And then there is safety to consider... Water - swiming pool - 230v. Proper design, construction and protection needed.

The 230v is a red herring - the OP has acknowledged that they are stepping 230V down to 12v with two transformers - one off the controlled 230V feed and the other an always on 230V. I think it's fair to assume these will be in safe places like a pump shed.

IIRC the valve documentation suggests that it has limit switches by saying that power can be applied for periods in excess of the 15 secs to activate, but being a cheap AliX valve, it may not be true. For a unit like that, I can't imagine how you would retrofit limit switches. 

Gordy7:

 

And then there is safety to consider... Water - swiming pool - 230v. Proper design, construction and protection needed.

 

 

 

And outside of the homeowners exemption for electrical work.

To clarify...

You have two separate 230 supplies, each with a 230VAC-12VDC converter. When supply A is good, you want to 12V from its converter to the switch. When supply A fails, you want to feed 12V from supply B's converter to the switch, inverted?

So, in fact, you would only be switching with 12VDC? And the 12VDC current would be less than 10A?

stuffed: All sorted! My Sparkie mate hooked up the Double Pole Double Throw (DPDT) with 1 x 12VDC being feed from the 230VDC and 1 x 12VDC feed on all the time.
So simple he said and it works brilliantly.

Spot the typo.

Spyware:

 

stuffed: All sorted! My Sparkie mate hooked up the Double Pole Double Throw (DPDT) with 1 x 12VDC being feed from the 230VDC and 1 x 12VDC feed on all the time.
So simple he said and it works brilliantly.

 

Spot the typo.

 

A smoking transformer... :-)

Gordy7:

 

Spyware:

 

stuffed: All sorted! My Sparkie mate hooked up the Double Pole Double Throw (DPDT) with 1 x 12VDC being feed from the 230VDC and 1 x 12VDC feed on all the time.
So simple he said and it works brilliantly.

 

Spot the typo.

 

 

A smoking transformer... :-)

 

 

 

Maybe not. Modern switch mode power adapters can probably handle 230VDC....