Introduction
As electric vehicle (EV) adoption accelerates, the safety of EV charging stations becomes increasingly important. One of the most critical aspects of electrical safety for EV chargers is ensuring proper residual current protection. This is especially essential in regions such as Europe, Asia, and Latin America, where IEC 61851-1 sets stringent standards for EV charger design. Proper protection ensures both the safety of users and compliance with regulations.
In 2017, the IEC 61851-1 standard was updated to specify the required residual current protection measures for Mode-3 EVSE, which have remained unchanged as of the post of this blog:
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Option 1: RCD Type B
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Option 2: RCD Type A, combined with appropriate equipment to disconnect the supply in case of DC fault currents exceeding 6mA.
The IEC 62955 standard, firstly introduced in 2018, further specifies the product type of RDC-DD (Residual Direct Current Detecting Device) for the protective measure of option 2.
A common question engineers face during the design process is: Should I integrate RDC-DD or a Type B RCD into my EV charger design? Both options are valid, but understanding their differences, advantages, and limitations is key to making an informed decision. In this guide, we’ll compare RDC-DD and Type B RCD, helping you select the most suitable solution for your Mode-3 EVSE design.
What is RDC-DD?
RDC-DD stands for Residual Direct Current Detecting Device, specifically designed for Mode 3 charging of electric vehicles in compliance with the IEC 62955 standard. This standard defines a new type of residual current device that ensures the proper functionality of Type A or Type F RCDs, even in the presence of DC residual currents exceeding 6 mA.
According to IEC 62955:2018, RDC-DDs are categorized into two types:
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RDC-MD (Residual Direct Current – Monitoring Device): Primarily used for monitoring DC residual currents to prevent Type A or Type F RCDs from malfunctioning due to DC leakage above 6 mA.
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RDC-PD (Residual Direct Current – Protection Device): Provides both monitoring and protection against AC residual currents and pulsating DC residual currents, in addition to protecting against DC residual currents above 6 mA.
The primary difference between RDC-MD and RDC-PD lies in their protection capabilities. While the RDC-MD can monitor DC residual currents to protect Type A or Type F RCDs from being impaired by DC leakage, it does not offer protection against AC residual currents or pulsating DC residual currents, making it unsuitable for directly protecting human life. For this reason, RDC-MD is sometimes referred to as DC RCM (Residual Current Monitor) by engineers.
What is Type B RCD?
A Type B RCD (Residual Current Device) is an advanced protection device designed to detect and trip a wide range of residual currents, including:
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Sinusoidal AC residual current
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Pulsating DC residual current
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Composite residual currents with frequencies up to 1000 Hz
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Smooth DC residual current
There are several types of Type B RCDs, each designed for different applications and functionalities:
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Type B RCCB (Residual Current Circuit Breaker): Used for general protection against residual currents in compliance with IEC 62423 and IEC 61008-1. Examples include Siemens 5SV3 B-Type and ABB F200 B-Type,commonly used in Europe and Asia.
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Type B RCBO (Residual Current Circuit Breaker with Overcurrent Protection): A combination of residual current protection and overcurrent protection, in accordance with IEC 62423 and IEC 61009-1. An example is the Doepke DRCBO 4 B NK.
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Type B mRCD (Modular Residual Current Device): Compliant with IEC 60947-2 Annex M and is typically used as an accessory for MCCB (Molded Case Circuit Breakers) to interrupt faulty circuits. These are often used in industrial applications.
Figure 1: Operating schematic of residual current protection with mRCD and circuit breaker.
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Type B CBR (Current Breaker Relay): Circuit breakers with integrated Earth Leakage Protection for industrial applications. compliant with Annex B of IEC 60947-2, such as Compact™ NSXm.
Difference Between RDC-DD and Type B RCD
The table below compares the responses of RDC-MD (one type of RDC-DD), RDC-PD (the other type of RDC-DD), and Type B RCD to various types of residual currents.
| Residual current form | RDC-MD | RDC-PD | Type B RCD |
|---|---|---|---|
| Sinusoidal AC residual current | N | Y | Y |
| Pulsating DC residual current | N | Y | Y |
| Smooth DC direct currents | Y (3–6mA) | Y (3–6mA) | Y (15–60mA) |
| Composite residual currents with frequencies up to 1000 Hz | N | N | Y |
Notes:
1) Type B 30mA RCDs respond to smooth DC residual currents ranging from 15mA to 60mA (0.5 IΔn to 2 IΔn, where IΔn = 30mA), while RDC-DD devices respond to smooth DC currents between 3mA (0.5 IΔdc) and 6mA (IΔdc).
2) Composite residual currents with frequencies up to 1000 Hz are generally not found in EV charging systems where the onboard charger (OBC) is properly isolated.
Why RDC-DD Dominates in Mode-3 EVSE protection design?
Since the update of IEC 61851-1 in 2017, which requires the detection of smooth DC fault currents, real-world experience has shown that RDC-DD is generally the preferred solution for Mode-3 EVSE protection. While some engineers may assume that RDC-DD is cheaper than Type B RCD, cost is not the core factor. Both solutions, when implemented with the proper residual current sensor based on the tripping characteristics of IEC 62955 for RDC-DD and IEC 62423/60947-2 for Type B RCD, are comparable in terms of manufacturing costs.
Mr. Zhou Gang, Product and Project Management Head at Bituo Technik, explains that the simplicity of electrical installation is the main reason EVSE manufacturers prefer RCD Type A combined with RDC-DD over Type B RCD. This configuration efficiently meets the requirements of IEC 61851-1 while minimizing installation complexity.
