Island mode and anti-islanding

Anti-islanding prevents a solar inverter, battery or other generator from continuing to energise the public network after the distribution supply has been disconnected. Grid-connected generating equipment must detect the loss of the network and separate from it in accordance with its G98 or G99 protection.

A deliberate backup island is different. A correctly designed system first disconnects the circuits it will support from the public network, then supplies those isolated circuits from the battery or other source. It does not defeat anti-islanding because the network side remains separated and de-energised.

Why an uncontrolled island is dangerous

If generation continued feeding a disconnected section of network, conductors expected to be dead could remain live. It could also leave local generation unsynchronised with the wider system when the DNO reconnects the network.

The safe outcomes are therefore:

• a normal grid-connected inverter stops supplying power when the grid fails
• a backup-capable installation separates the intended circuits from the grid before energising them as a controlled island

Owning a battery does not by itself provide backup. The inverter, switching, protection, earthing and circuit arrangement must all support island operation.

Connected mode and island mode follow different rules

The IET describes electrical energy storage as a type of generator and maps its two operating modes to the Electricity Safety, Quality and Continuity Regulations:

• Connected mode is parallel operation with the distribution network. Regulation 22 applies, and the power-conversion equipment follows G98 or G99 as appropriate.
• Island mode uses the storage system as a switched alternative to the network. Regulation 21 applies, and the backed-up part of the installation must be prevented from operating in parallel with the public supply.

G98 and G99 provide the connection and loss-of-mains requirements for parallel generation. They should not be confused with G100, which concerns export or import limitation while the network is present.

What happens in a power cut

In a grid-connected system without an EPS or backup output, loss-of-mains protection disconnects the inverter. Solar generation and the battery may then remain unavailable to household circuits until the network returns, even if the sun is shining and the battery is charged.

In a backup-capable system, the sequence is broadly:

1. The system detects that the public supply has failed.
2. The island-mode switching separates the circuits to be supported from the network.
3. The installation establishes the correct neutral and earthing arrangement for island operation.
4. The inverter energises the supported circuits within its backup limits.
5. When the network is suitable again, the system reverses the sequence and returns to connected mode.

Transfer time is product- and design-specific. “EPS”, “backup” and “UPS” should not be treated as interchangeable promises. Equipment that cannot tolerate an interruption needs a design assessed for that requirement, not an assumption based on a marketing label.

All live conductors must be separated

The IET guidance says that the conductors supplying the islanded circuits, including neutral, must be disconnected from the grid for island operation. The switching arrangement is commonly called an island-mode isolator.

This separation is a core safety function. A software instruction to stop exporting is not equivalent to physical or otherwise compliant electrical separation. G100 export limitation also does not provide the changeover needed for backup.

The designer must confirm:

• which circuits are included in the island
• the switching device and its ratings
• the position and treatment of neutral
• interlocking between grid separation and island energisation
• the return-to-grid sequence
• what happens if the switching mechanism or control supply fails

These details belong on the single-line schematic and commissioning record.

Earthing changes in island mode

In connected mode, a TN installation may use the distributor’s means of earthing. The neutral-to-earth reference is normally established on the network. Once the backed-up circuits are disconnected from all live network conductors, that network neutral reference cannot be relied upon for island operation.

The IET’s published arrangement therefore includes:

• an island-mode isolator
• a switched neutral-to-earth bond for the island
• a consumer earth electrode

The neutral-to-earth bond must be coordinated with the island-mode isolator. It is made only in the islanded condition and removed before parallel connection is restored. The resulting island arrangement is treated as TN-S in the IET guidance.

The exact design must follow the current edition of BS 7671, the equipment instructions and the conditions at the property. Existing TT electrodes, outbuildings, EV charging equipment and circuits with different earthing arrangements can change the protection calculations. An electrode value quoted from a generic diagram is not a substitute for design and testing.

Fault protection can behave differently on battery power

An inverter may supply much less fault current than the public network. A protective device that disconnects quickly on grid supply may not operate in the same way when the installation is islanded.

The design must verify automatic disconnection of supply in both modes. This includes the operation of RCDs, overcurrent devices, the neutral arrangement and earth-fault loop conditions. Whole-home backup is not simply a larger version of a socket labelled EPS; it can change the fault and selectivity study across the installation.

Backup power and energy limits

The backed-up demand must remain within the inverter’s island-mode power, surge and phase limits. The battery also needs enough usable energy for the intended duration.

A design should state:

• continuous and short-duration backup power
• whether solar can operate while islanded
• maximum load on each supported phase
• circuits deliberately excluded from backup
• battery reserve and expected duration assumptions
• manual reset or black-start requirements
• behaviour when the battery becomes empty

Large loads such as EV charging, electric showers, immersion heaters and heat pumps may be excluded or controlled. The answer depends on the actual system, not on the fact that a battery is installed.

Commissioning and handover

Commissioning should demonstrate connected operation, loss of grid, correct separation, establishment of the island neutral and earth, protection in island mode and safe return to grid. Test results should be recorded for the installed arrangement.

Keep:

• the as-built single-line schematic
• G98 or G99 connection records
• changeover and neutral-to-earth bond details
• earth-electrode and protection test results
• the supported-circuit schedule
• backup power and energy limits
• instructions for shutdown, reset and maintenance

Do not test anti-islanding by opening DNO equipment or altering protection settings. Use the manufacturer’s commissioning method and suitably competent electrical personnel.

Related entries

• Backup EPS output
• G98 notification pathway
• G99 application pathway
• G100 export limitation
• TT earthing arrangements
• Earth electrode and rod installation