Most low voltage switchgear has three contacts in the main circuit, which switch three-phase loads on all poles. In some applications, switchgear with four main poles is required, either for safety reasons or for an optimal solution of the application.
This may require various device configurations.
Switchgear applications with 4 NO contacts
Four NO contacts are necessary or at least very advantageous for the applications below:
Request n ° 1
Applications requiring the interruption of the neutral line to switch off or disconnect loads. This may be the case in power supplies subject to unfavorable earthing conditions, in TT power supplies, for protective disconnection in computer networks or to impedance earth. Care must be taken 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 changing non-linear consumers, special attention should be paid to the current load of the neutral line.
The effect of higher frequencies on the performance of low-voltage devices should be taken into account both in networks with higher base frequencies (for example 400 Hz) and in cases where current harmonics occur. These current harmonics occur if the supply voltage contains harmonics or if non-linear consumers are connected.
These consumers can, for example, be compensation for devices for luminescent lamps operating in the saturation range or devices with phase shift control. With consumers with phase angle control and with frequency converters, harmonics with frequencies up to several khz may appear in the power supply. The harmonic content can be increased by capacitors connected to the power supply, the current consumption of which increases with increasing frequency.
This factor should be given special attention in individually compensated motors and it may be necessary to correct the current settings of the protection relay.
In applications where current harmonics occur, the effect of the harmonics ( eg additional heating effects ) is added to that of the base frequency. This can be particularly critical in devices containing coils of ferromagnetic materials (bimetal heating coils, magnetic triggers, etc.).
In the case of loads with connection to the neutral conductor ( eg. Single-phase loads such as luminescent lamps, small current adapters, etc. ), the formation of a zero-sequence system can lead to thermal overload can lead to high harmonic content. Should also be taken into account when using 4-pole switchgear. To get training of Auto
Application n ° 2
Switching of supply systems ( eg for emergency power supplies ), where complete separation of the two power systems is required.
Switching of several single-phase loads (heaters, lamps) with a switching device.
Application n ° 4
Switching of direct current loads with a higher nominal voltage requiring the series connection of four contacts.
Switchgear designed for alternating current can carry at least the same rated rated operational direct 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 operating at low voltage can be switched by AC switchgear without difficulty , since their switching capacity for low voltage DC is practically the same as for alternating current.
With voltages above approx. 60 V, the DC switching capacity of the AC switchgear with double-break contacts (eg contractors) decreases sharply. By connecting two or three circuits in series this limit can be increased to two or three times the voltage.
The reason for the reduced switching capacity with direct current compared to alternating current is the absence of the current zero crossover, which in alternating current favors the quenching of the electric arc. The electric arc in the contact system can continue to burn under higher forward voltages and thus destroy the board.
With forward voltages, contact erosion and hence contact life also differs from that of alternating voltages. The possible values for direct current are specifically tested and documented. With direct current, the load influences the switching capacity more strongly than with alternating current. The energy stored in the inductance of the load must be largely dissipated in the form of an electric arc.
Therefore with a strongly inductive load ( large time constant L / R ) the permissible switching capacity for the same electrical lifetime is lower than that of an ohmic load due to the much longer breaking times.
Switchgear applications with 2 NO contacts and 2 NC contacts
Devices with two NO contacts and two NC contacts are useful in applications where one of the two circuits must always be closed.
These are, for example:
Switchgear applications with 3 NO contacts and 1 NC contact
Devices with three NO contacts and one NC contact are used in applications where, when the main load is turned off, for example the motor, another single-phase load must be turned on.
These applications could include:
- Safety circuits
- DC braking systems activated when a driver is stopped
- Clutches that must be released when the drive is turned off