Engineering Guide
IEC 62955 RDC-DD explained: DC 6mA detection for Mode 3 EV chargers
A plain-language walk through IEC 62955: what an RDC-DD is, the difference between RDC-MD and RDC-PD, how it pairs with a Type A RCD, and how to design it into a wallbox.
If you design or source Mode 3 AC chargers for Europe, IEC 62955 is the standard that decides how your charger handles DC fault current. It is also the reason most wallboxes on the market can be installed behind an ordinary Type A RCD instead of a more expensive Type B device. This guide explains what IEC 62955 asks for, the two device types it defines, and what that means when you choose and integrate a sensor.
Where IEC 62955 fits
IEC 61851-1 sets the general safety rules for conductive EV charging. One of them is that each Mode 3 charging point must be protected against DC fault currents above 6 mA. The reason is simple: smooth DC leakage from the vehicle’s on-board charger can saturate the core of a Type A RCD and make it blind to a real AC fault. We explain that mechanism in Type A vs Type B RCD for EV chargers.
IEC 61851-1 accepts two ways to meet that requirement:
- A Type B RCD, or
- A Type A RCD plus equipment that disconnects the supply when smooth DC residual current exceeds 6 mA.
IEC 62955 is the product standard for that “equipment” in option 2. Its full title refers to residual direct current detecting devices (RDC-DD) to be used for Mode 3 charging of electric vehicles. In other words, IEC 62955 tells you how the 6 mA DC detection function must behave and how it must be tested.
RDC-MD vs RDC-PD
IEC 62955 describes two forms of RDC-DD. The difference is who opens the circuit.
| Form | What it contains | Who disconnects the supply | Typical use |
|---|---|---|---|
| RDC-MD (monitoring device) | Detection and evaluation electronics | A separate switching device in the charger, normally the main relay or contactor, driven by the RDC-MD signal | PCB-mounted sensors inside wallboxes and AC charging stations |
| RDC-PD (protective device) | Detection electronics plus its own switching contacts | The RDC-PD itself | DIN-rail devices installed in the distribution board |
For a charger manufacturer, the RDC-MD architecture is the natural choice. The charger already has a main relay for normal start and stop of charging. A compact sensor around the charging conductors detects the DC residual current, gives a digital trip signal to the controller, and the controller opens the relay. No extra switching device is needed, and the controller knows exactly why charging stopped, so it can log the fault and report it to the back office.
The 6 mA DC threshold
The central number in IEC 62955 is 6 mA of smooth DC residual current. This is the level at which a Type A RCD can no longer be relied on, so the RDC-DD must make sure the supply is disconnected before the DC leakage gets there.
Because no measurement is perfect, a practical sensor trips inside a window below 6 mA. EVsense Type A + DC 6mA sensors publish the following DC trip current:
- 3 mA min / 4.5 mA typ / 6 mA max (smooth DC)
The lower limit matters too. A sensor that trips at very low DC levels would cause nuisance trips from small, harmless leakage currents in the vehicle and installation. A window between 3 mA and 6 mA gives protection at the limit while keeping charging sessions stable.
The same sensors also detect AC residual current (15 / 22 / 30 mA min / typ / max at 50 Hz) and pulsating DC waveforms, so the charger can react to AC faults directly as well.
Response time: faster for bigger faults
IEC 62955 does not only ask whether the device trips, but how fast. The required disconnection times become shorter as the DC residual current increases: a current just at the 6 mA threshold may take a longer time to clear, while tens or hundreds of milliamps must be cleared in a fraction of a second. Always check the latest edition of the standard for the exact limits that apply to your product.
For reference, here are EVsense’s published trip times for smooth DC residual current (from the 1R20 Series datasheet; the 3R14X and 5R15Z publish the same values):
| DC residual current | Typical trip time | Maximum trip time |
|---|---|---|
| 6 mA | 380 ms | 650 ms |
| 60 mA | 63 ms | 80 ms |
| 300 mA | 11 ms | 28 ms |
Remember that the sensor’s trip time is only one part of the chain. Your controller firmware must react to the TRIP signal promptly, and the relay has its own opening time. Test the complete charger, not just the sensor.
