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As far as I can tell, on my van which is negative-grounded, all fuses are somewhere between the positive terminal and the load being protected by the fuse.

However, some switches are on the positive, some on the negative side. For example:

  • The ignition switch switches the positive.

  • The sliding door switch switches the negative. This controls the light in the load bay, which is otherwise un-switched directly to the battery's positive via a fuse.

I'd have thought the positive should ideally be switched, since otherwise if a mechanic or user assumes there is no potential around the switched item while it is off, he would be wrong if it is ground switched. (The wires to the load still have positive potential with respect to the chassis).

What reasons might there be for negative connected switches?

I can give more info if required. (I'm following the Haynes manual which has pages and pages of circuits. I'm not entirely sure what's relevant for this question)

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  • It's really just a question of whether it's more convenient to attach the switch or the load to ground. – Dave Tweed Apr 20 '15 at 11:58
  • Is it? As the question says, conventionally switches are only in the positive side. – pjc50 Apr 20 '15 at 12:22
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    I've always assumed it's for pretty much what Dave said, the chassis is ground so for a door switch for example the ground is already taken care of and you have a single wire to run back. Also if that wire shorts to something (most likely the chassis) it doesn't cause a short circuit. – PeterJ Apr 20 '15 at 12:31
  • Generally the voltages used in cars/vans/etc. is either 12 or 24V. Neither of these is considered dangerous (your body would have to be <4k resistance for it to kill you, which it isn't). Also, any sensible mechanic doing anything to a vehicle which involves going near the electrics will disconnect the battery beforehand anyway, so there will be no voltages anywhere while working. – Tom Carpenter Apr 20 '15 at 12:48
  • Switching on the ground saves extra wire and wiring complication. An example Break Lights/4Way Flashers/Turn signals operating some of the same bulbs. – Optionparty Apr 21 '15 at 0:57
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It's really for convenience, or economical reasons.

Let's have a look at the light inside a car, which is controlled by switches at the four doors. If you want to switch the positive side, you need cables from the fuse box to each of the switches, as well as cables back to the lamp (upper schematic). If you switch the negative side, you need a single cable to the lamp, and just one cable to each switch - the circuit is closed via the chassis. So, you can save several meters of cable.

schematic

Most cars also have a switch for the lamp: normal operation / always on / always off. This can easily be implemented in the second design. For the first, you also need a cable from the fuse box to the lamp.

Now think about the starter:

It's shorter to have a cable from the fuse box below the steering wheel to the ignition lock to a relay near the motor than it is when having a cable from the fuse box to the relay, back to the key lock (locations may differ for different cars).

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    Would it be worthwhile to note that fuses have to be on the positive side to ensure that shorts to ground (the most common form of short) can't bypass a fuse, or to note the consequences of a short to ground bypassing switches (for cars with a light-mounted on/off/door switch, having one of the door switches short to ground would mean someone would have to manually switch the dome light but could still use it), but if door switches controlled the positive lead they would blow the dome-light fuse and make it completely useless. – supercat Apr 20 '15 at 15:35

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