Solar inverter anti-islanding

Why an ordinary grid-connected PV inverter stops during a power cut and what extra equipment is required for safe backup operation.

Anti-islanding protection makes an ordinary grid-connected solar inverter stop energising the installation when the public supply is lost or moves outside permitted conditions. This protects network workers and equipment and prevents a small local generator from trying to control an unintended section of the distribution network.

It is why a standard grid-tied PV system usually shuts down during a power cut even in bright sun.

What an unintended island is

An island is a section of electrical system that continues to be energised by local generation after it has been separated from the normal grid source. Without controlled isolation, voltage and frequency may be unsafe or unstable, and the network can be energised when it is expected to be dead.

The inverter monitors grid conditions and uses its loss-of-mains protection to disconnect or cease to energise within the applicable requirements. It remains offline until acceptable reconnection conditions and timing are met.

The UK connection framework

As at 22 July 2026, small generating installations in Great Britain connect under the ENA’s G98 or G99 framework, according to the installation and connection route. Type-tested inverter protection and the installed settings form part of compliance.

IEC 62116 defines a test procedure for islanding-prevention measures in utility-interconnected PV inverters. Passing relevant product tests does not remove the need for correct commissioning, DNO process and site protection.

Do not change loss-of-mains, voltage or frequency settings to keep an inverter generating through a weak grid. Those are protected engineering settings, not performance tweaks.

Why turning off the grid does not create backup

An ordinary grid-following inverter uses the public supply as its voltage and frequency reference. When that reference disappears, anti-islanding requires it to stop.

Simply opening the main switch does not create a safe private grid. The home would still need:

an approved changeover or island-mode isolator separating all required live conductors
a grid-forming source that establishes voltage and frequency
a suitable neutral and earthing arrangement
protective devices proven to operate with inverter fault current
load and surge management
controlled reconnection to the public supply

Improvised backfeeding through a socket or manual lead is unsafe.

Backup-capable systems

A battery inverter, hybrid inverter or dedicated gateway can be designed to form an intentional island. It isolates the backed-up circuits from the grid and then supplies them from stored energy and, where supported, PV.

The solar inverter must be compatible with that island. Some systems keep DC-coupled PV operating through the hybrid inverter. Some coordinate an AC-coupled PV inverter through frequency or communications. Other ordinary inverters stay off throughout the outage.

Having both solar and a battery does not guarantee this function. The exact topology, gateway, firmware and commissioning determine it.

Anti-islanding and island mode are complementary

Anti-islanding prevents uncontrolled energisation. Intentional island mode creates a controlled alternative supply after verified separation.

A correctly designed backup system still uses anti-islanding at its grid boundary. When grid conditions return, it verifies them and reconnects through the approved control rather than allowing the local island and grid to meet unsynchronised.

What remains live after shutdown

Stopping the inverter’s AC output does not remove daylight voltage from PV modules and the DC cables leading to the inverter. DC isolators separate defined sections but do not make the modules themselves stop generating.

Labels, diagrams and firefighter information should identify the local generation and isolation points. Anyone working on the roof or wiring needs a PV-specific safe system of work.

Normal outage behaviour

During a grid outage, an ordinary system may show a grid-lost or voltage/frequency code and zero generation. It should return only after the supply is stable and the inverter’s reconnection process completes.

A short delay after grid restoration is normal protective behaviour. The applicable setting and product implementation govern the delay; there is no universal time.

If the inverter reconnects and trips repeatedly, record the code, grid conditions and time. Do not cycle isolators repeatedly or alter protection limits.

Testing and commissioning

Commissioning verifies the configured country or network settings, protection interface, DNO documentation and normal disconnect and reconnect behaviour. A backup installation also needs tests in both connected and island modes.

The commissioning record includes:

inverter make, model and type-test evidence
G98 or G99 route and settings
AC and DC isolation
loss-of-grid and reconnection test
backup changeover, neutral and earthing arrangement where fitted
protected circuits, power and surge limits
behaviour of PV during an islanded outage

Do not simulate a grid failure by removing conductors or defeating interlocks. Use the system’s approved commissioning procedure.

Faults and escalation

Persistent loss-of-mains alarms when the grid appears present can have several causes:

actual voltage or frequency excursions
a loose or failed AC connection
excessive voltage rise during export
a defective isolator or protective device
inverter sensing or configuration fault

A competent contractor can measure the installation and preserve event logs. The DNO investigates network conditions where the evidence points beyond the property.