RCBOs and how they connectWhere does an RCBO's neutral go?

An RCBO looks almost identical to a circuit breaker. The difference is inside: it has an RCD built in, using a toroid coil rated in milliamps.

• Ours is a B32, 30 mA: a Type B 32 A circuit breaker with a 30 mA RCD included, which makes it right for additional protection.
• The RCD part is Type A: the symbol is a sine wave plus a pulsating-DC (half-wave) symbol. A plain sine wave on its own is the Type AC mark. Type A is now the most common type and is right for most circuits. Types B and F exist for more specific circuits.
• You can still buy Type AC, but note that Chapter 53 of BS 7671 limits where AC can be used. Type A is the go-to.

BS 7671 · Chapter 53 · Reg 531.3.3 · RCD types (Type AC only where the load has no DC components)

Wiring a CB versus an RCBO

• The RCBO clips on and is tightened exactly like a CB (with a torque screwdriver).
• The blue fly lead connects to the neutral bar, in the terminal matching its way (ours is in way 2, so neutral terminal 2). It comes with slack. You can shorten it, but many people leave it so the RCBO can be moved around the board later. That's personal preference.
• The CPC still goes to the earth bar, the same as every other circuit.

Why both line and neutral go through it

The toroid coil only works if both the line and the neutral pass through it. By Kirchhoff's current law, a healthy circuit has the same current in the neutral as in the line.

Healthy:      line 10 A → neutral 10 A   (Kirchhoff: in = out)
                difference = 10 − 10 = 0 A  → stays on
Earth fault:  1 A leaks into an earthed part
                neutral = 10 − 1 = 9 A
                difference = 10 − 9 = 1 A = 1000 mA
                1000 mA ≫ 30 mA  → trips very quickly
A CB has no toroid coil, so it can't see this imbalance.