Why is 6 mA DC RCM used in AC EV charging pile?
6 mA DC RCM is not a new term for electricians and electronic engineers that have experiences in the EV charging business. This article introduces why a 6 mA DC RCM is requested in a AC EV charging pile.
6 mA DC RCM (Residual Current Monitor) is nothing new for professional electricians installing EV charging piles in Europe. The device is widely integrated in EV charging piles to detect smooth DC residual current. However, some people may ask why a DC RCM is used in a AC EV charging pile rather than in a DC EV charging pile. In order to better answer this question of “WHY”, we turned the complex “Why”  question to three specific “What” questions and their answers in this article.

Question 1: What can cause DC residual current at EV charging application? 

The following picture shows the electrical diagram of a typical onboard charging system(OBC), which consists of EMI filter at the input side, AC/DC rectifier, PFC, DC/DC Power converter and EMI filter at the output side. In the use of electrical vehicle (EV), it inevitable to have vibration and aging for components and parts, which might result in OBC insulation fault. 
Electrical diagram of a typical onboard charging system(OBC)
Diagram 1: Electrical diagram of a typical onboard charging system (OBC)

The common scenarios of ground fault with regard to OBC are as following
  • Ground fault occurred at the grid AC input side, which results in a sinusoidal AC residual current at grid frequency
  • Ground fault occurred at the AC/DC rectifier part of the OBC, which results in a pulsating DC residual current
  • Ground fault occurred at the DC/DC converter part of the OBC, which results in a smooth DC residual current

Question 2: What risk does DC residual current at EV charging application have?

The smooth DC residual current resulted from the insulation fault of an OPC can impair the proper function of type-A 30 mA RCD installed at the power line supplying AC EV charging piles or installed in the AC EV charging piles. In other words, both the response time and the response level of type-A RCD can be negatively affected in the occur of DC fault currents above 6 mA. The worst case is that a type-A RCD will no longer be tripped in the occur of electrical accident.  

Question 3: What measures can be used to manage the risk brought by DC residual current ?

According to IEC 61851-1:2018 (Electric vehicle conductive charging system – Part 1: General requirements) and IEC 60364-7-722:2018 (Low-voltage electrical installations – Part 7-722: Requirements for special installations or locations – Supplies for electric vehicles), both RCD type B complying with IEC 62423 / 60947-2 and RDC-DD complying with IEC 62955 can be used to as mitigate the electrical risk brought by DC residual current. 

RDC-DD is the abbreviation of Residual Direct Current Detecting Device (RDC-DD) to be used for mode 3 charging of electric vehicles. According to IEC 62955:2018, RDC-DDs are classified as the following two classes of Residual Direct Current - Monitoring Device (RDC-MD) and  Residual Direct Current - Protection Device (RDC-PD).  The experts in the IEC committee called the device monitoring DC residual current in accordance with IEC 62955 standard as RDC-MD (Residual Direct Current - Monitoring Device), rather than DC RCM. In the use of built-in DC RCM at EV charging pile, a system consisting of DC residual current sensor, relays / contactors and charging controller shall be able to disconnect charging current flow meeting IEC 62955 standards. E.g. Our BRCS01/02 residual current sensors in diagram 2 have been integrated into their charging controllers by many EV charger manufacturers in IEC market.

BCS01 and BRCS01 DC Residual Current Sensors for IEC 62955 DC RCM (RDC-MD)   

Diagram 2: BCS01 / 02 DC Residual Current Sensors used for Built-in IEC 62955 RDC-MD at EV Chargers