TT earthing arrangements

How TT earthing works, why it normally depends on RCDs, and the design issues for modern home-energy equipment.

A TT earthing arrangement uses an earth electrode at the installation for its protective connection to Earth. The supply transformer’s neutral is also earthed, but there is no metallic protective-conductor path between the two earthing points.

TT is commonly used where the network operator does not provide a suitable TN earth, or where a TN-C-S earth is deliberately not used for a particular installation or part of one. A property can therefore be TT even if a network earth might have been available. What matters is the designed and verified arrangement, not the age or rural appearance of the supply.

How fault protection works

If a line conductor touches exposed metalwork, fault current has to travel through the installation electrode, the ground and the supply transformer’s earthing arrangement. That path normally has much more impedance than the metallic return path in a TN system.

A fuse or circuit-breaker may therefore see too little current to disconnect quickly, or at all. TT installations normally use one or more RCDs to provide automatic disconnection for earth faults.

For RCD fault protection, the central BS 7671 relationship is:

RA × IΔn ≤ 50 V

RA includes the installation electrode and the relevant protective conductor. IΔn is the rated residual operating current of the RCD providing the fault protection. The electrode resistance should also be as low as practicable and remain effective as soil conditions change.

This is not a reason to accept any measurement that just passes the equation. The design must also meet the required disconnection times, coordinate the protective devices and account for parts of the installation before an RCD.

The whole fault path has to be protected

Fitting 30 mA RCBOs to final circuits can give good separation between circuits, but it does not automatically protect every possible fault. Meter tails, a main switch, distribution circuits and the conductive enclosure of a consumer unit may sit upstream of them.

The designer must prevent an earth fault in those upstream parts from leaving metalwork live. Depending on the installation, that can involve suitable insulation, cable entry and restraint, equipment construction, placement of RCD protection and a coordinated hierarchy of devices. There is no single consumer-unit layout that is right for every TT system.

Metal consumer units are not prohibited on TT. The IET’s guidance is that the tails and their entries must be arranged so a fault to the enclosure cannot occur on an unprotected section. The solution should be designed and inspected, not assumed from the enclosure material alone.

TT is an arrangement, not just an earth rod

A local rod fitted to PME equipment does not necessarily create TT. It may be a supplementary electrode or a reference for an open-PEN protection device while the equipment remains connected to the PME earth.

Likewise, an islanded battery system does not automatically become TT. During island operation, a correctly designed system may establish a local TN-S arrangement with its own source neutral-to-earth connection and consumer electrode. Its switching has to disconnect the required live conductors from the grid and provide a dependable earth-fault path in each operating mode.

The label should follow the current path, switching and protective conductors, not the presence of a rod.

Whole-property and local TT

A whole property can use TT, or a deliberately separate part of a larger installation can use it. Local TT is sometimes considered for a detached garage, garden building or outdoor installation where exporting PME would introduce an unacceptable open-PEN risk.

The separation is the difficult part. A TT zone can be unintentionally reconnected to PME by:

cable armour or an unused protective conductor
metallic water, heating or gas pipes
structural metal and buried services
screened data or control cables
equipment spanning the two areas
conductive parts from both systems that can be touched at the same time

If a shared metallic pipe runs between the house and outbuilding, fitting a rod and disconnecting one cable core may not create a separate TT zone. The complete site needs to be assessed.

Solar, batteries, heat pumps and EV charging

New equipment does not change the property’s earthing label just because it has power electronics. It does, however, make verification more important.

Solar PV and grid-connected batteries. The inverter’s protective measures, leakage characteristics and RCD compatibility must suit TT. The designer should check both AC and DC fault behaviour and follow the manufacturer instructions.

Battery backup. Connected mode and island mode have different current paths. The changeover, neutral treatment, electrode and protective devices must operate as one system and be tested in both modes.

EV charging. A charger on an existing TT installation still needs the protection required for the charging circuit. A rod fitted as one element of an open-PEN measure on a PME installation is not the same thing as TT. The current edition of BS 7671, the charger instructions and the chosen protective-device specification all need to agree.

Heat pumps and outdoor equipment. Being outside does not automatically demand TT. The design depends on the supply earthing, exposed and extraneous conductive parts, manufacturer’s requirements, circuit protection and touch conditions.

Electrode location, testing and upkeep

The electrode must be positioned only after buried services and other earthing systems have been considered. Ground plans and cable-location equipment both have limitations, particularly for private and non-metallic services. Driving a rod without a safe ground-work plan can be fatal.

Commissioning should record the electrode type and location, test method, resistance, relevant RCD results and the parts of the installation the arrangement protects. Periodic inspection should check the connection, conductor, labelling and test result. Seasonal changes, corrosion or later ground works can alter a previously satisfactory result.

Signs that need investigation

Arrange an electrical inspection if:

the electrode or conductor is loose, damaged, heavily corroded or inaccessible
an RCD will not reset, trips repeatedly or fails its test button
no one can identify which RCD protects the installation before the final circuits
building work has taken place around the electrode
a new outbuilding, metal service, charger, inverter or backup system has been added without the earthing design being recorded
certificates do not state the earthing arrangement and electrode result

Do not disconnect the electrode to see what happens. Earthing and bonding conductors can carry current under abnormal network conditions even after the main switch has been turned off.

Earthing design and test results

For work involving TT, the earthing arrangement, each RCD’s duty, upstream fault protection and the electrode test method need to work across every normal and backup operating mode. The completed electrical certificate records the measured results.

“Install earth rod” is a component description. It is not a TT design.

Applies to

Solar, Battery, EV charging, Heat

Last reviewed

22 Jul 2026