Protection from contact voltage and corrosion present a particular problem in the case of DC powered railway vehicles. On the one hand, direct electrical currents should not be connected to ground in order to prevent corrosion, but on the other hand, strict electrical separation can result in hazardous voltage potentials between the two areas.
Dangerous contact voltages can occur if these areas are close together. Floating grounds are used to resolve this problem.
Low-voltage limiters – like voltage fuses – connect the different grounds when a threshold is exceeded.
This guarantees that no dangerous contact voltages can occur and that short circuits cannot trigger when different grounds are connected (for detailed information, please refer to DIN EN 50122-1 and DIN EN 50123-5 – VDE 0115 Part 3 and Part 300-5).
A complete solution with voltage fuse is described below. The voltage fuse is controlled over the current flowing through the fuse (for a complete solution including control of the voltage fuse over the potential differences on the fuse, see Brochure 8900).
The arrangement consists of 2 core elements:
1. Voltage fuse type 8960
2. Current relay type 8545.
The voltage fuse operates on he principle of a Zener diode (see Brochure 8960). This is the only way the threshold voltage can be adjusted precisely during manufacture – as opposed to the method of voltage flashover between 2 electrodes.
The voltage fuse is set to a value that is sufficiently below the permissible contact voltage (per DIN EN 50122-1, Section 7.3.3 DC 120 V – VDE 0115 Part 3).
Control of the voltage fuse over the flow of current through the fuse is always recommended when an immediate signal is desired and sufficient current (larger than 15 A) flows through the fuse.
In contrast to voltage control, current control triggers
immediately. Since the signal is only available while the current flows, it should be saved. The memory module including test and acknowledge elements is a part of this complete solution.
To check the actual status of the voltage fuse without removing it, continuity testers are recommended with a tone pitch that reacts in a characteristic manner to voltage drops that deviate from zero on the resistors to be checked. In the installed state lockout voltage fuses with one of their poles on the rail grounding always show a potential difference between their connections that consists of a direct current with a superimposed AC component.
A voltage fuse that has failed in the conductive state cannot demonstrate a potential difference between its connections because the continuity resistance is nearly zero.
Continuity testers that respond to these operating conditions in the manner described above can be used to test installed fuses.
For more information on this system, see “Technical Data” and information in the brochures for current relay type 8545, bi-stable relay type 8587, and voltage fuse type 8960.
Complete solutions are available in many different designs (send us your requirements).
Data sheet [PDF]
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