Single-phase and three-phase RCDs: 2-pole or 4-pole, and which rating?

Short answer: Use a 2-pole residual current device (RCD/RCCB) on a single-phase supply (230 V, phase + neutral) and a 4-pole device on a three-phase supply (400 V, three phases + neutral). The RCD's rated current must not be lower than the circuit breaker in front of it; 40 A, 30 mA is the most common choice in homes. The neutral must always pass through the RCD. Press the test button once a month and have a qualified electrician do the wiring.

What an RCD does and does not do

An RCD compares the current going out on the live conductors with the current coming back. When the difference reaches its sensitivity (30 mA or 300 mA), it disconnects the circuit, so the supply is cut if someone touches a live part or insulation fails.

An RCD does not protect against short circuits or overloads. That is the job of a circuit breaker (MCB), and an RCCB is always installed with a suitable breaker. See type B and type C breakers and which breaker for which circuit.

2-pole vs 4-pole

• 2-pole (1 phase + neutral): For single-phase installations and circuits. Phase goes through one pole, neutral through the other. Takes about 2 modules on the DIN rail.
• 4-pole (3 phases + neutral): For three-phase installations, the main supply after a three-phase meter and three-phase loads such as an 11 or 22 kW EV charger, motors or compressors. Takes about 4 modules.

A 2-pole RCD cannot be used on a three-phase circuit because it does not see the current of all three phases. If you are not sure what supply you have, read how to tell if you have three-phase power and single-phase vs three-phase.

Three-pole (no neutral) devices exist for special installations without a neutral; homes and businesses normally use 4-pole devices.

Wiring: why the neutral must go through the RCD

The RCD compares the current in every conductor passing through it. On single-phase, phase and neutral must go through; on three-phase, all three phases and the neutral. Common mistakes:

• Neutral bypassing the RCD: it sees an imbalance whenever a load is switched on and trips constantly.
• Mixed neutrals: the outgoing neutral must not be joined to the neutral of another RCD or an unprotected circuit.
• Neutral joined to earth after the RCD: it either trips or cannot measure leakage correctly.
• Supply and load reversed: follow the line and load markings.

The wiring diagram is printed on the device or in its instructions, and the neutral terminal is marked "N". On the Legrand XC10 4-pole devices, for example, the neutral is on the left. An RCD does not replace earthing; see our earthing guide.

Can a three-phase RCD be used on a single-phase circuit?

Some manufacturers allow a 4-pole RCD to be used on one phase + neutral and show which terminals to use. The critical point is the test button: the test circuit is connected between two specific poles inside the device. If phase and neutral go to the wrong terminals, the RCD may still trip on a fault but the test button may not work, so you can no longer check it.

The general rule: buy a 2-pole RCD for a single-phase circuit. If you must use a 4-pole device, wire it only according to the manufacturer's diagram and try the test button afterwards.

Which current rating?

The rated current is what the RCD can carry continuously; it does not trip at that value. Choose it according to the breaker in front of it:

• The rating must not be lower than the upstream breaker. A 25 A RCD does not go under a 40 A main breaker.
• 40 A is the most common rating for a home's main circuit; 63 A where the main breaker is 50 or 63 A.
• A device protecting a single circuit (bathroom, socket group, EV charger) may be 25 or 40 A, again depending on the breaker.

For a home EV charger, a common practice is 40 A, 30 mA, at least type A (2-pole single-phase, 4-pole three-phase). See type A RCD for EV chargers.

30 mA or 300 mA, type AC or type A?

Two more choices matter as much as poles and amps:

• Sensitivity: 30 mA for personal protection in homes, 300 mA for fire protection. See 30 mA or 300 mA.
• Type: type AC detects alternating residual current only; type A also detects pulsating DC. Prefer type A for inverter appliances and EV chargers. See RCD types.

Test button and monthly check

Every RCD has a "T" or "TEST" button that creates an artificial leak. Devices that never trip can stick over time, so a common recommendation is to test monthly.

1. Switch off sensitive equipment such as computers.
2. Press the test button; the RCD should trip immediately.
3. Switch it back on.
4. If it does not trip, it is not protecting you: have it checked and replaced.

Frequent tripping is usually caused by moisture, damaged insulation or the neutral mistakes above. Replacing it with a less sensitive device is not a fix.

RCDs in our shop

Our RCD category has the Legrand XC10 series; ENGN is an authorised Legrand dealer in Bursa, Türkiye.

• Legrand XC10 type AC: 2 and 4-pole, 25, 40 and 63 A, 30 and 300 mA, from {price_LEGRAND_XC10_AC}. Example: 2-pole 40 A 30 mA {vprice_433183}, 4-pole 40 A 30 mA {vprice_433191}.
• Legrand XC10 type A: same options, also detects pulsating DC, from {price_LEGRAND_XC10_A}. Example: 2-pole 40 A 30 mA {vprice_433225}, 4-pole 40 A 30 mA {vprice_433233}.

XC10 devices comply with EN/IEC 61008-1; 2-pole models are 230 V, 4-pole models 400 V. Breakers and surge protectors are in electrical supplies.

This guide follows general IEC practice; local wiring rules always apply. Work inside a consumer unit is dangerous: have a qualified electrician install and test the device.

• → Residual current devices
• → RCD types
• → 30 mA or 300 mA?