If you are speccing protection for an EV charger, one of the most common points of confusion is the difference between an RDC-DD and a Type B RCD. They both deal with DC fault current risk, but they are not the same device and they are not used in the same way.
In simple terms, an RDC-DD is usually built into the EV charger and monitors for DC fault currents above 6 mA, while a Type B RCD is a standalone protective device used where that DC protection is not built in or where the charger manufacturer specifically calls for Type B protection.
For wider EV protection options, browse our EV-Ready Consumer Units, Type B RCDs, EV RCBOs and Surge Protection ranges.
Quick Answer: Use a Type A or Type F device plus RDC-DD when the EV charger includes compliant 6 mA DC fault detection. Use a Type B RCD when that DC detection is not built into the charger, or when the manufacturer’s instructions specifically require Type B. An RDC-DD is a detection function. A Type B RCD is the protective device itself.
| Option | What It Is | Typical EV Use |
|---|---|---|
| RDC-DD | A 6 mA DC fault detection function, usually built into the charger | Lets you use an upstream Type A or Type F device where appropriate |
| Type B RCD | A standalone RCD designed to detect AC, pulsating DC and smooth DC residual currents | Used where no RDC-DD is built in, or where the manufacturer specifies Type B |
🧭 Jump to:
The Quick Answer
- RDC-DD is a 6 mA DC fault detection function, usually built into the charger.
- Type B RCD is a standalone protective device that detects AC, pulsating DC and smooth DC residual current.
- If the charger includes compliant RDC-DD, a Type A or Type F device upstream may be the correct route.
- If the charger does not include RDC-DD, or if the manufacturer specifically asks for it, use a Type B RCD.
Related reads: Type A vs Type B RCDs for EV Chargers · What Type of RCBO Do You Need for an EV Charger Circuit? · What Is an EV Consumer Unit?
What Is RDC-DD?
RDC-DD stands for Residual Direct Current Detecting Device. In EV charging, it refers to a function that detects DC fault current above 6 mA and initiates disconnection or control action inside the charging equipment.
In practice, this is often built into the EV charger itself. That is why many modern domestic chargers do not require a separate standalone Type B RCD at the board.
The key point is that RDC-DD is not usually the main upstream RCD sitting in the consumer unit. It is a DC fault detection layer within the EVSE, working alongside the upstream protective device.
What Is a Type B RCD?
A Type B RCD is a standalone residual current device designed for circuits where DC-sensitive protection is needed. It can detect:
- AC residual current
- Pulsating DC residual current
- Smooth DC residual current
That is why Type B RCDs are used on certain EV charging circuits, solar inverters, UPS systems and other inverter-driven loads where smoother DC fault conditions may appear.
Installer’s Pick: If the charger does not include RDC-DD, or the manufacturer specifically requires standalone Type B protection, our WEV240B-030 2 Pole B-Type RCD and WEV263B-030 63A B-Type RCD are strong options for single-phase EV circuits.
The Main Difference Between RDC-DD and Type B RCD
This is the part that trips people up:
| Feature | RDC-DD | Type B RCD |
|---|---|---|
| Role | DC fault detection function | Standalone protective device |
| Typical location | Inside the EV charger | At the board / upstream protection position |
| Main purpose | Detects DC fault current above 6 mA | Provides full RCD protection including smooth DC sensitivity |
| Used with | Usually Type A or Type F upstream | Used directly where separate DC detection is not provided |
So the choice is not really “which one is better?” It is more a case of which protective strategy the EV charger and installation call for.
When RDC-DD Is Enough
If the EV charger includes compliant 6 mA DC detection, you do not usually need a separate Type B RCD on that circuit. In that setup, the charger’s internal RDC-DD deals with the DC fault risk, and the upstream protection can usually be a Type A or Type F device where appropriate.
- Common on modern domestic EV chargers
- Often used with Type A EV RCBOs
- Keeps the board simpler and usually cheaper than fitting a standalone Type B
This is why many installers now start by checking the charger datasheet before ordering protection. If the charger already has RDC-DD, the spec path is often straightforward.
When You Need a Type B RCD Instead
You are usually into Type B RCD territory when:
- The charger does not include RDC-DD
- The charger manufacturer specifically says Type B RCD required
- The installation is more specialist, commercial or three-phase
- You are protecting a wider inverter-driven circuit where standalone Type B protection is the safer fit
For single-phase EV charger installs, that may mean a dedicated IP65 Weatherproof EV Kit with B-Type RCD. For wider standalone options, browse the full Type B RCD collection.
Installer’s Pick: Our IP65 Weatherproof EV Kit with 63A B-Type RCD + 40A C MCB is ideal where the charger does not provide integrated RDC-DD and you need an outdoor-ready, compliant solution.
Common Mistakes Installers Make With RDC-DD and Type B
- Assuming RDC-DD and Type B are the same thing – they are not. One is a detection function, the other is a standalone protective device.
- Ordering a Type B RCD before checking the charger datasheet – many chargers already include RDC-DD.
- Assuming all chargers include RDC-DD – some do, some do not, and some still specify separate protection.
- Ignoring the manufacturer’s instructions – charger requirements always matter.
- Forgetting the wider board spec – EV circuits may still need surge protection, suitable enclosure space and, on TN-C-S supplies, PME fault protection.
Regulation Reminder: For EV charging equipment, protection against DC fault current can be achieved by a Type B RCD, or by a Type A or Type F device used with RDC-DD where the EV charging equipment provides that protection. Always follow the charger manufacturer’s instructions as well as the current BS 7671 framework.
Products & Kits That Fit the Job
- Type B RCDs: WEV240B-030 40A 30mA · WEV263B-030 63A 30mA · WEV463B-030 4 Pole 63A
- Outdoor EV Protection: IP65 Weatherproof EV Kit with B-Type RCD
- Browse By Category: EV-Ready Consumer Units · EV RCBOs · Type B RCDs · PME Fault Detection
FAQs
Is RDC-DD the same as a Type B RCD?
No. RDC-DD is a DC fault detection function, usually built into the charger. A Type B RCD is a standalone protective device.
Do I need a Type B RCD if the charger has RDC-DD?
Usually not, unless the charger manufacturer specifically requires it. Where compliant RDC-DD is built into the EV charger, a Type A or Type F device upstream is often the accepted route.
What does RDC-DD actually do?
It detects DC fault current above 6 mA within the EV charging equipment and triggers the required protective response.
Why would I still choose a Type B RCD?
You would use Type B where the charger does not provide RDC-DD, where the manufacturer specifically requires Type B, or where the wider installation makes standalone DC-sensitive RCD protection the correct choice.
Are most modern EV chargers using RDC-DD now?
Many are, especially on domestic installs, but not all. Always check the charger specification before ordering protection.
Final Word
RDC-DD and Type B RCD solve the same broad problem from two different angles. RDC-DD is the charger’s internal 6 mA DC fault detection. Type B is the standalone upstream RCD used where that built-in protection is absent or the spec says Type B is required.
So before you order protection, check the charger datasheet first. If the EVSE includes compliant RDC-DD, a Type A or Type F route may be fine. If it does not, a Type B RCD is usually the right answer.
👉 Ready to spec? Compare our Type B RCDs, browse EV-ready consumer units, or choose an outdoor-ready B-Type EV kit for chargers without integrated RDC-DD.