Why relays?

[SPYDERLK]

Residential wiring and 120v is a different animal. I am not an electrician, and dont know much of the code and stuff, so I dont know a good reason as to why you cannot switch the neutral. Maybe someone else does???

Switching neutral will turn off an appliance, but visualize that switch and every part of the resulting broken circuit. One side of the switch is now HOT because the entire current path thru the appliance remains/becomes an unbroken hot wire. So any part of the appliance circuit you touch will try to get to neutral/ground thru you. The tiny current you draw will cause no voltage drop in the appliance circuitry so youll get the full effect ... maybe lethal.
larry

 

[LD1]

Switching neutral will turn off an appliance, but visualize that switch and every part of the resulting broken circuit. One side of the switch is now HOT because the entire current path thru the appliance remains/becomes an unbroken hot wire. So any part of the appliance circuit you touch will try to get to neutral/ground thru you. The tiny current you draw will cause no voltage drop in the appliance circuitry so youll get the full effect ... maybe lethal.
larry

I knew that, but figured there had to be other reasons other than that. Because I am sure somewhere in the osha and NEC laws states that you shouldnt rely on a switch to isolate power. Regardless of how its wired, you should flip the breaker and VERIFY with a VOM that NO voltage is present before touching anything.

 

[SPYDERLK]

I knew that, but figured there had to be other reasons other than that. Because I am sure somewhere in the osha and NEC laws states that you shouldnt rely on a switch to isolate power. Regardless of how its wired, you should flip the breaker and VERIFY with a VOM that NO voltage is present before touching anything.

You knew, but just posed the question to see if anyone else did? OK [] That is the whole reason barring mechanical failure of the switch. Virtually all the other precautions are because the wire switching may indeed be wrong, or other flip switchers may be present. ... Also lawyers.
larry

 

[Qapla]

Relay's are more of a positive contact then a switch. Therefore, all the bouncing and jostling that you get in a vehicle, especially a tractor, will not cause a relay to loose contact ... but, over time a switch that handles that much current may.

Also, in your scenario, by hooking additional light to the headlight circuit you may overload the fuse, leaving you in the dark. By using a relay, you can still use the headlight switch to operate the added lights, but the lights themselves will pull their power from a different circuit/fuse (perhaps an in-line fuse you add into the line going to the new lights)