Figure-2 below shows a Type B RCD (30mA) installed in a final power distribution system. As discussed earlier, this Type B RCD will not trip in the event of a 6mA to 15mA DC fault current—such as the DC fault current caused by OBC insulation faults. However, this range of DC fault currents (6mA to 15mA) can impair the functionality of the upstream Type AC/A RCD.
Figure 2: Type B RCD (30mA) installed in a final power distribution system
In order to mitigate this risk, two common installation approaches are typically used when integrating a Type B RCD:
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Approach A: Replacing the upstream Type-AC/A RCD with a Type B RCD.
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Approach B: Changing the incoming power supply of the Type B RCD to come from the incoming terminal of the upstream Type-AC/A RCD, rather than from the outgoing terminal.
However, for home charging applications, neither of these approaches is ideal, especially when considering product cost and installation convenience. As a result, most EV charger manufacturers opt for a built-in RDC-DD solution rather than a built-in Type B RCD design for residential applications.
Why Built-in RDC-DD within EV Chargers is Favored?
The following images illustrate the difference between Built-in RDC-DD and Traditional RDC-DD solutions. The text below further explains the characteristics of both types of RDC-DD.
Figure 3: Built-in RDC-DD Example
Figure 4: Traditional RDC-DD Example
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Built-in RDC-DD (also known as Integrated RDC-DD): An EV charger designed to meet the IEC 62955 RDC-DD requirements. The key function is achieved through seamless interaction between the residual current sensor, EV charging controller MCU, relays, and auxiliary components. This approach results in a streamlined design with a focus on efficient electronic integration.
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Traditional RDC-DD: A standalone protective or monitoring device, typically installed on a DIN rail, also compliant with IEC 62955. This design is generally more complex, incorporating electro-mechanical components for its operation.
Due to considerations such as size, cost, and the complexity of reset mechanisms after leakage protection, traditional DIN rail-mounted RDC-DDs are seldom integrated into charging stations. In fact, most home charging stations predominantly feature built-in RCD solutions instead.
Shall I Design Built-in RDC-PD or RDC-MD?
An RDC-PD is a protective device that combines AC, pulsating DC, and 6mA DC detection with evaluation and mechanical switching, all within a single unit. It is suitable for isolation and must comply with the requirements of IEC 61008 or IEC 61009, as outlined in Annex O (normative) of IEC 62955:2018. However, due to limitations in components such as power relays, achieving full compliance with IEC 61008-1 or IEC 61009-1 for a built-in RDC-PD is challenging.
Therefore, a more accurate question is: Should I design a built-in Type-A mRCD & RDC-MD combination, or should I opt for a built-in RDC-MD design and include guidance in the installation manual for electricians to ensure the use of a Type-A RCCB/RCBO? We recommend making this decision based on the target market and the region where the charger will be used.
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Built-in Type-A mRCD (IEC 60947-2) & RDC-MD (IEC 62955): Suitable for markets like the UK, Nordic countries, and China.
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Built-in RDC-MD (IEC 62955): More commonly used in the European continental countries like Germany, France, Italy, and Austria.
If your EV charger is to be sold across multiple regions, we suggest implementing both built-in designs on a single charging control board. For instance, by selecting Bituo Technik’s BRCS01C, BRCS01V, or BRCS03C residual current sensors, which feature two independent fault alarm output pins (for Type-A 30mA & DC 6mA and DC 6mA detection), you can easily achieve a dual built-in design on a single control board.
How to Declare Conformity for “Integrated” or “Built-in” Residual Current Protection in EVSE
We advise against declaring RDC-PD or Type-A RCCB solutions (in accordance with IEC 61008-1) unless they fully meet the criteria specified in IEC 61008-1. Misrepresenting conformity can lead to significant compliance issues down the road.
A relevant example comes from Easee, a prominent charging station manufacturer in Norway. Easee was initially prohibited from selling its products in Sweden by Elsäkerhetsverket (the Swedish Electrical Safety Authority) because their declaration of conformity with IEC 61008-1 did not meet the necessary requirements. As a result, Easee had to revise their declaration, switching to IEC 60947-2 and incorporating the DC detection requirements of IEC 62955. Here is the excerpt from Easee’s website:
“When Easee launched the Home and Charge products in 2019, they initially declared conformity with the RCD standard EN 61008-1 as part of their overall compliance with the charging standard EN IEC 61851-1. In the updated declaration of conformity, Easee will now declare compliance with IEC 60947-2, in addition to the DC detection element specified by IEC 62955. All aspects of the products have been tested and found in compliance with relevant requirements by accredited test laboratories.”
Since launching the first RDC-PD and RDC-MD solutions certified by TÜV Rheinland in 2019 and 2020, The Bituo Technik team consistently recommend the following declaration options for EVSE manufacturers using our residual current sensors:
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Built-in Type-A mRCD (IEC 60947-2) & RDC-MD (IEC 62955)
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Built-in RDC-MD (IEC 62955)
These declarations ensure that your product meets the relevant standards and avoids potential issues with misrepresentation.
Conclusion
Choosing the right residual current protection for your EV charger design is crucial for ensuring both safety and regulatory compliance. Integrating RDC-DD offers the best balance of cost, ease of installation, and safety. Whether you opt for Built-in Type-A mRCD & RDC-MD or Built-in RDC-MD, ensure that your design fits the requirements of your target market and provides the necessary protection.
If you need help navigating these options, Bituo Technik offers a range of residual current sensors for EVSE that can facilitate your design process and ensure compliance with industry standards.
