G98 And G99: What UK Standby And Prime Generator Owners Need To Know
Imagine a hospital switching to a closed-transition ATS so there’s no interruption to critical systems — then discovering during commissioning that the change makes the installation a G99 connection. If you design, specify or manage standby or prime generators in the UK, this guide explains exactly when G98 or G99 applies, what you must submit to the DNO, and practical steps to avoid delays and extra cost. In our experience, early decisions on transfer type and control philosophy save weeks and significant expense.
G98 Versus G99: Clear Thresholds And Practical Impact
G98 covers small microgeneration up to 16 A per phase (roughly 3.68 kW single-phase, ~11 kW three-phase) and typically uses type-tested inverter kits. Anything larger or any generator that can parallel with the public network falls under G99. A common issue we see is assuming a large standby set is non-parallel simply because it’s for resilience — if it can synchronise for no-break transfer or testing, G99 applies.
If you’re choosing a commercial or industrial set, decide early whether you need parallel operation. That decision drives switchgear, protection and DNO engagement. Use our kVA calculator to size the plant before specifying control gear or protection.
Standby Versus Prime Power: Which Scenarios Trigger Compliance?
Open-transition (break-before-make) ATS means the generator never parallels the DNO, so G98/G99 is usually out of scope. Closed-transition/no-break ATS, synchronised transfers, peak-lopping, or on-load testing that momentarily parallels the network all trigger G99.
If you’re in a UK hospital, data centre or manufacturing plant where seamless transfer is required, plan for G99 from the outset. For remote or temporary off-grid sites that never form a parallel connection, G98/G99 will generally not apply — but you still need documented interlocks and correct earthing.
What Most People Get Wrong
Most teams underestimate how common “unintentional paralleling” is during commissioning or monthly tests. A closed-transition ATS, certain controller defaults or a test routine can create a brief parallel condition that brings the whole installation under G99.
When This Doesn’t Apply
If the generator cannot physically parallel the network — true open-transition ATS with proven mechanical and electrical interlocks and documented commissioning records — G98/G99 usually does not apply. Temporary isolated supplies are also out of scope provided no parallel path exists.
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Permitting And DNO Process: A Practical Step-By-Step
Pre-application: confirm maximum export (including zero-export), prepare a single-line diagram, protection concept and relay type-test certificates, and check site capacity. Submit the G99 application to your local DNO and expect a connection offer detailing scope, costs and any required reinforcements.
Build to the agreed design, complete FAT/SAT, and plan for DNO witness testing if required. PowerTech Generators manages design, submissions, installation and commissioning end-to-end. See our installation and maintenance services for compliant delivery and handover documentation.
Protection Relays, Settings And Practical Choices
Interface protection must isolate the plant on over/under voltage, over/under frequency and Loss of Mains (including RoCoF where required). Vector shift is commonly disabled in ENA guidance; matching relay settings to the DNO offer is essential. A common issue we see is mismatched time delays that cause nuisance trips.
Choose type-tested interface relays or fully engineered panels from known vendors and record every setpoint. Keep calibration certificates, test reports and as-built single-line diagrams for audits and future modifications. For ATS selection and control philosophy, review options for ATS panels early in the project.
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Witness Testing, Commissioning And Handover
The DNO offer will state whether witness testing is required. For larger or bespoke schemes, expect DNO attendance. Typical checks include functional I/O, protection operation (secondary injection or on-set tests), synchronising trials and export-limit proof where a G100 is specified.
PowerTech Generators coordinates site safety, energisation and test scripts, and supplies as-built drawings, settings sheets, relay certificates and commissioning records on completion. For scheduling and site-specific advice, contact our team.
Safe Operation: Practical Differences Between Non-Parallel And Parallel Systems
Non-parallel systems need a proven open-transition ATS, correct neutral/earth treatment and routine verification to prevent backfeed. Document the scheme and include checks in annual maintenance.
Parallel systems require sync-check relays, breaker interlocks and robust control logic. Protect resilience with regular load testing using a load bank where site load is insufficient, and align protection re-tests with your generator servicing plan.
Costs, Lead Times And Common Pitfalls
Budget for DNO application fees, interface protection panels, export limiters (if used), and DNO witness or commissioning time. Lead times depend on DNO workload and network reinforcement needs. Common pitfalls include accidentally specifying a closed-transition ATS, missing type-test certificates, unclear single-line diagrams, and weak zero-export strategies.
PowerTech Generators can audit existing installations and upgrade legacy G59 protection to meet G99 requirements with minimal downtime. For specification guidance, see our pages on commercial diesel generators and choosing an industrial generator.
Quick Checklist
- Decide transfer type (open or closed) at project start
- Prepare single-line, protection concept and export limit strategy
- Confirm relay type-tests and supplier traceability
- Plan FAT/SAT and DNO witness windows
- Keep commissioning records, calibration certificates and SLDs
How PowerTech Generators Delivers Compliant Projects
We offer full-service delivery: design, permit submissions, supply, switchgear, controls, installation and commissioning across the UK. We work with major engine brands, support new builds and upgrades, and provide 24/7 spares and support to keep critical power online.
Next Steps: Plan Your Compliant Standby Power System
Start with a site survey and single-line review. Confirm operating mode (non-parallel or parallel), transfer type, maximum export and available switchgear space. Use our kVA calculator to validate sizing, then engage early on protection and DNO submission to avoid rework.
FAQs
How Do I Know If My ATS Makes My Project G99?
If the ATS can perform a closed-transition or synchronised transfer, or if commissioning/tests can momentarily parallel the network, the installation is G99. Check mechanical interlocks and controller defaults and get site evidence recorded.
What Documents Will The DNO Expect With A G99 Application?
Prepare a single-line diagram, protection philosophy, relay type-test certificates, export figure (or zero-export plan), site capacity data and site contact details. These reduce follow-up questions and speed approval.
Can Zero-Export Be Achieved Without Full G99 Works?
No. Zero-export requires a G99 application and a G100-compliant export-limiting scheme with approved controls and testing to prove no export occurs.
How Long Should I Allow For DNO Approval And Commissioning?
Timescales vary by DNO and scope. Allow several weeks to a few months from application to offer and commissioning; engage early to identify reinforcement or witness testing that could extend the programme.
Can Older G59 Installations Be Upgraded To Meet G99?
Yes. We routinely survey legacy systems, design required interface protection and controls, and manage the DNO process to bring installations up to current G99 requirements with controlled downtime.