Main Protective Bonding Regulations

Main Protective Bonding Regulations

Main Protective Bonding is a very important part of the 18th edition wiring regulations

Indeed, the main protective bonding regulations are often misunderstood or simply overlooked by contractors when completing work on existing installations

Ensuring that the requirements are met is an urgent safety issue, and electricians should make sure they know what is required and why bonding is so important.

Earthing vs Bonding

The most overlooked part of these regulations is the difference between earthing and bonding. As the bonding connects to the main earthing terminal, it can be easy to think that they are one and the same. However the reason behind them is actually quite different.

Earthing is the act of providing a low impedance path, such that in the event of a fault to an ‘earthed’ item (class 1), sufficient current will flow to disconnect the protective device for that circuit. In this way, earthing is designed to limit the duration of a fault

Main protective bonding is designed to connect items together which may introduce a potential (usually earth potential) into an area. In this way, they ensaure that should an earth fault occur, everything connected to the MET rises in potential at the same rate. By doing this, minimising potential differences, bonding is designed to limit the severity of a fault

For example, if an end user is holding a class 1 item which becomes live in one hand and a metal tap in the other hand, they can be exposed to a potential difference of 230v <> 0v (in the case that the tap is connected to buried pipework) between both hands. This can give a very dangerous, possibly lethal, shock across the heart.

By doing this, the severity of the shock will be lowered to an end user if they experience an earth fault.

The whole concept of earthing and bonding can be difficult to grasp at first, but it is worth remembering that anything connected to the main earthing terminal (MET) will become live until the protective device for the circuit disconnects the supply.

So if the kettle has a fault and the body becomes live (exposed conductive part) then the washing machine, dishwasher, boiler and any other class 1 items will become live at the the same potential until the protective device for the circuit to which the kettle is connected cuts the power.

In a nutshell, anything that is connected to true earth (buried pipework in the form of gas/water supplies or buried structural metal) will introduce an earth potential into the building (called an extraneous conductive part). This means that it needs bonding.

By doing this it creates an ‘equipotential zone’, an area where everything is interconnected to each other to ensure that the potential stays equal in the event of a fault occuring.

Whilst there are different types of bonding, electrical installations typically include main protective bonding and where required, supplementary bonding. This guide focuses particularly on main protective bonding regulations.

Where Does Main Protective Bonding Need Fitting?

Now we have established what main protective bonding is, it is important to be sure where exactly it needs fitting.

Main protective bonding needs fitting to anything that introduces a different potential (earth potential in this instance) into the equipotential zone.

Generally speaking, this is anything metal which is either buried in the floor or can be connected (bolted to the floor).

Quick examples are:

  • Main water pipework which is buried in the floor before entering the property (metal)
  • Main gas pipework which is buried in the floor (metal)
  • Structural steelwork, can either be buried or bolted to the floor
  • Oil pipework (from an outside tank)

If there are metal pipes which serve heating or water in an outbuilding, this would also require main bonding.

It should be noted that if a pipe travels through the building, entering and then exiting again, it should be bonded at both ends. This isn’t very usual in domestic installations, however commercial premises can have this type of arrangement quite often.

The bonding connection should be made to the pipework within 600mm of the entry to the property. It should also be made before any ‘branch’ pipework, such that alterations do not affect the continuity of the bonding.

In newer buildings, the main water pipework is quite often MDPE plastic, which according to the 18th edition DOES NOT require bonding. Equally, the main protective bonding regulations in the 18th state that if a pipe has an insulating section at the entry (for example a plastic section in the water pipework where a meter is fitted) then again, bonding is not required.

Main bonding is also required to be connected to lightning protection (if present), such as a lightning conductor. This is quite specialist and is dealt with by it’s own set of regulations, however be aware that a property which does have this fitted will need bonding fitted here.

What Size Cable Should I Use For Main Protective Bonding?

There are 2 ways to size the main protective bonding cables, one is by selection in table XX of BS7671.

This method MUST be used where the installation earthing arrangement is TN-C-S (PME), and you are free to use this method with any other earthing arrangement supply.

The table states that:

The alternative method is from regulation XXX.XX and basically requires that the bonding cables are at least half of the required size of the earthing conductor. It should be noted that this doesn’t mean the size of the earthing conductor which is actually fitted, but the required size (as per the adiabatic equation).


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