Why is a Type B RCD more expensive than Type A?

Type B devices require a more complex detection circuit to identify smooth DC residual currents. They are manufactured in lower volumes than Type A devices, which drives cost up.

What is a Type B RCD? Why EV Charging Needs It

Q: Does every EV charger need a Type B RCD?

No. Single-phase EV chargers with integrated DC leakage monitoring (RDC-DD) do not require an external Type B RCD — a Type A RCBO is sufficient. Always check the charger manufacturer's installation documentation.

Q: What is an RDC-DD and how does it relate to Type B?

An RDC-DD is an internal monitoring circuit built into many modern EV chargers. It detects smooth DC leakage within the charger and trips before it can desensitise an external Type A RCD. Where an RDC-DD is present, a Type B RCD is not required.

A Type B RCD detects all the residual currents a Type A device detects, plus smooth DC leakage — the type generated by 3-phase EV chargers and industrial equipment. It is required when the connected load can produce DC leakage currents that a standard Type A device would not reliably trip on.

For most of electrical installation history, a Type A RCD was sufficient for virtually everything. Modern EV charging — particularly 3-phase AC charging — changed that. The power electronics inside a 3-phase EV charger can generate smooth DC leakage currents as a byproduct of normal operation. A Type A RCD exposed to these currents can become magnetised and desensitised, causing it to fail to trip at the rated 30 mA threshold. In the worst case, it may not trip at all.

This is not a fault in the charger or the RCD — both are operating as designed. The issue is one of compatibility. Type B RCDs are engineered to detect and respond to smooth DC leakage, making them the correct specification where this type of current may be present.

What makes Type B different from Type A?

Feature	Type A RCD	Type B RCD
AC residual current detection	✅	✅
Pulsating DC detection (up to 6 mA)	✅	✅
Smooth DC residual current detection	❌	✅
Suitable for standard domestic circuits	✅	✅ (but over-specified)
Required for 3-phase EV chargers (no built-in DC protection)	❌	✅
Standard	BS EN 61009-1	BS EN 62423

Why 3-phase EV chargers specifically need Type B

Single-phase EV chargers (Mode 3, up to 7.4 kW) typically use a single-phase rectifier circuit. The leakage this produces is pulsating DC — detectable by a Type A device. Most single-phase chargers also include an internal monitoring circuit (RDC-DD) that detects smooth DC leakage before it can affect the external RCD, and trips the charger before the threshold is reached.

Three-phase EV chargers (Mode 3, 11–22 kW) use a 3-phase bridge rectifier internally. This produces smooth DC leakage currents that neither a Type A RCD nor an internal RDC-DD in a single-phase configuration can reliably handle. A 4-pole Type B RCD is therefore required on the supply side of a 3-phase EV charger circuit where no equivalent protection is built in.

When do I actually need a Type B RCD?

Situation	Type B required?	Reason
Single-phase EV charger with integrated RDC-DD	No — Type A sufficient	Internal protection handles DC leakage
Single-phase EV charger without integrated DC protection	Yes	No internal DC monitoring
3-phase EV charger (11–22 kW)	Yes — 4-pole	3-phase rectifier produces smooth DC leakage
Heat pump (most domestic models)	Often Type A	Check manufacturer — some specify Type B HP RCD
Solar PV inverter	Depends on inverter	Transformer-less inverters may need Type B
Standard domestic circuits	No — Type A sufficient	No DC leakage generated

The WCED Type B RCD range

The WCED WEV series provides Type B RCDs in both 2-pole (single-phase) and 4-pole (3-phase) configurations. These are designed specifically for EV charging circuit protection and are available in 25A, 40A, and 63A ratings at 30 mA sensitivity. The WHP240B-030 is a dedicated heat pump variant compatible with Vaillant and similar systems.

2-pole (single phase) — 25A, 40A, 63A

4-pole (3-phase) — 25A, 40A, 63A

30 mA sensitivity — personal protection

BS EN 62423 compliant

Heat pump variant available (WHP series)

UKCA marked

WCED WEV 2-Pole Type B RCD

Single-phase Type B RCD for EV charging circuits. 25A, 40A, and 63A at 30mA. For installations where a Type A is not sufficient or where charger documentation specifies Type B.

View Type B RCDs →

WCED WEV 4-Pole Type B RCD

Three-phase Type B RCD for 11–22kW EV charging installations. 4-pole for complete 3-phase protection. Required for 3-phase chargers without integrated DC protection.

View 4-pole Type B →

WCED WHP Heat Pump RCD

B-type 2-pole 40A 30mA RCD designed specifically for heat pump protection — Vaillant compatible. The correct specification for most domestic heat pump installations.

View HP RCD →

Shop WCED Type B RCDs

2-pole and 4-pole Type B RCDs for EV charging and heat pump installations. Significantly more competitive than branded equivalents.

View Type B RCD range →

Frequently Asked Questions

Does every EV charger need a Type B RCD?

No. Single-phase EV chargers that include integrated DC leakage monitoring (RDC-DD) — which includes most modern Mode 3 chargers including Simpson & Partners units — do not require an external Type B RCD. A Type A RCBO is sufficient for these installations. Always check the charger manufacturer's installation documentation to confirm what type of RCD protection is specified.

What is an RDC-DD and how does it relate to Type B?

An RDC-DD (Residual DC Detection Device) is an internal monitoring circuit built into many modern EV chargers. It detects smooth DC leakage within the charger and trips the charger before the leakage current can reach a level that would desensitise an external Type A RCD. Where an RDC-DD is present and functioning correctly, a Type B RCD on the supply circuit is not required. Where it is absent, a Type B is required.

Why is a Type B RCD so much more expensive than Type A?

Type B devices require a more complex detection circuit to identify smooth DC residual currents alongside AC and pulsating DC. The internal design is more sophisticated and manufactured in lower volumes than Type A devices, which drives cost up. WCED Type B RCDs offer substantially better value than branded equivalents while meeting the same BS EN 62423 standard.

Can I use a Type B RCBO instead of a Type B RCD?

Type B RCBOs (combining Type B RCD + MCB in one module) are available but less common and more expensive. For EV charging circuits, the standard approach is a Type B RCD on the supply with a separate MCB for overcurrent protection, or a fully assembled EV protection board. A dedicated Type B RCBO can be used where a single device is preferred.

Further reading:
RCBO types: A, B, AF — explained
4-pole vs 2-pole RCDs for 3-phase
PME vs RCD vs RCBO vs RDC-DD — EV charging protection explained
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