RCD and RDC-DD Trip Testing for EV Chargers: Why 30 mA AC Alone Is Not Enough

The problem: DC leakage current can blind a standard RCD

A conventional Type AC or Type A Residual Current Device works by detecting an imbalance between the current flowing out on the line conductor and the current returning on the neutral, using a current transformer wound around both conductors. This works reliably for AC fault currents, and Type A devices are also designed to detect pulsating DC fault currents (the kind produced by simple half-wave rectifier faults).

The problem is specific to EV chargers: many onboard vehicle chargers use power electronics (rectifiers and inverters) that, under certain internal fault conditions, can produce a smooth DC residual current rather than a pulsating one. A sustained smooth DC current through the current transformer core of a Type A RCD can partially or fully saturate that core, meaning the RCD becomes progressively less able to detect a subsequent genuine AC earth fault, exactly when its protection is needed most. This is not a theoretical risk; it is the specific failure mode that led standards bodies to mandate additional protection for EV charging circuits.

The fix: a dedicated Residual DC Current Detecting Device (RDC-DD)

IEC 62955, "Residual direct current detecting device (RDC-DD) to be used for mode 3 charging of electric vehicles," defines a device specifically built to detect smooth DC residual current and disconnect the supply before it can desensitise the upstream RCD. The standard sets the maximum DC current at which the RDC-DD must operate at 6 mA. In practice, this protection is delivered in one of two ways:

• A stand-alone RDC-DD module fitted alongside a conventional Type A RCD, monitoring specifically for smooth DC leakage and tripping the associated contactor or breaker if it exceeds 6 mA.
• A Type B RCD, which combines AC, pulsating DC, and smooth DC residual current detection in a single device, meeting the RDC-DD function without a separate module.

SS 638 and IEC/HD 60364-7-722 require this level of protection on EV charging circuits specifically because of how vehicle onboard chargers are built internally, and Type B RCDs (or an equivalent Type A plus RDC-DD combination) are mandated wherever Mode 4 DC components are present, and are the recommended practice for Mode 3 AC circuits given the DC fault current risk described above.

Why a standard RCD tester is not enough

A conventional handheld RCD tester, of the type used for general electrical installation testing, injects an AC test current (typically at 0.5x, 1x, and 5x the rated residual current) and measures the trip time, in line with CP 5 requirements. This correctly verifies the AC trip function of a Type A or Type AC RCD. It does not, by itself, inject a smooth DC test current, and so it cannot verify whether the RDC-DD function, whether stand-alone or built into a Type B RCD, will actually operate at the required 6 mA DC threshold.

This is the specific gap an EV charging station analyzer closes. The Fluke FEV350 includes a combined 30 mA RCD plus 6 mA RDC-DD trip test as one of its standard measurements, injecting both an AC test current to verify the RCD's AC trip behaviour and a smooth DC test current to verify the RDC-DD trips at 6 mA, from the vehicle side of the connector where the actual fault current would originate during real use. A commissioning test that only exercises the AC function with a generic tester has not actually confirmed the installation's real protective performance against the failure mode the standard exists to prevent.

What happens during the combined trip test

During commissioning, the sequence run by an analyzer such as the FEV350 typically covers both aspects of the protective device in one pass: verifying the 30 mA AC residual current trips the device within the required time, then separately verifying the 6 mA smooth DC test current also trips (or, for a stand-alone RDC-DD, that the RDC-DD's associated switching device operates correctly). Both results are captured as a clear pass or fail, and where the analyzer is paired with Fluke's TruTest™ software, both are recorded against the specific charging point being commissioned, giving a facilities manager or charge point operator documented proof that both protective functions were verified, not just assumed to be present because a Type B RCD is fitted.

Where this fits in the wider commissioning sequence

RCD and RDC-DD trip testing is one step in a defined commissioning sequence that also includes insulation resistance, earth continuity, earth fault loop impedance, polarity, and functional protocol testing. We set out the full sequence, and where the RCD/RDC-DD test sits within it, in our EVSE commissioning checklist for Singapore. Getting this one test wrong, or skipping the DC component of it entirely, is one of the more common gaps we see referenced in our article on common EV charger installation faults caught by testing.

Understanding RCD types: why "Type A" is not automatically wrong, just incomplete

It is worth being precise about the different RCD classifications, since the confusion around EV charging protection usually comes from conflating "Type A" with "inadequate" rather than understanding what each type is actually rated to detect. Type AC RCDs detect only sinusoidal AC residual currents and have no place on an EV charging circuit at all. Type A RCDs detect AC residual current and pulsating DC residual current, which makes them the general-purpose standard for most modern domestic and commercial electrical circuits, but they are not required to reliably detect smooth (non-pulsating) DC residual current, the specific fault condition an EV's onboard charger and power electronics can produce. Type B RCDs detect AC, pulsating DC, and smooth DC residual currents in a single device, making them a complete solution on their own for EV charging circuits without needing a separate RDC-DD module. Type F RCDs sit between A and B, adding detection of certain mixed-frequency residual currents but still not smooth DC, and are not considered adequate on their own for EV charging protection. The practical takeaway: a Type A RCD is not wrong to install on an EV charging circuit, provided it is paired with a genuine stand-alone RDC-DD module; what is wrong is installing a Type A RCD alone and assuming its rating alone provides complete protection.

