Limit switch wiring question ...

[Jonathan]

I agree, leave the other end disconnected to avoid a ground loop.

Lets us pretend that the interference is actually a snowstorm containing various electromagnetic waves, from motors, radio, TV, sparks, transformer, random noise etc. Your shield, or coat in this analogy, is some sort of metal shroud that creates a low resistance path to ground. It should stop the snow from getting in, or out however this depends on the type/size of snow and coat. If the shield is completely closed then not much is going to affect it, however if there are holes in the shield, such as in an open braid it's different. These holes are effectively closed if the snow is too big (the source of interference has a long wavelength) and vice versa.[It'd be interesting to compare this with the frequency stepper driver uses.] Snow settling on you whilst walking outside probably wont bother you, however if you never wipe it off or it doesn't melt then it could impede your walking. However if your coat allows the snow to slide off easily to ground then there's no problem. If you walked under a tree and a significant amount of snow fell on you (motor switched on causing lots of noise) then you fall over (signal disrupted). With the stepper motors it's not so bad, since you're walking back and fourth with the snow falling evenly (wires are all the same length (resistance), and common ground), so no snow will accumulate and prevent the small amount of snow (background noise/interference) being conducted to ground. So there's no ground loop...

Hopefully that helps.

With regards to aluminium extrusion, I think that seems reasonable as long as you don't put multiple wires down the hole since that might induce crosstalk (snow falling off you onto someone else!).