Fuse vs
Circuit Breaker
The difference between a fuse plus a circuit breaker, why UK consumer units moved from one to the other plus what an old fuse board means for your home in 2026.
A fuse is a one-shot sacrificial wire that melts plus breaks the circuit during an overload or short circuit. A circuit breaker (MCB) is a reusable mechanical device that trips plus can be reset. Fuses were the UK domestic standard until the 1990s. MCBs replaced them through the 1990s plus 2000s. Modern UK consumer units use RCBOs which combine an MCB plus an RCD on a single device. Fuses remain in service in many older UK homes plus are not retrospectively illegal but typically fail an EICR.
Same job, two
different approaches
Both protect circuits from overload plus short circuit. Only one can do that without being replaced. The shift from fuses to MCBs is the single biggest change UK consumer units have been through.
The Fuse
A short length of wire calibrated to melt at a known current. Once melted the fuse must be replaced. Used as the standard UK domestic protective device from the 1900s until the 1990s.
- ✗Single use. Once blown the fuse element must be physically replaced. No reset.
- ✗Slow response to short circuit current compared with a magnetic MCB.
- ✗Inconsistent protection after rewiring the rewireable type with the wrong gauge wire.
- ✗No earth fault protection. Cannot detect 30mA shock-level currents.
- ✗Typical EICR result: C2 (potentially dangerous) on a rewireable BS 3036 fuse board today.
The Circuit Breaker
A mechanical device with a thermal plus magnetic trip mechanism. Trips on overload or short circuit plus can be reset by hand. Standard on UK domestic consumer units since the 1990s.
- ✓Reusable. Reset by flipping the switch back up after the fault is cleared.
- ✓Fast magnetic trip on short circuit (within milliseconds) plus calibrated thermal trip on overload.
- ✓Consistent protection across its rated life. No on-site rewiring possible.
- ✓Earth fault protection available when paired with an RCD or built into an RCBO.
- ✓BS 7671 compliant on every UK consumer unit fitted since 1990s. Mandatory device on new installs.
Both do the same job. One does it once.
Every protective device on a consumer unit has the same fundamental job: open the circuit before damage occurs. Damage in this context means overheated cables, ignited insulation or destroyed appliances. The two technologies achieve the goal through completely different mechanisms.
How a fuse works
A fuse is a length of wire (the fuse element) sized so that it carries the rated current indefinitely without overheating but melts cleanly when the current rises significantly above that rating. Three types of fuse have been common in UK domestic boards:
- BS 3036 rewireable fuses. The classic UK domestic fuse from the 1900s to the 1980s. A porcelain or ceramic carrier with a screw at each end plus a length of bare fuse wire stretched between them. When blown the homeowner unscrews the carrier plus rewires it with fresh wire of the correct rating.
- BS 1361 cartridge fuses. Sealed ceramic cartridge with the element inside. More predictable plus safer than rewireable. Used in UK consumer units from the 1960s plus still found in some older 1980s boards plus in service cut-out fuses on the supply.
- BS 88 HRC fuses. High Rupturing Capacity cartridge fuses used in industrial plus commercial distribution boards. Rated to safely interrupt very large fault currents.
The fundamental disadvantage of all three is the one-shot operation. After a fuse blows the circuit stays dead until someone replaces the element. In a domestic context this often means torch-lit fumbling at the consumer unit at night, sometimes with the wrong gauge wire to hand. The BS 3036 rewireable design was particularly prone to being repaired with whatever wire was available which compromised the protection.
How a circuit breaker works
A miniature circuit breaker (MCB) replaces the sacrificial fuse element with two parallel trip mechanisms inside a switching device:
- Thermal trip. A bimetallic strip that bends as it heats. Calibrated to bend far enough to release the trip mechanism only after a sustained overload at typically 1.13 to 1.45 times the rated current. Slow plus deliberate.
- Magnetic trip. A solenoid coil that produces a strong electromagnetic pull at very high current. Calibrated to trip almost instantly (within milliseconds) at typically 5 to 10 times the rated current depending on the MCB curve.
