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Three-phase distribution boards, MCBs and RCDs
How a 400/230 V board feeds single-phase final circuits from three phases, why the neutral current depends on balance, and how MCBs and RCDs protect people and cables.
From three phases to 230 V circuits
A TN-S supply brings three phases L1, L2 and L3, a neutral N and a separate protective earth PE. Between two phases there are 400 V; between a phase and neutral there are 230 V. Final circuits such as lighting, sockets and water heaters are connected from one phase to neutral.
A load connected by mistake between two phases receives 400 V and is destroyed, so the wiring and labelling of each circuit matter.
Balancing the phases
Single-phase circuits are spread over L1, L2 and L3 so that each phase carries about the same current. When the loads are balanced, the three neutral currents cancel because they are 120° apart. When one phase carries more, the difference flows back in the neutral. Good balance keeps the neutral current, the voltage drop and the supply transformer loading low.
MCBs protect the cables
Each final circuit has a miniature circuit breaker sized for its cable. On overload the thermal release trips after a delay that gets shorter as the current rises; on a short circuit the magnetic release trips at once. The B curve trips magnetically at 3–5 times its rating, C at 5–10 times and D at 10–20 times.
With good selectivity a fault on one circuit trips only that circuit’s MCB, and the rest of the board stays on.
RCDs protect people
A residual current device compares the current going out on the lines with the current coming back on the neutral. If the difference, which is current leaking to earth, reaches its rated residual current IΔn, it trips. A 30 mA RCD gives additional protection against electric shock for socket and lighting circuits.
An RCD also trips if a neutral is connected to earth after the RCD, because the return current then bypasses it. Press the test button regularly. If an RCD trips at every reset, switch off the final circuits one by one to find the faulty one, then test its insulation.
Try it yourself
Choose “Distribution board” in the simulator. Read the phase and neutral currents, switch one MCB off and watch the neutral current rise, then press the RCD test button and simulate an earth fault.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.