what's different between type B, Type B+, and Type B-HP

Within the UK Wiring Regulations (BS 7671) and IEC standards (IEC/EN 62423), these types are distinctly defined. Their core difference lies in the types of residual current waveforms they can detect and respond to.

To build a quick conceptual model:

• Type B: The foundational all-rounder, covering AC, pulsating DC, and smooth DC.

• Type B+: Enhances the Type B by improving its response to high-frequency residual currents.

• Type B-HP: Builds on Type B+ by offering superior resistance to nuisance tripping from high-frequency currents, representing the current peak of performance.

Let's delve into the details.

1. RCD Type B (Standard Type B)

Type B is the foundational "all-rounder" RCD for handling complex DC fault components. It is a superset of Type A.

• Waveforms it can detect:

  1. AC Residual Current (Type AC functionality): Standard power frequency (50Hz) leakage.

  2. Pulsating DC Residual Current (Type A functionality): e.g., from half-wave rectification or simple switch-mode power supplies.

  3. Smooth DC Residual Current (Core Type B functionality): This is the key differentiator from Type A. It can detect and interrupt a persistent smooth DC fault current, preventing the RCD from being "blinded."

• How it works: It uses a special internal design (often involving a tuned circuit) to reliably operate for AC residual currents from DC up to 1kHz, and can also detect smooth DC.

• Typical Applications (according to BS 7671):

  • Equipment driven by three-phase variable speed drives (elevators, industrial pumps, AC compressors).

  • Electric Vehicle (EV) Charging Points (often a mandatory requirement under BS 7671 Section 722).

  • The output side of large UPS systems and PV inverters.

  • X-ray equipment, medical imaging equipment.

2. RCD Type B+ (Type B Enhanced)

Type B+ is an evolved version of Type B. It inherits all the standard Type B functions and significantly enhances its sensitivity to high-frequency residual currents.

• Waveforms it can detect: Includes all Type B functions, plus:

  • Reliable response to high-frequency residual currents up to 20kHz. The response of a standard Type B drops off sharply above 1kHz.

• Why is this needed? Modern switch-mode power supply technologies (like LLC resonant converters, Totem-Pole PFC) operate at increasingly higher frequencies, commonly found in:

  • Latest generation of high-efficiency EV charging points

  • Server power supplies, high-end IT equipment

  • LED drivers

  • Industrial variable speed drives (more advanced topologies)
    If these devices develop a high-frequency earth fault, a standard Type B might not detect it or may not trip quickly enough. Type B+ fills this safety gap.

3. RCD Type B-HP (High-Performance Type B)

Type B-HP is the top-tier product in the current RCD technology landscape. It builds upon Type B+ by adding one critical feature: immunity to nuisance tripping from high-frequency currents.

• Waveforms it can detect: Fully encompasses all Type B+ functions.

• Core Additional Feature:

  • It can not only reliably detect fault currents at frequencies up to 20kHz (or even higher, e.g., 50kHz), but it can also differentiate between dangerous fault currents and tolerable high-frequency leakage currents present during normal equipment operation.

  • Modern equipment, especially high-power chargers and industrial drives, inherently generates some high-frequency common-mode leakage during normal operation. A less performant RCD might misinterpret this normal leakage as a fault, causing a nuisance trip and resulting in an unnecessary power outage.

• How it works: B-HP RCDs incorporate more complex filtering and signal processing circuits. Their frequency-tripping current characteristic curve is meticulously designed for greater stability at high frequencies, ensuring operation only under genuinely dangerous fault conditions.

• Typical Applications:

  • High-frequency immunity for inverter/heat-pump leakage

  • Latest, highest-power Electric Vehicle charging equipment (this is the primary application for B-HP).

  • Critical facilities where power continuity is paramount (e.g., data centers, operating theatres).

  • Production lines and large machinery utilizing the most advanced variable speed drive technology.

Summary and Comparison

For a clear, at-a-glance comparison, please refer to the table below:

Advice for UK Electrical Engineers

1. Comply with Regulations: When designing an installation, your primary and mandatory duty is to follow the latest edition of BS 7671 and any relevant equipment manufacturers' technical requirements. For instance, selecting an RCD for an heat pump should using Type B-HP.

2. Consult Manufacturers: The manufacturer of electrical equipment (especially drives, chargers) will explicitly specify the required RCD type in its installation manual. Always treat the manufacturer's instructions as the final authority.

3. Trend: As technology evolves, Type B+ and Type B-HP are increasingly becoming the standard choice for protecting advanced power electronic equipment, particularly in commercial and industrial settings. Due to its superior stability, Type B-HP is becoming the preferred choice for high-end applications to minimise operational disruptions from nuisance tripping.