UK G99 Loss of Mains Protection and Relay Setting Guide

Key Takeaways

• Loss of Mains (LoM) protection is mandatory for all grid-connected generators in Great Britain.
• Vector shift LoM protection must be replaced with Rate of Change of Frequency (RoCoF) detection.
• The required RoCoF setting is 1Hz/s with a 500 millisecond time delay.
• Non-compliance can lead to de-energisation of your generation assets by the Distribution Network Operator (DNO).

What is Loss of Mains Protection and Why is it Essential?

Loss of Mains (LoM) protection is a critical safety feature for any electricity generator connected to the distribution network in Great Britain. Its primary purpose is to "check if a generator is still connected to the supply network." If the main grid supply is lost, LoM protection ensures that your generator quickly disconnects. This prevents a dangerous condition known as "islanding," where a section of the grid remains energised by local generation even after the main supply has failed. Islanding poses a significant safety risk to utility workers who may be working on what they believe to be a de-energised network.

The Electricity Act 1989, specifically Section 16, places a duty on electricity distributors to connect generators. However, Section 17 of the same Act outlines exceptions to this duty, often related to safety and network stability, which LoM protection addresses.

The Evolution of Grid Connection Standards: G59, G98, G99, and G100

Grid connection requirements for generators in Great Britain are defined by Engineering Recommendation (EREC) standards published by the Energy Networks Association (ENA). These standards ensure the safe and reliable operation of distributed generation.

Historically, EREC G59 was the primary standard for connecting generators. However, "G98 and G99 superseded the previous ENA standard G59 on 17 May 2019."

• EREC G98 applies to small-scale generation (typically up to 16 A per phase).
• EREC G99 applies to larger generation systems.
• EREC G100 details "the technical requirements for customers' export and import limitation schemes applicable after 1 May 2023."

For more details on these standards, you can refer to our guides on /uk/guides/technical/uk-g98-micro-generation-connection-guide, /uk/guides/technical/uk-g99-grid-connection-requirements, and /uk/guides/technical/uk-g100-export-limitation-schemes.

Understanding LoM Detection Methods: RoCoF vs. Vector Shift

Older LoM protection schemes often relied on "vector shift" detection. However, this method has been identified as less reliable under certain network conditions. For this reason, "LoM provided by vector shift must be removed/replaced with Rate of Change of Frequency (RoCoF)."

Rate of Change of Frequency (RoCoF) protection monitors how quickly the grid frequency changes. A rapid change in frequency is a strong indicator that the generator has become isolated from the main grid. This method provides a more robust and dependable detection of a loss of mains event.

Required RoCoF Settings and Compliance Deadlines

For compliant LoM protection, specific RoCoF settings are mandated. The "RoCoF setting requirement" is "1Hz/s with a definite time delay of 500 milliseconds." These settings are crucial for ensuring that your generator disconnects promptly and safely when required.

The compliance date for updating LoM protection was "1 September 2022." If your site was previously compliant with G59/3, you should "notify us by completing this form" to your Distribution Network Operator (DNO).

RoCoF Protection Settings

What You Can Check Yourself, and What You Cannot

As a generator owner, you are responsible for ensuring your system's compliance. You can:

• Verify your system's LoM protection method: Check if your inverter or protection relay uses RoCoF detection. If it uses vector shift, it requires an update.
• Confirm RoCoF settings: If your system uses RoCoF, check its settings against the required "1Hz/s with a definite time delay of 500 milliseconds."
• Notify your DNO of changes: "It is a requirement of G59, that any material change to the G59 protection settings or generating equipment are communicated to the DNO which supplies the site."

You cannot:

• Change protection relay settings yourself: Adjusting these settings requires specialist knowledge and accredited personnel. Incorrect settings can compromise safety and grid stability.
• Install or modify protection equipment without accreditation: All work on grid-connected generation protection must be carried out by qualified and accredited installers. A "contractor directory can be found here" for G59 installers.

Consequences of Non-Compliance

Non-compliance with LoM protection requirements carries significant risks and penalties. "Any generators who are not compliant and therefore require updates to their protection will be subject to an enforcement programme which could ultimately result in the de-energisation of generation assets."

The de-energisation process can be swift. "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" by Week 26 of the enforcement process. This means your generation equipment could be disconnected from the grid, preventing you from exporting electricity.

For submitting G98 forms, you can email g98notifications@enwl.co.uk. For general application forms, send them to connectionapplications@enwl.co.uk. The Pre-Application Customer Engagement (PACE) Team can be reached at 0800 988 1730 (8 am - 5 pm Monday to Friday) or pace@enwl.co.uk.

What the Published Sources Do Not Tell You

While the ENA standards and DNO guidance provide clear requirements, some specific details are not publicly available or vary by DNO:

• The exact content and format of the "form" for notifying DNOs of G59/3 compliance.
• A comprehensive, publicly accessible list of all recognized contractors for G59 generation installers across Great Britain.
• The full, detailed technical specifications of G98, G99, and G100 beyond their general descriptions, which are typically available through paid subscriptions or industry bodies.
• The complete text of "Distribution Code DGC12" referenced in the de-energisation process.
• Specific details or access to the "self-serve tool on the Futureproof Your Power website" for checking inverter/relay compliance.