Interesting point, but is be looking to consider the chances of a diyer working on that versus losing a phase of a three phase supply for a significant length of time.
Losing one phase of a small TP supply is pretty rare in my experience, especially for the length of time it takes a cylinder full of hot water to go cold.
Think about what the loss of a phase would entail, if it's a supply issue the DNO will be on it pretty quickly I'd have thought!
It could be the fuse in the head which goes I guess, but since it would be the fuse for your SP supply which goes then the whole house will be off so the hot water not heating will be secondary to the lack of lights and power.
Yes you have to label anything with a Uo above 230V where you would not normally expect it to be, but nothing in your install has a Uo above 230V
I agree that the changeover switch should be labelled regardless of that reg, due to fact anyone could open it up unwittingly.
You must have done more than change the contactor, the old one worked and this one didn't initially.
But regardless of that if the coil cct is unfused and just tapped off of the L1 terminal then you have the cables running to the timeswitch fused at 63A (in the exel) they don't look like 16mm to me
Obviously you don't always fuse down a coil cct, think about a basic DOL starter, but in this situation I would expect to see the coil cct fused down.
It's din mount contactor in a din rail enclosure so you could have easily popped a 1A MCB in there for the coil cct.
I'll see if I can post a pic of a recent lighting control I put together to illustrate this setup.