Fuse box vs consumer unit: what’s the difference?

The short answer

They are the same piece of equipment: the box where the incoming supply is divided into the separate circuits in a building, each with its own protective device. “Consumer unit” is the technical term, and the one used on reports and certificates. “Fuse box” is the older everyday name from when every circuit was protected by a fuse, and people still use it for any board. “Fuse board” and “distribution board” are used too.

The names are interchangeable in conversation. What differs between one board and another is the type of protection inside, and that is worth understanding. For replacement work, see consumer unit replacement.

Three generations of circuit protection

Rewireable fuses

The oldest boards use pull-out carriers holding a length of fuse wire. When too much current flows, the wire melts and the circuit goes dead. The protection only works if wire of the correct rating is fitted, and a fuse that keeps blowing is a sign of a fault on that circuit rather than a fuse problem.

Cartridge fuses

Some boards use sealed cartridge fuses in their carriers instead, similar in principle to the fuse in a plug. They are more consistent than fuse wire, but a blown cartridge still has to be replaced.

Miniature circuit breakers

A miniature circuit breaker, or MCB, is a switch that turns itself off when a circuit is overloaded or short-circuited. Once the cause has been dealt with, it is switched back on; nothing needs replacing. Fuses and MCBs protect the wiring from too much current. Neither is designed to protect a person from an electric shock.

RCDs and RCBOs

A residual current device, or RCD, does a different job. It compares the current going out along a circuit with the current coming back. If some is missing, it is flowing to earth somewhere it should not, possibly through a person, and the RCD disconnects the supply very quickly. RCDs used to protect people are rated at 30 mA.

An RCBO combines both jobs in one device for a single circuit: overcurrent protection like an MCB, and residual current protection like an RCD. A standalone RCD, by contrast, gives no overcurrent protection and relies on the breakers beside it for that.

The difference is felt when something goes wrong. Where one RCD covers a group of circuits, leakage on one of them switches off all of them. Where every circuit has its own RCBO, only the affected circuit goes off.

How to tell what you have

You can usually tell from the front of the board, without opening or removing anything:

• Pull-out carriers, often white, brown or black, mean fuses: either fuse wire or cartridges.
• A row of small switches means circuit breakers.
• A device with a button marked T or Test is an RCD or RCBO.
• One or two wider test-button devices with several breakers beside each usually means circuits sharing RCDs in groups.
• A test button on every breaker usually means RCBOs.

Keep the cover on. Parts behind it stay live even with the main switch off, and working out what type of board you have never requires opening it.

Does the difference matter?

Day to day, not very much. It matters in three situations:

• When something trips or blows. A breaker or RCD that trips again straight after being reset, or a fuse that blows repeatedly, is pointing to a fault that needs finding.
• When you add to the installation. New circuits generally need RCD protection, which a board of fuses cannot provide.
• When you read a report or ask for a quote. Knowing the type explains observations about protection on an EICR, and helps an electrician understand what they are pricing.

Having an older type does not by itself mean the board needs replacing. The reasons that do are covered in When should a fuse box or consumer unit be replaced?

Whatever you call it

Describe what you can see on the front of the board, and I can tell you what type it is and whether anything about it needs attention.