Mains Electric Box: What is it and what does it do?

The mains electric box is a square or rectangular box made from plastic or metal that is installed somewhere in our homes. The incoming live electricity feed enters the home and passes through the electricity meter that monitors how much electric we use. The cables then leave the electricity meter and enter the MAINS ELECTRIC BOX. This box has several functions:

• Distribute the mains electric supply around the home
• Break down the electric supply into smaller more usable supplies called electrical circuits
• Provide some degree of electric shock protection to the users and homeowners
• Provide protection against electrical overload and short circuits
• Protects the electrical cables and system from overheating and fire

The Mains Electric Box is also known as the following:

Electrical Panel, Main Panel, Electrical service panel, consumer unit, fuse box, fuse board, circuit breaker panel and circuit breaker box. All these words are used to refer to essentially the same thing.

1) How is the supply broken down into circuits?

The electricity supply is supplied from the utility company into the electric meter then in to the mains electrical box by thick power cables. These cables enter into an isolating device called “The Main Switch” or “Main Breaker”. This switch isolates BOTH the Live and Neutral cables together so when it is turned off there is no connection or continuity through the switch. 

The Main Switch then usually has a large copper bar that connects underneath and feeds the electricity supply to each of the switches inside the box. Each of these switches will operate a different part of the electrical installation and provide isolation to each part. Each of these switches will have a different electrical current rating to limit the amount of electrical current that can flow through the switch before it turns off. 

A professional electrician that installs the wiring and mains electrical box will carry out design calculations for the entire house along with other considerations to decide which switches control which parts of the home, the ratings of each switch and the cables that should be installed. Each switch controls a separate “electrical circuit” and each circuit should be designed to capably deliver the required electrical demands of the installation. The home’s electrical system should always be designed properly by a licensed electrician to ensure that electrical problems are kept to a minimum.

2) How is the electrical supply distributed around the home?

Once the electrical supply has been divided into electrical circuits using individual breakers, the electrical cables carry the electrical current around the home to the sockets, lights and other electrical equipment as required. These cables are carefully selected for type and size to ensure that no electrical danger will arise from the normal use of the electrical system. The cable will be selected to safely supply the demands of the system considering any factors that may affect the ability of the cable to deliver the electrical current safely.

Such factors could include:

• Thermal insulation
• Grouping of cables together
• External resistances of the electrical system
• Length of electrical circuit
• The protective device installed in the mains electrical box

On a plug socket circuit, the cable or cables leave the fuse box / mains electrical box and enter the rear of the first plug socket, they then leave the rear of the first plug socket and travel to the second plug socket and so on until each of the plug sockets on the electrical circuit is fed. On a “RING MAIN” (Ring Final Circuit) the cable that enters the final plug socket on the circuit then returns to the consumer unit/fuse box/Mains electrical box creating a continuous loop of cable or RING. 

On a Radial Circuit the cables enter the last point of the circuit or plug socket and simply remain there without returning to the consumer unit.

3) How does the mains electrical box offer electric shock protection?

The mains electrical box contains an Earthing Terminal for all of the electrical systems earthing connections. This Earthing terminal will usually connect metal pipework, earth pins of plug sockets and therefore metal cases of electrical appliances, metal parts of light fittings, metal switches and other exposed conductive parts of the electrical system to the means of earthing for the electrical installation. 

The means of earthing in many cases is an Earthing Conductor. The earth connection in most parts of the UK, USA and other countries is often provided by the electricity company via a cable supplied with the other incoming electrical cables. 
In some cases where the installation is more rural the electricity company may not supply the EARTH. The customer is often required to have a separate EARTH ROD which must be installed correctly to provide a sufficiently low earth resistance path in the case of faults. 

The other form of electric shock protection is by way of a special “Trip Switch” known as an RCD/RCB/RCCB/RCBO or GFCI. This type of trip switch monitors one or more electrical circuits and the electrical current flowing throughout the circuit. When someone receives an electric shock, electrical current leaves the electrical circuit and flows through their body instead, the RCD device notices the missing electrical current and turns the circuit or circuits OFF to save the life of the person receiving the shock.

4) How is electrical overload protection done?

To protect from electrical overload occurring we need to make sure that the electrical cables are not supplied with more electrical current than they can handle. Each electrical cable will have a maximum safe current rating which changes depending upon how it is installed. The electrician installing the cables will make sure that the electrical cable can NEVER be overloaded. 