Why RCD mechanisms degrade and periodic re-testing matters

An RCD's trip mechanism is an electromechanical device, and like any mechanical component, it can degrade with age, environmental exposure, and the number of times it has actually operated. Contact contamination, spring fatigue in the trip mechanism, and, in outdoor or semi-exposed EV charging installations common in Singapore, humidity and temperature cycling can all gradually affect trip time and trip current threshold even where the device has not visibly failed. A device that tripped correctly at commissioning is not guaranteed to trip correctly two or three years later without a genuine re-test, which is why periodic re-verification (commonly annual, aligned with the broader electrical safety re-testing cadence covered in our EVSE commissioning checklist) is standard practice, not just a one-time commissioning requirement. This periodic re-testing is also the practical way a facilities team catches a degrading device before it fails to trip during an actual fault, rather than discovering the gap only after an incident.

The physical mechanism: why smooth DC actually blinds a Type A device

Understanding the underlying electromagnetic mechanism makes the Type A limitation concrete rather than an abstract classification rule. A standard RCD detects a residual current fault by sensing an imbalance between the current flowing out through the line conductor and the current returning through the neutral conductor, using a toroidal current transformer that the conductors pass through; any imbalance induces a small signal in the transformer's sensing winding, which triggers the trip mechanism once it exceeds the rated threshold. This detection method works reliably for AC and pulsating DC fault currents because both continue to vary over time, inducing a detectable signal in the transformer. A smooth, steady DC fault current, by contrast, does not vary, and because a transformer only responds to changing current, a steady DC fault current can effectively saturate or fail to induce any usable signal in a standard AC-sensing transformer core, rendering the RCD blind to that specific fault condition regardless of its rated trip current. A genuine RDC-DD device (or a Type B RCD's built-in DC detection stage) uses different sensing technology, often a Hall-effect or fluxgate sensor, specifically capable of detecting steady DC current, which is the actual physical reason a Type A device alone cannot be relied upon for this fault type, not simply a regulatory labelling convention.

Trip time requirements and what the combined test actually verifies

Beyond simply confirming a device trips at all, the combined RCD/RDC-DD test verifies the device trips within the time specified by the relevant standard for the rated residual current, since a device that eventually trips but takes too long to do so still exposes a person or the installation to fault current for longer than considered safe. For the standard 30 mA AC residual current test, IEC/HD 60364-7-722 and the underlying RCD product standards specify a maximum trip time, and a genuine test verifies both that the trip occurred and that it occurred within this window, not merely that the device eventually responded. The same principle applies to the 6 mA smooth DC test current for the RDC-DD function. An analyzer such as the FEV350 measures and records the actual trip time achieved, not just a pass/fail against the current threshold, which is meaningfully more informative than a simple continuity-style pass/fail test, since a device trip time trending slower across successive annual tests, even while still within the standard's maximum, is itself useful early-warning data about a mechanism that may be degrading, consistent with the periodic re-testing discussion above.

Shared protective devices on multi-charger circuits: a specification trade-off

On multi-charger installations sharing an upstream distribution circuit, a design decision arises between fitting a single shared RCD/RDC-DD protecting multiple charging points versus individual protective devices dedicated to each charging point. A shared device is lower cost and simpler to install, but introduces a genuine operational trade-off: a fault on any single charging point trips protection for every charger sharing that device, taking multiple charging points out of service simultaneously for a fault that only actually affected one of them, and complicates fault diagnosis since the tripped device does not by itself indicate which specific downstream charger caused the trip. Individual dedicated protective devices per charging point cost more and require more panel space, but isolate a fault to the single affected charging point, keeping the rest of a multi-charger site operational, and make fault diagnosis considerably more straightforward since the specific tripped device directly identifies the affected charger. For any multi-charger commercial site where charger uptime and straightforward fault diagnosis matter, individual protection per charging point is generally the better-engineered choice despite the higher upfront cost, a trade-off worth raising explicitly at the design stage rather than defaulting to a shared device purely to minimise initial installation cost.

A specification decision guide: RDC-DD module versus Type B RCD

When designing a new EV charging circuit, the choice between a Type A RCD plus a separate stand-alone RDC-DD module, versus a single Type B RCD covering both functions, generally comes down to cost, available panel space, and existing standardisation on a site. A stand-alone RDC-DD module added to an existing Type A RCD installation is often the lower-cost retrofit option where the panel already has a compliant Type A device installed and simply needs the DC detection gap closed. A single Type B RCD is the cleaner specification for a new installation, since it consolidates both protective functions into one device with one set of wiring and one point of testing, at a generally higher unit cost than the Type A plus RDC-DD combination. Neither approach is inherently superior from a protection standpoint, both, correctly installed and tested, meet the intent of SS 638 and IEC/HD 60364-7-722; the decision is a practical installation and cost question, not a compliance one, provided the combined AC and DC leakage detection function is genuinely present and verified either way.

Retrofitting protection on older EV charging installations

A meaningful share of Singapore's EV charging installations were commissioned before the specific RDC-DD requirement became a widely enforced expectation, and facilities managers reviewing an older installation's compliance status sometimes discover a Type A RCD alone protecting a charging circuit with no genuine DC leakage detection at all. Addressing this retrospectively is generally straightforward from an engineering standpoint, a stand-alone RDC-DD module can usually be added within the existing panel without requiring a full rewiring of the circuit, provided adequate panel space exists, but it does require a deliberate audit of existing installations to identify the gap in the first place, since a Type A RCD alone continues to pass a basic AC-only trip test indefinitely without ever revealing the missing DC protection. Facilities managers responsible for EV charging infrastructure installed more than a few years ago, particularly before SS 638's current requirements were widely adopted across the industry, should specifically commission a review confirming genuine combined AC and DC leakage protection is present, rather than assuming an older installation that has operated without incident to date is necessarily compliant with current expectations.