RDC-DD — detecting smooth 6 mA DC fault current at charge points

The RCD types guide (AC/A/F/B) covers why some charge points and appliances require a Type B RCD, because smooth (pure) DC fault currents can "blind" a Type A RCD. This article covers an alternative mechanism from IEC 62955 that solves the same problem without necessarily requiring a full Type B RCD: the RDC-DD (Residual Direct Current Detecting Device).

The problem: smooth DC blinds Type A

An electric-vehicle charge point contains power electronics (rectifiers, DC/DC converters) that, in the event of an internal fault, can leak a smooth, non-pulsating DC current to earth. A conventional Type A RCD is designed to detect pulsating DC fault currents, but a sufficiently large smooth DC component can saturate the magnetic core of that Type A RCD, causing the device to lose its sensitivity to the actual AC fault — the device then no longer trips on a fault it would otherwise detect. See also the RCD types guide for the full background on this blinding mechanism.

What an RDC-DD does

An RDC-DD is a device that specifically monitors smooth DC fault current, separately from the RCD's AC fault-current detection. As soon as the smooth DC component exceeds a threshold of 6 mA, the RDC-DD disconnects the supply to the charge point — before that DC current can blind the upstream Type A RCD. This means a Type A RCD (instead of the more expensive Type B) can still be used safely, provided an RDC-DD with a 6 mA threshold is integrated into the charge point itself — which is now standard on most modern electric-vehicle charge points.

RDC-MD and RDC-PD

IEC 62955 also distinguishes variants of the detection mechanism:

RDC-MD (Monitoring Device) — monitors the DC fault current and issues a signal or warning, but does not necessarily disconnect the supply itself; actual disconnection then happens via a coupled switching device.
RDC-PD (Protective Device) — contains the switching function itself and interrupts the supply directly as soon as the 6 mA threshold is exceeded, without depending on an external switching device.

When assessing a charge point, it is therefore important to check which variant is integrated, and whether that variant actually interrupts the supply or merely issues a signal that must be acted on by another part of the installation.

Note: the presence of an RDC-DD (or RDC-MD/RDC-PD) in a charge point does not replace the need for a suitable RCD for the remaining (AC) fault currents on that circuit — it specifically solves the smooth-DC blinding problem, not the overall earth-fault protection.

Practical relevance

When assessing or designing an electric-vehicle charge point circuit under NEN 1010 §722, it is important to check whether the charge point itself contains an RDC-DD (or RDC-MD/RDC-PD) with a 6 mA threshold. If not, an upstream Type B RCD is required to prevent blinding by smooth DC; if it does, a Type A RCD is often sufficient in many cases, which is significantly cheaper.

Common mistakes

Assuming every charge point automatically contains an RDC-DD without verifying this against the charge point's documentation or rating plate.
Applying a Type A RCD to a charge point without an integrated RDC-DD, risking that a smooth DC fault blinds the RCD and the protection effectively drops out.
Confusing RDC-MD with RDC-PD and assuming a signalling device (RDC-MD) also independently disconnects, when that depends on a coupled switching device.
Confusing the 6 mA threshold with the regular residual-current threshold (often 30 mA for personal protection) of the RCD itself — these are two different protective functions with different thresholds and purposes.