Main earth bonding

The difference between earthing and protective bonding, what normally needs bonding, and why it must be checked before home-energy work.

Main protective bonding connects extraneous-conductive-parts, such as certain incoming metallic services, to the installation’s main earthing terminal. Its purpose is to reduce dangerous voltage differences between metal parts that a person could touch during a fault.

It is related to earthing, but it is not the same thing:

earthing connects exposed conductive parts of electrical equipment to the protective system so a fault can be disconnected
main protective bonding deals with conductive parts that can introduce a potential from outside the electrical installation
supplementary bonding connects conductive parts locally where an additional protective measure is required

Calling every green-and-yellow conductor an “earth” hides these different jobs.

What normally needs to be assessed

Incoming metal water and gas services are familiar examples, but the rule is not based on the name of the service. The question is whether the part is an extraneous-conductive-part: a conductive part that is not part of the electrical installation and is liable to introduce a potential, including Earth potential.

The assessment can include:

water, gas, oil and other fuel services
metallic district-heating or cooling pipes
structural metal that can introduce Earth potential
other incoming pipework or ducting with a conductive path to ground
services shared with another building or installation

A plastic incoming pipe may mean the internal metal pipework is not extraneous through that route. It must not be assumed from a short visible section alone. Metallic inserts, buried transitions, shared systems or another path can change the result. Conversely, bonding metal simply because it looks substantial can introduce a potential where none existed before.

The determination should be made by inspection and appropriate testing by a competent person.

Where the connection is made

Main bonding is normally connected as near as practicable to where the service enters the building, on the consumer’s side of any insulating section and in a position that remains accessible for inspection. The correct point depends on the service layout, meters, branches, insulating joints and the service provider’s requirements.

A general distance copied from a photograph is not a substitute for identifying the incoming conductive path. The clamp should be suitable for the pipe and environment, secure without damaging the service, correctly labelled and accessible. It must not be attached to a removable section in a way that leaves the installation unbonded during routine service work.

Gas and other utility pipework has its own safety rules. An electrician should not move a meter, disturb a gas fitting or use an unsuitable clamp to force a convenient connection.

Conductor size is a design result

The required bonding-conductor size depends on the supply earthing arrangement and the relevant supply conductor. PME systems have specific sizing requirements because bonding can carry diverted neutral current if the network PEN conductor is damaged.

That is why “10 mm² is normal” is not a safe specification. It may be a familiar domestic size, but the electrician must confirm the earthing arrangement, conductor material, supply details and current BS 7671 requirements. A bonding conductor that was suitable before a supply alteration may need reassessment after conversion to PME or a service upgrade.

The conductor should run continuously or use compliant, accessible joints. It needs protection from mechanical damage and should not be concealed in a way that prevents verification.

Bonding on a PME supply

In a healthy PME installation, the combined PEN (protective earth and neutral) conductor is separated into neutral and earth at the supply position. If that upstream PEN conductor opens, return current can find another route through bonded metal services.

Main bonding helps keep accessible metalwork within the building at a similar potential, but it can then carry substantial diverted current. It does not make outdoor contact with true Earth safe, and it does not provide complete open-PEN protection for an EV or other equipment used outdoors.

Never disconnect a bonding conductor because it appears to be carrying current. That can create a dangerous voltage on the pipe or other metalwork. A competent electrician should use safe-isolation procedures and suitable instruments; suspected neutral-current diversion must be reported to the network operator, normally through 105.

Why it is checked before new equipment is connected

The safety of a new solar, battery, heat-pump or EV circuit depends on the existing earthing and bonding. Electrical Safety First advises that these arrangements should be checked when electrical work is carried out.

The check is not an automatic instruction to add more bonds. It should establish:

the confirmed supply earthing arrangement
the location and condition of the main earthing terminal
which incoming parts are genuinely extraneous-conductive-parts
whether the bonding connections and conductor sizes are suitable
whether diverted current or an abnormal touch voltage is present
whether new metallic services or an outbuilding change the original assessment

Solar-panel frames, battery enclosures, heat-pump casings and EV chargepoints are normally dealt with through the protective measures for their circuits and the relevant product or installation standard. They are not all “main bonded” merely because they are metal.

Outbuildings and separate earthing zones

Exporting a PME earth to a detached building can turn its incoming metallic services into main-bonding paths. The protective conductor or cable armour must be suitable for every duty assigned to it. A conductor adequate as a circuit protective conductor is not automatically adequate for main protective bonding.

Changing the outbuilding to TT is not just a matter of fitting a rod. Any water pipe, structural metal, armour or data screen that links the TT area back to PME can transfer the very potential the separation was intended to avoid. The complete site needs to be designed as one system.

Handover records

The installation certificate or condition report should identify the earthing arrangement and record the presence and condition of main protective bonding. Where a service has been assessed as not requiring bonding, the reasoning and test evidence are useful records for future work.

Arrange further investigation if a bonding conductor is missing, loose, corroded, inaccessible, clearly undersized for the confirmed arrangement or connected on the wrong side of an insulating section. The appropriate classification on an EICR depends on the actual danger and installation conditions; a missing-looking clamp does not have one automatic code in every property.

Do not reconnect, move or remove bonding as a DIY task. Even with the main switch off, a conductor can carry current from a network fault or another installation.

Applies to

Solar, Battery, EV charging, Heat

Last reviewed

22 Jul 2026