Helpful video reference. Mark Allison's "Selectivity (Discrimination) between MCBs Tested! More flash bangs!" demonstrates exactly what happens when protective devices do and do not coordinate correctly, using live short-circuit tests. Mark is a time-served UK electrician with over 25 years in the industry, covering domestic, commercial and industrial work with a specialist focus on renewables.
1. What selectivity means, in plain English
Selectivity, sometimes called discrimination, is a design requirement from BS 7671 Regulation 536.4. It means that when a fault occurs, only the protective device nearest to the fault operates. Everything else stays on.
Think of it like a fuse in a plug. If your kettle element burns out, the fuse in the 13 A plug blows. The socket stays live, the rest of the kitchen ring stays live, and the consumer unit does not see anything unusual. That is selectivity working perfectly at appliance level.
The same principle applies at every level: circuit breaker, RCD, and main switch should each only operate when nothing downstream has already cleared the fault.
2. Where older split-load boards fall down
A typical split-load consumer unit from the 1990s or 2000s has one main switch, then two RCDs, each feeding a group of MCBs. The left-hand RCD might cover sockets; the right-hand one covers lighting.
Now imagine a socket has a tiny earth fault causing 40 mA to flow to earth. The MCB does not trip because 40 mA is nowhere near its overcurrent threshold. The shared RCD on that side, which is set to 30 mA, sees the current and trips. Every socket in the house goes dark.
That is a selectivity failure. The correct device (the circuit's own protection) did not operate because plain MCBs have no earth fault detection. The backup device (the shared RCD) operated instead, and took everything down with it.
3. Time-delay RCDs: one solution in older boards
Before RCBOs became affordable, one common fix was to fit an S-type (time-delayed) RCD as the upstream device. An S-type RCD deliberately delays its trip by around 60--100 milliseconds. Instant G-type devices downstream trip faster, so in theory the downstream device wins the race and the S-type never needs to operate.
This works reasonably well for overcurrent faults, but it has limits. The short delay means the upstream S-type still provides protection if a downstream device fails, but that 60--100 ms delay is long enough for some nuisance trips to persist. It also does not help where the downstream protection is a plain MCB with no earth-fault sensing at all.
4. Why all-RCBO boards give inherent selectivity
An RCBO (Residual Current Breaker with Overcurrent protection) is a single module that combines a 30 mA RCD and an MCB. Every circuit gets its own RCBO. When a fault causes 40 mA to flow to earth on the socket circuit, the socket circuit RCBO trips. Its 30 mA threshold is breached, it opens in milliseconds, and every other circuit stays live.
No shared RCD is involved. Each RCBO operates independently. You get inherent selectivity on every circuit without needing a time-graded scheme at all.
Modern consumer unit manufacturers design their boards around this. The metal-cased boards now required by BS EN 61439-3 almost always ship in all-RCBO configurations.
5. How to look at your own board
Open the door of your consumer unit (the cover plate, not the inner enclosure). Look at the individual circuit devices:
- If you see wide modules each labelled with a circuit name and both an ampere rating and "30 mA RCD" or similar, those are RCBOs. Each circuit is individually protected.
- If you see small, thin modules (MCBs) in groups underneath one or two larger modules (the shared RCDs), your board uses a split-load arrangement.
- If you see very old rewireable fuses, the board pre-dates RCD protection entirely and is overdue for replacement.
Identifying which type you have takes two minutes and tells you whether selectivity is built in.
6. What to ask when getting a replacement quote
When an electrician quotes for a consumer unit upgrade, ask specifically for an all-RCBO board. Respectable quotes will already include this as standard. If a quote offers MCBs under shared RCDs at a lower price, ask why.
An all-RCBO board costs a little more in materials, but it is simpler to wire (no separate RCD rail), faster to fault-find (one tripped RCBO tells you exactly which circuit), and more convenient to live with (a fault on one circuit does not affect the others).
When to call us
If your board keeps tripping circuits that should not be related, or you are getting a consumer unit replacement quote and want an honest second opinion on the specification, call Richard. Sandwich is a 10-minute drive and small consultations are charged at the standard £10 per 10-minute rate.
Thinking about a consumer unit upgrade in east Kent?
Richard specifies all-RCBO boards as standard. Get a fixed written quote for your property.
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