The split-load boardHow many neutral bars on a split-load board?

A split-load board shares RCDs between groups of circuit breakers. Ours is a 10-way board with the same main switch as before, plus two RCDs:

• Both RCDs are 30 mA, Type A (the sine wave plus pulsating-DC symbol), and each is rated as an 80 A isolator: it can switch 80 A through. (The main switch carries 100 A.)
• The RCDs can be slid along the rail to space the board out.
• The busbars come loose in the box: two with 4 spines and one with 5. Read the manufacturer's instructions before cutting any down to suit. If they say don't, doing it voids the warranty.
• Plastic busbar end covers are included to go over unused spines. Leave the plastic sleeve on the busbar itself.
• The thick cables between the main switch and the RCDs come pre-fitted from the factory.

How it's fed

• Two brown tails leave the bottom of the main switch: one to the top of each RCD. So both RCDs are fed directly from the main switch, and turning the main switch off turns everything off.
• On the neutral side, one cable goes from the main switch to a neutral bar, and another goes to the RCDs. Each RCD's neutral out goes up to its own neutral bar.
• The loose busbars fit into the line terminal at the bottom of each RCD. Then the CBs drop onto the spines and hook onto the DIN rail.
• A circuit is just line joined to neutral through a load. Line comes from the RCD's busbar through the CB. Neutral must go back to that same RCD's neutral bar.

The unprotected side

Our example: an SWA (steel wire armoured) cable out to a garage or shed, with its own small consumer unit there.

• The circuits in the shed may well need RCD protection, so you fit RCBOs in the shed board.
• If the shed's feed cable were on RCD 1 in the house, a bit of damp in the shed could trip RCD 1 and knock off every circuit in the house on that side. That isn't against the rules, just frustrating.
• So you might decide the distribution circuit from house to shed doesn't need an RCD (the SWA has an earthed metallic covering), and put its CB on the unprotected side, between the main switch and RCD 1. Its neutral goes in neutral bar 1.

RCBOs on a split-load board

• An RCBO can go on the unprotected side. Its fly lead goes into neutral bar 1, and the circuit's line and neutral go into the RCBO.
• Never put an RCBO under an RCD. That puts two 30 mA RCDs in series and they end up in conflict: an earth fault on that circuit might trip the RCD as well as, or instead of, the RCBO, depending on which one is slightly more sensitive from the factory. You lose selectivity and waste the money you spent on the RCBO.