How an RDC-DD works with the Type A RCD
In a typical installation the two devices share the job:
- Type A RCD (30 mA) in the distribution board: protects against AC and pulsating DC residual current, the classic shock-protection function.
- RDC-DD inside the charger: watches for smooth DC residual current and disconnects the charger before 6 mA is reached, so the upstream Type A RCD is never blinded.
This is why the charger’s instruction manual usually says “install behind a Type A RCD”. If the charger did not have built-in DC 6 mA detection, the installer would need a Type B RCD instead. That difference is often what makes a charger easier to sell. See our article on RCD sensor vs Type B RCD cost for the commercial side.
GB/T 40820: the China counterpart
Charger makers that also sell into China will meet GB/T 40820, which covers DC residual current detecting devices (RDC-DD) for Mode 3 charging of electric vehicles. Its purpose is the same as IEC 62955. EVsense RCD sensors are positioned for both standards, which lets one sensor family serve export and domestic charger platforms. For a side-by-side look, read GB/T vs IEC leakage protection for EV chargers.
Designing an RDC-MD into your charger
When you integrate an RCD sensor as the RDC-MD function of a Mode 3 charger, a few points make validation much easier:
- Route both line and neutral (all phases) through the sensor aperture. The sensor measures the sum of the currents; leaving out a conductor creates a false residual current.
- Run the self-test. EVsense sensors require a CLK&CHK self-test calibration once at power-on and again before each charging session. The sensor also calculates its zero point during these steps.
- Allow for start-up time. The sensor does not respond during the first 800 ms after power-on, so the controller should not close the relay before the sensor is ready.
- Keep the sensor away from the relay. The datasheets recommend at least 3 cm distance from the relay and a clean 5 V supply from an LDO.
Our PCB integration guide covers these steps in more detail.
Which EVsense sensors suit Mode 3 chargers
- 1R20 Series: 20 mm aperture, available with harness connection (1R20X) or PCB mounting (1R20Z). A good fit for higher-current and three-phase AC charging stations.
- 5R15Z: 15 mm aperture, PCB-mounted, used in GB-standard charging piles and Mode 3 chargers.
- 3R14X: 14 mm aperture with harness connection, a compact option where board space is limited.
Not sure which one fits your enclosure and conductor size? Start with how to choose an RCD sensor for an EV charger.
Summary
- IEC 62955 defines the RDC-DD, the device that handles smooth DC residual current above 6 mA in Mode 3 charging.
- An RDC-MD signals the charger to open its own relay; an RDC-PD switches the circuit itself. Charger-integrated sensors use the RDC-MD approach.
- RDC-DD plus a Type A RCD is an accepted alternative to a Type B RCD under IEC 61851-1.
- GB/T 40820 is the corresponding standard for China’s Mode 3 charging equipment.
Frequently asked questions
What does RDC-DD stand for?
RDC-DD means residual direct current detecting device. IEC 62955 defines it for Mode 3 charging of electric vehicles: a device that detects smooth DC residual current and causes the supply to be disconnected when that current reaches the 6 mA level.
What is the difference between an RDC-MD and an RDC-PD?
An RDC-MD (monitoring device) detects the DC residual current and gives a signal that makes a separate switching device in the charger, such as the main relay or contactor, open the circuit. An RDC-PD (protective device) contains its own switching contacts and disconnects the circuit itself. Charger-integrated PCB sensors are normally used in the RDC-MD architecture.
Does an RDC-DD replace the Type A RCD?
No. The RDC-DD handles smooth DC residual current above 6 mA. A Type A RCD (30 mA) is still needed to provide AC and pulsating DC residual current protection. Together they are an accepted alternative to a Type B RCD under IEC 61851-1.
Which Chinese standard corresponds to IEC 62955?
GB/T 40820 covers DC residual current detecting devices (RDC-DD) for Mode 3 charging of electric vehicles in China. EVsense RCD sensors are positioned for both IEC 62955 and GB/T 40820 projects.
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