As is well known, the standard for electric vehicle charging is IEC 61851 - ELECTRIC VEHICLE CONDUCTIVE CHARGING SYSTEM - Part 1: General requirements. In the early version of IEC 61851-1:2010, the protection against residual current was merely required to follow the safety regulations for building electrics, without specifying any detailed analysis of the system or clear protection levels.

It was not until 2017, with the release of the new version, that substantial requirements for electric vehicle charging were introduced, and the protection requirements for residual current in the case of smooth DC residual current were clarified. In the IEC 61851-1:2017 (EN 61851-1:2017) version, the requirements for residual current protection are as follows: Each AC connection point shall be protected by a residual current device (RCD) individually. The RCD protecting each connection point shall at least meet the requirements of Type A RCD, and its rated residual operating current shall not exceed 30 mA. When an EV charging station contains sockets or vehicle connectors in accordance with IEC 62196, additional protection measures against smooth DC fault current are required. The residual current protection shall at least be Type B or Type A, and it shall be capable of disconnection upon detection of a current exceeding 6 mA/DC. This standard introduced for the first time the concept of RCD-DD devices for protection against smooth DC residual current.

Figure 1: IEC 61851-1

Since the promulgation of the IEC 61851-2017 standard, regions including the United Kingdom (BS 7671:2018) and New Zealand (AS/NZS 3000:2018) took the lead in implementing the new standard in 2019. For Mode 3 charging piles, it became mandatory to install residual current protection modules with smooth DC residual detection capabilities. Thanks to the maturity of Type B residual current devices (RCDs) in applications such as photovoltaics and elevators, and the lack of suitable RCD-DD options at that time, Type B RCDs, despite being slightly more expensive, remained the top choice for many charging station companies or users to enhance their electrical safety and comply with safety regulations.

Fast forward to 2021, the current year. The EN standard stipulates that all EU countries covered by the standard must comply with IEC 61851. This means that more charging station companies or users who originally chose Type A residual current protection solutions will have to add smooth DC residual protection mechanisms starting this year. The market's incremental demand has forced the relatively expensive Type B RCD solutions to gradually reduce their prices, while new solution ideas are also being conceived.

Figure 2: RCD types

The first approach is the wall-box solution led by Tesla. The device itself does not have smooth DC residual detection. Under the upgrade of the European standard IEC 61851 safety regulations, the internal AC/DC Sensor has not been correspondingly upgraded and integrated. This means that European users, under the mandatory requirements of safety regulations, must have a Type B residual current device (RCD) in order for an electrician to complete the installation and grid connection of the charging station. Of course, the esteemed Tesla owners in 2019, while complying with the regulations, were also shocked by the high price of Type B RCDs.

Figure 3: TESLA solution

The second approach is a cost-effective integrated solution, characterized by its distinctive features. It utilizes the AC/DC Sensor to detect both AC and DC currents, and combines this with relay control and the computational capabilities of a microcontroller unit (MCU) to provide comprehensive residual current protection. The core of this solution lies in the detection and processing capabilities of the AC/DC Sensor. Two sets of design philosophies are presented:

The first approach involves using traditional analog signal output for real-time monitoring of residual current. This method preserves the true state of the detected residual current to the greatest extent possible, offering significant flexibility. Manufacturers can develop a customized protection mechanism based on their understanding of residual current values and protection timing, providing a highly adaptable and versatile solution.

Figure 4: Magtron’s solution

The second approach leverages the latest digital signal output for residual current protection. This method integrates the residual current protection requirements of IEC 62752 (Mode 2) and IEC 62955 + Type A (Mode 3). It eliminates the need for customers to perform complex calculations or processing, as protection is determined by simple high and low voltage levels, enabling a plug-and-play solution. This approach significantly reduces the design workload for engineers and the computational burden on the MCU.

In AC charging pile systems, the forms of residual current include AC, pulsating DC, pure DC, and some high-frequency residual. There are several reasons why electric vehicles may pose a risk of direct electric shock during charging:

(1) Plugs: The danger of discontinuity in the protective earthing conductor (PE).

(2) Cables: The risk of mechanical damage to the cable insulation layer (due to rolling tires, repeated operations causing crushing, etc.).

(3) Electric vehicles: Due to the destruction of basic protective measures (accidents, car repairs, etc.), there is a possibility of contact with the active parts of the vehicle charger (Class I).

