ENA Engineering Recommendation G100: Customer Export and Import Limitation Schemes
Instrument type: ENA Engineering Recommendation
Issuer: Energy Networks Association
Current version: Issue 2 Amendment 1, May 2022 (mandatory from 1 May 2023)
Canonical source: sources/energy-networks/ena-er-g100.md
PDF: https://dcode.org.uk/assets/uploads/G100_Issue_2_amendment_1__2022_.pdf
What It Is
EREC G100 defines the technical requirements for Customer Export or Import Limitation Schemes (CLSs) installed at electricity consumers' premises. A CLS is a control system that monitors current flow at the connection to the DNO's network and throttles the customer's generation or load to stay within an agreed limit.
The limits are: - MEL (Maximum Export Limit): the maximum current a customer may export onto the DNO's network - MIL (Maximum Import Limit): the maximum current a customer may draw from the DNO's network
CLSs are how DNOs enable generation or large loads to connect without network reinforcement. Instead of upgrading the network to full unconstrained capacity, the DNO agrees a lower limit with the customer. The CLS enforces that limit in real time.
Why It Exists
Distribution networks were not designed for bidirectional power flows. As solar PV, battery storage, EV chargers, and heat pumps proliferated on LV networks, DNOs increasingly faced requests for connections that would breach thermal or voltage limits. Full reinforcement is expensive and slow. G100 provides a third option: connect at reduced capacity with a control system guaranteeing that the limit is not breached.
The original Issue 1 (2016) covered export limitation only - primarily for solar PV and small embedded generation wanting to export more than their connection agreement permitted. Issue 2 (2022) extended the framework to import limitation, reflecting the growth of EV charging and heat pumps creating large demand connections constrained by LV network capacity.
How It Works
G100 structures CLS operation around four states:
State 1 - Normal operation. The CLS modulates connected Devices (inverters, chargers, storage units) in real time to keep current at the connection point within the agreed limits. For a solar farm with MEL of 200A, the CLS curtails inverter output whenever aggregate generation would push export above 200A.
State 2 - Occasional excursion. Brief breach of the MEL or MIL. G100 permits short excursions (under 10 s) as an inevitable consequence of switching events. Longer excursions must be corrected within 1 minute. If excursions become frequent or prolonged, the CLS escalates to state 3.
State 3 - Fail Safe. Any CLS failure, communication failure, or excessive state 2 pattern must trigger state 3 within 5-10 s: all controlled Devices are tripped or set to a defined low-power state. The CLS stays in state 3 until explicitly reset - for domestic installs, up to 3 resets in 30 days before lockout; for commercial and industrial, no reset for at least 4 hours.
State 4 - Operation without CLS. A pre-agreed DNO arrangement for operating some Devices while the CLS is out of service. Not available by default.
The fail-safe requirement is the core of G100. A CLS that fails open - that is, fails in a way that removes the limit - is not compliant. The standard requires that any failure mode, including loss of power, loss of communications, and internal component failure, must result in Devices being constrained or tripped, not released.
Relationship to G98 and G99
G100 is a companion to G98 and G99, not a replacement. The relationship is:
- G98 / G99: Govern whether generation equipment may connect to the distribution network at all, and what protection and notification requirements apply.
- G100: Governs how the Customer's control system enforces an agreed limit on what flows at the connection point.
A solar farm connecting under G99 may also need a G100-compliant CLS if the agreed MEL is lower than the farm's total installed capacity. Both ERs must be complied with simultaneously.
Practical scope
G100 is commonly encountered in:
- Rooftop solar with export cap. A domestic or commercial site with a large PV array whose DNO connection agreement sets MEL at 16A or 32A per phase. The G100 CLS limits inverter output to prevent export exceeding the agreed level.
- Rural sites with constrained HV connection. Farm, school, or industrial estate with CHP, wind, or solar where the local HV feeder cannot accommodate full export. MEL set below total generation capacity; CLS manages curtailment.
- EV charging hubs with constrained import. A fleet depot or public charger site where the local network can only supply 200A but 400A of charge points are installed. G100 MIL limits total simultaneous charging to avoid tripping the DNO's upstream protection.
- Battery storage with flexible connection. Battery connected under a flexible connection agreement with a variable set point issued by the DNO's ANM system. G100 CLS accepts and enforces the set point.
Domestic installations
For domestic installs (LV, up to 100A), G100 is intended to be low-friction. Manufacturers produce Fully Type Tested CLSs registered on the ENA Type Test Register. The DNO does not witness commissioning. The installer selects the MEL or MIL setting (from a limited range: 16A, 32A, 60A, 80A, 100A for export; 60A, 80A, 100A for import). The CLS must protect the setting from casual change.
The ENA Type Test Register is the compliance gateway for domestic-scale products. Manufacturers who register an FTT CLS provide customers with the ENA reference number; the installer records it on Form C. This replaces the need to submit the full Form B product declaration.
Voltage and thermal design limits
Before agreeing a MEL or MIL, the DNO must verify that worst-case state 2 operation stays within:
- LV connection: voltage between 87% and 112% of nominal (200 V to 258 V at 230 V nominal); current below 145% of upstream fuse rating
- HV connection: voltage between 92% and 108% of nominal; current within relay settings
The 112% cap for LV aligns with the overvoltage trip setting in G98 and G99 (114% with tolerance), preventing generation tripping on other customers' connections as a consequence of state 2 export.
Connection to wider distribution policy
G100's extension to import limitation in Issue 2 reflects Ofgem's Access SCR direction to move demand connections to a shallow charging boundary. Where DNOs cannot economically reinforce immediately, a constrained connection at lower cost - enforced by G100 - becomes a viable route to market for time-sensitive projects. The regime is essentially a bilateral contract (Connection Agreement) with a technical standard (G100) specifying the enforcement mechanism.
NESO's connections reform (Connections Action Plan 2024) and the move to a queue-based connection process are likely to increase use of constrained connections, making G100 compliance more common across commercial and industrial sites.
See also: ena-er-g98.md - Type A micro-generator connection requirements | ena-er-g99.md - Type B/C/D generation connection requirements | access-scr.md - Ofgem's connection boundary reform