Solar Inverter Loss of Mains Nuisance Tripping in the UK: Diagnosis & RoCoF Settings

Key Takeaways

• All generators that are connected to the distribution networks in Great Britain have Loss of Mains (LoM) protection, one of the many protection requirements on the electricity system.
• Legacy Vector Shift relays are vulnerable to network transmission disturbances and must be replaced with Rate of Change of Frequency (RoCoF) set to 1Hz/s with a definite time delay of 500 milliseconds.
• National grid statutory parameters require a declared frequency of 50 hertz with a permitted variation not exceeding 1 per cent at 230 volts.
• Modifying protection settings on grid-connected inverters or external relays must be conducted by certified commissioning engineers to comply with ENA EREC G98 / G99.

What is Loss of Mains protection and why does it trip?

When operating a solar photovoltaic installation in Great Britain, your inverter generates power in synchronism with the wider distribution grid. If an overhead line is struck by lightning, a substation transformer trips, or a car hits a telegraph pole, the regional Distribution Network Operator (DNO) safety systems disconnect that section of the network.

At that instant, your solar system must stop generating immediately. If it continued running, it would create an isolated electrical "island," posing a fatal electrocution hazard to utility linesmen and risking severe out-of-phase re-closure damage when the grid reconnects.

To prevent this, Loss of Mains is designed to check if a generator is still connected to the supply network. This applies to generating plants that run in long term parallel with the distribution network and are within scope of Engineering Recommendation (EREC) G59 , regardless of the type of generator, such as solar, wind or gas.

However, when LoM protection is calibrated too sensitively, normal power grid disturbances—such as large power stations tripping in Scotland or heavy industrial loads switching on—cause harmless frequency ripples that trick the inverter into shutting down.

The ALoMCP reform: why Vector Shift was outlawed

Historically, British distributed generators used two primary methods to detect grid disconnection: Vector Shift (measuring instantaneous voltage waveform angle changes) and RoCoF (measuring how fast frequency changes over time).

As the UK grid incorporated more renewable generation and less heavy spinning thermal inertia, Vector Shift relays began nuisance tripping during distant transmission faults. In extreme scenarios, widespread false tripping risked cascading national blackouts.

In response, National Grid ESO and the Energy Networks Association (ENA) created the Accelerated Loss of Mains Change Programme (ALoMCP):

• Under the reformed rules, where LoM is provided by vector shift, it must be removed and/or replaced with Rate of Change of Frequency (RoCoF).
• By definition, RoCoF is the rate at which frequency changes and is used to detect the absence of the connection to the system and disconnects the site if that RoCoF exceeds a specific value.
• Across all parallel generators, the RoCoF setting requirement has been changed to 1Hz/s with a definite time delay of 500 milliseconds.
• The mandatory national deadline for compliance with these updated EREC requirements was 1 September 2022.

Modern domestic inverters compliant with ENA EREC G98 already integrate this 1Hz/s 500ms delay curve within their certified factory firmware. However, systems installed under older G83 or G59 standards often retain legacy, hair-trigger settings that cause regular mid-afternoon dropouts.

DNO enforcement and de-energisation process

For commercial solar farms, schools, and larger three-phase systems operating under G59 or G99, DNOs enforce rigorous legal compliance timelines:

• Week 0: Written notification sets out to generator owner the non-compliance or unknown status of their generator site and sets out the actions to address non-compliance.
• Week 4: Written notification sets out whether a response has been received to the first notice.
• Week 8: Repetition of second notice: written notification sets out whether an appropriate response to non-compliance has been provided.
• Week 16: Repetition of second notice.
• Week 22: Repetition of second notice.
• Week 26: Week 26: If no evidence of compliance provided or evidence is not sufficient to demonstrate compliance with the Distribution Code, the DNO will make arrangements to de-energise the generation site's Connection Point in accordance with Distribution Code DGC12.

If a commercial installation fails to submit commissioning test sheets demonstrating the 1Hz/s 500ms RoCoF standard, the DNO will physically isolate the incoming high-voltage or low-voltage circuit breaker, disconnecting the site entirely.

Diagnostic checklist for recurring LoM trips

If your solar system regularly reports "Grid Loss", "Island Fault", or "Frequency Out of Range" during clear weather:

1. Check local outage status

Verify whether the outage is genuine. If your home loses mains lighting and neighbours report blackouts, the inverter has operated correctly. Dial 105 to report the incident to your regional DNO.

2. Verify inverter grid code profile

Check the front display or commissioning app to confirm the inverter is configured to "UK G98" or "UK G99". If an installer left the grid profile on "Default 50Hz" or an unapproved international standard, the inverter will use tighter frequency trip curves that drop out during minor National Grid balancing fluctuations.

3. Review firmware and protection relays

For older systems commissioned prior to September 2022, request an installer inspection. For string inverters, a firmware update usually updates internal anti-islanding algorithms to the 1Hz/s 500ms profile. For commercial arrays with external G59/G99 protection relays (such as Deep Sea, ComAp, or Woodward units), an engineer must connect via software to recalibrate RoCoF slope parameters.

Safety rules: protection relay commissioning

• Never attempt to rewire or adjust internal DIP switches on external mains protection relays without secondary injection test equipment.
• Always isolate AC and DC supplies before opening electrical control cabinets.
• Under the Distribution Code, generator owners are legally responsible for maintaining protective devices; unauthorized tampering with anti-islanding parameters creates extreme safety liabilities.