What Is an Automatic Transfer Switch?
An automatic transfer switch (ATS) automatically selects between two AC power sources — typically campsite shore power and your inverter — so that your 230V sockets always have power without manual switching. When you plug into a campsite bollard, the ATS detects mains voltage and switches your circuits to shore power. When you unplug, it switches back to inverter power within milliseconds.
Without an ATS, you would need to manually flip a changeover switch every time you connect or disconnect shore power, which is inconvenient and easy to forget.
Victron Transfer Switch Options
Victron offers two main transfer switch solutions, each suited to different system configurations:
Victron Filax 2
The Filax 2 is a dedicated automatic transfer switch designed specifically for systems where the inverter and mains charger are separate components (for example, a Phoenix inverter paired with a separate Blue Smart IP22 charger).
Key specifications:
- Transfer time: Less than 20 milliseconds (no interruption to sensitive electronics)
- Maximum current: 16A per input (3,680W at 230V)
- Inputs: 2x AC input (typically shore power + inverter)
- Output: 1x AC output to consumer unit
- Priority: Input 1 has priority — when both sources are present, Input 1 is selected
- DIN rail mounting: Fits standard consumer unit rail
MultiPlus / MultiPlus-II Built-in Transfer Switch
If you are using a Victron MultiPlus or MultiPlus-II inverter/charger, you do not need a separate transfer switch. These units have an integrated automatic transfer switch that handles the changeover internally. When shore power is connected to the MultiPlus AC input, it passes mains through to the AC output while simultaneously charging the batteries. When shore power is disconnected, it automatically switches to inverter mode.
The MultiPlus-II transfer switch handles up to 32A (on 32A models) or 50A (on 50A models), with a transfer time under 20 milliseconds.
When Do You Need a Filax 2?
You need a Filax 2 (or similar ATS) when your system uses separate inverter and charger components:
- Phoenix Inverter + Blue Smart IP22 charger — The Phoenix is a pure inverter with no AC input, so there is no way to pass shore power through it. The Filax 2 switches your 230V circuits between the inverter output and direct shore power
- Phoenix Inverter + Skylla-i charger — Same situation as above, just with a higher-capacity charger
- Budget systems — A Phoenix inverter plus a separate charger can be cheaper than a MultiPlus for smaller systems
You do not need a Filax 2 if you have:
- A MultiPlus or MultiPlus-II (built-in transfer switch)
- A Quattro (built-in dual transfer switch)
- A system that never uses shore power
Installation Guide
Filax 2 Wiring
The Filax 2 has two AC inputs and one AC output. The wiring is straightforward but must be done carefully as you are working with 230V mains electricity.
Always isolate all power sources before working on AC wiring. If you are not confident working with mains electricity, hire a qualified electrician.
Step 1: Plan Your Circuit Layout
Decide which circuits should be on the transfer switch output. In most motorhome installations, all 230V circuits go through the Filax 2. The wiring path is:
- Shore power inlet → RCD → Filax 2 Input 1 (priority)
- Inverter output → Filax 2 Input 2
- Filax 2 output → MCB consumer unit → 230V sockets and appliances
Step 2: Mount the Filax 2
The Filax 2 mounts on a standard 35mm DIN rail. It fits inside most motorhome consumer units or can be mounted on a separate DIN rail nearby. Allow adequate ventilation around the unit.
Step 3: Wire the Connections
Input 1 (Shore Power — Priority):
- Live (brown) from RCD output → Filax 2 Input 1 L terminal
- Neutral (blue) from RCD output → Filax 2 Input 1 N terminal
- Earth (green/yellow) → common earth bar
Input 2 (Inverter):
- Live (brown) from inverter AC output → Filax 2 Input 2 L terminal
- Neutral (blue) from inverter AC output → Filax 2 Input 2 N terminal
- Earth from inverter → common earth bar
Output (to your circuits):
- Live (brown) from Filax 2 Output L → MCB consumer unit live busbar
- Neutral (blue) from Filax 2 Output N → consumer unit neutral bar
- Earth → common earth bar
Step 4: Set the Priority
By default, the Filax 2 prioritises Input 1. Connect shore power to Input 1 so that whenever mains is available, it is used in preference to the inverter. This saves battery power and ensures the charger receives mains power to recharge the batteries.
Step 5: Test
- With shore power connected, verify that 230V appears at your sockets
- Disconnect shore power and verify the inverter takes over (you should hear a brief click from the Filax 2)
- Reconnect shore power and verify it switches back to mains
- Check that the LED indicators on the Filax 2 show the correct active input
Filax 2 vs Manual Changeover Switch
| Feature | Filax 2 (Automatic) | Manual Changeover |
|---|---|---|
| Transfer time | Under 20ms | Manual operation |
| Convenience | Fully automatic | Must remember to switch |
| Sensitive electronics | No interruption | Brief power loss during switch |
| Risk of error | None | Forgetting to switch can backfire |
| Cost | Higher | Lower |
| Complexity | Simple wiring | Simple wiring |
Safety Considerations
- RCD protection: Always install an RCD (residual current device) on the shore power input. Most motorhomes have this already as part of the mains inlet. The RCD should be rated at 30mA for personal protection
- Earth bonding: The inverter output neutral must be bonded to earth for the RCD to function correctly when running on inverter power. Some inverters have a built-in ground relay that handles this automatically — check your inverter's manual
- MCB ratings: Ensure your MCB (miniature circuit breaker) ratings do not exceed the Filax 2's 16A maximum. A single 16A MCB on the output is appropriate for most motorhome installations
- Never back-feed: The Filax 2 prevents back-feeding by design, but always verify that your inverter output cannot feed back into the shore power inlet, which could be lethal to anyone working on the campsite electrical supply
Troubleshooting
Filax 2 keeps switching back and forth
This usually indicates unstable shore power — the campsite supply voltage is fluctuating near the Filax 2's detection threshold. Some older campsites have poor electrical infrastructure. Try a different bollard or pitch. If it persists at multiple sites, check your shore power cable for damage or loose connections.
No output when shore power is connected
Check the RCD has not tripped. Verify that the shore power cable is properly connected and that the campsite bollard is switched on. Test with a multimeter at the Filax 2 Input 1 terminals to confirm voltage is present.
No output when running on inverter
Check that the inverter is switched on and producing AC output. Test with a multimeter at Filax 2 Input 2 terminals. If voltage is present at the input but not the output, the Filax 2 may be faulty.
Frequently Asked Questions
Can the Filax 2 handle a 3kW inverter?
The Filax 2 is rated at 16A (3,680W) so it can handle any inverter up to that output. For higher-power systems, you would typically use a MultiPlus or MultiPlus-II which has a built-in transfer switch rated for the unit's full output.
Do I need a Filax 2 with a MultiPlus?
No. The MultiPlus and MultiPlus-II have a built-in automatic transfer switch. Adding a Filax 2 would be redundant.
Can I use the Filax 2 with a generator?
Yes. Connect the generator to Input 1 (priority) and the inverter to Input 2. When the generator runs, it will power your circuits and charge your batteries via a separate charger. When the generator stops, the Filax 2 switches to inverter power automatically.
What happens during the 20ms transfer time?
Most electronic devices have capacitors that bridge this brief gap. Lights may flicker imperceptibly. Clocks, computers, and fridges continue operating normally. The transfer is fast enough to meet the requirements for uninterruptible power supplies.