Together these two mechanisms cover the full range of fault scenarios: slow overloads from too many appliances on one circuit (thermal trip) plus fast short circuits from damaged cables or faulty appliances (magnetic trip). After the MCB trips the homeowner identifies plus removes the fault then resets the device by flipping the switch back up. No tools, no wire, no loose components.
Where the modern RCBO fits
An RCBO is the natural evolution of the MCB. It combines:
- The MCB thermal plus magnetic trip mechanisms for overload plus short circuit protection.
- An RCD residual current sensor that detects earth fault current at 30mA threshold for additional shock protection.
Modern UK consumer units increasingly use RCBOs as the standard device on every circuit. This is the technology stack that BS 7671:2018+A2:2022 effectively assumes. The journey from fuse to MCB to RCBO is the journey from "blow plus replace" to "trip plus reset" to "trip plus reset plus shock protection."
From rewireable fuse
to modern RCBO
UK domestic protection moved through four broad eras. Each step added something the previous one lacked.
Rewireable Fuses
BS 3036 rewireable fuses dominate UK domestic boards. Homeowners replace fuse wire by hand. Inconsistent protection if wrong gauge wire is used.
Cartridge Fuses
BS 1361 sealed cartridge fuses become more common in domestic consumer units. Safer plus more predictable than rewireable. Still single-use.
MCBs Take Over
Miniature circuit breakers (BS EN 60898) replace fuses as the UK domestic default. Resettable, consistent, faster on short circuits.
RCDs plus RCBOs
17th Edition mandates 30mA RCD protection on most circuits. Full RCBO boards become the modern UK standard combining MCB plus RCD on each circuit.
Where each device
shows up in 2026
Fuses have not vanished from UK homes entirely. The places they remain are narrow but worth knowing.
Older boards plus the supply head
BS 3036 rewireable fuse boards remain in service in pre-1990s UK housing that has not been upgraded. The service head supply fuse (the one in the meter cabinet sealed by the DNO) is also a cartridge fuse on every UK property regardless of board age. UK plug tops use BS 1362 cartridge fuses inside the plug body itself.
Every UK consumer unit fitted since the 1990s
Every domestic consumer unit fitted in the UK from the 1990s onward uses MCBs as a minimum. Boards fitted from 2008 onward typically combine MCBs with RCDs (split-load) or use RCBOs throughout (full RCBO). Modern installs from 2018 onward are normally full RCBO with optional AFDD plus SPD layered on top.
Four things every homeowner
should know
A fuse board is not retrospectively illegal
An existing rewireable fuse board fitted before BS 7671 changed is not illegal just because the rules tightened. It will however normally fail an EICR.
The plug fuse stays
BS 1362 cartridge fuses inside UK plug tops are not going away. They protect the appliance flex which the consumer unit MCB cannot do alone.
RCBO is the modern default
A modern UK consumer unit fitted in 2026 will typically use one RCBO per circuit. RCBO combines MCB plus RCD into one device.
An old fuse board is the strongest upgrade trigger
If your board still has rewireable fuses you almost certainly need a full upgrade. The board itself is the limiting factor regardless of what else is fine.
Fuse Board Replacement in Milton Keynes
C-Lec Electrical replaces old rewireable plus cartridge fuse boards with modern BS 7671 compliant RCBO consumer units across Milton Keynes, Bedford plus the surrounding Bedfordshire area. EIC plus Building Control notification handled in full.
For the wider context on consumer units, RCBOs, AFDDs plus the regulations behind UK protective devices, head back to our full guide to consumer units where every common question is answered in one place.
Back to the Consumer Units Guide
This article sits inside our complete Consumer Units knowledge base. The hub covers everything from board types plus RCBOs through to landlord requirements plus BS 7671 wiring regulations.
More on consumer
unit protection
If you are still nailing down what a consumer unit actually does, the explainer on what is a consumer unit covers every part of the box in plain English. To work out whether your existing board needs upgrading regardless of whether it has fuses or MCBs, head to do I need to upgrade my consumer unit for the practical decision points. To understand the older split-load RCD design that came after fuses but before full RCBO boards, see what is a dual RCD board. If you need a fuse board replaced in Milton Keynes or Bedford, our consumer unit upgrades service page is the fastest route to a quote.