G99 Witness Testing: What It Is And When Your Standby Generator Needs It
ENA EREC G99 sets the UK requirements for any generation that can parallel with the grid. Your standby diesel generator needs a G99 witness test if it can parallel at any time, including closed transition, soft loading, peak‑lopping, or export‑limited operation. Open‑transition ATS with zero overlap is usually outside scope, but always confirm against your DNO’s guidance, your connection agreement and project scope. See our overview of G99 and G98 compliance.
Who’s involved: the DNO engineer (or ICP), the client, and the generator/ATS integrator and commissioning team. Outcomes: verified interface protection, a completed DNO test schedule, and approval to energise in parallel. If you are considering closed‑transition ATS options, plan G99 early to avoid outages and rework.
Pre‑Assessment And Site Readiness: Pass First Time
Arrive with a complete, accurate package so there are no surprises. Confirm the single‑line diagram, protection study, DNO‑issued settings, CT/VT data, earthing and neutral switching and earthing method, metering schematic, ATS sequencing, breaker trip coils, interlocks, and the G99 relay I/O map. Check generator make/model (Cummins, Perkins, FG Wilson, Baudouin), governor/AVR type and control philosophy. Drawings must match “as‑built”.
- Document pack: SLD, settings, RAMS, permits, access and LOTO plan.
- Hardware: labels, cable IDs, CT orientation, test links, safe isolation points.
- Operations: UPS/VFDs, essential loads, ventilation/noise limits, fuel readiness and safe test windows. Plan generator installation and any servicing tasks in advance.
Deriving And Validating Protection Settings
Translate the DNO settings pack into the interface protection relay. Apply current ENA guidance: use the specified ROCOF method, typically disable Vector Shift, and configure staged under/over‑voltage and under/over‑frequency. Validate CT/VT ratios, scaling and polarity before you inject. Record both “proposed” and “as‑left” settings for traceability.
Integrate export limits, run‑up/run‑down delays, breaker fail, anti‑islanding and ATS interlocks. Confirm the trip path opens the point of common coupling. For coordination and selectivity, review generator output and interface protection. Keep notes clear if you are managing multiple sets or mixed controllers.
This image was generated with AI and may not always represent the product or service exactly.
Test Equipment And Calibration Requirements
Minimum kit: a calibrated secondary injection set (e.g., Omicron or Megger), multimeter, clamp meter, phase rotation tester, isolation tools, test leads/adaptors, laptop, and a camera/printer for evidence. Carry current calibration certificates and include them in the evidence pack. Time‑sync your laptop, test set and relay so timestamps align.
- Prove CT/VT polarity and burdens before any injection.
- Verify trip coil health and breaker status feedback.
- Standardise file formats and naming for quick DNO review. See related guidance on ATS compliance testing.
Loss‑Of‑Mains And Interface Trip Verification
Load the ROCOF test profile from the DNO sheet. Inject events, capture pick‑up value and trip time, and verify alarm/trip logic. Confirm the protection operates the correct ACB/trip coil to open the point of common coupling. Prove loss‑of‑voltage blocking and breaker status inputs. Disable Vector Shift where the DNO requires.
Check reclose/return‑to‑service delays and permissives match the schedule. Do not use defaults; always follow the latest ENA G99 updates and the issued test sheets. Capture relay event records and screenshots alongside your test set report for a clear audit trail.
Voltage And Frequency Protection: Staged Tests And Timing
Test in sequence. First, verify UV/OV Stage 1 ride‑through (no trip), then confirm Stage 2 trips with measured clearing time. Repeat for UF/OF. Record the setpoint, measured trip and total clearing time, with screenshots and relay event logs. Confirm deadbands and delays match the DNO schedule.
Watch interaction with UPS and VFDs, especially during under‑frequency or shallow voltage dips. If nuisance trips occur, review coordination and harmonic behaviour. Our guide to UPS–generator integration helps avoid repeat faults.
