A G99 commissioning test is not a paperwork exercise. It is a live electrical test of the protection relay functions built into the grid-tied inverter — the same functions that disconnect the system from the grid in abnormal conditions to protect both the network and the people working on it.
Understanding what gets tested, which settings apply, and what documentation to retain is essential for any installer working on systems above the G98 threshold. This guide covers the full G99 commissioning test sequence, the correct protection settings (including the commonly misunderstood RoCoF value), and the post-commissioning notification requirements.
For background on whether G98 or G99 applies to a specific system, see the G98 vs G99 guide.
G99 Issue 1 Amendment 6
These commissioning requirements are based on G99 Issue 1 Amendment 6, the current version as of 2026. ENA updates G99 periodically. Always download the current version from the ENA website before commissioning, and check whether the DNO’s acceptance letter specifies any amendments to the standard settings.
G98 vs G99 Commissioning: The Key Difference
Before working through the G99 test sequence, it is worth being clear about why G99 commissioning is more demanding than G98.
G98 commissioning requires only that the installer:
- Tests the inverter’s loss of mains (anti-islanding) protection
- Records the result
- Sends a post-commissioning notification to the DNO within 28 days of energisation
Because most modern grid-tied inverters perform an automatic anti-islanding test at start-up and log the result, G98 commissioning is largely self-certifying for standard UK-certified inverters.
G99 commissioning requires a full protection relay test covering every protection function — not just loss of mains. The results must be formally documented and retained for DNO inspection. The inverter’s protection relay settings must be explicitly verified and recorded against a protection settings schedule, not assumed from factory defaults.
This difference reflects the larger potential impact of a G99 system on the distribution network. A fault in a G99 system’s protection relay could affect a wider area of the network — so the DNO requires verified test results, not just a start-up self-test.
G99 Protection Relay Settings: The Full Schedule
The G99 standard specifies default protection relay settings for inverter-based generating units connected to the UK low-voltage distribution network. These are the values you must verify are configured on the inverter before testing.
| Protection Function | Setting | Trip Time |
|---|---|---|
| Under Voltage (UV) | 0.87 pu — 200V | 2.5 seconds |
| Over Voltage Stage 1 (OV1) | 1.14 pu — 264V | 1.0 second |
| Over Voltage Stage 2 (OV2) | 1.19 pu — 273V | 0.5 seconds |
| Under Frequency (UF) | 47 Hz | 20 cycles (approximately 0.4s) |
| Over Frequency (OF) | 52 Hz | 0.5 seconds |
| Rate of Change of Frequency (RoCoF) | 1 Hz/s | 500 milliseconds |
| Anti-islanding (Loss of Mains) | Per type test | Per type test |
RoCoF: The Setting That Causes Most Failures
The correct G99 RoCoF primary setting is 1 Hz/s, not 0.5 Hz/s. Some inverters — particularly older models and some imported from continental Europe — ship with a default RoCoF setting of 0.5 Hz/s, which was the standard under the previous G59 recommendations. Using 0.5 Hz/s on a G99 installation is non-compliant. Before commissioning, verify the RoCoF setting on the inverter’s commissioning screen. If the inverter does not allow RoCoF settings above 0.5 Hz/s, it may not be suitable for G99 use in the UK without a firmware update from the manufacturer.
DNO-Amended Settings
The DNO’s G99 acceptance letter may specify settings that differ from the defaults above. This is more common in:
- Areas with high renewable penetration where the network is sensitive to voltage variation
- Networks with specific power quality conditions
- Areas where the DNO has published supplementary standard conditions
Always read the acceptance letter before configuring protection settings. If the DNO has specified tighter trip settings (e.g., OV1 at 253V instead of 264V), configure the inverter to the DNO-specified values and record the amended settings in the protection settings schedule.
Anti-Islanding Protection Testing
Anti-islanding, or loss of mains (LoM) protection, is the most safety-critical protection function on a grid-tied inverter. If the grid supply is interrupted — planned outage, fault, DNO switching — the inverter must disconnect from the grid before the grid is re-energised. If the inverter continues to operate on an isolated section of the network (islanding), it presents a danger to DNO engineers working on what they believe is a de-energised line.
