Breakers and switches on positive or negative leg?

DC circuit breakers (a bit older type) tend to be polarity sensitive. One example of why--They can use a magnet to "blow" the arc into the arq/quench chute. If wire them backwards (i.e., do not connect the + of the breaker should connect to the positive of the battery bus), If you connect the breaker backwards, the magnet will pull the arc into the switch guts and (as I understand) ruin the breaker (or even catch it on fire).

As Wellbuit says, the method for grounding (+ or - or both) for breakers depends on the grounding of the system. It applies to both DC and AC systems. In some ways, it is easier to talk about "Hot" and "Return" and "Neutral" leads.

Breakers (and fuses) are designed to prevent excessive current flow in wiring and reduce the chances of wiring overheating or catching/setting fire--Breakers are not there to protect the equipment from damage (other than too much current setting a failing device on fire).

There are "floating" power systems. Such as a simple 12 VDC battery, or an isolated transformer output. For these systems, "safety wise", you need double pole breakers on the +/- leads/buses (or AC "Hot and Return") to the loads. More or less, you are protecting against the first short circuit to ground (which in a "floating power system" does not "short circuit". It is the second short to ground (or to other power wire) that causes excessive current to flow. It is not that complex, but if you have heavy wires (like to a fuse panel) and lighter branch circuit wires out, you need 2x fuses/breakers on each pair of wires... That is so that both the Hot and Return wires are protected against excessive current flow.

For "floating power systems", there can be requirements for (example) monthly checks that the outputs are not shorted (like in a lab or assembly line). Or there an be automatic/lights that show if the output is floating or shorted (such as used on ships).

When you "ground" one of the power leads (negative or positive does not matter) to "electrical ground" (such as water pipes, vehicle sheet metal chassis), then that "grounded lead" can never go above "zero volts" with respect to metal around the house, in the vehicle, etc... So there is no need for a breaker on the "return lead". Save money, and grounded neutrals do help a bit in lightning situations.

There is a "reason" to pick positive or negative grounding. In some cars older than 1960 or so in the US, and in many foreign cars from europe, the battery was positive grounded. In the US, negative grounding was more common and seem to become the standard when it "mattered more". DC Generators, you can polarize them so that they output positive or negative voltage with respect to ground--So they were a bit more flexible--And few things in the car really "cared" about + or - ground. When Car Radios and AC alternators came about--It seemed like negative ground because the standard (reason why, not really sure).

Now, there is a reason for positive ground is used in some cases... For example in Telecom (Telephone) systems, positive grounding is used for the battery bank and wires to your phone at home. This was to lessen corrosion issues for terminals/wet conditions/etc... If negative grounded, then the wiring did not last as long.

There is also "cathodic protection" for things like Gas Pipe Lines--Charging the gas lines with a low DC voltage so that electron flow does not corrode metal pipe lines.

https://www.solar-electric.com/ae15ampsocap.html

Other corrosion systems used for metal towers, ships too (zinc anodes--You want the sacraficial zinc anodes to corrode and be replaced years down the road rather than your ship/propeller/etc. get holes in them).

https://www.corrosionpedia.com/definition/4326/zinc-anode

So--While electron flow direction can matter--It is usually because of specific requirements (like corrosion control).

The typical home/cabin/grounding is for two reasons... Ground bonding the Neutral and/or DC return allows the safe use of breakers only the Hot/+ wiring. And provides path through the green wire (safety ground) and/or metal plumbing and such back to the power source (Utility power, AC inverter, DC battery bank).

Typically, the AC and DC "ground bonding" takes place together in one location (typically a ground rod or cold water pipe)... That way if there is a short circuit on the AC side, the ground bond to neutral can trip a breaker if there is short. And same thing on the DC side (+ to ground short).

There are reasons for single point grounding (mostly for short circuit return current flow). But there is hand waving involved.

The "Ground Rod"--Is not there to trip a breaker in case of a short circuit. It is typically to high of resistance from rod to earth to do that (30 Ohm max per NEC--Which is about 4 amps @ 120 VAC--Not enough to even trip a 15 amp branch circuit breaker).

The reason for ground rod (other than the "common point" to connect AC and DC grounds together for single point grounding--Is for lightning strike grounding. Have the ground rod at the outside of the wall/foundation, and direct any lighting current from hitting a solar array, AC wiring, phone wiring, Internet wiring directly to earth--Rather than let the lightning "bounce around the home".

For "code" (like the USA National Electric Code and similar)--Things are done in a "standard method" based on experience (make the wiring "safe") and so that people years down the road can use the common rules for the original construction was done "to code" and they can continue on with new additions/repairs/etc. without having to figure out "what strange things" were done before (like the house my friends grew up in when their father built the home and put all the fuses on the "neutral" wires, not the Hot wires--He found out decades later).

-Bill