by MtnDon » Thu Oct 08, 2015 5:23 pm
Disconnecting the negative wire is the convention used when servicing electrical items on autos and boats. The idea being that on a switch connected through the positive side, the exposed terminals on the back of the switch or a broken positive cable could contact the sheetmetal or frame if something goes wrong, creating a short to ground and safety hazard. The negative cable connects to the body ground already—so there’s no additional danger if it rubs. It’s the same principle as why you should always disconnect the battery’s negative side first before performing electrical service.
Note that disconnects that are made to attach directly to a battery terminal are sized to fir the smaller diameter negative post, not the positive. Disconnecting the positive also interrupts the power but a negative terminal disconnect is safer. That said, a fuse or breaker used to protect the wiring is normally in the positive wire.
Not all breakers are made to be used as a switch or disconnect. I can't point to a list of ones that are and are not. I have a single purpose disconnect (big red handle thing) in our CTC negative wire and a fuse as the wiring protection device in the + wire..
Take your choice as to how you look at it.