A Type B RCD protects a circuit with two sensing systems. A summation transformer watches for AC and pulsating fault currents; an electronic DC-sensitive stage watches for the smooth DC leakage a transformer alone cannot see. When either channel measures a residual current at the device’s rated level, a release opens the contacts and disconnects the supply.
Key Takeaways
- An RCD trips on imbalance: current that leaves and does not return becomes residual flux in the core.
- Smooth DC gives a transformer nothing to sense and can saturate the core, blinding a standard RCD to later AC faults.
- Type B adds an actively driven DC-sensitive channel and electronics that judge AC, pulsating and smooth DC residuals together.
- No EVDANCE listing states a built-in RCD type — upstream protection is set by the manual, local rules and the installer.
The Basic Idea: Watching for an Imbalance
Every RCD works on a balance test. All load conductors pass through a ring-shaped magnetic core. In a healthy circuit, current out and current back are equal and opposite, so their magnetic effects cancel and the core sees nothing. If current leaks to earth — through damaged insulation or a person — the flows no longer cancel. The leftover current, the residual current, creates a magnetic flux in the core. A sensing winding picks up that flux, and a large enough signal trips the device.
| Circuit state | What the summation transformer sees | What the DC-sensitive stage sees |
|---|---|---|
| Healthy | Out and return currents cancel; no residual flux | No steady offset |
| AC or pulsating fault | A changing residual flux — the trip signal | Evaluates the waveform |
| Smooth DC fault | A steady offset: no changing flux, so no signal — and the core can saturate | A measurable DC residual current |
Our guide to RCD protection for EV chargers compares the types; this article stays on the mechanism.
Why Smooth DC Defeats a Standard RCD
A transformer only responds to change. A smooth DC residual current flows steadily, so it produces a steady flux, which induces no signal. The standard RCD is blind to that fault.
Worse, the steady flux biases the core towards saturation, so a genuine AC fault at the same time may produce too weak a signal to trip. Electricians call this blinding: the device looks healthy and cannot do its job. EV charging raised the question because a car’s onboard charger can, in a fault, put smooth DC current onto the supply side — the waveform a plain Type AC or Type A device cannot be trusted with.
The Second Sensor: How Type B Sees DC
Type B adds a second channel built for steady currents. Its DC-sensitive stage — commonly a fluxgate arrangement — works differently from a passive transformer: its magnetic element is actively driven, so a steady residual field appears as a measurable asymmetry rather than an induced voltage. Electronics then evaluate the combined picture:
- The transformer channel reports AC and pulsating residual currents.
- The DC channel reports the smooth DC component.
- The electronics compare the residual current, across its waveforms, against the rated trip level and release the contacts at that level.
A separate approach is 6 mA smooth-DC detection, built into some chargers; it can allow a Type A RCD upstream instead of a Type B. A different component does the DC watching, as the protection guide explains.
What This Means on an EV Charging Circuit
The protection hardware is part of the installation design, not a property to assume about a charger. No EVDANCE listing states a built-in RCD type, so the installer specifies the circuit: a Type B device, or the combination the manual and local rules allow. For a fixed installation such as the EV-ACT3204PE wallbox (22 kW three-phase, Type 2, RFID and app, IP66, CE marked, hardwired), upstream protection is confirmed from the manual and the electrician’s design before the circuit is energised. The test button proves the trip mechanism moves; it does not calibrate the sensing, so periodic testing follows the device maker’s instructions.
Frequently Asked Questions
Is a Type B RCD the same as a Type B circuit breaker?
No — the letter means different things on different devices: on an RCD it names the residual waveforms detected; on a miniature circuit breaker, an overcurrent trip curve.
Does the test button prove a Type B RCD detects DC faults?
No. The button creates an artificial imbalance to show the mechanism works; it does not verify the DC channel’s calibration — that is what the maker’s test schedule and an electrician’s instrument testing are for.
Does my EV charger already include Type B protection?
Do not assume it. EVDANCE listings state no built-in RCD type: read the model’s manual for what it includes and requires upstream, and have the installer confirm the design in writing.
Related Reading
- RCD Protection for EV Chargers: Type A, Type B and 6 mA DC Detection Explained
- Mode 2 vs Mode 3 Charging: What IEC 61851 Means for Portable EV Chargers
- IP Ratings for EV Chargers: IP54, IP65 and IP66 Compared