Test Method
For G99 commissioning, anti-islanding testing must confirm that the inverter disconnects from the grid within the required time when the grid supply is removed. Two methods are accepted:
1. Type test reference method: The inverter is certified to BS EN 62116 (the international standard for utility-interconnection protection devices for PV systems). The installer documents the inverter’s type test certificate reference number in the commissioning test report. No further live anti-islanding test is required where the inverter has a valid BS EN 62116 certificate.
2. Live test method: With the system generating at or near rated output, the grid connection is interrupted (using the site’s main isolator or a portable open-circuit test set). The time from grid disconnection to inverter shutdown is measured and recorded. G99 requires disconnection within the time set by the relevant protection setting — typically within 2.5 seconds for UV protection.
Most installers use the type test reference method, as it avoids the need for specialist protection relay test equipment. However, if the inverter’s type test certificate cannot be located or verified, the live test method must be used.
Keep a Record of Inverter Type Test Certificates
The inverter manufacturer’s type test certificates (BS EN 62116 for anti-islanding, BS EN 61000-3-2 for harmonics) should be included in the product documentation provided with the inverter. Download and retain copies from the manufacturer’s website for each model used — these are required for both G99 applications (submitted before commissioning) and commissioning test reports. For common UK inverter brands (Fronius, SMA, SolarEdge, Enphase, Solis, GivEnergy), the certificates are available on the manufacturer’s website or on request from the UK distributor.
RoCoF Testing
Rate of Change of Frequency (RoCoF) measures how quickly the grid frequency is changing, not just whether it has exceeded a threshold. RoCoF protection trips the inverter when frequency change is occurring too rapidly — a sign of a major generation or load imbalance on the network that may indicate an islanding condition or a large network fault.
Why 1 Hz/s, Not 0.5 Hz/s
The UK moved from a 0.5 Hz/s RoCoF setting (under G59) to a 1 Hz/s setting (under G99) for several reasons:
- As the UK grid has reduced its synchronous generation (coal, gas), RoCoF events have become more frequent during normal operations
- The 0.5 Hz/s setting was causing nuisance trips of solar installations during normal frequency variations, particularly in winter when grid inertia is lower
- A 1 Hz/s setting provides adequate protection against real islanding while reducing nuisance trips
The G99 standard explicitly specifies 1 Hz/s as the primary RoCoF setting for inverter-based generation. This is not a typo or a liberal interpretation — it is the defined value.
Test Method for RoCoF
Testing RoCoF protection at the correct trip point requires either:
- An inverter commissioning mode that injects a synthetic frequency ramp to test the RoCoF function
- An external protection relay test set (e.g., Megger MPRT series, Omicron CMC 356) capable of generating a test frequency ramp
Where the inverter has a commissioning test mode for RoCoF, use it and record the result. Where it does not, the type test certificate that confirms factory RoCoF testing at 1 Hz/s can be used in the commissioning report in place of a live site test — provided the protection settings are verified on the commissioning screen.
Vector Shift Testing
Vector shift protection detects a sudden change in the phase angle of the grid voltage — another indicator of islanding. When the grid supply is interrupted, the remaining generation (the solar inverter) cannot maintain exactly the same phase angle as the pre-fault grid — a phase jump occurs. Vector shift protection detects this jump and trips the inverter.
The G99 default vector shift setting is typically 6 degrees. As with RoCoF, vector shift testing can be performed using:
- An inverter commissioning test mode
- An external protection relay test set
- A type test certificate confirming factory testing
Many modern UK inverters (SolarEdge, Fronius, SMA, Enphase) combine their anti-islanding, RoCoF, and vector shift protection into a single integrated anti-islanding function certified to BS EN 62116. In this case, the type test certificate for the combined function covers all three protection methods.
