RCD for EV Charger Circuits: Type B and 6mA DC (RDC-DD)
Does an EV charger need a Type B RCD, or will a standard RCBO do? An EV charge point can present a smooth DC fault current from the vehicle's onboard rectifier that ordinary Type AC and Type A devices, per IEC 62423, cannot detect above about 6 mA. That undetected DC component saturates the toroidal core and desensitizes the whole differential path, so a real AC earth fault downstream may fail to trip a 30 mA RCD that would otherwise clear it in time. This article covers the DC-blinding mechanism, the IEC 62955 6 mA RDC-DD threshold, Type B versus Type A-plus-RDC-DD, circuit sizing and poles, and brand options from Schneider, ABB and Siemens.
Why an EV Charger Produces a Fault Current Other Loads Don't
A domestic socket load leaks AC current, if it leaks at all. An EV on-board charger is different: its input rectifier and DC-DC stage can, under an internal insulation fault, push a smooth, unidirectional DC residual current onto the earth conductor. No half-cycle reversal, no zero crossing. A conventional Type A or Type AC device senses residual current by inducing a signal in a sense winding as the core flux changes; a steady DC offset from the vehicle can pre-magnetize that core toward saturation. Once saturated, the core stops responding proportionally to a genuinely dangerous AC leakage on the same circuit. The RCD does not fail loudly. It just quietly stops doing its job.
What we see in the field: installers assume "Type A covers everything modern," which is true for most SMPS and drive loads, but not for a vehicle traction inverter feeding fault current back through the charge cable. That is the one case IEC 62423 draws a hard line around.
IEC 62955 and the 6 mA RDC-DD Threshold
IEC 62955 defines the residual direct current detecting device (RDC-DD): a sensor, standalone or built into the charger electronics, that monitors smooth DC residual current and opens the supply contactor before that current can blind an upstream Type A RCD. The nominal detection threshold is 6 mA DC. Below it, a Type A RCD's AC and pulsating-DC sensitivity stays intact; at or above it, disconnection is mandatory before the core desensitizes further.
Formula: RDC-DD Disconnection Rule — Source: IEC 62955 / IEC 62423
Idc ≥ 6 mA → disconnect supply before IΔn sensitivity is compromised
| Symbol | Description | Unit |
|---|---|---|
| Idc | Smooth DC residual current detected at the charge point | mA |
| IΔn | Rated residual operating current of the backup Type A RCD (typically 30 mA) | mA |
| ttrip | Disconnection time of the RDC-DD once Idc is exceeded continuously | ms |
Type B RCD: One Device Covers Every Waveform
A Type B RCD detects AC residual current, pulsating DC (Type A's territory), and smooth DC in one enclosure, satisfying the charge point requirement without adding a separate RDC-DD module. ABB's F200 B and Siemens's 5SM3 are built for exactly this duty; Schneider's Acti9 iID range also extends to Type B in its higher-sensitivity variants. Fit one at the origin of the charge point circuit and the DC-blinding question is closed structurally, not by relying on a second component staying wired correctly for the life of the installation.
Type A RCD Plus RDC-DD: The Modular Alternative
Many commercial EVSE units now ship with an RDC-DD built into the charger's own electronics, in which case the upstream board-side protection only needs to be a Type A RCD or RCBO sized to the circuit, with the DC-fault duty already handled inside the equipment. This costs less at board level. It also shifts the compliance question to documentation: the installer has to confirm, from the charger's datasheet or certification, that the 6 mA RDC-DD is actually present and active, not assumed. Some electricians default to Type B on every job to avoid chasing that paperwork, and on a multi-vendor site with mixed charger models, that is a reasonable simplification even if it costs slightly more per circuit.
