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Backup gateways vs EPS: wiring and earthing

Short answer

In island mode the house must become a TN-S system with its own earth electrode. Isolate every line and the neutral from the grid, never the grid earth. Bond neutral to earth at one point only, via the gateway's N-E relay, while islanded. Never rely on the DNO's PME earth. Choose RCDs to suit electrode resistance and test both modes.

Last checked 24 September 2026 · 12 sources

What should I know before deciding?

Only one system referencing point at any time; SRCSD switching <5 s without reference acceptable (BS EN IEC 63445:2025). SDFI and SRCSD interlocked (BS 7671 826.1.1.5). ADS: Ze <= 1.2UsU0/(S_PCE*Ia) - (R1+R2). Tesla Backup Gateway 2: local earth rod required; internal contactor re-bonds N-PE when L and N open. PEN fault risk: with PME supply, the island earth must not depend on the supplier's PEN; keep electrode separate per CoP 9.4.10.

Which home batteries give whole-home backup in a power cut?

Whole-home backup needs a gateway or backup box that isolates the house from the grid: Tesla Backup Gateway 2, Sigen Gateway HomePro, Fox EPS Box SP or the Giv-Gateway. Many hybrids only offer an EPS output for a few circuits, and some, like the UK Enphase IQ Battery 5P, offer no backup (checked 24 Sept 2026).

Whole-home via gateway: Tesla PW3 + Backup Gateway 2 (100 A single-phase); SigenStor + Sigen Gateway (SP 12/22 kW, TP 30 kW); Fox EVO/H1-G2/KH + EPS Box SP (launched 18 July 2025); GivEnergy AIO + Giv-Gateway (100 A, 20 ms). EPS circuits only: GivEnergy Gen 3 hybrid (3.6 kW/16 A, 10 ms), Solis S6 (<10 ms), SolaX X1 G4 (<10 ms), Dura-i (10-20 ms), EcoFlow PowerOcean backup port, AlphaESS SMILE-G3 backup port. Socket/lighting circuit only: myenergi libbi optional blackout backup.

What tests prove a backup island is safe?

Test continuity, insulation, polarity and earth electrode resistance, then measure Ze/Zs in island mode to prove the neutral-earth switching works. Test RCDs in both grid and island modes, check disconnection with the IET formula because inverter fault current is limited, and functionally test switchover, interlocks and test buttons (IET Wiring Matters, July 2026).

MIS 3012 cl.4.1.3: loop impedance and PFC in all modes. Electrode test method per IET Guidance Note 3.

Where does the grid CT go, including with a backup gateway?

Put the grid CT or meter at the connection point: on the meter tails between the meter and everything else, so all load, generation and the battery sit downstream. Orientation varies: Fox says arrow toward the grid, SolaX says never pointing at the inverter, Sunsynk and GivEnergy say toward the house. Tesla gateway CTs must read positive on import.

With a gateway, the site CTs sit on the grid side of the gateway contactor (or in the gateway's own meter). Split/multiple tails or a second supply: every import path needs a CT, or use an aggregated site meter (Tesla 'virtual aggregated meter'). Three-phase: one CT per phase (Tesla allows 1CTx3 for PV only via a toggle).

What to do next

Record the design assumptions, protection settings and commissioning results. Keep the DNO approval and final settings with the handover pack.

Sources

Last checked: 24 September 2026