MYTH: Type B RCDs Are Unnecessary for Most Installations
VFDs, EV chargers, and solar inverters produce DC fault components that Type A RCDs cannot detect. Type B is increasingly mandatory, not optional.
February 26, 2026
The Myth
"Type A RCDs cover most situations. Type B is overspecified for typical installations."
This was true 15 years ago. With the proliferation of VFDs, EV chargers, solar inverters, and modern switched-mode power supplies, it's now dangerously outdated.
The RCD Type Hierarchy
| RCD Type | Detects | Typical Use |
|---|---|---|
| Type AC | AC sinusoidal residual current only | Legacy installations, purely resistive loads |
| Type A | AC sinusoidal + pulsating DC residual current | General use, single-phase rectifier loads |
| Type F | Type A + composite residual currents from VFDs | Single-phase VFD applications |
| Type B | All above + smooth DC residual current up to 6mA | Three-phase VFDs, EV chargers, PV inverters |
The critical gap: smooth DC residual current. When a ground fault occurs on the DC side of a three-phase rectifier (inside a VFD, EV charger, or inverter), the fault current flowing back through the earth path is smooth DC — not pulsating, not sinusoidal. Type A and Type AC RCDs are physically incapable of detecting smooth DC because their current transformers are designed for time-varying currents only.
Where Type B Is Now Required
EV Chargers (Mode 3) IEC 61851-1 Clause 6.4.3.3 requires either:
- A Type B RCD upstream of the charger, OR
- A Type A RCD with an integral DC fault detection device (RDC-DD) built into the charger
BS 7671:2018 Section 722 (EV installations) aligns with this requirement. Most Mode 3 chargers include an internal RDC-DD, but Mode 2 portable chargers on a standard socket do NOT — making the upstream RCD type critical.
Variable Frequency Drives (Three-Phase) A three-phase VFD with a 6-pulse rectifier front end produces smooth DC fault current during a ground fault on the DC bus or motor winding. Type A RCDs see this as "no residual current" and fail to trip.
Solar PV Inverters Transformerless (TL) PV inverters can produce DC fault components during certain ground fault scenarios. IEC 60364-7-712 and AS/NZS 5033 specify RCD requirements accordingly.
The Blinding Effect
Worse than failing to detect a DC fault, smooth DC current through a Type A RCD can magnetically saturate its current transformer, reducing its sensitivity to AC faults as well. A 6mA smooth DC leakage current can increase the AC tripping threshold of a 30mA Type A RCD to above 50mA — outside the safety threshold.
This means a DC-producing load can compromise the protection of OTHER circuits sharing the same RCD, even if those circuits have only AC loads.
The Economic Objection
Type B RCDs cost 3-5× more than Type A. For a single EV charger circuit, that's $120 vs $30. But:
- The EV charger itself costs $1,500–15,000
- The installation labour is $500–1,000
- An undetected DC ground fault can damage the charger, the installation, or injure a person
The $90 premium for a Type B RCD is trivial relative to the overall installation cost and the consequences of an undetected fault.
Bottom Line
If your installation includes any three-phase rectifier loads (VFDs, EV chargers, PV inverters, large UPS systems), you need Type B or Type B+ RCDs on those circuits. Type A is no longer sufficient for modern electrical installations.
Design your protection: Specify correct RCD types for EV installations with the EV Charging Calculator.
Frequently Asked Questions
What standards govern cable sizing calculations?
The primary standards are AS/NZS 3008.1.1:2017 (Australia/NZ), BS 7671:2018 (UK), IEC 60364-5-52 (International), and NEC Article 310 (USA). Each has different assumptions for ambient temperature, installation methods, and derating factors.
Why do different standards give different cable ratings?
Standards differ in reference ambient temperature (AS/NZS uses 40°C, BS 7671 uses 30°C), test conditions, grouping factor calculations, and installation method classifications. A 50mm² XLPE cable can vary by 15% between standards.
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