Switching 230V source in the motorhome

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Updated - June 22, 2025

Why - automatically - switch the 230V source in the motorhome? However, if you are aiming for a certain degree of energy self-sufficiency, you also have several options for feeding AC consumers and switching between them as required.

230 V sources

The normal AC voltage supply (AC - alternating current) is powered by shore power, provided this is available, of course. On the road, there are two other options: supply via your own inverter or via a built-in (gas) generator.

As very few generators are designed to feed an inverter and it must be ensured that the generator or inverter cannot be switched on by mistake when shore power is connected, it would be helpful to interlock the three possible power supplies so that parallel operation cannot take place.
Of course, this also applies to applying voltage from the generator or inverter to the external socket for incoming shore power!

The selected energy supply should also automatically connect to the consumers, i.e. the motorhome network.

Network priority circuit

Electronic mains priority circuits usually have the disadvantage that they can only handle around 1,600 ... 2,000 W, which is why they cannot be used here, even though the advantage is switching at zero point.

Zero crossing

The zero crossing describes the moment at which the sine wave of the alternating voltage changes to the positive or negative half-wave, i.e. twice per sine wave, or at 50 Hz mains frequency, 100 times per second or every 10 ms.

At these moments, the current is zero, as the voltage is also zero, which means that load-free switching is possible.

The angle shown on the x-axis represents the phase shift starting from the zero point.

Neutral (N) and neutral conductor (PEN)

Potential for confusion offer the electrotechnical layman the common term Zero-conductor, often equated with the blue wire marking.

However, the conductor marked with the blue wire color is NOT the Zero-leader, but the Neutral-conductor (N). The current flows back to the neutral point of the three transformer coils of the electricity supplier via this conductor. Because it is located in the "center" of the three transformer coils, their neutral point, it was also called the Medium-ladder.

The yellow-green conductor, known in layman's terms as the "earth" conductor, is correctly labeled with Zero-or Protection-Manager (PEN – protective earth neutral).
Although the name "zero" conductor was in use until 1991, it was changed to PEN conductor as early as 1973, a combination of the designations "PE" and "N".

The phases (L1, L2 and L3), on the other hand, are often recognized correctly: black, brown and grey each represent one of the three phases (also called Exterior-conductors - because they are located on the outside, at the end, of the transformer coils). They supply the current from the three transformer coils to the end consumer.

This illustration shows the star connection of the three transformer coils: 230 V is measured between each phase (Lx) and the neutral point (N). However, 400 V is measured between each phase, not including the neutral point.
This explains why the neutral conductor (N) must never be disconnected with a time difference, as it keeps the voltages of the three phases at a common level of 230 V regardless of the load.

The alternative

The ABB Contactor ESB40-22N-06 can cope with 9,200 W, corresponding to 40 A, and should therefore have a good service life for the 16 A fuse usually used in motorhomes with more than double the power reserve.

Electronic high-performance relays (solid-state relays) SSR - Triac circuits) are ruled out due to the high heat development and the required, at least passive, cooling - even if they always switch at zero point, which would be desirable.

The obvious question of whether it might be possible to use two individual changeover relays to switch the neutral conductor and phase must be answered categorically with NO:
The neutral conductor must NEVER be connected at different times to the phase in order to avoid the risk of a load difference on the phases, Unbalanced load and the resulting Star point shift to be avoided. This could lead to a 230 V load being subjected to up to 400 V and thus being damaged.
Therefore, BOTH conductors MUST be switched at the SAME TIME via a (positively driven contactor and thus reliably).

Practical example

But how do you interconnect the whole thing?

The diagram below shows the wiring of the components (without the PEN drawn in):

materials

There are two pieces ABB ESB40-22N-06 needed.

The wiring should be connected with 2.5 mm² in order to keep cable losses close to zero. The use of ferrules for multi-core flex cables is taken for granted, but is explicitly mentioned here.

The values shown in the above diagram with S_Ext and S_Int designated contactors are each responsible for the external, or. internal responsible for voltage switching.

Shore power connected

The contactor S_Ext switches when the shore power supply is connected and connects zero to terminals 1 and 2, or phase to terminals 7 and 8. This establishes the power supply to the motorhome and the inverter (which thus charges the connected batteries in charging mode).

Shore power switched off - inverter supply

If the shore power connection is removed, the contactor drops out, disconnects the aforementioned connections and instead establishes a connection between the R3 and R4 (neutral) or R5 and R6 (phase) connections, which are connected to the second contactor. S_Int to the connections 1 / R3 and 7 / R5 and connect them via the connections R4 (neutral) and R6 (phase) to the output of the inverter, which in turn now supplies the motorhome with 230 V in inverter mode.

Shore power switched off - generator use

When the generator is started, the contactor S_Int disconnects the connection to the inverter and connects its AC output via contacts 1 and 2 (neutral) and 7 and 8 (phase) to the 230 V mains of the motorhome.

Shore power switched on

If shore power is connected, the contactor pulls S_Ext disconnects the currently active connection R3 and R4 (neutral) or R5 and R6 (phase) and routes neutral via contacts 1 and 2 or phase via 7 and 8 into the motorhome network.

Automated generator stop

If you use SmartHome, you can have the generator switched off automatically with incoming shore power by switching the generator ignition to ground (switch on the relay in parallel with the ignition switch for 5 seconds).
Switching off the relay after the above-mentioned time period ensures that the generator is ready to start again once this period has elapsed.

Notice

As there is no electronic switch-on delay of the power supply and no detection of the zero crossing of the incoming AC voltage, apart from the short switching time of the contactor, switching should not be carried out under significant load. This protects the contactor contacts on the one hand and the equipment on the other, as computers in particular do not like the phase shift that occurs during switching (difference in the time of the zero crossings of the AC voltage).

- Attention -

All electrical installation work must be carried out by qualified personnel in compliance with applicable regulations. When working on electrical installations, they must always be kept safely disconnected from the power supply!
To inform uninvolved third parties, appropriate information signs must be attached to the shutdown devices during the work.
The above information is intended as a suggestion and makes no claim to completeness, compliance with relevant regulations, standards, etc.. Any liability is excluded.