waiting opinions for motorhome internal / external mains switching circuit

[wanek]

Hello guys!

We are located in Europe, and building a motorhome. Currently I'm designing the electrical wiring for the mains 230V AC.

We will use a 24V / 230V 2kW inverter from battery, but when we have the possibility we would also use external AC. For the safe switching between the internal and external power, I designed a circuit, with the following points in mind:

- for external power, we plan to use the EU standard mains plug, because this is what we can find on most places in Europe

- the inverter and the external source is not allowed to be connected at the same time

- when the external power is used, we would like to automatically charge the motorhomes battery bank

- if the external power source has any faulty or abnormal wiring (eg. the order of the L, N, E is incorrect, or any of them is missing) the isolation relays should not engage

- the motrohome must have his own RCB and MCB, which we will use also for the inverter and external power

So I've came up with the following schematic, which I also built and tested for all the possible cases: it should work only if all 3 wires are present and all are in the correct order. Please take a look and tell your opinion.

For making things a bit more easy to follow:

0)
All the relays have small current coils (less than 6mV), so they will not trip any existing RCB (the lowest rated RCB on the market is 15mA, this coil uses 3 times less current)

1)
R7 is there for the case when the L and N are present and are in the correct order, but E is missing. In this case the current goes through both relays, K2 and K1. But because the current is weak, causes an abnormal functionality, sometimes engages the relays, sometimes not. For a safe function, if there is no earth, it must not engage K2, and must engage K1. R7 helps for both relays, in this aspect.

2)
K0 is used to completely isolate the external power, not only L and N, so if from any reason it is high voltage on the E line (faulty earth), it should not output on the whole vehicle. It is connected only if the external power has a safe ground

Thanks!

 

[HaldorEE]

Why wouldn't you use an inverter/charger that has a built-in ATS switch to manage automatically switching between shore and battery power?

All of the better quality inverter vendors sell products that do this safely and efficiently. The Victron Multiplus adds a power assist mode that provides additional power from the batteries to start high surge loads like motors, if the shore power supply is inadequate to do so. This is a very nice feature to have here in the USA because a common residential power source is 120VAC @ 15A which can be insufficient to start an air conditioner, but is enough to run the air conditioner, once it is started.

 

[wanek]

Why wouldn't you use an inverter/charger that has a built-in ATS switch to manage automatically switching between shore and battery power?

All of the better quality inverter vendors sell products that do this safely and efficiently. The Victron Multiplus adds a power assist mode that provides additional power from the batteries to start high surge loads like motors, if the shore power supply is inadequate to do so. This is a very nice feature to have here in the USA because a common residential power source is 120VAC @ 15A which can be insufficient to start an air conditioner, but is enough to run the air conditioner, once it is started.

i'm aware of those models, but:
- they are exaggeratedly expensive
- they do not care about missing / faulty shore ground connection, which in turn can be dangerous in certain situations
- we already have a victron phoenix 24/2000 smart inverter, and we can't afford to just buy a smarter one
- the cost for the components of the above mentioned setup are less than 80 euro, and it took me around 1 day work (design + build), compared with the 430 euro price difference between a phoenix vs multiplus of the same capacity
- with diy you can always explore new ways of problem solving and learn new stuff