Direct connection between the return conductor and the earth has to be prevented within DC traction systems due to the risk of stray currents as stray currents would lead to corrosion of buildings, bridges and other metallic (electrically conductive) structures and equipment. Moreover, hazards might arise from impermissible high contact voltages. One of the precautions for protection of human beings against impermissible contact voltages is the temporary short-circuiting of the various earthing systems according to EN 50122.
The ESC is an automatically resettable earthing short-circuiting device according to EN 50526. It is parameterisable and is monitoring the voltage differences between the return conductor and the earth. The combination of thyristors arranged anti-parallel and a power contactor ensures a short switching period and high current carrying capacity.
If the set voltage waveform is exceeded, the earthing short-circuiting device is activated, i.e. if the criteria of a closing operation are fulfilled, the thyristors are triggered and simultaneously the contactor is switched. The thyristors switch in a few microseconds, the contactor closes in approx. 90 ms. When the thyristors have been triggered, the voltage collapses to a few volts. The thyristors remain triggered until the contactor has been closed, which is detected by way of the position of the auxiliary contacts of the contactor. Thereafter, the thyristors are turned off and the contactor takes over the load.
The main assemblies of the earthing short-circuiting device are in a dual-channel redundancy design. Moreover, the earthing short-circuiting device has got self-testing functions. In that way it is ensured that malfunctions hardly occur and that a malfunction is always reported if it should occur once nevertheless.
The following products are available in this context:
A use case is desribed in this technical description: