Checking whether a wire is live, neutral or switched live...

Hi,
Please see below for full story...Another suggested all should be fine to plug the wires I have into the Salus stat as long as I can confirm which is live, which is switched and if the earth is connected. Any ideas how I can do this based on the facts below?

My old Honeywell T40 room thermostat has lost the will to live so the plan is to upgrade it to the Salus RT300.
My problem is this. Currently the T40 is connected as follows - a black wire goes to terminal 1, a red wire goes to terminal 3 and the earth goes to the earth. That's it, no other wires, no other colours. First concern is that, despite the T40 seeming to work fine in the past, the red and the black wires are connected round the wrong way - is this the case? Second concern is that the Salus wants me to connect a live wire and a switched live wire - what's happened to red is live and black is neutral? I'd assumed that the red wire is live and that the black wire is neutral - is this not the case? To confuse me even more, a plumber (who had been in servicing our boiler) said that the black wire was in fact a load wire... So is it neutral, load or switched live? Or are load and switched live the same thing?
Please help...

 

Thank you for that. How can I check what each wire is doing? - bearing in mind that I'm your bog standard DIYer!!
What is the worst that could happen if I put the red wire onto the live terminal, the black into the switched live and the green into the earth?
Regards

 

Okay, that makes sense thank you.
So I'm safe to connect red to live, black to switched live and green to earth?
And to confirm - load and switched live mean the same thing?

 

What is "you know what"?

 

I can confirm that the green wire is connected to the earth on the old stat.

 

I'm going in - fingers crossed...

 

Yes.

 

Thanks for your help. Nothing blew up and everything seems fine. Lights and numbers on. Let's just hope we don't freeze in the night...