RCD types and what they respond to
Residual current devices are classified by the type of fault current they are designed to detect. The classification matters because a device that cannot respond to a particular fault current waveform provides no protection against it — and in an EV charging context, the fault current characteristics can be quite different from those in a conventional electrical installation.
Type AC RCDs respond to sinusoidal alternating fault currents only. They are the most basic type and the most widely installed in general domestic and commercial work. Type A RCDs respond to both sinusoidal AC and pulsating DC fault currents — including the half-wave rectified waveforms produced by single-phase loads with simple rectifier circuits. Type F devices extend this to include composite-frequency currents, relevant where variable-speed drives and inverter loads are present. Type B RCDs respond to the full range: sinusoidal AC, pulsating DC, smooth (pure) DC, and AC fault currents up to 1,000 Hz.
The hierarchy matters. A Type A RCD cannot safely replace a Type B in an installation requiring Type B protection — it will not operate under the fault conditions Type B is specified for.
DC blinding — what it is and why it occurs
DC blinding describes the failure of an RCD to operate when a DC component is present in the fault current. The mechanism is straightforward. The sensing element in a conventional RCD is a toroidal transformer whose core saturates in the presence of even a modest DC flux. When a DC fault current flows — or when a fault current contains a significant DC offset — the core saturates before it can respond to the AC residual current. The device appears to be functioning but will not trip under a fault condition it cannot detect. The result is loss of protection at the point where it is most needed.
DC charging equipment — whether a stand-alone rapid charger or the central power unit in a distributed hub architecture — performs AC to DC conversion within the unit. During certain fault conditions, particularly an internal fault within the charger or a fault on the DC output wiring, DC current can flow back through the protective conductor and the RCD sensing element to the supply. A Type AC or Type A RCD will saturate and fail to trip. A Type B RCD, which uses a different detection principle, will operate correctly.
TT earthing systems and why the risk is higher
In a TN-C-S (PME) earthed installation — the most common arrangement in UK commercial and industrial premises — fault current has two paths to operate protective devices: through the RCD and through the overcurrent protective device (fuse or circuit breaker). If the RCD fails to operate, there remains a possibility that overcurrent protection will clear the fault, depending on fault impedance and device rating.
In a TT-earthed installation, the fault current path to earth relies entirely on the earth electrode resistance. This resistance is typically high enough that overcurrent protective devices will not operate under a high-impedance earth fault — the fault current is simply insufficient. The RCD is therefore the only device that will clear the fault. If that RCD is blinded by a DC component and fails to trip, there is no fallback. The fault persists, with all of the associated risks of a live exposed conductor or a sustained heating fault.
TT earthing is used — and is often mandatory — at standalone eHGV charging sites, public charging hubs fed from private HV substations, and any installation where connecting the protective earth to the PME network would introduce a simultaneous contact hazard (see our related article on on-street charging). In all of these contexts, correct RCD selection is not an optional refinement — it is the only mechanism providing fault protection.
Type A versus Type B coordination and the cost of getting it wrong
Type B RCDs are significantly more expensive than Type A devices and are physically larger — a relevant constraint where panel space is limited. The economic temptation to specify Type A and review later is understandable, but misguided. Retrofitting Type B RCDs into an installed panel that was not designed to accommodate them can require a complete board replacement. In a public charging hub, downtime during retrofit is a commercial cost on top of the direct equipment and installation cost.
Correct RCD selection at the design stage costs nothing beyond the incremental device price. In a protection discrimination study — which should be part of any competent EV charging electrical design — the type, rating, and time-current characteristics of every RCD in the installation are confirmed at the outset, ensuring that each device is appropriate for the fault conditions it may encounter, and that the overall discrimination scheme operates as intended.