What a residential consumer unit should contain

A consumer unit is not just a box full of switches. It is the brain of your home's entire electrical installation. We have already written about the signs that your panel needs replacing. The obvious next question is: if I am replacing it anyway, what should a properly built consumer unit actually contain?

The short answer is: a great deal more than a row of breakers sitting side by side. A modern panel, built to current practice and standards (the IEC/HD 60364 series), is a layered protection system in which every component has a clear job and a well-defined area of responsibility.

1. The main switch

This is the single point from which power to the whole property can be cut. It has to be easy to reach, clearly labelled, and rated for the total current the installation draws. In modern panels this is usually either a main RCD-type incomer or a circuit breaker paired with a front-end RCD, depending on the protection scheme you agree with your electrician.

2. MCB — Miniature Circuit Breaker

The MCB is the classic breaker fitted to each individual circuit: lighting, sockets, cooker, water heater and so on. Its job is to protect the cable and the equipment against:

• overload — when a circuit carries more current than it was sized for, over a sustained period;
• short circuit — a very large current appearing instantly, usually between line and neutral or line and earth.

An MCB does not protect people against electric shock, and it does not detect arc faults. It only "sees" the current flowing through it, and acts on two thresholds: a thermal one for overload and a magnetic one for short circuit. That is where the tripping curve comes from (B, C or D), which we come back to below.

3. RCD — Residual Current Device

An RCD measures the difference between the current going out on the line conductor and the current returning on the neutral. Normally that difference is zero. If current starts leaking — through the body of someone who touches a metal enclosure made live by an insulation fault, for example — the RCD detects the imbalance and breaks the circuit within a fraction of a second.

The essential difference from an MCB: an RCD protects people, not the cable or the appliance. An RCD will not trip on overload, and will not trip on a line-to-neutral short circuit. You still need the MCB for that. The two devices work together; neither replaces the other.

For dwellings, 30 mA sensitivity is the standard requirement for shock protection, while "wet" circuits — bathroom, kitchen, outdoors — can be covered by a 10 mA device.

4. AFDD — Arc Fault Detection Device

This is the newest component, and for many homeowners the least familiar. An AFDD analyses the current waveform to detect electrical arcing: intermittent sparking caused by, say, a loose conductor in a socket, degraded insulation, a cable nicked by a nail driven into a wall, or a poor connection inside a junction box.

Arcing is dangerous precisely because it often does not draw enough current to trip an MCB, and does not produce an earth leakage that would trip an RCD. It can smoulder for hours or days inside a wall, and it is one of the common causes of fire in installations that appear perfectly healthy. The AFDD is the device specifically designed to recognise the electrical signature of these arcs and cut the circuit before it becomes a fire.

In short, the three devices are not interchangeable. They complement each other:

A well-built modern panel uses all three where they are relevant, not just MCBs, which is still what you find in most older installations.

5. The sizing principle most people ignore

This is probably the most important thing to take away from this article, and something any serious electrician checks every time:

Both the rated current (I_n) and the tripping curve (B, C or D) of a protective device are not chosen according to how "important" the circuit is, or what the client would prefer. They are chosen according to the weakest element in that circuit — whether that is the accessories fitted (sockets and switches), the cross-section of the cable, or the appliance connected directly at the end of the circuit.

In practice, the protective device has to be sized so that it trips before the most vulnerable element in the chain is damaged or becomes a hazard. A few concrete examples:

• A 1.5 mm² cable carries considerably less current than a 2.5 mm² one. If you fit a 16 A breaker on a 1.5 mm² circuit "to leave some headroom", the cable can heat beyond its permitted limit long before the breaker trips, because the breaker only knows its own limit, not the cable's.
• If an appliance with its own lower current limit is connected directly at the end of a circuit — a fixed appliance, a pump, a water heater — then the device must be chosen according to whichever limit is lower, the cable's or the appliance's. Not the cable's, if the appliance is the weaker one.
• The tripping curve also has to match the real behaviour of the load and the capability of everything downstream, rather than being picked out of habit. Curve B is the sensitive one, for resistive circuits such as lighting and sockets. Curve C suits loads with a higher inrush, such as small motors or certain appliances. Curve D is for loads with very high starting peaks.

6. Other components not to forget

• Separate earth (PE) and neutral (N) bars, clearly labelled, with every conductor terminated individually rather than doubled up under the same terminal with other wires.
• Surge protection devices (SPD), particularly if the property has an overhead supply, sensitive electronics, or sits in an area exposed to lightning.
• A contactor or time relay, if you are on a differential tariff or want automatic control of certain circuits such as a water heater or heating.
• Clear labelling of every circuit, either directly on the panel or on an attached schedule. It is a cheap detail that saves hours on any future work.
• Spare capacity for later additions such as an EV charge point, solar panels or extra air conditioning. A panel filled to the last way on the day it is installed is a panel that will need replacing again sooner than it should.

7. Using AI instead of a real inspection

The progress in this area is remarkable, but an AI assessment is made purely through image recognition and processing, and that limitation matters:

• Misreading a value on an RCD, or the breaking capacity marked on a protective device, can create a false sense of safety.
• A technician can identify conductor cross-section and cable type reliably, and can feel whether a device is running warm.
• A technician can spot corrosion on terminal screws more easily, and can assess the conditions the installation actually operates in.

For those reasons we always recommend a check carried out by a technician over an "AI inspection".