GFI for 24 volt DC?

andyrudandyrud Solar Expert Posts: 60 ✭✭
I want to run a 24 volt DC pump using 2 dedicated panels to circulate water in a small swimming pool. Do you need some sort of Ground Fault Interrupter in a low voltage 24 volt circuit ? If so, how about in a 12 volt DC circuit?

Thanks in advance,

Andy

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,485 admin
    Re: GFI for 24 volt DC?

    The typical GFI at the hardware store measures the "AC leakage current" (basically a very small current transformer).

    DC is much more expensive and difficult. Basically, one way that this is done is to connect to DC circuit breakers together (just like a double pole circuit breaker for 240 VAC home wiring). Except one is XX amps (load) and the other is 1-5 amps (the "sense" breaker).

    Wire the XX amp to your load wire. And connect the 1-5 amp in the ground wire (between DC battery negative post) and the earth ground (or well casing?).

    If you get a current flow in the ground wire (say the negative wire from the pump hits the well casing), there will now be return ground current in the safety ground lead. That will trip the 1-5 amp breaker, which then turns of the XX Amp load breaker.

    Is this a good idea? I am not sure. Normally "safety grounds" do not have circuit breakers in them. Lightning, and other grounding where you want a "solid ground" are now compromised by the circuit breaker in the ground lead.

    Midnite Solar 63 amp 150VDC DC Ground Fault Protector
    OutBack Power 80 Amp PV Ground Fault Protection
    80 amp 125VDC DC Ground Fault Protector

    I guess the thinking is that if there is a short in the solar array output to earth ground... Normally, the array will current limit (not trip any fuse or breaker) and can overheat the shorted component and start a fire.

    If you have to meet NEC (National Electric Code) for your installation--then you need a DC GFI setup.

    If you are installing this to prevent protect swimmers against electric shock--this really will not do anything.

    A typical AC GFI is set up for 5 mAmps (0.005 amps) or so... A DC interrupter is setup for 0.5-5.0 amps or so. Much more current than "is safe".

    Low voltage will reduce the change of shocks greatly... With wet/sweaty hands, I have gotten small electric shocks on 24 volt systems. I have not receive any shocks on 12 volt. Perhaps that is your answer.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • andyrudandyrud Solar Expert Posts: 60 ✭✭
    Re: GFI for 24 volt DC?
    BB. wrote: »
    The typical GFI at the hardware store measures the "AC leakage current" (basically a very small current transformer).

    DC is much more expensive and difficult. Basically, one way that this is done is to connect to DC circuit breakers together (just like a double pole circuit breaker for 240 VAC home wiring). Except one is XX amps (load) and the other is 1-5 amps (the "sense" breaker).

    Wire the XX amp to your load wire. And connect the 1-5 amp in the ground wire (between DC battery negative post) and the earth ground (or well casing?).

    If you get a current flow in the ground wire (say the negative wire from the pump hits the well casing), there will now be return ground current in the safety ground lead. That will trip the 1-5 amp breaker, which then turns of the XX Amp load breaker.

    Is this a good idea? I am not sure. Normally "safety grounds" do not have circuit breakers in them. Lightning, and other grounding where you want a "solid ground" are now compromised by the circuit breaker in the ground lead.

    Midnite Solar 63 amp 150VDC DC Ground Fault Protector
    OutBack Power 80 Amp PV Ground Fault Protection
    80 amp 125VDC DC Ground Fault Protector

    I guess the thinking is that if there is a short in the solar array output to earth ground... Normally, the array will current limit (not trip any fuse or breaker) and can overheat the shorted component and start a fire.

    If you have to meet NEC (National Electric Code) for your installation--then you need a DC GFI setup.

    If you are installing this to prevent protect swimmers against electric shock--this really will not do anything.

    A typical AC GFI is set up for 5 mAmps (0.005 amps) or so... A DC interrupter is setup for 0.5-5.0 amps or so. Much more current than "is safe".

    Low voltage will reduce the change of shocks greatly... With wet/sweaty hands, I have gotten small electric shocks on 24 volt systems. I have not receive any shocks on 12 volt. Perhaps that is your answer.

    -Bill

    Fantastic explanation. I think I will use a 12 volt pump and I think this will probably aleave my concerns. I usually turn off the existing 12 volt pump when somebody is going into the pool, but I just want everything to be safe in case somebody gets in and I don't know it. Thanks again for the info. Off Grid really has it's own unique problems to overcome. Makes you think out of the box.

    Andy
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