Under/Over Voltage Trip Testing
Voltage protection testing verifies that the inverter trips at the configured voltage thresholds — 200V (UV), 264V (OV1), and 273V (OV2) — within the specified trip times.
Test Method
| Test | Method |
|---|---|
| UV trip (200V) | Simulate a voltage drop below 200V using the inverter’s test mode, or verify from type test certificate |
| OV1 trip (264V) | Verify configuration and type test certification; live testing requires injection equipment |
| OV2 trip (273V) | Verify configuration and type test certification |
| Trip timing | Record the configured trip time from the protection settings schedule; confirm against type test |
Under and over voltage trip testing in the field typically relies on type test certification for the specific trip voltage thresholds, combined with a live verification that the protection relay settings are correctly configured in the inverter.
230V ± 10% Is the Normal UK Voltage Range
The UK LV distribution network operates at a nominal 230V with a permitted range of +10%/-6% (217V to 253V) under EN 50160. The Over Voltage Stage 1 trip at 264V sits above the maximum permitted supply voltage — it is intended to trip the inverter when the grid voltage rises above normal bounds, not during normal voltage variations. The UV trip at 200V is well below the minimum normal supply voltage, providing margin against normal low-voltage conditions without nuisance tripping.
Under/Over Frequency Trip Testing
| Test | Setting | Trip Time |
|---|---|---|
| Under Frequency (UF) | 47 Hz | 20 cycles (~0.4s) |
| Over Frequency (OF) | 52 Hz | 0.5 seconds |
The UK grid operates at 50 Hz ± 1% (49.5 Hz to 50.5 Hz) under normal conditions. Under frequency at 47 Hz indicates a major generation deficit. Over frequency at 52 Hz is extremely rare in normal UK grid operation but would indicate a large unexpected load disconnection or generation surplus.
Frequency trip testing follows the same approach as voltage trip testing — type test certification supplemented by verification of the configured settings in the inverter commissioning interface.
Commissioning Test Report: What to Document
The G99 commissioning test report is the formal record of the commissioning test. The DNO can request to inspect it at any time. It must contain:
| Document | Content |
|---|---|
| Protection Settings Schedule | All configured protection settings as read from the inverter commissioning screen — UV, OV1, OV2, UF, OF, RoCoF, vector shift |
| Equipment Installation Records | Inverter make, model, serial number, firmware version; panel make, model, power rating; generation meter serial number |
| Test Results | Anti-islanding test method and result (live test result or type test certificate reference); RoCoF test result or type test reference; voltage and frequency trip verification |
| Type Test Certificates | BS EN 62116 anti-islanding certificate; BS EN 61000-3-2 harmonic emissions certificate |
| Installer Details | Installer company name, MCS number, installer name, date of commissioning |
| DNO Application Reference | G99 application reference number from the acceptance letter |
Document Retention
Retain the complete commissioning documentation file for a minimum of 2 years following commissioning. In practice, many installers retain documentation for the full life of the installation, as commissioning records may be requested by:
- The DNO at any time during network inspections
- Building insurers following a claim
- Mortgage lenders or conveyancers during a property sale
- The customer seeking to verify the installation specification
Post-Commissioning Notification: The Step Many Miss
After the system is energised and commissioning tests are complete, the installer must send a post-commissioning notification to the DNO within 28 days of energisation.
This notification is separate from and in addition to the G99 application submitted before installation. It confirms:
- The system has been commissioned and is generating
- Protection settings are configured per the G99 application and acceptance letter
- Commissioning test results are available for inspection
Missing the Post-Commissioning Notification Is a Common Compliance Gap
The 28-day post-commissioning notification is a G99 condition of connection — it is not optional. Failure to submit it means the DNO’s records show the G99 application as outstanding, not completed. This can cause problems during property sales, SEG registration reviews, and DNO network inspections. Build the post-commissioning notification into your installation process checklist so it is not missed.
