Zapped, it all depends on what the "low voltage" side is feeding. If this is driving relays or even SSRs the current required to drive the "controlled" load is far beyond anything that could be coupled by running next to power conductors. This started becoming an issue when we started using CMOS "Field Effect" transistors. They respond to microamps so a little noise will be seen. You can relate to this when you see that random number generator we call a digital meter when it is connected to an open circuit. A well designed driver/receiver set will actually present a load on the line (usually 100-150 ohms) that matches the characteristic impedence of the wire so it does not respond to those low current transients. If they are really serious they use a twisted pair like CAT5 which is not going to see that 60 cycle hum at all.
I have seen some poorly designed stuff that does have problems with ingress of noise but if that is true you find the world is full of noise. Wiring practices may make it better but you will still have problems.
