Three-phase systems need different leakage protection to single-phase circuits. The loads behave differently, neutral currents can circulate under imbalance, and inverter-driven equipment introduces DC leakage that standard RCDs can’t always detect. That’s why 3-phase RCDs are almost always 4-pole devices - they disconnect all live conductors, including the neutral, under fault conditions.
Quick Answer: A 3-phase RCD is a 4-pole leakage protection device (L1-L2-L3-N) used on three-phase installations to disconnect all live conductors when earth leakage occurs. Most systems use Type A for standard loads, High-Immunity Type A for noisy inverter circuits, and Type B when smooth DC leakage is possible (EV chargers, solar inverters, VFDs, heat pumps). Sensitivity is typically 30mA for final circuits and 100mA or 300mA time-delay for upstream protection and discrimination.
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What a 3-Phase RCD Actually Does
A three-phase RCD monitors the combined current across L1, L2, L3 and the neutral. Under balanced load, the vector sum of these currents equals zero. If leakage occurs - to earth or through a fault path - the imbalance triggers the RCD to disconnect all live conductors.
This matters because three-phase systems can generate:
- Circulating neutral currents under imbalance
- Harmonic distortion from motors, drives and inverters
- Pulsating DC leakage from modern electronic loads
- Smooth DC leakage on renewable and EV systems
Why 3-Phase RCDs Must Be 4-Pole
On a TN-S or TN-C-S system, the neutral is a live conductor. Failing to disconnect it can leave equipment energised under fault conditions, especially when fault current returns through unintended paths. A 4-pole RCD isolates L1 + L2 + L3 + N simultaneously.
Regulation Reminder (BS 7671): Three-phase RCDs must disconnect all live conductors including neutral unless the neutral is proven to be reliably disconnected elsewhere. For most distribution boards, this means using a 4-pole RCD.
Type A vs Type B on 3-Phase Systems
Different loads produce different leakage signatures. Choosing the wrong type can lead to nuisance tripping or, worse, failure to trip under smooth DC leakage.
Type A (Most 3-Phase Industrial & Commercial Loads)
Type A detects AC and pulsating DC leakage. Suitable for:
- Standard three-phase distribution circuits
- Motors, pumps, compressors
- Basic inverter drives (low DC component)
- Heat pump systems with internal DC monitoring
High-Immunity Type A
Best for noisy circuits with harmonics or fluctuating leakage, such as:
- Long cable runs
- Industrial machinery
- Motors starting under heavy load
- Installations with high-frequency noise
Type B - When Smooth DC Leakage Is Possible
Type B is mandatory when smooth DC leakage may occur, because Type A can become blind to faults above 6mA DC.
Use Type B for:
- Three-phase EV chargers
- Solar PV inverters (3-phase)
- VFDs & motor drives with DC components
- Large heat pumps / GSHP systems
- Battery storage / hybrid inverters
Choosing Sensitivity: 30mA vs 100mA vs 300mA
The correct sensitivity depends on the circuit’s position in the installation and its purpose:
30mA – Shock Protection (Final Circuits)
Used where additional protection is required. Typical for three-phase socket circuits, small machinery, or equipment accessible to users.
100mA Time-Delay – Selectivity Upstream
Used where the RCD sits upstream of 30mA devices. The time delay prevents unnecessary disconnection on earth faults downstream.
300mA Time-Delay – Fire Protection / Main RCD
Typical for commercial distribution boards where fire protection is required over large circuits or submains.
Regulation Reminder: BS 7671 requires discrimination between RCDs in a cascade. Use time-delay (S-type) RCDs upstream where 30mA devices protect final circuits.
Installer-Favourite 3-Phase RCDs
All devices below are UK-stocked, 4-pole, and suitable for commercial, industrial and renewable-ready installs.
High-Immunity 4-Pole Type A RCDs (40A–100A)
- 63A Type A High-Immunity RCD
- 40A Type A High-Immunity RCD
- 80A Type A High-Immunity RCD
- 100A Type A High-Immunity RCD
4-Pole Type A Time-Delay (S-Type) RCDs – 100mA
Type B 4-Pole RCD – For EV, Solar, VFDs and Heat Pumps
Explore full range: Browse all 3-Phase RCDs
FAQs
Installers frequently ask these when choosing a 3-phase RCD.
Does every 3-phase consumer unit need a 4-pole RCD?
Yes - unless neutral is disconnected elsewhere (rare). A 4-pole device ensures all live conductors are isolated.
When should I use a Type B RCD on 3-phase?
When smooth DC leakage is possible: three-phase EV chargers, solar inverters, VFDs, heat pumps and battery systems.
Can a 3-phase RCD protect mixed single-phase loads?
Yes. A 3-phase RCD can protect multiple single-phase circuits, provided the neutral path remains consistent.
Why do 3-phase RCDs nuisance trip?
Common causes include N-E faults, harmonic distortion, long cable runs, and inverter-driven equipment creating leakage spikes.
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