What is a Main Linked Switch? Why do we need one?

There is normally always one of these devices inside the consumer unit and occasionally there is an external Main Linked Switch in a small enclosure on the incoming power cables. This Main Linked switch on the incoming power cables I often known as “the isolator”. This can be found inside your house or on newer building, in the external electricity meter cupboard. 

An Isolator external to the consumer unit is quite useful in the event of an electrical fault affecting the consumer unit, fuse box or internal installation. It is also useful if there is the need to replace the consumer unit as this can be carried out without too much extra work. 

There are a few things to look for when checking the Main Linked Switch:

• The switch is likely to often be a different colour to the other switches in the consumer unit
• The Switch should appear to be a double width switch 
• It will be ‘linked’, that is, will disconnect BOTH Line and Neutral together (DOUBLE POLE)
• It will usually be at the Left, or Right-hand side of the fuse box or consumer unit

1) What does a Main Switch Do?

Main Switches are responsible for isolating (disconnecting power or supply to) a portion of or the entire electrical installation in electrical systems. It does this by disconnecting BOTH the Line and Neutral conductors (wires that supply electricity). 

Where a Main Linked Switch is installed in a consumer unit , fuse box or distribution board, the purpose of the switch is to prevent the electrical supply (and therefore preventing current flow) from reaching all the different types of switches, circuit breakers and RCD devices that are present in the consumer unit. 

These electric switches individually control different parts of the electrical system such as plug socket outlets, lights, and electric cookers. Cutting the supply to these devices means that the electrical system as a whole will have no electricity supply. 

Where a Main Linked Switch is installed in the incoming power cables then there should be no power to ANY consumer units, fuse boxes or distribution boards in the property. Whilst there should be an individual Mains Switch in each of these boxes, the Main Switch installed on the incoming power cables should isolate the supply to everything in the property. If in doubt check with a qualified electrician before assuming the supply is dead.

2) What happens if a Main Switch is OFF?

If the Main Switch is off then there should be no power to the electrical system. If the Main Switch is installed on the incoming supply cables, then the Main Power supply should not be able to reach the electrical system in your home or property. If the main switch is installed in a fuse box, electrical panel, or consumer unit then there should be no supply to anything downstream of the Main Switch inside that enclosure. 

3) Does a Main Switch Trip Off?

Certain types of Main Switch do indeed trip off. The tripping main switch is of a different design than the one pictured. Where the main switch is an RCD (Residual Current Device) it will trip off in the event of various different electrical faults. The RCD is an electric shock prevention device that detects tiny amounts of missing electrical current from an electrical system and trips off when it detects someone might be having electric shocks. 

The Main Switch pictured is NOT one that trips. It is there for isolation, emergency power shut off and for maintenance purposes. 

4) Why won’t my Mains Switch Stay On?

If your main switch WON’T stay on then check the type of switch it is. If its an RCD/RCB/GFCI Main Switch then it is likely an automatic main switch that is tripping because there are one or more electrical faults within your system that require attention. 

If the Main Switch is NOT an RCD/RCB/GFCI type switch and it appears to not turn on then it likely needs to be replaced by an electrician without delay. These switches have been known to fail due to loose connections or overheating causing internal failure. 

5) How do I turn off Mains Electricity UK?

The supply can be turned off at the main switch for the installation or the consumer unit, fuse box or distribution board. 

Great care should be taken when working on electrical systems and we strongly recommend that the correct test equipment and procedures are used to ensure that the supply is isolated safely before carrying out any work.

6) What are the reasons a Main Switch might fail?

Where there are many power-hungry electrical circuits, large appliances, powerful household appliances or other high current draw electrical devices there can be a strain on the mains switch board and in particular the main switch. Most Main Switches have a maximum current rating written on them, the one pictured says ‘100A’ meaning 100 amps. Once this rating is exceeded there can be failures that occur. 

Loose connections can develop over time due to the heating, cooling, expansion, and contraction of the metals due to electrical current flow. Loose connections create a high electrical resistance and lead to burning out of electrical equipment. This can occasionally just take place internally to the switch and no outward signs may be visible or there may be signs of browning, blackening or general discolouration to the external of the switch indicating problems. 

The first signs of heat damage are normally to the area surrounding the input terminals or output terminals of the switch. 

Repair work would normally be to replace the affected component or components if heat has spread to a number of circuits/circuit breakers inside the consumer unit. 

7) Popular brands of Main Switch

The most common brands of Main Switch found in UK consumer units tend to be:

• Wylex
• Contactum
• BG
• MEM
• Hager
• Fusebox (increasing in popularity now)
• Crabtree

8) Are all main switches compatible?

No, in accordance with 536.4.203 (BS7671 Wiring Regulations) the manufacturer of the original assembly should be contacted to ascertain compatibility. The likelihood is that all manufacturers equipment is incompatible with all the rest DESPITE any ease of fitting or similarities in physical dimensions of each switch. 

Only the original manufacturers equipment should ever be fitted when a replacement is required and even within that, the same range of equipment should normally be used.