If the maximum safe rating of the cable is exceeded this will result in premature failure of the electrical system, overheating and possible fire! 

In order to ensure that this does not happen, the MAINS ELECTRICAL BOX will have several fuses or switches that prevent too much electrical current from passing down the cables. In the case that too much electrical current does flow down the electrical circuits, the fuse for that circuit will blow or the trip switch will trip off and stop the flow of electrical current. 

The incoming electrical cables into the property will also have a maximum safe current rating and these are often protected by a MAIN INCOMING FUSE. 

5) What is a trip switch?

A trip switch is any switch in the consumer unit / fuse box / mains electrical box that is designed to trip off under fault conditions.

Such trip switches include:

• MCB
• RCBO
• RCD/RCB/RCCB/GFCI (https://www.electricalfaultsfixed.co.uk/blog/trip-switch-rcd )
• AFDD

The MCB or miniature circuit breaker is a device that only really offers protection against gradual overloading of electrical circuits and short circuit conditions where large electrical fault currents are present. When this circuit breaker trips it is either a gentle click indicating a small overload or a loud bang indicating a larger flow of electricity. 

The RCBO is an MCB and RCD device all in one. Residual Current Breaker with Overload. It offers electrical overload protection and electric shock protection in one device.  These will trip under earth leakage (like electric shock) conditions and under short circuit or gradual overload problems. 

The RCD (Residual Current Device)/RCB (Residual Current Breaker)/RCCB (Residual Current Circuit Breaker)/GFCI (Ground Fault Circuit Interrupter) all protect against earth leakage conditions (electric shock) but not OVERLOAD or short circuit. 

The AFDD is discussed below.

If you have a trip switch problem, check out our article to see if you can solve it yourself. We are always happy to help.

6) How are cables protected from overheating and fire?

Along with the overload protection discussed above, cables are additionally protected from damage by use of an AFDD or Arc Fault Detection Device. These devices sense changes in the supply frequency such as would occur under arcing conditions. When these electrical devices detect arcing, they disconnect the supply to the electrical circuit to prevent overheating and electrical fires.

In 2024 the local store prices for these devices is quite high although since their introduction, the cost of the technology has dropped slightly. 

RCD devices will also offer a small degree of fire protection under specific fault conditions and also Fuses and MCB trip switches will assist to some degree. 

7) Is a plastic mains electric box illegal?

The photograph above shows a plastic MAINS ELECTRIC BOX. This box contains a main switch, an RCD device and several MCB devices. The plastic box is NOT ILLEGAL. It is currently not permitted to fit a plastic box in the UK and all new consumer unit installations must use a box that is constructed from non-combustible materials such as metals. 

8) Does a plastic box need to be replaced?

There is NO REASON to replace a plastic consumer unit UNLESS there are signs of thermal damage. Such signs could include discolouration of the plastic, melting or distorting of the case or any internal damage that would warrant a change. 

Simply because a consumer unit is plastic IS NOT a reason to swap it! 

9) Do you have more questions? 

Check out our massive question and answer article on consumer units and fuse boxes here.

10) How long does a Mains Electric Box last?

If correctly installed and subject to regular maintenance, these boxes will last a real long time. Several decades is not uncommon. It is more likely that the wiring regulations will change long before these modern boxes fail. 

A qualified electrician is the best person to carry out maintenance on the main electrical box. They will check the circuit breaker switches, electrical wiring, the residual current devices and other safety measures. They will alert you of any necessary electrical repairs or electrical work that needs to be carried out and produce an electrical report for you at the end of the inspection. 

11) What is a sub panel?

A sub panel or standalone consumer unit is often a small separate fuse box that is fed from the mains cables separate to the main breaker panel/Consumer unit. 

These smaller units are often installed as an add-on to the larger consumer unit to keep the total price of a job down and prevent additional costs when adding an electrical circuit to an older consumer unit. Common issues such as; old consumer unit technology, lack of compatible components and adherence to the wiring regulations can mean that the addition of a separate smaller panel is necessary rather than upgrading the main service panel or consumer unit. 

These smaller consumer units are often installed where a garage supply is required, an electric shower is installed after the original electrical installation has been carried out in the property. This is common in older homes where the electrical boxes may be old or older systems may require replacing in order to comply with modern standards. 

Electricians often install these standalone smaller boxes to keep costs down by avoiding major work whilst still ensuring compliance with the modern regulations.