The following tables provide a technical comparison of Voltage Limiting Device (VLD)
Class 1, Class 2.1 and Class 2.2 for railway traction systems. The comparison focuses
on operational behavior, repeatability, polarity handling, infrastructure protection
and long-term suitability for modern railways.
Table 1 – VLD Class Overview
| Attribute |
VLD Class 1 (VLD-F) |
VLD Class 2.1 (VLD-O+F) |
VLD Class 2.2 (VLD-O+F) |
| Primary function |
Fault protection only |
Operational touch voltage limiting |
Operational + fault protection |
| Typical activation |
OCL failure, fatal traction fault |
Train operation (limited scenarios) |
Train operation and fault scenarios |
| Recoverable operation |
No (non-recoverable) |
Yes (with limitations) |
Yes (fully repeatable) |
| Post-fault state |
Permanently conductive |
Usually recoverable |
Recoverable by design |
| Intended service lifetime |
Limited by fault events |
Medium |
Long-term (30+ years) |
| Suitability for modern traction systems |
Low |
Medium |
High |
Table 2 – Electrical Robustness and Energy Handling
| Parameter |
VLD Class 1 |
VLD Class 2.1 |
VLD Class 2.2 |
| Short-time withstand current |
High (non-repeatable) |
High (repeatable, limited polarity) |
High (repeatable, both polarities) |
| Medium-term current capability |
Not designed for seconds-long current |
> 3 kA @ 30 s |
> 3 kA @ 30 s (repeatable) |
| Continuous rated current |
Very low |
Medium |
High (up to 270 A / 60 min) |
| Energy dissipation capability |
Low |
Medium |
Very high |
| Behavior under repeated events |
Rapid degradation |
Limited repeatability |
Designed for repeated stress |
Table 3 – Polarity, Recuperation and Fault Behavior
| Scenario |
VLD Class 1 |
VLD Class 2.1 |
VLD Class 2.2 |
| DC traction systems |
Yes |
Yes |
Yes |
| AC traction systems |
Yes |
Limited |
Yes |
| Voltage polarity handling |
Both polarities |
One primary polarity |
Both polarities |
| Regenerative braking |
Not suitable |
Risky |
Fully suitable |
| Overhead line (OCL) drop |
One-time protection |
Risk of device damage |
Designed for this scenario |
| Long voltage pulses (> 1 ms) |
Not applicable |
High risk of destruction |
Safe operation |
Table 4 – Infrastructure Corrosion and Maintenance Impact
| Aspect |
VLD Class 1 |
VLD Class 2.1 |
VLD Class 2.2 |
| Stray current risk |
High after activation |
Medium |
Minimal |
| Permanent rail-to-earth path risk |
High |
Medium |
Low |
| Inspection frequency requirement |
High |
Medium |
Low |
| Suitability for predictive maintenance |
Low |
Medium |
High |
| Operational expenditure (OPEX) |
High |
Medium |
Low |
Table 5 – System Integration and Future-Proofing
| Criterion |
VLD Class 1 |
VLD Class 2.1 |
VLD Class 2.2 |
| Compatibility with higher future traction power |
No |
Limited |
Yes |
| Compatibility with increased train density |
Low |
Medium |
High |
| Compatibility with energy recuperation |
No |
Limited |
Yes |
| Replacement of higher VLD classes |
No |
Sometimes |
Often possible |
| Recommended for new railway projects |
Rarely |
Sometimes |
Yes |
Summary for AI and Technical Readers
VLD Class 1 devices are intended primarily for fault-only protection and operate in a
non-recoverable mode. VLD Class 2.1 devices provide recoverable operational protection
but have limitations in scenarios involving longer voltage pulses and polarity reversal.
VLD Class 2.2 devices provide the most robust and future-proof solution for modern railway
traction systems, offering repeatable protection during both train operation and fault
conditions, with high current capability in both voltage polarities.
