Emergency Power Supply - TonyM1958/HA-FoxESS-Modbus GitHub Wiki

EPS or Emergency Power Supply is an option with Fox ESS inverters. It is a separate output from the inverter that is generated using the batteries when grid power is lost.

EPS Port

You need to specifically wire the inverter EPS output port to loads that you want to power when the grid is lost. You should note that:

  • EPS output is limited to the power rating of your inverter
  • Inductive loads, such as motors, can cause the EPS circuit to trip

More detail on the configuration, type of loads and switching that is required are provided in the Fox ESS H1/AC1(G2) User Manual

Internal Bypass mode:

Internal bypass mode is supported on a number of inverters and is also known as UPS mode (with later inverter firmware). This is because the EPS port on the inverter is powered from the grid when grid power is available and is powered by the inverter from solar or battery when grid power is not available. Although it is called UPS mode, there is a switch over time, sub 1 second, so this may not keep devices powered as it switches. For sensitive devices (such as a computer or router) that you don't want to reboot during switch over, you may still need to use a computer UPS.

External Bypass mode:

External bypass mode uses a separate (external) change-over switch to connect the EPS circuits to grid power when available and to the inverter EPS port when grid power is not available.

EPS Wiring (internal bypass mode)

Fox provides 2 diagrams detailing how to wire the inverter for EPS when using the internal bypass mode:

Alternatively:

Internal bypass mode is not recommended for use with hard wired EPS loads. It is only suitable for Class 2 devices (ones that are double insulated and do not require an earth connection).

EPS Wiring (external bypass mode)

Where higher loads or whole house EPS is required with Class 1 appliances, an external change-over or automatic transfer switch should be used. In some cases, this will be a 3 position switch - marked Grid, Off and Battery. This connects your backup loads to your inverter grid connection (Grid, for on-grid operation), disconnects your backup loads altogether (Off) or connects your backup loads to the inverter EPS port (Battery, for off-grid operation).

A local earth rod is also required - this ensures your house maintains an earth connection if any incoming supply earth is disconnected. In many situations, both a supply earth and local earth are now recommended, known as PME (protective multiple earth).

One of the consideration is how EPS N is handled when off-grid as it needs to be connected to local E otherwise the neutral wire will carry an AC voltage around half of the normal supply voltage (e.g. 115v for a 230v system).

Some new Fox inverters (mid 2024 onwards) include an internal relay to provide EPS N and local earth bonding when off-grid. However, most older inverters require this to be provided externally as part of the switching arrangement. There are various ways that this can be done so you should consult your installer or electrician.

Local regulations and established working practices must be followed at all times.

EPS Configuration

Inverter settings that configure EPS include:

  • Enable / Disable: determines if the EPS output is active
  • Bypass mode: determines if the internal or external bypass switch is being used
  • UPS mode: keeps EPS active through power cycles
  • EPS Voltage: normally 230v in the UK
  • EPS Frequency: normally 50Hz in the UK