Understanding your consumer unit (fuse box)

Helpful video reference. John Ward of Flameport Electrical, Poole, explains old and new consumer units in his video "John Ward Talks About Old Consumer Units (Fuse Boards)". John is a widely respected UK electrician and educator whose content is often cited by professionals in the trade. His explanation of old rewireable boards and why they fall short of current requirements is particularly useful.

1. The main switch

The large double-pole switch — usually on the far left of the consumer unit — isolates every circuit in the house in one action. Turning it off is the first step before any electrical work. It does not isolate the meter tails: the thick cables connecting the board to the meter are still live at the meter end regardless of switch position.

Some older properties have a separate main isolation switch elsewhere, sometimes near the meter itself.

2. MCBs — miniature circuit breakers

Each MCB in your consumer unit protects one circuit. The number on the front is its current rating. Typical ratings and what they protect:

• 6A — lighting circuits
• 16A — immersion heater, individual appliances
• 20A — radial socket circuits, garage
• 32A — ring final socket circuits (the ones feeding most of your plug sockets)
• 40A or 50A — electric shower
• 32A or 40A — electric cooker or oven

When a circuit is overloaded or short-circuits, the MCB trips to OFF. Resetting it is straightforward — see the guide to resetting a tripped breaker. The MCB itself is fine after a trip; it is designed for this.

3. RCDs — residual current devices

An RCD is the wider device with the small button marked T (for test). It monitors the difference between current flowing out on the live conductor and returning on the neutral. If those differ by more than 30 milliamps — because current is leaking to earth, possibly through a person — the RCD trips in under 40 milliseconds. That speed is what keeps a shock survivable.

On older boards, one large RCD often covers half the board. This means a single faulty appliance can take out multiple circuits at once. It is one of the reasons modern boards use RCBOs instead.

4. RCBOs — combined protection

An RCBO (residual current circuit breaker with overcurrent protection) does the job of both an MCB and an RCD in a single device. Each circuit gets its own RCBO, so a fault on one circuit only trips that circuit and nothing else. Modern consumer units increasingly use RCBOs throughout. They cost more, but the improvement in usability and fault diagnosis is significant.

5. Spotting an old rewireable fuse board

If your board does not have rows of toggle switches but instead has ceramic holders with small porcelain pull-out carriers inside, you have a rewireable fuse board from the 1960s, 70s or earlier. The thin wire inside each carrier blows when that circuit is overloaded.

These boards have no RCD protection at all. They predate Part P Building Regulations and BS 7671's current requirements. They are not automatically illegal to keep, but they offer dramatically lower protection than a modern board. If you have one and are considering any electrical work, a fuse box upgrade is worth discussing at the same time.

6. Reading the circuit labels

The inside of the consumer unit cover should have a chart identifying every circuit: which breaker covers which rooms, which socket ring, which appliance. If it is blank, ask an electrician to label it. It takes twenty minutes and means you can isolate the right circuit without guessing.

It also means the emergency services know what they are dealing with if they ever need to isolate your installation quickly.

When to call us

If you have an old rewireable board, have never had an EICR, or are planning any electrical work and want to know where you stand, a fuse box assessment is the right starting point. In Sandwich and east Kent, Richard can inspect the board and give you a straight answer: working well, needs attention, or replace it now.