What the DNO Can Inspect
The DNO has the right to inspect any G99-connected installation and its commissioning documentation at reasonable notice. During an inspection, the DNO may:
- Request the commissioning test report and protection settings schedule
- Verify the protection relay settings configured on the inverter match the documentation
- Test protection relay functions using portable test equipment
- Check that the generation meter, isolation switches, and labelling comply with G99 requirements
If the DNO finds that protection settings are incorrectly configured or commissioning documentation is unavailable, they can require the system to be disconnected from the network until the issue is resolved.
Common G99 Commissioning Failures
| Failure | Cause | Fix |
|---|---|---|
| RoCoF set to 0.5 Hz/s instead of 1 Hz/s | Inverter shipped with G59-era default setting | Update to 1 Hz/s via inverter commissioning interface or manufacturer firmware update |
| OV1 set to 253V instead of 264V | Confusion with EN 50160 upper voltage limit | Set to 264V per G99 default (or DNO-specified value from acceptance letter) |
| Missing post-commissioning notification | Not included in installer’s post-commissioning workflow | Add to commissioning checklist; submit within 28 days of energisation |
| No type test certificates | Certificates not included in inverter documentation | Download from manufacturer website before commissioning |
| Protection settings not recorded | Not checked/recorded during commissioning | Access inverter commissioning screen, record all settings in protection settings schedule |
| Wrong MPAN on commissioning notification | Taken from incorrect document | Verify MPAN against the electricity bill or smart meter display — not from a previous notification |
Using Solar Design Software for G99 Documentation
Solar design software that supports UK compliance documentation can pre-populate protection settings schedules with the correct G99 default values, reducing the risk of settings errors. For solar designing on G99-threshold projects, having equipment specifications and single-line diagrams ready from the design stage means the commissioning documentation is largely complete before the installer arrives on site.
The generation and financial tool also provides system output modelling that can be included in the commissioning file as a reference performance baseline — useful when comparing actual system performance against design values.
Generate G99-Ready Protection Settings Schedules in SurgePV
SurgePV produces pre-populated protection settings schedules and commissioning documentation formatted for UK DNO submissions — covering G98 and G99 projects.
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Related UK Compliance Guides
For the G99 application process (submitted before installation), see the G99 application guide. For battery storage commissioning requirements — including G99 requirements for AC-coupled battery inverters — see the battery storage compliance guide. For the complete UK solar compliance framework, see the UK compliance hub.
Frequently Asked Questions
What is the RoCoF setting for G99 commissioning in the UK?
The correct G99 Rate of Change of Frequency (RoCoF) primary setting is 1 Hz/s, with a trip time of 500ms. This is a common source of commissioning failures — some inverters ship with a default RoCoF setting of 0.5 Hz/s, which was the previous G59 standard. Using 0.5 Hz/s on a G99 installation is incorrect. Verify this on the inverter’s commissioning screen and record it in the protection settings schedule before submitting the commissioning report.
What voltage trip settings are required under G99?
G99 requires the following default voltage protection settings: Under Voltage at 0.87 pu (200V) with a 2.5 second trip time; Over Voltage Stage 1 at 1.14 pu (264V) with a 1 second trip time; Over Voltage Stage 2 at 1.19 pu (273V) with a 0.5 second trip time. The DNO may specify amended settings as a condition of acceptance — always check the acceptance letter for any network-specific conditions.
How long after commissioning must I notify the DNO under G99?
Under G99, the installer must send a post-commissioning notification to the DNO within 28 days of the system being energised. This notification confirms that the system has been commissioned, the protection settings are correctly configured, and commissioning test results are available. The post-commissioning notification is a separate submission from the pre-installation G99 application and is a condition of G99 compliance.
What is the difference between G98 and G99 commissioning testing?
G98 commissioning requires testing the inverter’s loss of mains (anti-islanding) protection and recording the result. This is relatively straightforward — most modern UK-certified inverters perform this test automatically. G99 commissioning requires a full protection relay test: anti-islanding, Rate of Change of Frequency at 1 Hz/s, vector shift, under and over voltage trip testing across all stages, and under and over frequency trip testing. The results must be documented in a formal commissioning test report retained for DNO inspection.
