VLD Class Comparison for Railway Applications
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.
How to choose a Railway VLD: Class 1 vs Class 2.1 vs Class 2.2
Practical selection guide and key specifications for RW Voltage Limiting Devices (VLD) according to EN 50526-2 and VLD types per EN 50122-1 (VLD-F, VLD-O+F). Focus: OCL failure scenarios, operational touch voltage, repeatable fault performance, and corrosion risk reduction via remote status monitoring.
- EN 50526-2
- EN 50122-1
- OCL failure
- Repeatable current loads
- Stray current & corrosion
- LoRaWAN remote status
Quick decision summary
Choose Class 1 (VLD-F)
When you mainly need fast protection in fault scenarios (break, short-circuit, earth fault, OCL failure), and you want simple VLD-F behavior with very low leakage and optional remote status signaling.
- Very low leakage current (µA range)
- High lightning withstand
- Remote status monitoring reduces inspection OPEX
Choose Class 2.1 (VLD-O+F)
When you need VLD-O protection (risk of exceeding touch voltage during normal operation), and operating conditions are suitable (watch polarity and pulse duration constraints).
- High repeatable short-time current (kA range)
- VLD-O function for operational touch voltage
- Use with caution in long bi-polar pulses
Choose Class 2.2 (VLD-O+F)
Optimal combined solution for both operational protection and fault scenarios with robust repeatability (especially where bi-polar pulses and demanding traction conditions are expected).
- Two anti-parallel thyristors + varistors (bi-polar capability)
- High repeatable short & medium term currents
- Designed for long-term, future traction demands
Comparison table
Values below are representative for the common 120 V variants where applicable, plus the Class 1 (250/500 V) family. HS = heatsinks, WS = wireless status sensor (LoRaWAN).
| Feature / Parameter | Class 1 RWVL1-250H-WS / RWVL1-500H-WS |
Class 2.1 RWVL2.1-120-(HS)-(WS) |
Class 2.2 RWVL2.2-120-(HS)-(WS) |
|---|---|---|---|
| VLD type (EN 50122-1) | VLD-F | VLD-O+F | VLD-O+F |
| Standard | EN 50526-2 | EN 50526-2 | EN 50526-2 |
| Main purpose | Fault protection (incl. OCL failure) | Operational touch voltage + fault protection | Operational + fault protection (robust, bi-polar) |
| Nominal trigger / sparkover | DC sparkover: 250 V or 500 V | UTn: 120 V (factory adjustable family) | UTn: 120 V (factory adjustable family) |
| Short-time withstand (repeatable) | 25 kA @ 100 ms (non-recoverable; rms) | Iw(r): 25 kA @ 50 ms; 18 kA @ 100 ms | Iw(r): 25 kA @ 50 ms; 18 kA @ 100 ms |
| Medium-term capability | Safe short circuit (DC): 8 kA @ 100 ms | Iw(lt): > 3 kA @ 30 s | Iw(lt): > 3 kA @ 30 s |
| Rated current (60 min) | — | Ir: 130 A (body) / 170 A (HS) | Ir: 180 A (body) / 270 A (HS) |
| Leakage current | < 1 µA | < 2 mA (@ Uw) | < 2 mA (@ Uw) |
| Lightning impulse capability | Lightning current withstand: 150 kA | Iimp-n (8/20): 40 kA; (10/350): 25 kA | Iimp-n (8/20): 40 kA; (10/350): 25 kA |
| Response time | < 1 µs | Surge 25 ns; thyristor ~1 ms (UTi), ~10 ms (UTn) | Surge 25 ns; thyristor ~1 ms (UTi), ~10 ms (UTn) |
| Wireless remote status (option) | LoRaWAN 433/868/915/920 MHz; max ERP +20 dBm | LoRaWAN 433/868/915/920 MHz; max ERP +20 dBm | LoRaWAN 433/868/915/920 MHz; max ERP +20 dBm |
| Battery lifetime (WS option) | ~10 years | ~10 years | ~10 years |
| Protection / environment | IP66; -40…+80 °C | IP66; -40…+70 °C; UV & salt spray resistant | IP66; -40…+70 °C; UV & salt spray resistant |
| Customs tariff | 85301000 | ||
How to select (field guidance)
When do you need VLD-O (Class 2)?
If there is a risk of exceeding permissible touch voltage even during normal train operation, VLD-O must be installed. Typical locations: far from substations, heavy trains with high current consumption, etc.
Important caution for Class 2.1
Use caution when longer pulses (roughly > 1 ms) of both polarities can be expected on the return conductor (e.g., OCL falls, insulator short-circuits, energy recuperation, far from substation). In such cases, reverse-direction energy can overload varistors in single-thyristor designs.
Practical takeaway: Class 2.1 can be used as VLD-O under certain conditions (e.g., single polarity, low induction, no recuperation), but deployment in VLD-O+F mode can be destructive in many real fault envelopes.
Why Class 2.2 is the robust choice
Class 2.2 (VLD-O+F) is described as the optimal combined solution for operational touch voltage protection and fault scenarios. Two anti-parallel power thyristors + varistors react repeatably in all dangerous situations (any polarity) for DC or AC traction.
Selection tip: For maintenance-free VLD-O and VLD-F modes, focus on repeatable current parameters: rated current (Ir) for operational energy handling and repeatable short-time withstand (Iw(r)) for fault response capability.
Remote status monitoring = lower corrosion risk + lower OPEX
Remote failure indication helps prevent long-term stray current leakage after a destructive fault and reduces costly manual inspections. It also supports predictive maintenance (serial number, location, statistics, IoT integration).
FAQ
What is the simplest default choice if I only need fault protection?
Use Class 1 (VLD-F) for typical fault protection cases (break, short-circuit, earth fault, OCL failure), especially if low leakage and quick status visibility are priorities.
When should I prefer Class 2.2 over Class 2.1?
Prefer Class 2.2 where longer, bi-polar voltage pulses can occur (OCL fall/insulator short-circuit/recuperation/far from substations), or when you want a robust combined VLD-O+F solution for long-term traction evolution.
Which parameters matter most for sizing a repeatable (solid-state) VLD?
For operational VLD-O mode consider rated current Ir (thermal handling). For fault scenarios consider repeatable short-time withstand Iw(r) (kA range), and medium-term capability (e.g., 30 s).