Whole-home backup changeover

Whole-home backup connects the normal household distribution system to a battery inverter when the public supply fails. It needs a changeover arrangement that makes the home a safe electrical island and prevents any part of the backup supply energising the public network.

A battery described as “backup ready” does not provide this by itself. The inverter output, switching, neutral treatment, electrode, protection and consumer-unit connections all have to be designed and tested together.

“Whole home” is a connection, not a power promise

The term means the normal consumer unit is on the backed-up side of the changeover. It does not mean the inverter can supply the full service-head rating or every household load at once.

The design must check:

• inverter continuous, short-duration and starting capability
• battery discharge limit at the expected state of charge and temperature
• maximum pass-through current while the grid is present
• the consumer unit and changeover assembly ratings
• motor, compressor and transformer inrush
• low inverter fault current and protective-device operation
• single-phase and three-phase load allocation
• which loads are automatically shed or manually restricted

High-demand circuits can remain physically connected yet be unavailable in an outage. The handover should make that distinction explicit.

The sources must never be paralleled accidentally

The island-mode isolator separates all required live conductors of the backed-up installation from the grid before the inverter supplies it. Restoration reverses the process only after the grid and inverter controls are in the correct state.

The transfer device must be suitable for isolation and for its current, voltage, fault and switching duty. The sequence is break-before-make unless the complete system is specifically designed and authorised for controlled parallel operation. A consumer must not improvise a changeover with plugs, interlocked-looking breakers or a generator lead.

Grid-connection type testing under G98 or G99 supports the inverter’s parallel operation and loss-of-mains protection. It does not, on its own, prove that a separate gateway provides compliant isolation, neutral switching or island fault protection.

Neutral-to-earth reference in island mode

On-grid, the installation normally relies on the supply transformer’s neutral-to-earth connection. Once the required live conductors are disconnected, the island cannot rely on that remote reference.

For the usual home EESS connected to a low-voltage public supply, the island-mode arrangement requires:

• an isolator that disconnects the backed-up live conductors from the grid
• controlled neutral-to-earth switching for the island source
• a suitable consumer earth electrode

These functions are coordinated so a local neutral-to-earth link exists when needed in island mode but does not remain as an uncontrolled parallel link when connected to the public supply. The IET describes the resulting island arrangement for most such systems as TN-S.

The distributor’s earth can remain physically connected. In a PME installation, PME conditions therefore continue to matter even though the island does not rely on the DNO earth as its source reference.

An existing TT electrode may be usable if it meets the EESS design requirements. An electrode fitted merely because “backup needs a rod” is not enough; its location, resistance, conductor and role must be verified.

Fault protection must work twice

The public supply can deliver far more fault current than a battery inverter. A fuse or circuit-breaker that trips promptly on-grid may not see enough current in island mode.

The designer must prove automatic disconnection in both modes, taking account of:

• inverter fault-current magnitude, duration and shutdown behaviour
• RCD type, placement and selectivity
• the neutral-to-earth switching state
• earth electrode and protective-conductor paths
• circuits on separate TT or PME earthing zones
• bidirectional current through consumer-unit devices

Testing only the on-grid loop impedance does not prove island protection. The commissioning record should identify the method and results used for each operating state.

Transfer time and sensitive equipment

Transfer time varies by product, load, operating state and firmware. A fast inverter EPS output does not prove that an external whole-home gateway has the same performance.

Use the technical specification for the installed configuration and confirm it during commissioning. If equipment cannot tolerate the stated interruption, provide an appropriate local UPS. Marketing words such as “instant”, “seamless” or “uninterruptible” are not measured results.

Load shedding and restart

A whole-home design should state which loads are blocked in island mode and how. Common candidates include EV charging, immersion heating, electric showers, cooking appliances and sometimes heat pumps.

Local hard-wired control is generally more dependable than a cloud command during a power cut. The design should define the safe state if a relay, CT, communications link or controller fails.

Restoration also needs thought. Several thermostatically controlled loads can all restart when grid power returns, while the battery is resynchronising or recharging. Staggered restart or continued load limits may be required.

Bypass, isolation and equipment failure

The owner needs a safe way to isolate the battery and, where the design provides it, bypass failed gateway equipment so the house can be returned to grid supply. Bypass switching must preserve source separation and neutral arrangements; it is not an informal rewiring instruction for an emergency electrician.

Keep a single-line diagram beside the main equipment and in the handover pack. It should show the grid, inverter, gateway, neutral-to-earth switching, electrode, backed-up distribution and non-backed-up loads.

The labels should remain understandable when the app and internet are unavailable.

Commissioning tests

Commissioning should demonstrate at least:

1. correct connected-mode import, export and protection
2. isolation from the public network before island energisation
3. correct line and neutral switching sequence
4. creation and removal of the island neutral-to-earth link
5. effective fault and RCD protection in both modes
6. electrode test and protective-conductor continuity
7. transfer under representative load
8. load shedding, overload and low-battery shutdown
9. restoration, resynchronisation and restart behaviour
10. manual isolation and any bypass procedure

The customer should see a controlled outage test before handover. Record firmware and settings because later changes can alter transfer, reserve and load-control behaviour.

Backup design boundaries

The backup design defines:

• whether backup covers the whole home or requires additional equipment
• continuous and starting loads in island mode
• circuits shed by local control
• transfer performance for the installed configuration
• neutral, earth electrode and RCD operation in each mode
• grid-supply operation if the inverter or gateway fails
• tests and diagrams retained at handover

Battery capacity without these system details does not define a whole-home backup installation.

Related entries

• Off-grid switchover and circuit grouping
• Battery location and ventilation
• Backup EPS output
• Earth electrode and rod installation
• Island mode and anti-islanding