Engineering Guide
Type A vs Type B RCD for EV chargers: when is Type A + DC 6mA detection enough?
Why smooth DC leakage blinds a Type A RCD, what the charging standards accept instead of a Type B device, and how charger makers choose between the two architectures.
Every AC charger designer eventually meets the same question: should the charging point be protected by a Type B RCD, or is a Type A RCD plus DC 6mA detection enough? The answer decides your bill of materials, your enclosure size and what the installer has to buy. This guide explains the difference in plain engineering terms.
Why EV chargers need special leakage protection
A residual current device (RCD) compares the current flowing out on the line conductors with the current returning on neutral. If some current leaks to earth, for example through a damaged cable or a person, the difference is the residual current, and the RCD disconnects the supply.
Most household RCDs are Type AC or Type A. They work by sensing the alternating magnetic flux that a residual current creates in a toroidal core. That works well for ordinary AC loads.
An electric vehicle is not an ordinary load. Its on-board charger contains rectifiers and power-factor correction stages. If insulation fails on the DC side of that electronics, the fault current that flows to earth can be smooth DC rather than AC.
The problem: smooth DC blinds a Type A RCD
Smooth DC residual current does not produce an alternating flux. Instead, it pushes the RCD’s magnetic core towards saturation. A partly saturated core responds much less to AC, so the RCD becomes less sensitive or stops tripping at all, even for a dangerous AC fault that happens at the same time.
That is why the charging standards treat DC fault current as a separate hazard:
| RCD type | Detects | Typical use |
|---|---|---|
| Type AC | Sinusoidal AC residual current | Older general-purpose installations |
| Type A | AC + pulsating DC residual current | Standard residential circuits |
| Type F | Type A + mixed-frequency residual current | Single-phase inverter loads |
| Type B | Type F + smooth DC residual current | EV charging, PV, three-phase drives |
A Type A RCD is designed to keep working with only a small amount of superimposed smooth DC, which is where the 6 mA figure comes from. Above that level, its behaviour is no longer guaranteed.
What the standards allow
IEC 61851-1 (conductive charging systems) requires each Mode 3 charging point to be protected against DC fault currents above 6 mA. It accepts two ways of doing that:
- A Type B RCD for each charging point, or
- A Type A RCD together with equipment that disconnects the supply when smooth DC residual current exceeds 6 mA.
The second route is defined in IEC 62955, which covers residual direct current detecting devices (RDC-DD) for Mode 3 charging. In China, the corresponding requirements are found in GB/T 40820. For Mode 2 portable chargers, the in-cable control and protection device (IC-CPD) in IEC 62752 must itself detect both AC and smooth DC residual current. We explain those standards in separate guides on IEC 62955 RDC-DD and IEC 62752 IC-CPD.
In short: the standards do not say “Type B is required”. They say “DC fault current above 6 mA must be handled”. Type B and Type A + 6 mA DC detection are both accepted ways to do it.
Type A + DC 6mA vs Type B: practical comparison
| Criterion | Type B RCD at each charging point | Type A RCD + DC 6mA detection in the charger |
|---|---|---|
| Where the DC protection sits | In the distribution board | Inside the charger, on the controller PCB |
| What the installer buys | One Type B RCD per charger | A standard Type A RCD |
| Size | Larger DIN-rail device | Compact sensor on or near the PCB |
| Integration with charger logic | None; trips independently | Digital TRIP signal to the controller, which opens the relay and logs the fault |
| Self-test | Manual test button | Automatic self-test at power-on and before each session |
| Typical choice for | Installations where the charger has no built-in DC detection | Wallboxes, AC charging stations and portable chargers built for volume |
For a charger manufacturer, building the DC detection into the product has two benefits. The installer can use a common Type A RCD, which makes the charger easier to sell. And the charger can report the DC fault to its controller and to the back office, rather than simply losing power.
How a Type A + 6mA sensor works inside a charger
An integrated RCD sensor such as the EVsense 1R20 Series or 5R15Z is placed around the charging conductors (line and neutral pass through its aperture). It measures the residual current with a fluxgate-type magnetic circuit, which can detect smooth DC as well as AC. The sensor gives the charger controller a digital TRIP output; the controller then opens the main contactor or relay.
Typical published thresholds for EVsense Type A + DC 6mA sensors are:
- Smooth DC trip current: 3 mA min / 4.5 mA typ / 6 mA max
- AC (50 Hz) trip current: 15 mA min / 22 mA typ / 30 mA max
- Supply: 5 V; operating temperature −40 °C to 85 °C; start-up time 800 ms max
Some models add other functions. The 2R4Z integrates voltage and current transformer functions, which saves separate magnetic parts on the controller board. If you are interested in the physics, read how 6mA DC residual current detection works.
When a Type B RCD still makes sense
Type A + DC 6mA detection is the usual choice for chargers built in volume, but a Type B RCD can still be the right answer when:
- the charger you install has no built-in DC fault detection,
- the local wiring rules or the site operator specifically ask for Type B devices, or
- the circuit also supplies other equipment that can generate smooth DC fault currents.
Always check the national installation rules (for example the local version of IEC 60364-7-722) and the charger manufacturer’s instructions.
Summary
- EV on-board chargers can produce smooth DC fault current, which can blind a Type A RCD.
- IEC 61851-1 requires DC fault current above 6 mA to be handled, either by a Type B RCD or by a Type A RCD plus DC 6 mA detection (RDC-DD to IEC 62955).
- Building DC 6mA detection into the charger lets installers use a standard Type A RCD and lets the charger report faults.
- To choose a sensor for your platform, see our RCD sensor selection guide for wallboxes or compare all EVsense RCD sensors.
Frequently asked questions
Can I use a Type A RCD alone to protect an EV charger?
Usually not. A Type A RCD can be desensitized by smooth DC residual current from the vehicle side. IEC 61851-1 requires protection against DC fault current above 6 mA, which is provided either by a Type B RCD or by a Type A RCD combined with a DC 6mA detecting device such as an RDC-DD to IEC 62955.
Is Type A + DC 6mA detection as safe as a Type B RCD?
For the purpose of EV charging, both architectures are accepted routes in IEC 61851-1 for handling DC fault current. Type A + 6mA DC detection disconnects the supply when smooth DC residual current reaches 6 mA, before it can blind the upstream Type A RCD.
Why do most AC wallbox manufacturers choose Type A + 6mA DC detection?
Because the DC detection can be built into the charger as a compact PCB-mounted sensor, the installer only needs a standard Type A RCD in the distribution board. This is usually smaller and less expensive than a Type B RCD for every charging point.
What does an RCD sensor such as the 1R20 or 2R4Z do?
It measures the residual current of the charging conductors and gives a digital TRIP signal to the charger controller when AC or smooth DC residual current exceeds its thresholds (for EVsense sensors, DC trip at 3 mA min, 4.5 mA typical and 6 mA max). The controller then opens the main relay.
Need help choosing a sensor?
Send your charger platform, target standard, rated current and PCB space. Our engineers reply with a model recommendation, datasheets and sample options.