Fitting an MCB and RCD in a consumer unit

In the price list, this is the electrician's labour to fit and connect one device in an existing or new consumer unit. For both the MCB and the RCD the work typically includes:

• isolating the circuit, locking it off so it cannot be switched back on, and proving that it is dead,
• clipping the device onto the DIN rail in the consumer unit,
• connecting the incoming and outgoing conductors, or linking the device to other devices with a busbar,
• tightening the terminals to the torque the manufacturer specifies,
• labelling the circuit on the consumer unit chart,
• switching on and carrying out a functional check, which for an RCD includes pressing the test button.

The work does not include the device itself, a new consumer unit, new cables to the rooms or the certification. If the whole consumer unit is being replaced or a new one is being installed, that is a separate item, installing and connecting a consumer unit in a flat. Testing and certification are priced separately.

MCB

A miniature circuit breaker monitors how much current flows through the circuit. Under a sustained overload, for example when a kettle, an oven and a fan heater all run on one radial socket circuit, it trips after a delay. In a short circuit it trips almost instantly. The MCB is marked with a code such as B6 or B32. The letter is the tripping curve and the number is the rated current in amps. Homes normally use type B, while type C suits motors and appliances with a high inrush current. In UK homes, lighting circuits are usually on 6 A, a ring final circuit for sockets on 32 A, and radial socket circuits on 16 A or 20 A, depending on the cable. The MCB rating is always chosen to suit the cable size and how the cable is installed. The MCB must never be rated higher than the cable can carry.

RCD

An RCD compares the current flowing into the circuit with the current returning. If the difference is greater than its sensitivity, it trips. Homes use a sensitivity of 30 mA. Under the IET Wiring Regulations (BS 7671), additional protection by a 30 mA RCD is required for socket outlets in dwellings, for circuits in bathrooms and shower rooms, for outdoor sockets and for cables buried shallow in walls. An RCD on its own does not protect against overload or short circuit, so an MCB must always be fitted in series with it.

RCBO, the combined device

There is also a device that combines an MCB and an RCD in one unit, the RCBO. The advantage is that a fault on one circuit only trips that circuit, not half the home. It takes up little space, but costs more. Many new consumer units in the UK are now fitted with an RCBO on every circuit.

• An older property without an RCD. Many flats and houses wired decades ago have a consumer unit with no RCD, sometimes even an old fuse box with rewireable fuses. Adding RCD protection is one of the most sensible investments in safety.
• A new circuit. An induction hob, cooker, electric shower, air conditioning, immersion heater or underfloor heating mat gets its own MCB or RCBO.
• Replacing a worn device. The MCB trips for no reason, the RCD does not respond to its test button, or the device is burnt or physically damaged.
• Rewiring. In a new installation, circuits are split up and RCD protection is added according to the design.
• New equipment with electronics. A heat pump, EV charger or solar PV system may require a different type of RCD, as stated in the manufacturer's instructions.

The main fuse in the cut-out before the meter is a different matter. It belongs to the distribution network operator (DNO), it is sealed, and only the DNO or someone it authorises may remove it. Any upgrade of the main fuse, for example for an EV charger, has to be arranged with the DNO. This is also related to the external low voltage connection to the meter.

A small job in the consumer unit can be done quickly if the electrician has all the information beforehand. It is worth preparing the following:

• A photo of the open consumer unit. A photo of the devices behind the front door is enough, with no cover removed. From it the electrician can judge whether there is a spare way for a new device, which make the board is and what condition it is in.
• A list of appliances. Write down what you want on the new circuit, for example an induction hob, oven, air conditioning or water heater, and their power ratings.
• A description of the problem. If something keeps tripping, note when it happens, with which appliance, and whether it is the MCB or the RCD.
• Previous certificates. If you have an earlier Electrical Installation Certificate or condition report, the electrician can see how the installation was wired and what values were measured.
• Access to the consumer unit. Clear the space in front of it and expect part or all of the home to be without power during the work. Keep the fridge and freezer closed.

In a block of flats, the meter and the isolator for each flat are often in a communal meter cupboard. If the whole flat has to be switched off there, the electrician needs access, so find out from the managing agent or caretaker who holds the key.

Inspecting the consumer unit

The electrician first checks the earthing arrangement, the cable sizes, whether there is a spare way for a new device, and whether the condition of the existing devices and terminals allows anything to be added. New devices must be the same make and range as the consumer unit, because mixing brands is not acceptable. In older boards it often turns out that adding one RCD also means altering other circuits or replacing the whole consumer unit, for instance when the existing board is plastic and current practice calls for a non-combustible enclosure.

Isolating and proving dead

Before any work, the relevant MCB or main switch is turned off, locked off, and a voltage indicator is used to prove that the conductors are dead. In practice there may be a supply from another source in the board, so after switching off one MCB some conductors can still be live. That is why testing is always done.

Fitting and connecting

The device is clipped onto the DIN rail. With an RCD, both the line and neutral conductors of every circuit it protects are routed through it. The neutral of a protected circuit must not be connected to the neutral of an unprotected circuit, or the RCD will trip for no obvious reason or fail to work at all. The protective (earth) conductor does not pass through the RCD. The conductors are stripped to the correct length and tightened to the specified torque. A loose terminal heats up over time and is one of the common causes of fires in consumer units.