(4) Damp or corrosive environments (snow in the vehicle's water inlet, rain, etc.).

The smooth DC component of the residual current will be fed back to the AC input side through paths such as the front end of the OBC and the vehicle chassis, PE line, etc. The rear DC side of the OBC is a floating ground system, and its insulation resistance value needs to be monitored by an IMD.

Figure 5: EVSE Circuit topology

Direct current (DC) residual protection is mainly divided into two parts. The first is the smooth DC residual current of 6 mA. To understand why it is necessary to detect a smooth DC residual current of 6 mA, one must start from the standards and safety requirements. The essence of residual current protection for charging piles is introduced in the IEC 61851 standard under the section "Protection against electric shock." No safety standard has set 6 mA of smooth DC residual current as a safe current for human protection because such a small current would not cause harm to the human body. The reason for detecting it is straightforward: an excessive presence of the smooth DC component can cause premature saturation of Type A toroidal cores, ultimately leading to inaccurate detection of AC residual current by Type A residual current devices (RCDs).

Different types of RCDs have varying requirements for resistance to smooth DC residual current according to international standards:

Type AC: Cannot detect residual current in the presence of smooth DC residual.

Type A: Can only operate in environments where smooth DC residual current is below 6 mA.

Type F: Can only operate in environments where smooth DC residual current is below 10 mA.

Type B: Is sensitive to smooth DC residual current.

It can be understood that the purpose of detecting 6 mA of smooth DC residual current is to ensure the accuracy of Type A RCDs. The other part of smooth DC residual protection concerns the potential harm to the human body from smooth DC residual current exceeding 6 mA. In both RDC-DD and Type B standards, it is observed that the protection value set for smooth DC residual current is higher than the 30 mA for AC residual current, with a defined safety protection threshold of 60 mA.

Figure 6: IEC 62955

Therefore, it is essential to detect and protect against 6 mA of smooth DC residual current in addition to Type A detection, because Type A RCDs (Residual Current Devices) were not originally designed for AC charging scenarios. Excessive smooth DC components can cause Type A RCDs to become "blind" to residual currents. Thus, a "patch" is needed to address this detection flaw.

Type B RCDs, on the other hand, already provide a comprehensive residual current protection solution. They are inherently sensitive to smooth DC residual and do not require any additional patches. In fact, adding extra protection for 6 mA of smooth DC residual could increase the likelihood of nuisance tripping during electric vehicle charging, due to the overly low protection threshold for smooth DC residual.

The layout of charging piles in use includes single, independent private piles (or Wall-boxes) as well as public piles with multiple charging points arranged in parallel. When a charging pile meets the residual current safety requirements on its own, if it is a single unit, only a Type A residual current device (RCD) is needed upstream. However, if multiple piles are connected in parallel and share a common busbar, the upstream RCD must be Type B, or its design must be capable of withstanding smooth DC currents exceeding the specified values required by the IEC standards without affecting its AC protection performance.

Figure 7: Magtron’s cost-effective solution

From a long-term perspective, integrating an AC/DC Sensor within the charging pile to achieve Type A + 6 mA protection offers higher integration and better cost-effectiveness. Meanwhile, the price of Type B RCDs is decreasing year by year, potentially reaching a reasonable level. This makes them suitable for use in charging piles that do not have an internal smooth DC detection module, as well as in the upstream locations of multiple parallel charging piles.

[1]Climate and Sustainability Research & Analysis-BPenters European EV charging partnership with BMW, Daimler》

[2]《Recharge EU：how many chargepoints will Europe and its Member States need in the 2020s》

[3]《IEC61851-ELECTRIC VEHICLE CONDUCTIVE CHARGINGSYSTEM - Part 1: General requirements》

[4]《BS 7671:2018-Requirements for Electrical Installations》

[5]AS/NZS 3000:2018-Electricalinstallations ”Wiring Rules”》

[6]《EV charging - electrical installation design - ElectricalInstallation Guide》

[7]《Tesla Motors Club-Tesla Wall Connector - Type B / TypeA-EV RCD》[8]《IEC62955-Residual direct current detecting device(RDC-DD) to be used for mode 3 charging ofelectric vehicles》

[9]《IEC62423-Type F and type B residual current operatedcircuit-breakers with and without integral overcurrent protection for householdand similar uses》