The short answer
A rewireable fuse contains a piece of rated wire that melts when the current through it exceeds its design limit, breaking the circuit. It protects against overload and short circuit but provides no protection against earth leakage (the fault behind most fatal electric shocks) and must be manually rewired after operation. An MCB (miniature circuit breaker) does the same overload and short-circuit job electromechanically and resets by flipping a switch, but still provides no earth-leakage protection. An RCBO combines the MCB function with an RCD (residual current device) in one unit — it protects against overload, short circuit and earth leakage on a single circuit and resets by flipping a switch. Modern consumer units under BS 7671 use MCBs with shared RCDs, or RCBOs per circuit; rewireable fuses are no longer installed in new domestic work.
The device in each circuit position has evolved from simple fuse wire to sophisticated electronic protection. Here is what each generation does and what the practical differences mean.
Device comparison
- Rewireable fuseOverload/short protection only; manual rewire
- MCBOverload/short protection; resettable switch
- RCDEarth leakage protection; no overload/short
- RCBOOverload + short + earth leakage; per circuit
- Absent in rewireable fuse boxesEarth leakage (shock) protection
Rewireable fuses — how they work and what they do not do
A rewireable fuse is the oldest of the three technologies. It consists of a ceramic or porcelain carrier holding a length of rated fuse wire. When the current through the circuit exceeds the wire's rating, the wire heats up and melts, breaking the circuit and stopping current flow.
What this protects against:
- Overload: if too many appliances run on a circuit and the total current exceeds the fuse rating, the wire melts.
- Short circuit: if a live conductor touches a neutral or earth conductor directly, the resulting high current melts the fuse.
What it does not protect against:
- Earth leakage / residual current: a dangerous leakage to earth — such as a person touching a live conductor, or a faulty appliance with a live-to-casing fault — may not produce enough fault current to blow the fuse. This is the scenario behind the majority of fatal electric shocks in domestic settings. A rewireable fuse provides no protection in this situation.
After the fuse operates, it must be manually rewired with new fuse wire of the correct rating — a process that requires the correct wire and some practical skill. A wrong-rated wire (too thick) can mean a circuit is no longer protected at its correct current rating.
MCBs — the resettable overload breaker
A miniature circuit breaker (MCB) performs the same overload and short-circuit protection as a rewireable fuse but does so electromechanically, by a bimetallic strip (for sustained overload) and a magnetic coil (for instantaneous short-circuit disconnection). When it trips, it flips visibly to an 'off' position and can be reset by switching it back on once the fault is cleared.
MCBs are classified by their tripping characteristic — Type B (trips at 3–5 times rated current) is standard for most domestic circuits; Type C (5–10 times) is used where a higher surge current is expected at switch-on, such as motor loads.
What MCBs do not do: they provide no earth leakage protection. A standalone MCB board (with no separate RCD) offers no residual-current protection. The 16th and 17th Editions of BS 7671 progressively required RCD protection to be added to consumer units, which led to the dual-RCD board arrangement — MCBs for overload, a shared RCD for earth leakage protection across the circuits in its group.
| Device | Overload / short circuit | Earth leakage (RCD) | Resets how? |
|---|---|---|---|
| Rewireable fuse | Yes (fuse wire melts) | No | Manual rewire |
| MCB | Yes (electromechanical) | No | Flip switch |
| RCD (standalone) | No | Yes | Flip switch |
| RCBO | Yes | Yes (per circuit) | Flip switch |
| MCB + shared RCD (dual-RCD board) | Yes (MCB) + Yes (shared RCD) | Yes (group protection) | MCB or RCD flip switch |
Functional comparison. Sources: IET BS 7671 18th Edition; Electrical Safety First guidance.
RCBOs — combined protection per circuit
An RCBO (residual current breaker with overload protection) is a single device that combines the MCB function — overload and short-circuit protection — with an RCD function — detection of current leaking to earth — in one unit. It is fitted per circuit and provides:
- Overload protection (bimetallic strip or equivalent).
- Short-circuit protection (magnetic coil).
- Earth leakage detection, typically at 30 milliamps sensitivity (the threshold below which the risk of a fatal shock is substantially reduced).
Because each circuit has its own RCBO, a fault on one circuit trips only that device. The rest of the board continues to supply power. This is the key practical advantage over the dual-RCD arrangement, where an earth leakage fault trips the RCD protecting the whole group.
RCBOs are physically wider than MCBs but fit into standard consumer unit busbar systems. A full RCBO consumer unit has a row of RCBOs — each with its own test button — replacing what was previously a row of MCBs plus separate RCDs.
Frequently asked questions
Why don't new consumer units use rewireable fuses?
Rewireable fuses provide no protection against earth leakage — the fault condition most associated with fatal electric shocks. BS 7671 has required residual-current protection (RCDs or RCBOs) on most domestic circuits for many years. A new installation with only rewireable fuses would not pass testing or receive an EIC.
Can I replace an MCB with an RCBO?
Often yes, provided the board's busbar system is compatible — many modern consumer unit designs accept either MCBs or RCBOs in the same slot. This allows an upgrade from a shared-RCD arrangement to per-circuit RCBO protection without replacing the whole board. A registered electrician can assess whether your specific board supports this and carry out the work safely.
What does the test button on an RCD or RCBO do?
The test button simulates an earth leakage fault to verify that the device operates and trips correctly. Testing is part of the commissioning and periodic inspection process. You should not press the test button without being aware that it will cut power to the circuits that device protects — make sure any sensitive equipment (medical devices, fish tanks, computers with unsaved work) is not affected.
Sources & further reading
- IET — BS 7671 18th Edition wiring regulations
- Electrical Safety First — consumer unit replacement best practice guide
- NICEIC — understanding your consumer unit
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property and installation. They are guidance, not a quotation.