Actually the opposite happens... if you ground out high voltage side nothing happens, if the low voltage side shorts out then the accessory comes on and tells you where the problem is. With the relay actuating the positive side, and the switch shorts to ground you have a fire, blow a fuse, or both, if the high side shorts out then sparks and smoke for sure.
Hi Mike,
Can you clarify what you mean by "high voltage side" and "low voltage side"?
The diagram in the original post shows +12v from the battery going through a fuse and then to both the switch (30) and coil (86) terminals of the relay. The other side of the coil (85) is I assume connected to ground through the switch that controls the relay. The other side of the switch (87) connects to the load and then to ground.
My comment was that given that the chassis is grounded on most cars it is much easier to short a wire to ground than it is to short to +12v. In the diagram there are multiple things that can happen if a wire gets grounded out.
1. If the wire from the battery to the fuse grounds then you get sparks and perhaps a fire.
2. If the wire from the fuse to terminals 30/86 grounds then you blow the fuse.
3. If the wire from 85 to the switch grounds the load turns on.
4. If the wire from 87 to the load grounds then then nothing happens, until the relay is activated and then you blow the fuse.
#3 is the state I'd worry about. Personally I don't want a load to turn on if there is a short to ground since I might not be near the truck or I might not notice it while driving.
I suspect Michael used this setup since it can save running one wire to the switch. If you were to switch the positive voltage to the relay's coil then you'd need two wires to the switch, one with +12v and one to the coil. In a negative ground setup you can just run one wire from the relay to the switch, and then ground the other switch terminal anywhere on the body. The tradeoff is the issue in #3.