Battery Energy Storage Systems (BESS) are rapidly transitioning from pure energy-arbitrage devices to essential whole-home or critical-load backup power systems. Designing an Emergency Power Supply (EPS) circuit requires careful management of earthing arrangements, neutral switching, and anti-islanding protection.

The Anti-Islanding Protection Requirement

Under Energy Networks Association (ENA) Engineering Recommendation G98/G99, when the grid supply fails:

  • The battery inverter must disconnect from the public electricity network within 0.5 seconds.
  • This prevents the battery from back-feeding a de-energized street cable, which would electrocute DNO linesmen working to restore power.

The Neutral-Earth Earthing Trap in EPS Mode

On standard UK TN-C-S (PME) supplies, the installation relies on the DNO incoming neutral for its earthing reference.

When an Automatic Transfer Switch (ATS) or EPS contactor opens during a power cut, it isolates the incoming Live and Neutral:

  • Floating Neutral Hazard: If the inverter supplies backup power without creating a local neutral-earth bond, the EPS output has no earth reference (floating system). Standard RCDs and RCBOs on the backup board will fail to operate during an earth fault!
  • Automatic N-E Bonding Relay: Modern UK-compliant hybrid inverters incorporate an internal automatic earth-neutral bonding relay that closes only when operating in islanded EPS mode, recreating a safe TN-S earth reference.
  • Independent Earth Electrode (TT): An independent earth rod must be installed to ensure the property maintains an effective earth reference even if the incoming supply cable is physically severed.

Dedicated EPS Backup Sub-Board vs Whole-House ATS

Design Best Practice:

• Critical Load Board (Recommended): Wire essential circuits (refrigeration, lighting, WiFi, medical equipment, gas boiler electronics) through a dedicated EPS sub-board within the inverter's continuous backup rating (typically 3 kW to 6 kW).

• Heavy Loads: Keep high-demand loads (EV chargers, heat pumps, electric showers, induction hobs) outside the backup circuit to prevent instant inverter overload shutdown upon grid failure.