Testing and labelling

After switching on, the electrician presses the RCD test button and confirms that it trips. The insulation resistance and RCD operation are also measured with a test instrument, which shows the actual tripping time at the rated current. The test button alone does not tell you how fast, or at what current, the RCD trips. Finally the circuit chart is filled in, so it is clear which device controls what. Depending on the work, the electrician issues a Minor Electrical Installation Works Certificate or, for a new circuit, an Electrical Installation Certificate.

• Type AC RCD. Responds to alternating residual current. Current wiring rules restrict it to fixed equipment where the load has no DC component, so it is no longer suitable for most circuits in modern homes full of electronics.
• Type A RCD. Also responds to pulsating DC from washing machines, induction hobs and power supplies. It is now the usual minimum for homes.
• Type F or B RCD. Used for equipment with variable speed drives and for some heat pumps and EV chargers. The required type is set by the equipment manufacturer's instructions.
• Double-pole and four-pole RCD. Double-pole for single-phase circuits, four-pole for three-phase supplies, which are less common in UK homes.
• Single-pole MCB. The most common type for lighting and socket circuits. Typically 6 A for lighting, 32 A for a ring final and 16 A or 20 A for radials, always according to the cable.
• MCB for high-power circuits. For a cooker, an electric shower or a heat pump, with a rating chosen to suit the load and the cable.

With several circuits, putting the whole home under one RCD is not recommended. If it trips, you lose lights and the fridge at the same time. It is better to split circuits between two RCDs, as in a split-load board, or to use RCBOs.

• A bigger MCB so it stops tripping. It happens that someone swaps a 16 A MCB for a 20 A or 32 A one to stop it tripping. The cable is still the same and overheats under overload. This is a serious fire risk.
• Borrowed neutrals. The neutral of a protected circuit is connected to the neutral of another circuit, and the RCD then trips at random.
• Type AC RCD for appliances with electronics. It may not respond to some faults.
• The whole home under one RCD. The normal leakage of many appliances adds up and the RCD trips even when there is no fault.
• Loose terminals. The terminal heats up, the device discolours and can catch fire.
• Mixed brands. A device from another manufacturer is squeezed into the board. It may not fit the busbar properly and the board no longer matches its tested design.
• DIY replacement. The homeowner swaps a device following a video, with no testing and no certificate. After a fault or an injury there is then no record that the installation is safe.
• No circuit chart. When something goes wrong, nobody knows which MCB controls what.

Electrical work must comply with BS 7671 and be carried out by a person competent to do it safely. In practice this means a qualified electrician registered with a competent person scheme such as NICEIC, NAPIT, ELECSA or Stroma. In Scotland, SELECT is a common trade body. In England and Wales, installing a new circuit or replacing a consumer unit is notifiable building work. A registered electrician can self-certify it and you then receive a Building Regulations compliance certificate. Replacing a single device like for like is not notifiable, but it still has to be tested and recorded on a certificate. In other parts of the UK, check with the local authority whether the work needs building approval. Keeping the installation in good order over the years is covered on the page electrical installation condition report for a flat. A change of the main fuse needs the DNO.

Keep all certificates. Your insurer may ask for them after a claim, buyers' solicitors ask for them when you sell, and landlords need to show the installation has been checked when letting a property.

Fitting is a job for the electrician, but as a homeowner you can safely check a few things without removing the consumer unit cover:

1. Look at the consumer unit and find out whether it has an RCD at all. It has a test button marked T or Test and a sensitivity rating, for example 30 mA.
2. Press the test button. The RCD must trip immediately. Then switch it back on. The standard RCD notice says to test six-monthly, and the device instructions may give their own interval.
3. If the RCD does not trip when the button is pressed, do not keep trying, do not use the circuit for anything sensitive, and call an electrician.
4. If the RCD trips on its own, unplug all appliances, switch the RCD back on and plug the appliances back in one by one. If it trips with one particular appliance, the fault is probably in that appliance.
5. If an MCB trips, check whether several high-power appliances are running on one circuit at the same time. If it trips even with no load, it is a fault for an electrician.
6. Label which MCB controls which room if the chart is missing. Switch the MCBs off one at a time and see which lights and sockets go off.
7. Never remove the consumer unit cover, swap devices or reconnect conductors yourself.

If the consumer unit is warm to the touch, smells of burning or the devices are discoloured, turn off the main switch and call an electrician.

• the devices sit firmly on the rail and the consumer unit cover closes without pressing on the conductors,
• the circuits are labelled and the chart matches reality,
• the RCD trips immediately when the test button is pressed,
• the MCB ratings suit the cables, and the electrician can explain why each circuit has the rating it has,
• you have received a certificate with the measured test results, signed by the electrician, and for notifiable work also the Building Regulations compliance certificate,
• nothing trips when ordinary appliances are switched on.

The labour price in the NUMIQ price list for fitting an RCD is €12 per device. For fitting a single-pole MCB it is €35 per device. This is the electrician's labour including small consumables, without the device itself, the consumer unit or the cables. Prices vary by region and city.

The final amount depends on:

• the number of devices and circuits being split or reconnected,
• the condition and size of the existing consumer unit, whether it has spare ways or a larger unit is needed,
• whether borrowed neutrals have to be traced and separated in an older installation,
• the type of RCD and MCB, for example a four-pole RCD or an RCBO,
• testing and certification after the work, and putting right any defects found,
• travel time, if it is a one-off small job.

All related items can be found in the section electrical installation work.