Main switch (railway)
In electric traction vehicles, a main switch is a circuit breaker which is located in the high-voltage line between the pantograph and the transformer in AC vehicles or the power control elements in DC vehicles . The main switch is used to switch the entire electrical system of the traction vehicle on and off. The main switch can also switch off the vehicle automatically when a protective device is triggered. Such protective devices depend on the vehicle type and can, for example, in the event of a response from the Buchholz protection , earth fault , short circuit , overload , overvoltage or undervoltage in the contact line (e.g. in front of protective lines or low contact lines at underpasses , but also when the traction current system at border stations and at Address swing entrances ). The main switch can also be switched off by the safety driving circuit and other train protection systems. Because with some technically triggered emergency brakes, the main switch is also switched off in order to switch off the drive power, and not just rapid braking is initiated.
In older traction vehicles, especially those with low power, so-called main fuses were sometimes installed instead of a main switch.
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Protective sections that have not been connected through and system separation points in general may only be entered with the main switch switched off. There are special contact line signals to announce such points . These show the driver where to turn off the main switch and where to turn it on again.
If you were to drive into such an insulated section without the “main switch off”, an electric arc could occur and the rail vehicle could be damaged , especially on the pantograph and the transformer . Even raising and lowering the pantograph with the main switch on could create a large arc. Therefore, the pantograph is only raised and lowered when the main switch is switched off. Acoustically, the activation of the main switch can be heard very clearly: It is the dull bang after lifting or before lowering the pantograph.
Design and function
The main switch must be designed as a circuit breaker because it has to safely disconnect the traction vehicle from the voltage source, i.e. the overhead line or the side busbar, even under full load . Above all, this requires reliable spark quenching in order to collapse the resulting arc . The main switch is usually located on the roof or in the machine room between the high-voltage converter and the transformer . Whereby it often also takes on the function of the roof duct.
Main switches for the traction current systems commonly used today are suitable for either switching alternating current or direct current. With alternating current, relatively high voltages and low currents have to be switched. In addition, alternating current has a zero crossing , which makes it easier to extinguish the arc. With direct current, however, relatively low voltages and high currents are usually switched. Furthermore, direct current does not have a zero crossing, which is why direct current makes different demands on a main switch than alternating current. Multi-system vehicles that run in both AC and DC networks are therefore usually equipped with two main switches.
Oil switch
The contacts of the oil switch are located in an airtight tank that is filled with oil. When the oil switch is actuated, part of the oil evaporates due to the arc, which extinguishes the arc. In the case of oil switches, the high voltage is usually interrupted at several points at the same time. The oil switch is switched on with the aid of a compressed air drive and held in place against spring pressure by a latch. It is switched off by loosening the latch, whereby the contacts are separated by the spring force. Since there is a risk of the main switch exploding with this type of construction, these were always placed on the roof of the traction vehicle. Further disadvantages are their large space requirements, the limited switching capacity and the high weight. Oil switches were widely used until the 1930s and are now out of date.
Expansion switch
Expansion switches are single contact pin switches. The switching process takes place in the expansion or steam chamber. The lower part of this is filled with a water-based extinguishing agent, which partially evaporates when switched off, which in turn extinguishes the arc. Expansion switches are switched on by a compressed air drive or by hand. They are switched off by spring force. They were used on new vehicles until the mid-1940s.
Compressed air switch
With the increasing power of the locomotives, the aforementioned types reached their limits. This led to the development of compressed air switches, which not only operate pneumatically , but also blow off the resulting arc with compressed air .
Vacuum switch
Vacuum switches are often used in modern traction vehicles. They consist of a switching chamber in which there is a pressure of around 10 −5 Pa, in which there are two cylindrical pressure contacts. One pressure contact can be displaced in relation to the other pressure contact. With the help of a toggle lever , the sliding pressure contact is pressed against the fixed pressure contact and locked when switched on. The toggle lever is driven by a compressed air drive. Due to the vacuum in the switching chamber, the switching distance is significantly shortened and the arc is extinguished.
Individual evidence
- ↑ a b c d e Bendel, Helmut: The electric locomotive: structure, function, new technology . 2., arr. and additional edition Transpress, Berlin 1994, ISBN 3-344-70844-9 .
- ↑ a b c GDL (Ed.): Manual of the electric traction vehicles of the German Federal Railroad . 1st edition. Asset management of the union of German locomotive drivers and aspirants GmbH, Frankfurt am Main 1959.