When it comes down to the basics of where to put a switch in a circuit there is no significant reason to put a switch in any particular point of the circuit.
Conventional wisdom says if I flip a switch I am turning off the power to the device. So if there is any possibility of someone else working on the circuit it is best to put the switch as close to source power as possible.
Turning off a circuit once something goes wrong is done by a fuse or circuit breaker. Generally the only potential damage that a person could prevent with turning off the switch is personal injury caused during diagnosis/repair. A persons reaction time would be too slow to prevent most electrical/mechanical damage that would occur during a failure and not something to be relied upon.
In several industries, automotive included, ground side switching has been used in most circuits. This is a decision made by financing. Until recently the cost of an NPN transistor(low side driver) was a good deal cheaper then a PNP transistor(high side driver). Using low side drivers made the cost of producing control modules a good deal cheaper and so everybody did it.
Recently though the cost of high side drivers have been coming down. This has lead many manufactures to start using high side drivers on high current draw circuits such as HID headlights, ABS, fuel pump, electronic power steering, and others. With high side drivers the load is directly connected to the control module instead of an output to a relay that then switches the load. This allows a diagnostic current to run during initial startup and whenever the device is turned off.
Low side drivers are still very common and can be seen side by side, in the same module, as they are still cheaper to produce.
Now of course this wouldn't be the first time I have been wrong and if someone has some sound reasoning to switch the ground side I would like to hear it.
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Originally Posted by cdwood2010
