Type A and Type B RCDs are residual current devices (RCDs) used to protect people and installations by disconnecting power when they detect dangerous earth leakage current. In EV charging, selecting the correct RCD type is critical because some charging faults can include DC leakage, which may “blind” certain RCDs and prevent them from tripping.
What Is an RCD?
An RCD monitors the balance between live conductors. If current leaks to earth (for example through a damaged cable, insulation failure, or internal fault), the outgoing and returning currents no longer match. The RCD detects this residual current and trips to reduce the risk of:
– Electric shock and injury
– Fire caused by insulation faults
– Hidden leakage that can damage equipment over time
RCDs are commonly specified by:
– Sensitivity (e.g., 30 mA for personal protection, higher values for upstream protection)
– Type (A, B, etc.), which defines what waveforms the device can detect reliably
Type A RCD: What It Detects
A Type A RCD is designed to detect:
– AC residual currents (sinusoidal)
– Pulsating DC residual currents (typical in many electronic loads)
Type A is widely used in general electrical installations and can be suitable for some EV charging configurations only when DC leakage is properly managed by the EVSE design.
Type B RCD: What It Detects
A Type B RCD detects a wider range of residual current waveforms:
– AC residual currents
– Pulsating DC residual currents
– Smooth DC residual currents (continuous DC leakage)
– Higher-frequency components that may appear in power electronics systems
Because EV charging systems can, under certain fault conditions, produce smooth DC leakage, Type B RCDs are often required for EVSE circuits unless an alternative protective method is used.
Why Type A vs Type B Matters for EV Charging
EV charging involves power electronics and vehicle onboard chargers that can create DC components in leakage current during faults. If smooth DC leakage exceeds certain levels, it can saturate the sensing core of Type A devices, reducing their ability to trip correctly.
Correct RCD selection supports:
– Safer user protection at the charging point
– Compliance with EV charging installation requirements
– Reduced nuisance tripping and clearer fault behavior
– Reliable protection in multi-charger sites and public installations
Typical EV Charging Approaches
Common compliant approaches used in AC EV charging include:
– Type B RCD protecting the circuit (covers AC + smooth DC leakage)
– Type A RCD + 6 mA DC leakage detection integrated into the EVSE (a common approach for AC chargers)
– Site-specific schemes depending on earthing arrangement and local rules (TN/TT, PME)
In many modern AC chargers, 6 mA DC leakage detection is built in specifically to allow the use of a Type A upstream device while still protecting against DC blinding risk.
Practical Selection Considerations
When specifying protection for EV charging circuits, consider:
– Whether the charger has integrated DC leakage detection (and to what standard)
– Local regulations and installer requirements (some markets mandate Type B in certain cases)
– Earthing system and additional protection requirements (e.g., PME fault protection)
– Number of chargers on a feeder and selectivity/coordination with upstream RCDs
– Likelihood and cost of nuisance trips in public or fleet operations
– Maintenance strategy and testing requirements (periodic RCD testing)
Limitations and Common Mistakes
– Assuming any RCD is “good enough” without considering DC leakage behavior
– Using Type A without confirmed EVSE DC leakage detection (increasing blinding risk)
– Poor coordination between upstream and downstream RCDs causing unnecessary site-wide trips
– Confusing RCD “type” (waveform capability) with sensitivity rating (mA trip threshold)
– Overlooking the need for proper earthing and bonding (RCDs are not a replacement for safe installation design)
Related Glossary Terms
Residual current device (RCD)
Leakage current detection
DC leakage detection (6 mA)
Earthing system
Protective earth (PE)
Equipotential bonding
PME fault protection
Overcurrent protection device (OCPD)
Insulation monitoring device (IMD)
Electrical safety compliance