G99 vs G100: which do you actually need?

If you're connecting generation to the grid in Great Britain — solar, batteries, wind, hydro, CHP or generators running in parallel with the mains — you'll meet two Engineering Recommendations from the Energy Networks Association: G99 and G100. They get mentioned together so often that they blur into one, but they do completely different jobs, and plenty of projects need only one of them.

G99 answers the network operator's first question: if something goes wrong, will your generation disconnect when the network needs it to? A G99 protection scheme watches the mains — voltage, frequency, loss of mains — and trips your generation off when conditions leave the limits written into your connection agreement.

Every generator that operates in parallel with the grid needs G99 protection appropriate to its size, from a single rooftop inverter (where it's built into the inverter under G98/G99 type testing) up to multi-megawatt sites with dedicated relay panels, CT/VT metering and witness-tested settings.

G100 answers a different question: how do we stop your site exporting (or importing) more than the connection agreement allows — continuously, automatically, and provably? That's an Export Limitation Scheme: measure true site export at the point of supply, and rein generation back — a simple scheme trips it, a dynamic scheme trims it — before the agreed limit is ever breached.

You need G100 when the DNO offers you less export capacity than your generation could produce. Without it, the choice is shrink the system or pay for network reinforcement. With it, you can install the array or battery the site deserves and cap what leaves the site instead.

Ask two questions about your project:

• —Will generation run in parallel with the mains? Then G99 protection applies — always.
• —Is your export (or import) capped below what the system could produce? Then you also need a G100 export limitation scheme.

EREC G100 — currently Issue 2 Amendment 2, G100/2 for short — lets manufacturers have a complete scheme independently tested against its requirements: settable limits, reaction times, fail-safe behaviour, lockout — and entered on the ENA Type Test Register that DNOs check. A registered, fully type-tested scheme means your application carries test evidence rather than assurances: connections are typically accepted on the documentation, with on-site testing only where the DNO chooses to attend.

That's the route ENICO took. The ENICO G100 Controller is fully type-tested to EREC G100 Issue 2 Amendment 2 and listed on the ENA Type Test Register — reference ENICT/19509/V1, checkable by any DNO or client. It's supplied as a standalone panel or built into the switchgear it belongs with, and the same scheme covers solar, batteries, wind, hydro and diesel generation.

A constrained connection usually means one panel doing two jobs: G99 protection to disconnect when the network demands it, and G100 limitation to stay inside the agreed capacity the rest of the time. We build them into a single assembly — shared metering where appropriate, one set of documentation, one witness visit — and for solar work, the same panel can carry the generation metering and monitoring too.