Description
There are special problems with railways operating with DC power concerning contact and anti-corrosion protection. On the one hand, an attempt is made to prevent the DC currents from connecting to earth as far as possible so as to avoid corrosion, while on the other hand there is the problem that dangerous voltage potentials can arise between the two areas if a strict electrical separation is made.
Dangerous contact voltages can arise if these areas are spatially close to one another. This problem can be solved by using open earthing.
Low-voltage limiting units such as voltage-limiting devices connect the various power supply networks with one another when the trigger value is exceeded.
This thus ensures that no higher contact voltages than those planned can arise and lead to short circuits when connecting the various power supply networks for triggering. (See DIN EN 50122-1 and DIN EN 50123-5 – VDE 0115 Part 3 and Part 300-5 for further details.)
The following describes a complete solution with voltage-limiting devices. The monitoring of the voltage-limiting device is done via the flow of current through the voltage-limiting device (see brochure sheet 8900 for a complete solution with monitoring of the voltage-limiting device via the voltage differences at the voltage-limiting device).
The arrangement consists of 2 core items
1. the type 8960 voltage-limiting unit
2. the type 8545 current relay.
The voltage-limiting device involves a fuse whose action is based on that of the zener diode (see brochure sheet 8960). Only in this way is it possible to achieve precise setting of the trigger voltage in manufacture, as opposed to the method using voltage monitoring between two electrodes.
The voltage-limiting device can be designed for various values (60V, 100V, 200V, 425V).
Monitoring of the voltage-limiting device via the flow of current through the voltage-limiting device is always recommended if a message is required without a delay and if sufficiently large currents (more than 15 A) flow through the voltage-limiting device.
As opposed to voltage monitoring, the current monitoring device reacts at once. It is recommended that you store the message, since it only exists while the current is flowing. The memory module that includes a test and acknowledgement unit is a constituent part of this complete solution.
When testing the voltage-limiting device to determine its current actual state without needing to dismantle it, we recommend the use of circuit continuity testers, where the pitch of the tone varies in a characteristic way in the event of voltage drops deviating from zero at the resistances to be tested. Blocking voltage-limiting devices that have a pole at the rail earth will in practice always indicate a potential difference that is made up of a DC voltage with a superimposed AC portion between their connections when in an installed state.
The voltage-limiting device that has been shorted cannot show between its connections any potential differences whose forward resistance is zero. Fuses can thus be checked while installed when using circuit continuity testers that react to these operating circumstances as described above.
See the »Technical data« and the details in the individual brochures for the type 8545 current relay, the type 8587 bistable relay and the type 8960 voltage-limiting device for further information on this unit.
Complete solutions in a wide variety of different configurations are available (please enquire).
