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Protective devicesWhat's the difference between an RCD and an RCBO?
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Consumer units contain devices that protect the final circuits (cable and equipment) against overloads, short circuits and earth faults.
| Device | Stands for | Overload | Short circuit | Earth fault (by ADS) | Earth leakage (residual current) |
|---|---|---|---|---|---|
| CB (MCB) | Circuit breaker, now just called "CB" | ✔ | ✔ | ✔ if Zs is low enough | – |
| Fuse | e.g. BS 88, BS 3036, BS 1362 | ✔ | ✔ | ✔ if Zs is low enough | – |
| RCD (RCCB) | Residual current device | – | – | ✔ | ✔ |
| RCBO | Residual current circuit breaker with overcurrent protection | ✔ | ✔ | ✔ | ✔ |
OCPD = overcurrent protective device. It's the family name for CBs, RCBOs and fuses.
How an RCD works
- It compares the current in the line with the current in the neutral. If some current is leaking to earth they don't match, and it trips.
- A domestic RCD is typically rated 30 mA for additional protection, e.g. an 80 A device protecting circuit breakers totalling up to 80 A.
- Press the test button six-monthly. That is what the RCD test notice at the board says (Reg 514.12.2): “Test six-monthly by pressing the relevant test button(s)”.
- A small amount of normal earth leakage is fine, but keep it below about 30% of the rating to avoid nuisance tripping. That's why too many circuits on one RCD cause problems.
Worked example: nuisance tripping
RCD rating IΔn = 30 mA 30% of 30 mA = 0.3 × 30 = 9 mA → keep total standing earth leakage on that RCD below ~9 mA
- Split-load board (CBs grouped under RCDs): if one circuit has an earth fault, the RCD trips every circuit on that side. An RCBO board trips only the faulty circuit. That costs more, but prices are coming down. A split-load board is still compliant, so you can't tell a client they must upgrade.
- Overload example: a 4-way extension lead full of 13 A appliances in one socket, or a kettle and a toaster on the same double socket. That causes overheating and melted accessories.
Protects against transient overvoltages from lightning strikes and from the power coming back on after a power cut. The kind of surge that kills laptop and games console chargers. There are three types, T1, T2 and T3. Worth looking up what each is for and where it goes. Once an SPD has diverted a big surge it may need replacing, and its indicator shows this.
Detects small arcs, where electricity jumps between loose or poor contacts, like a plug not fully pushed into a socket, which can start fires. AFDDs are required on single-phase AC final circuits supplying socket-outlets up to 32 A in higher-risk residential buildings, HMOs (houses in multiple occupation), purpose-built student accommodation and care homes. A higher-risk residential building is one “over 18 m in height or in excess of six storeys, whichever is met first”. In all other premises they are recommended.
BS 7671 · Reg 421.1.7 AFDDs · p.93BEAMA guides (free online) · AFDDs and SPDs: where and why they're installed
Breaking capacity (Icn) is the largest fault current a device can safely break. A typical domestic CB or RCBO is 6 kA (6,000 A), and some are 10 kA or more. The DNO's main cut-out fuse is rated much higher, e.g. 33 kA.
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