If your electrician has mentioned a “Type B RCD” before fitting your home EV charger, you are not alone in wondering what that means, or whether it will cost you an extra few hundred pounds. The short answer is that BS 7671:2018+A2:2022 requires specific protection against DC fault currents on EV charging circuits, and a Type B RCD is one of two ways to meet that requirement. Whether you actually need one installed at your consumer unit depends on the wallbox you choose.
What is a Type B RCD?
A Type B RCD is a residual current device that detects AC, pulsating DC, and smooth DC fault currents. It will trip at 6 mA of smooth DC residual current, a fault type that standard Type A and Type AC devices cannot see. For EV charging circuits, that smooth DC detection is precisely the protection BS 7671 Section 722 requires.
Type B RCDs detect three classes of fault: AC sinusoidal, pulsating DC, and, critically, smooth unidirectional DC currents down to 6 mA. This makes them more capable than the Type AC or Type A devices found in most domestic consumer units. A Type AC device detects only AC sinusoidal faults. A Type A adds pulsating DC to that, but stops there. Neither can see a smooth DC residual current flowing in a circuit, and that matters enormously when you connect an electric vehicle.
At consumer-unit level, a Type B RCD typically retails in the UK for between £80 and £160, compared with £15-£40 for a standard Type A. The price difference reflects the additional sensing circuitry required to detect smooth DC faults.
Why EV Chargers Produce DC Fault Currents
The internal electronics of an EV charger are specifically what creates the risk of smooth DC residual currents, something that no previous domestic electrical standard needed to account for.
Inside a mode 3 AC wallbox, the electronics include a diode bridge and IGBT (Insulated Gate Bipolar Transistor) switching components. If a fault develops here, an insulation failure for instance, the residual current leaking to earth may be smooth and unidirectional rather than alternating: a smooth DC residual current.
A conventional Type A RCD monitors for imbalances between live and neutral currents, but its sensing coil responds only to alternating and pulsating waveforms. A smooth DC fault current will not induce a sufficient response, and the RCD may not trip, even with a dangerous fault present. The IET Code of Practice for EV Charging Equipment Installation, 5th edition (2023) addresses this risk explicitly and requires appropriate mitigation on all EV charging circuits.
What BS 7671 Section 722 Actually Says
BS 7671:2018+A2:2022 (IET Wiring Regulations), Section 722, sets the specific protection requirements that all EV charging installations in the UK must satisfy.
Section 722 requires that EV charging circuits include protection against smooth DC residual currents of 6 mA or greater. It offers two routes to compliance:
- Install a Type B RCD: a single device that handles AC, pulsating DC, and smooth DC protection in one unit.
- Install a Type A RCD combined with a 6 mA DC detection device (RDC-DD): a Type A at the consumer unit, plus a separate DC monitoring device in the circuit or integrated into the wallbox itself.
Both routes are equally valid under the standard. The IET Code of Practice for EV Charging Equipment Installation, 5th edition (2023) confirms that a built-in RDC-DD within the wallbox satisfies this requirement when paired with a Type A at the board, provided the installation is designed and signed off by a competent person.
For a broader look at the standards that govern EV charging equipment safety, see our guide to EV charging cable safety standards.
RCD Type Comparison: Which Detects What?
The table below summarises how the four main RCD classes perform against EV charging fault scenarios, and whether each satisfies BS 7671:2018+A2:2022 Section 722 on its own.
- headers: [RCD Type, AC Sinusoidal Faults, Pulsating DC Faults, Smooth DC Faults (6 mA), EV-Charging Compliant (BS 7671 S722)]
- rows:
- [Type AC, Yes, No, No, No]
- [Type A, Yes, Yes, No, No (alone)]
- [Type A + 6 mA DC device (RDC-DD), Yes, Yes, Yes, Yes]
- [Type B, Yes, Yes, Yes, Yes]
Type AC and Type A alone are not sufficient for EV charging circuits. You need either the Type A + RDC-DD combination or a Type B, and your wallbox may already provide half of that solution.
The RDC-DD Alternative and Why It Matters
The RDC-DD (Residual Direct Current Detection Device) is the often-overlooked second route to BS 7671 Section 722 compliance, and for many homeowners it means no consumer-unit upgrade is needed at all.
An RDC-DD monitors the charging circuit continuously for smooth DC residual currents above 6 mA. If it detects one, it signals the wallbox to stop charging, removing the fault condition before it can become dangerous. Paired with a standard Type A RCD at the consumer unit, this combination satisfies Section 722 fully, per BS 7671:2018+A2:2022 (IET Wiring Regulations).
Wallboxes with built-in DC detection
Several of the UK’s most popular home wallboxes already include an RDC-DD on their internal PCB, per published product specifications and the IET Code of Practice for EV Charging Equipment Installation, 5th edition (2023):
- Zappi (myenergi) includes an internal RDC-DD; a Type A at the board is sufficient
- Ohme Home Pro includes an internal RDC-DD; a Type A at the board is sufficient
- Andersen A2 includes an internal RDC-DD; a Type A at the board is sufficient
- Pod Point Solo 3 includes an internal RDC-DD; a Type A at the board is sufficient
If your wallbox is on this list, or your installer can confirm a built-in RDC-DD from the product datasheet, you will not need a Type B RCD at your consumer unit. This is worth clarifying before accepting a quote that includes one, as it can represent a saving of £80-£160 on hardware alone, plus associated installation labour.
