Why Is 6mA DC RCM Used In AC EV Charging Pile?

The term "6mA DC RCM" (Residual Current Monitor) is widely used by EV charger manufacturers and installers in Europe. It refers to the component integrated into AC EV chargers to detect smooth DC residual current.

However, strictly speaking, a standard RCM only monitors leakage. In EV charging, the system must also disconnect the power when a fault occurs. Therefore, the IEC committee officially designates this device as an RDC-MD (Residual Direct Current - Monitoring Device) under the IEC 62955 standard.

Terminology aside, why is this 6mA DC detection so critical for AC EV chargers? We break down the technical reasons below.

Causes of DC Residual Current in EV Charging

The following diagram illustrates a typical onboard charging system (OBC), comprising an EMI filter at the input, an AC/DC rectifier, a PFC, a DC/DC power converter, and an output EMI filter. During EV operation, component vibration and aging inevitably occur, which can lead to OBC insulation faults.

Common OBC ground fault scenarios include:

• Grid AC input side: Results in a sinusoidal AC residual current at grid frequency.

• AC/DC rectifier: Results in a pulsating DC residual current.

• DC/DC converter: Results in a smooth DC residual current.

The Risk of Smooth DC Residual Current (RCD Blinding)

Smooth DC residual current resulting from an OBC insulation fault can severely impair the proper function of a Type-A 30mA RCD installed in the upstream power line or within the AC EV charger itself.

Specifically, DC fault currents exceeding 6mA can negatively affect both the response time and the tripping threshold of a Type-A RCD—a dangerous phenomenon known as RCD blinding. In the worst-case scenario, the blinded Type-A RCD will fail to trip during an electrical accident, leaving users unprotected.

Mitigating DC Residual Current Risks (IEC Standards)

According to IEC 61851-1:2018 and IEC 60364-7-722:2018, the electrical risks associated with DC residual current can be mitigated using either a Type B RCD (IEC 62423 / IEC 60947-2) or an RDC-DD (IEC 62955).

RDC-DD stands for Residual Direct Current Detecting Device, designed for Mode 3 EV charging. Under IEC 62955:2018, RDC-DDs are classified into two categories:

• RDC-MD: Residual Direct Current – Monitoring Device

• RDC-PD: Residual Direct Current – Protection Device

The IEC committee designates the DC residual current monitoring device compliant with IEC 62955 as an RDC-MD, rather than a generic DC RCM. When utilizing a built-in DC RCM in an EV charger, the system—comprising a DC residual current sensor, relays/contactors, and a charging controller—must be capable of disconnecting the charging current in accordance with IEC 62955 standards. For example, our BRCS residual current sensors are widely integrated into charging controllers by numerous EV charger manufacturers across the IEC market.