with 2 NO and 2 NC contacts

Devices with two NO and two NC contacts are useful in applications in which one of two circuits must always be closed.

These are, for example:

  • Switching a heater between two levels (Figure 2)
  • Switching-over between single-phase supplies – for example, emergency power supply systems (Figure 2)
  • Reversing motors for space saving arrangement of devices (Figure 3)
  • Reversing of 2-step motors with separated windings (Figure 4)
Four-pole contactors with 2 NO / 2 NC contacts
Figure 2 – Four-pole contactors with 2 NO / 2 NC contacts for switching single-phase loads (left) or switching-over between two supplies (right)

Slimline reversing starter with a 2 NO / 2 NC contactor for reversing
Figure 3 – Slimline reversing starter with a 2 NO / 2 NC contactor for reversing

Reversing of a two step motor with separate windings
Figure 4 – Reversing of a two step motor with separate windings

Applications of switchgear with 3 NO and 1 NC contact

Devices with three NO and one NC contact are used in applications in which, when the main load is switched off – for example the motor –, another single-phase load must be switched on.

Such applications could include:

  1. Safety circuits
  2. Direct current brake systems that are activated when a drive is switched off
  3. Clutches that must be released when the drive is switched off

Resource: Allen Bradley – Low-Voltage Switchgear and Controlgear

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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 facilities. Professional in AutoCAD programming.


  1. Zeeshan Khalid
    Feb 29, 2016

    Can some one help me regarding 400hz switchboard.

  2. romain
    Nov 02, 2012

    Hello all, Can 4-P breaker become a standard in the near future, even for application where today we have 3-P breaker as a acceptable solution ? In other words, will one day 4-P breaker replace 3-P as they already cover their functionnality ?

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