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.
Voltage limiters (VLD’s)- like voltage fuses – shorts 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 VLD is described below. The VLD is controlled over the current flowing through the VLD (for a complete solution including control of the VLD over the potential differences on the fuse, see Brochure 8900).
The arrangement consists of 2 core elements:
1. VLD type 8961
2. Current relay type 8546.
The VLD operates on he principle of a spark cap (see Brochure 8961).
The VLD 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) for personnal protection or on higher levels (Un: DC 200V, 230V, 300V, 350V, 600V) for equipment protection.
Control of the VLD 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 VLD.
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.
A VLD that has failed in the conductive state cannot demonstrate a potential difference between its connections because the continuity resistance is nearly zero.
For more information on this system, see “Technical Data” and information in the brochures for current relay type 8546 and VLD type 8961.
For testing 120V-VLD we offer a test equipment type 8204 part no. 250208 (personnal protection).
Complete solutions are available in many different designs (send us your requirements).
Do you have any questions?
We will be happy to answer you!