A plain-English guide to the heart of your home's electrical system — MCBs, RCDs, and how to know if your consumer unit needs upgrading.
Most people call it a fuse box, but the correct term is a consumer unit — and it's the most important piece of electrical equipment in your home. Everything in your property's electrical system flows through it, and it's the first line of defence against electrical faults.
A consumer unit is the main distribution board for your home's electricity. It takes the incoming supply from the meter and splits it into individual circuits — one for lighting, one for sockets, one for the cooker, one for the shower, and so on. Each circuit has its own protective device that will trip if something goes wrong.
The large switch at the top or side of the consumer unit is the main switch. This isolates the entire installation from the supply — useful when you're doing work on the electrics or in an emergency.
MCB stands for Miniature Circuit Breaker. These are the individual switches in your consumer unit — one for each circuit. They protect against overload from too many appliances on one circuit, and short circuits from a sudden surge of current.
When an MCB trips, you can reset it by switching it back on — but if it trips again immediately, there's a fault on that circuit that needs investigating.
RCD stands for Residual Current Device. Unlike an MCB which protects the wiring, an RCD protects people. It detects tiny imbalances in current and trips within 30–40 milliseconds — fast enough to prevent a fatal electric shock.
Signs that your consumer unit may need attention include a wooden back or old materials, rewirable fuse wire instead of circuit breakers, no RCD switches, circuits tripping without obvious cause, or the unit feeling warm to the touch.
Since 2016, BS 7671 has required that consumer units in domestic properties are housed in a non-combustible metal enclosure. If your consumer unit is made of plastic, it may not comply with current regulations.
At Zeno Electrics, we carry out consumer unit upgrades across Hertfordshire and surrounding counties. Get in touch for a free, no-obligation quote.