DC voltage

[Cleveland Apprentice]

Is it true that direct current flows (from negative batt post to positive batt post) the opposite than alternating current? If so, would it make more sense to fuse any negative terminals instead of positive in cars, marine and other DC applications? Thanks

 

[suemarkp]

Current flow is one way, and electron flow is another. In school, they showed current coming out of the positive pole and returning to the negative pole. Alternating current is the same way, however, the poles reverse multiple times per second, so the current flow reverses as the poles change.

You should fuse the side that is not grounded or common. Direction of current flow has nothing to do with it. If you touch a voltage terminal, you don't care if the current is flowing from feet to fingertip or finger tip to feet, either way will kill you. You fuse the ungrounded parts because it is assumed you are standing on the grounded parts and you want the ungrounded parts de-energized after a fuse blows or breaker is tripped.

 

[Cleveland Apprentice]

Thanks for the reply!

 

[dereckbc]

It flows in any direction you want it to in theory. Electron theory is neg to pos. Conventional theory it flows from pos to neg. Which ever way makes the math easier for you. For the test question there are two models ;Electron and Conventional.

As for which polarity gets fused makes no difference unless you have circuits that use earth as a conductor. If you use earth as a conductor you would prefer to bond the positive circuit electrode sticking into earth so as to form a cathodic protection of the electrode. With that said there is no NEC application where you would use earth as a conductor as it is forbidden. Only folks who can use earth as a conductor is utilities.. So in light of that it makes no difference which polarity you fuse or in some case you would use fuses in both polarities if the system in ungrounded.

FWIW AC current flows in both directions. That is why the call it Alternating Current. One half a cycle it flows the same direction as DC, the other half it flows in reverse direction.

 

[kingpb]

In school, many moons ago, I always had the same issue. For DC, most of the guys used conventional approach, "+" --> "-", but for some reason it was easier for me to do the math using electron flow which was "-" --> "+"

Results were the same as long as you are consistent with your signs.