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During the design of Mode 3 AC charging stations, DC residual current protection is a critical factor in ensuring both compliance and operational safety. Choosing between a Type A + RDC-DD combination and an integrated Type B RCD directly impacts system cost, reliability, certification efficiency, and supply chain compatibility. This article provides engineers with a clear selection guide based on standards, solution characteristics, and real-world application scenarios.
1. DC Leakage Risks and Mandatory Standards
During operation of Mode 3 AC chargers, the onboard charger (OBC) generates two types of leakage currents during rectification: pulsating DC residual current and smooth DC residual current. A standard Type A RCD can effectively protect against AC and pulsating DC leakage but has little to no detection capability for smooth DC leakage.
If smooth DC leakage enters the AC grid, it may lead to protection failure, posing risks such as electric shock, neutral conductor magnetization, and even fire hazards.
Relevant standards impose clear mandatory requirements:
- IEC 61851-1:2017 specifies that when DC residual currents may occur, additional protective measures must be implemented. If smooth DC residual current ≥ 6 mA is possible, only a Type B RCD is acceptable.
- RDC-DD devices, as dedicated detection components, must comply with IEC 62955:2018.
2. Key Features of the Two Protection Solutions
2.1 Type A + RDC-DD Combination
The RDC-DD (Residual Direct Current Detecting Device) is not a standalone protective device and must be used together with a Type A RCD. Their roles are clearly defined:
- Type A RCD: protects against AC and pulsating DC leakage
- RDC-DD: detects smooth DC leakage
When smooth DC leakage ≥ 6 mA is detected, the RDC-DD sends a trigger signal to disconnect the upstream Type A RCD or contactor.
Key advantages:
- Cost-effective solution
- Type A RCDs are significantly cheaper than Type B RCDs
- Mature supply chain for RDC-DD modules
- Widely adopted in cost-sensitive overseas charging projects
- Fully compliant with international standards
2.2 Integrated Type B RCD
A Type B RCD is an all-in-one protection device that independently detects:
- AC leakage
- Pulsating DC leakage
- Smooth DC leakage
- Mixed waveform leakage
With fewer components in the system, there are fewer potential failure points, resulting in improved reliability and simplified maintenance.
Key advantages:
- Preferred in European markets
- Meets stricter installation requirements in countries like Germany and France
- Eliminates the need for coordination and tuning between devices
- Compact design, suitable for space-constrained applications such as onboard (OBC-integrated) installations
3. Core Parameter Comparison
| Parameter | Type A + RDC-DD | Type B RCD |
|---|---|---|
| DC Detection Threshold | 6 mA DC (RDC-DD trigger) | 6 mA DC (direct tripping) |
| Protection Coverage | AC, pulsating DC, smooth DC (combined protection) | AC, pulsating DC, smooth DC, mixed waveforms (full coverage) |
| Device Count | 2 (combined use required) | 1 (integrated design) |
| Response Time | Depends on coordination between devices | Stable and predictable |
| System Complexity | Higher, requires coordination and tuning | Lower, plug-and-play |
| Typical Cost | Lower (over 30% less than Type B) | Higher upfront cost |
| Certification Standard | IEC 62955 compliant | IEC 62955 compliant |
| Main Application Markets | Cost-sensitive overseas AC charging projects | Europe, onboard applications, high-end chargers |
Important note:
RDC-DD testing requirements are stringent. According to IEC 62955:2018 Clause 9.9.2, the device must trip within a specified time when subjected to 6 mA smooth DC current, while also ensuring that AC leakage does not cause false triggering. This ensures reliable coordination with Type A RCDs.
4. Key Decision Factors
4.1 Cost Sensitivity
For cost-sensitive overseas AC charging projects, the Type A + RDC-DD combination is recommended. With declining prices for RDC-DD modules and the widespread availability of Type A RCDs, this solution offers a clear cost advantage.
4.2 Reliability Requirements
For onboard (OBC-integrated) applications or high-end DC fast chargers, the Type B RCD is recommended. Its integrated design reduces system-level coordination risks and lowers failure probability.
Additionally, automotive-grade Type B RCDs with AEC-Q100 certification are becoming more widely available, supporting IATF 16949 requirements and making them suitable for high-reliability applications.
4.3 Certification and Export Requirements
When exporting, it is essential to verify local regulations in the target market. Although IEC 61851-1 permits both solutions, many European countries (such as Germany and France) strongly prefer Type B RCDs. Early verification helps avoid certification delays and grid connection issues.
5. Key Engineering Considerations
Regardless of the selected solution, PCB layout and grounding design are critical.
- The current sensing circuit of RDC-DD or Type B RCD must include all live conductors (phase + neutral)
- Ground conductors must never be included in the sensing loop
For high-power chargers (≥22 kW), thermal effects on conductors must also be considered, as heat can affect sensor accuracy and potentially compromise protection performance.
Additionally:
- RCD installation must comply with IEC 62955
- Devices should be installed in accessible locations for routine testing
- In Mode 3 chargers, RCD or RDC-DD modules are typically integrated inside the charger enclosure, with a front-panel test interface for usability and compliance
6. Selection Summary
There is no absolute “best” solution—only the most suitable one for the application.
- Type A + RDC-DD: cost-effective, mature supply chain, widely used in price-sensitive AC charging markets
- Type B RCD: highly reliable, integrated design, increasingly adopted in Europe, onboard applications, and high-end charging systems
7. Magtron DC Residual Current Protection Solution
Magtron’s RCMU series residual current sensors are specifically designed for DC leakage protection in EV charging applications. They are fully compatible with Type A + RDC-DD solutions and include models such as RCMU-A6 and RCMU-A6+CT.
The series complies with IEC 62955:2018 and IEC 61851-1:2017, offering:
- High immunity to interference
- High measurement accuracy
- Automotive-grade reliability (for selected models)
These products provide a field-proven, mass-deployed DC leakage detection solution for overseas AC charger manufacturers, helping streamline compliance design and accelerate market deployment.