Choosing Between Type B and Type A + RDC-DD
The two approaches are functionally equivalent when correctly specified; the difference is where the DC-detection function physically lives and who is responsible for confirming it is there.
| Criteria | Type B RCD (standalone) | Type A RCD + Integrated RDC-DD |
|---|---|---|
| DC fault coverage | Built into the RCD core | Handled inside the charger, ahead of the board RCD |
| Board-level cost | Higher per pole | Lower, if RDC-DD is confirmed present in the EVSE |
| Compliance verification | Visible on the DIN rail, easy to audit | Requires charger datasheet or certificate check |
| Best fit | Unknown or mixed charger models, retrofits, public charging hubs | Single known charger model with documented RDC-DD |
Sizing, Poles and Backup Protection for the Charge Point Circuit
A domestic single-phase charge point runs on a 2P device; a three-phase commercial unit needs 4P. Rated current In is set to the charger's continuous draw with the usual cable-derating margin, not the EVSE's peak negotiated current alone. An RCBO combines the RCD and the overcurrent function in one module and is the common choice for a dedicated single-charger circuit; an RCCB plus a separate MCB is more common on multi-way commercial boards where discrimination between circuits matters. Either way, the device still needs a rated conditional short-circuit current (Inc) matched to the upstream backup fuse or breaker, typically 6 or 10 kA.
The earthing system on site changes the picture too. On a TT installation, the RCD is doing the primary earth-fault clearing job because loop impedance is too high for the overcurrent device alone; on TN, the MCB or MCCB backs it up, but 30 mA-class protection is still required for the socket-outlet or dedicated EV circuit in most codes. This is on top of, not instead of, the RDC-DD or Type B requirement above.
Brand Options for EV Charge Point Protection
ABB's F200 B RCCB and the DS200-series RCBOs cover the Type B and RCBO ends of this circuit respectively; both sit in the wider residual current devices range. Siemens's 5SM3 is its dedicated Type B for EV, PV and VFD DC fault currents, alongside the 5SU1 RCBO for the overcurrent side. Schneider's Acti9 iID B pairs with the Vigi add-on block approach if the board already standardizes on iC60 MCBs, and its RCBO equivalents sit in the RCBOs collection. None of the three differs on the underlying IEC 62955 threshold; the choice comes down to form factor and what the rest of the board already uses.
Frequently Asked Questions
Does every EV charger need a Type B RCD?
Not necessarily. A Type A RCD is acceptable if the charger has a certified 6 mA RDC-DD built in per IEC 62955. Without confirmed RDC-DD, Type B is required.
What is the difference between a Type B RCD and an RDC-DD?
A Type B RCD is a complete residual current device that also detects smooth DC. An RDC-DD is a narrower DC-only detection function, either standalone or built into an EVSE, that works alongside a Type A RCD rather than replacing it.
Can a Type A RCD alone protect an EV charge point?
No. A Type A RCD without a paired 6 mA RDC-DD does not meet the EV charging requirement, because a smooth DC fault current from the vehicle can desensitize it without tripping.
What DC leakage current triggers an RDC-DD?
6 mA, per IEC 62955. At or above that level the device disconnects the supply before the upstream Type A RCD's sensitivity is compromised.
Is an RCBO enough for a home EV charger circuit?
An RCBO handles overcurrent and residual current in one module, which is convenient for a single dedicated circuit, but its Type (A, B, or A+RDC-DD) still has to meet the same DC-detection requirement as a standalone RCCB.
Conclusion
An EV charge point circuit needs either a Type B RCD or a Type A RCD backed by a certified 6 mA RDC-DD; there is no third compliant option. Type B is the simpler specification when the charger model or its internal protection is not fixed for the life of the installation. Type A plus RDC-DD can cost less at the board when the charger's DC detection is documented and verified. Poles, In, and Inc still have to match the circuit and the backup device regardless of which route is chosen, and the earthing system on site adds its own requirement on top. For background on how Type A and Type B differ mechanically, see the Type A vs Type B RCD comparison, and for the wider device family the RCD protection guide and the MCB vs RCBO vs RCD vs RCCB differences article cover how this fits into the rest of the board.