Always verify the current specification with your installer. Hardware revisions and firmware changes can affect how a wallbox meets the standard.
Do You Need a Type B RCD? A Decision Guide
The straightforward decision path looks like this.
- nodes:
- START: Installing a home EV charger?
- Q1: Does your wallbox include a built-in RDC-DD (6 mA DC detection)?
- Q1_YES: COMPLIANT: A standard Type A RCD at your consumer unit is sufficient. Your wallbox satisfies BS 7671 S722.
- Q1_NO: Is a separate standalone RDC-DD being fitted in the circuit by your electrician?
- Q2_YES: COMPLIANT: Type A at the board plus standalone RDC-DD in circuit satisfies BS 7671 S722.
- Q2_NO: ACTION NEEDED: You need a Type B RCD installed at or near your consumer unit.
- flow: START → Q1 → (YES → Q1_YES) | (NO → Q1_NO → (YES → Q2_YES) | (NO → Q2_NO))
If your wallbox does not include a built-in RDC-DD and no standalone device is fitted, a Type B RCD at the consumer unit is not optional. It is the requirement under BS 7671:2018+A2:2022 Section 722.
Cost: Type B RCD vs Type A + RDC-DD Route
The cost difference between the two routes can be meaningful, and it is worth understanding before accepting a quote.
| Route | Device cost (approx. retail) | Notes |
|---|---|---|
| Type B RCD at consumer unit | £80-£160 | Replaces or supplements existing protection; labour additional |
| Type A RCD at board (already fitted in most homes) | £15-£40 | Often already present: no additional device cost |
| Standalone RDC-DD in circuit | £30-£80 | Fitted between board and wallbox if not built into wallbox |
| Wallbox with built-in RDC-DD | £0 additional | Cost absorbed in wallbox price; no board-level Type B needed |
Where your wallbox already includes an RDC-DD, the total additional cost for DC protection may be nil, because your existing Type A RCD at the consumer unit does the rest. Where you do need a Type B, budget for the device cost plus your electrician’s time to upgrade the consumer unit, which may involve a board inspection and certification.
Who Can Install a Type B RCD?
All electrical work on a dedicated EV charging circuit is notifiable under Part P of the Building Regulations. It must be carried out by a qualified electrician, not as a DIY task.
Your installer should be registered with one of the following:
- NICEIC (National Inspection Council for Electrical Installation Contracting)
- NAPIT (National Association of Professional Inspectors and Testers)
- ECA (Electrical Contractors’ Association)
Registration allows your installer to self-certify the work and issue a BS 7671 installation certificate, which you will need if you ever sell your home.
If you are uncertain about overcurrent protection alongside your residual current protection, our article on RCBO for EV charging circuits explains how a combined RCBO can simplify the board-level design.
Frequently Asked Questions
Do I definitely need a Type B RCD for my home EV charger?
Not necessarily. BS 7671:2018+A2:2022 Section 722 permits either a Type B RCD or a Type A RCD combined with a 6 mA DC detection device (RDC-DD). Many modern wallboxes include an RDC-DD on their internal PCB, which means a standard Type A at your consumer unit can be sufficient. Check with your installer and the wallbox datasheet before committing to a board upgrade.
What faults does a Type B RCD detect that a Type A cannot?
A Type A RCD detects AC sinusoidal and pulsating DC residual currents. A Type B detects all of those plus smooth (unidirectional) DC residual currents down to 6 mA: the fault type that EV charger electronics can produce through IGBT switching and the diode bridge.
Which wallboxes have a built-in DC detection device?
Zappi (myenergi), Ohme Home Pro, Andersen A2, and Pod Point Solo 3 all include an internal RDC-DD as of their current production specifications. Always confirm the current specification with your installer and check the product datasheet, as hardware and firmware can change between model revisions.
How much does a Type B RCD cost to install?
The device itself typically retails between £80 and £160 at consumer-unit level, compared with £15-£40 for a standard Type A. Your electrician will also charge for labour and any associated board work. If your wallbox has a built-in RDC-DD, you may not need a Type B at all.
Can I install a Type B RCD myself?
No. All work on EV charging circuits is notifiable under Part P of the Building Regulations and must be carried out by a qualified electrician registered with a competent person scheme such as NICEIC, NAPIT, or ECA. A DIY installation is not certifiable and may invalidate your home insurance.
What is an RDC-DD?
An RDC-DD (Residual Direct Current Detection Device) is a 6 mA DC monitoring device. When fitted in an EV charging circuit alongside a Type A RCD, whether built into the wallbox or as a standalone device in the cable run, it provides the smooth DC fault detection that BS 7671:2018+A2:2022 Section 722 requires, without needing a full Type B RCD at the consumer unit.
Does a Type B RCD replace overcurrent protection?
No. A Type B RCD handles residual current protection only. Your electrician will also fit appropriate overcurrent protection (an MCB or RCBO) on the dedicated EV circuit, and the two work together.
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