Functional, Synchronising And ATS Sequencing Checks
Prove synchronising permissives: voltage, phase angle and frequency windows. For closed transition, measure overlap time and confirm safe fallback to open transition. Validate breaker interlocks, status indications and fail‑safe logic. If specified, demonstrate reverse power and overcurrent under controlled test conditions.
Where safe, perform a step load or load rejection using site load or a temporary load bank. Observe governor and AVR stability, voltage dip and recovery. Capture controller trend logs or oscilloscope traces as evidence to inform future servicing and tests.
This image was generated with AI and may not always represent the product or service exactly.
DNO Liaison For G99: Booking, On‑The‑Day Process And Safety
Timeline: submit pre‑test evidence, agree the schedule and RAMS, then book the witness. Confirm access, permits and switching plans in writing. Hold a pre‑start briefing, complete a walkdown, and prove isolation and LOTO before energisation. Follow the agreed sequence and record each step.
If a result is borderline, collect data, propose controlled setting changes, and document any deviations for DNO acceptance. Conclude with sign‑off, export the as‑left settings, and issue a concise summary to stakeholders.
G99 Evidence Pack And Certification: What To Submit
Provide a complete, clearly named pack. Include the signed test schedule, calibration certificates, relay settings files and screenshots, CT/VT verification, wiring photos (before/after), synchronising checks, timing sheets and event logs. Add the as‑left SLD and interlock drawings.
- Use a clear structure: date_site_testcode folders.
- Update O&M manuals and reflect label changes on drawings.
- Record operator training and handover notes. Timely, complete packs accelerate DNO approval.
Common G99 Pitfalls And A First‑Time Pass Checklist
- Earthing/neutral strategy wrong (3‑pole vs 4‑pole ATS) or not documented.
- CT/VT ratios or polarity incorrect; scaling not verified before tests.
- Relay inputs mapped to the wrong breaker status or trip coil.
- Vector Shift left enabled when the DNO requires ROCOF only.
- Dead times/overlap timers not set per schedule; time sources not synced.
- Firmware out of date; no pre‑day dry‑run.
Sectors, Scenarios And Equipment We Support
Data centres: UPS interaction, harmonics and tight change windows. Healthcare: HTM constraints and zero unplanned interruptions. Manufacturing: large motors/VFDs, voltage dip and stability. Construction and utilities: temporary closed‑transition needs and safe switching.
PowerTech Generators supports Cummins, Perkins, FG Wilson, Baudouin and more. We handle modern controllers and relays across new and used generators UK, generator hire and export generators.
How PowerTech Generators Delivers First‑Time G99 Passes
We front‑load success with impartial design review, DNO liaison, settings engineering and a dry‑run before the witness. On site, calibrated kits, disciplined method statements and experienced engineers deliver clean results and complete evidence packs.
Beyond testing, we supply ATS panels, new and used sets, temporary load banks and generator hire, plus lifecycle generator installation, generator servicing and decommissioning. Need help now? Book a pre‑assessment via contact for a swift, practical plan.
FAQs
Do all standby generators need G99 witness testing?
No. If your ATS is open transition with zero overlap and there is no ability to parallel, witness testing typically does not apply. If any parallel is possible, G99 usually applies.
How long does a G99 witness test take?
Most single‑set sites complete in half to a full day if pre‑assessment items are in place. Complex sites or remedial work can extend this.
What if the DNO settings differ from our relay defaults?
You must implement the DNO settings and record as‑left values. Do not use defaults; validate CT/VT scaling and timers before injection.
Can we test with live loads on the system?
Yes, but only with an agreed plan, safe windows and LOTO. Many functional checks can be done with a load bank to protect essential loads.
Will UPS or VFDs affect the tests?
They can. Harmonics and ride‑through behaviour may influence timings and nuisance trips. Coordinate thresholds and verify stability during staged tests.
What evidence does the DNO expect after the witness?
A signed schedule, calibration certificates, settings files, timing reports, CT/VT checks, wiring photos and updated drawings. Clear file names speed approval.
Do you support multi‑set synchronising systems?
Yes. We engineer settings, interlocks and sequencing for single and multi‑set plants to ensure stable, compliant parallel operation.