What is a RDC-DD, RDC-MD and RDC-PD?
Since the publishing of IEC 62955 standard in 2018, RDC-DDs have been widely used in Mode-3 EV charging to detect DC residual current. The author introduces the function of RDC-DDs and its classification as RDC-MDs and RDC-PDs. Moveover, the main-stream RDC-DD design for mode-3 EV charger is also introduced.
RDC-DD is the abbreviation of Residual Direct Current Detecting Device (RDC-DD) to be used for mode 3 charging of electric vehicles. It is IEC 62955:2018 standard that specifies RDC-DD to ensure that the proper functionality of RCDs type A or type F is not impaired by DC residual currents above 6 mA. RDC-DDs are intended to disconnect the power supply to the EV in cases where a smooth residual direct current equal to or above 6 mA is detected.

According to IEC 62955:2018, RDC-DDs are classified as the following two classes
  • RDC-MD: Residual Direct Current - Monitoring Device 
  • RDC-PD: Residual Direct Current - Protection Device

The major difference between RDC-MD and RDC-PD is whether protection against AC 30 mA and pulsating DC residual current is integrated in the device. In other words, a RDC-MD can not protect humans from being injured or electrocuted by AC 30 mA and pulsating DC residual current, but it can monitor DC residual currents to avoid RCDs type A or type F being impaired by DC residual currents above 6 mA. Therefore, RDC-MD is also called DC RCM by some engineers. 

IEC 62955:2018 further classifies RDC-MD according to the type of construction as following: 
  • RDC-MD with mechanical switching in one unit
  • RDC-MD consisting of RDC-M-unit mechanically coupled to a separate protective device
  • RDC-MD consisting of RDC-M-module electrically coupled to a separate protective or switching device

Since the update of IEC 61851-1 (Electric vehicle conductive charging system - Part 1: General requirements) in 2017, RDC-DDs have been widely used together with type-A RCD to provide sufficient protection against electrical shock at electric vehicle charging. In another article,  we have discussed why the combination of type-A RCD and RDC-DD is better than type-B RCD for private charging.

The EV charging infrastructure market is full of fierce competition, which pushes EV charger manufacturers to use cost-effective design for RDC-DD. Since the publishing of IEC 62955 in 2018, Bituo team has supported hundreds of EV charger manufacturers in RDC-DD design and applications. We think the following three types of RDC-DD design are main-stream.  
  • EV charger as RDC-MD consisting of RDC-M-module electrically coupled to a separate contactor or relays
  • EV charger as RDC-PD
  • Independent RDC-PD
Popular design of RDC-DDs for mode-3 EV charging

The above diagram also shows how the three main-stream RDC-DD designs look like in practice. The author strongly suggests to install a type-A 30 mA RCCB / RCBO outside the EV charger- even the EV charger designed as a RDC-PD. A EV charger might provide a type-A 30 mA protection with a coordinated system of charging controller, AC/DC residual current sensor and switching devices, but the reliability of such a design in practical application has been argued in comparison with the traditional type-A 30 mA RCCBs / RCBOs in one unit.