>each of those signal wires needs only to be shorted to the power to close the circuit and activate a particular function.

 

Yes, exactly (I do not know what two-stage is, so likely I over-simplified something).

For years, here in England, I was frightened to mess with landine phone circuits until I noticed that all the wires excpt two are just snipped off.

 

By the way, by contrast, the thing a home user (here) should never try to mess with is ordinary electrical power sockets, because they have an unbelievably subtle configuration. All the live, neutral, and ground wires are connected in a intentional circles called 'ring mains' (with redundancy and an intentional ground loop) so you can depend on how it will work even if any wire is cut, and, secondly all the neutral wires and all the ground wires are connected together...but they are connected outdoors. Even before RCD there were almost never any fatalities because of this strange system. So for example, with two leads coming into a house, to connect two sockets, you need nine wires, with most in parallel with other wires, where only six, or actually four, would suffice. I won't try to explain the theory behind this but it is non-trivial. Essentially, neutral and ground are two independent versions of ground that are redundant but isolated from each other in such a way that significant current cannot flow between them without being noticed at the fusebox.

 

Another weird thing is, the ground wire of an electrical shower was bare, but covered by a decorative green/yellow striped tube which did not totally cover it. This meant, because of having that large bare wet electrical wire next to you, you were sure you were completely safe. And, it worked. Where, back in the USA with half the voltage (one-quarter the power into any given resistance) there was actual significant danger just because of people mowing their lawn or trying to install an electrical shower when users were allowed to wire things up. They may still have those polarized but ungrounded plugs which go in either way, but you have to press harder to get them to go in with the reverse polarity.

 

Actually, that is a sense in which any of the wires to the boiler/furnace could, in the USA, be dangerous because the ground of the furnace/boiler could well be live compared to the ground of some other room. Something that would be dangerous but you'd never notice it without stretching a wire between the rooms for some purpose.