DC Switch on Positive or Negative or Does it Matter?

RavenRaven Registered Users Posts: 15
Hi All. I've set up a few lights on DC circuits and am wondering if it matters if the switch for a given light is on the positive side or the negative. I know with AC you don't want the switch on the negative (non hot) because you could potentially hurt yourself when switching out a light bulb or something, but with DC I would think it wouldn't matter because there is no alternating current. Let me know if I need to rewire my circuit. Thanks!

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,765 admin
    Re: DC Switch on Positive or Negative or Does it Matter?

    If your system has the battery negative tied to earth/safety ground (or frame ground of a vehicle), then you want the switch in the positive leg.

    If your battery bank is floating (not tied to any sort of safety/frame ground), in theory, you want a double pole switch that turns off both + and - power at the same time.

    In practice, most people use a single pole switch in either lead--just fine.

    To avoid confusion, I would put the switches in the positive lead for all circuits just to avoid confusion down the road.

    You should also have a fuse or circuit breaker in the positive wire too, near the battery, rated no larger than the wire can handle (i.e., if you follow NEC, 14 AWG wire would have a 15 amp fuse/breaker maximum). Any decent sized Lead Acid battery can output a surprising amount of current into a dead short and fuses/breakers are very good for safety.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • paulskirockspaulskirocks Solar Expert Posts: 84 ✭✭✭✭
    Re: DC Switch on Positive or Negative or Does it Matter?
    Raven wrote: »
    Hi All. I've set up a few lights on DC circuits and am wondering if it matters if the switch for a given light is on the positive side or the negative. I know with AC you don't want the switch on the negative (non hot) because you could potentially hurt yourself when switching out a light bulb or something, but with DC I would think it wouldn't matter because there is no alternating current. Let me know if I need to rewire my circuit. Thanks!

    Well, in the 120 volt AC world, we always switch the hot, so that when you shut off that switch, there is no voltage potential in the device... So, same with a negative ground DC system... You need to switch the positive...

    If you switch the negative, then there will still be a positive voltage present the device you switched off, even though the device is not on, and the device loses is grounded leg, which can a safety issue... Also, if you switch the grounded leg off, you take away the path to ground for static discharge...
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: DC Switch on Positive or Negative or Does it Matter?

    Wouldn't make any difference - - except for the same reason as not being best practice to switch Neutral in an AC circuit. It's common to ground the Negative in DC systems used in North America, so if the negative (ground) and not the + is switched, the remote possibility exists where, with the switch switched off, it might be assumed the + was switched as usual, and under unusual circumstances, the hot + might come in contact with a ground, causing a short circuit.
    That said, many modern automobiles now switch the negative (ground). Headlights for instance - - Battery +, through a fuse/circuit breaker of course, goes directly to the headlights, but the headlights don't light until the other side of the light is grounded through the headlight switch inside the vehicle.
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: DC Switch on Positive or Negative or Does it Matter?

    Here's a real life example. A friend of mine takes his car into the shop for an electrical issue and the mechanic hooks up the battery in reverse polarity. Suddenly, no radio, no headlights and the dashboard is acting goofy. Later he realizes this and tells the shop manager what happened and the shop pays for the bill to fix all the shorted electronics in the car. One simple mistake = costly repairs.

    Like any switch, it's on the hot side that disconnects when off. In this case a short could fry all of any electronics, diodes, inverters and batteries in its path. But usually stops at the first electronic piece, severing it from the loop, just depends on where. Worst case scenarios: Fire & battery explosion if any of the cells in them are running dry if one of the wires heats up and creates the spark. Boom.
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