Why do I get a spark when the DC is off?

Jim PJim P Registered Users Posts: 5
I have a net-metering system. I am trying to install an additional 24V DC ceiling fan. Naturally I turned all power of at the main switch from the batteries before starting. I did test it and there is no DC to other circuits running fans. When I tested the wire at the fan with a voltmeter it sparked. How can it do this is the power is off? I'm concerned about the safety issue.
Thanks for any answers.

Comments

  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Why do I get a spark when the DC is off?

    You might just have been statically charged when you touched connection.

    Other then that you will have to give a lot more information. What does having net-metering have to do with you connecting a 24 vdc ceiling fan?
  • Jim PJim P Registered Users Posts: 5
    Re: Why do I get a spark when the DC is off?
    RCinFLA wrote: »
    You might just have been statically charged when you touched connection.

    Other then that you will have to give a lot more information. What does having net-metering have to do with you connecting a 24 vdc ceiling fan?

    Nothing really. I was just giving the context of my home wiring setup.

    I'm not sure what extra information I can give. I have solar panels and battery backup. The DC circuits come right off the batteries. They are only used in the house for a few items like the ceiling fans. I switched the circuit off at the breaker box, tested it and got a spark when I put the voltmeter across the ends of the wire. I then switched of the main switch for all DC to the house and still got a spark. Being statically charged is possible but surely everything in the meter is insulated and the wires still had the plastic coating on them as I have not stripped them back yet.

    I know enough about electricity to have done most of the AC wiring in the house but this is the first time I have tried to connect anything using DC wiring. I was told everything is pretty much the same in how it is wired except, of course, the ground is not used.
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Why do I get a spark when the DC is off?

    When you say you turned it off at breaker box, I assume you mean you have a separate breaker box for DC circuits and another for AC circuits.

    A meter in voltage mode is very high impedance so should not spark when leads are connected to measurement points regardless if wires are energized or not.

    Are you running DC wiring in parallel runs with AC wiring?. If so you may get some AC coupling into DC wires.

    Do you have some AC voltage reading across wires when breaker is open?
  • Jim PJim P Registered Users Posts: 5
    Re: Why do I get a spark when the DC is off?
    RCinFLA wrote: »
    When you say you turned it off at breaker box, I assume you mean you have a separate breaker box for DC circuits and another for AC circuits.
    That is correct. I have separate boxes for DC, grid AC and solar AC.
    A meter in voltage mode is very high impedance so should not spark when leads are connected to measurement points regardless if wires are energized or not.

    Are you running DC wiring in parallel runs with AC wiring?. If so you may get some AC coupling into DC wires.
    There are a lot of wires coming out of the breaker boxes and some run very close together. It is highly like that some DC and AC are running like that.
    Do you have some AC voltage reading across wires when breaker is open?
    I'd have to check. If that is the case I don't think I need to be as concerned. It was the potential danger of DC current I was worried about.

    Thanks for the responses.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Why do I get a spark when the DC is off?


    "Do you have some AC voltage reading across wires when breaker is open?
    "
    "I'd have to check. If that is the case I don't think I need to be as concerned. It was the potential danger of DC current I was worried about."

    no so fast here for if ac is coupling into your dc wiring you have a shock hazard. wires that run parallel to each other can induce a voltage and current from one to the other and is partly how a transformer works. you may need to separate the dc and ac wiring from each other and run them separately.
    the issue of the spark to the voltmeter is concerning, for as rc pointed out, as normally next to nothing in current or voltage should be going through a meter with a high input resistance. if we read the layout correctly nothing should be present to spark the meter unless you have the wrong wires being measured or ac voltages are being transformed into the dc wires. try shutting off the ac in those wires and see if the spark still occurs on the dc wires.
  • BB.BB. Super Moderators, Administrators Posts: 30,505 admin
    Re: Why do I get a spark when the DC is off?

    The typical way you could get a spark between the AC and DC wiring would typically be cause of voltage drop in the wiring/ground wiring... It is very easy to get a few volts difference between neutral/hot/and flowing DC current.

    Now--there can be a problem like DC return current going through the Safety Ground (for example, may 12 VDC units have sheet metal enclosures which is also the negative power return lead (chassis is car is both return and safety ground). So, it is easy to get differential voltages between the safety ground, neutral, and DC Return.

    Also broken grounds/neutrals can cause weird voltages in different points of your electrical system.

    Otherwise, a spark from the meter sounds like the meter was set for something other than volts (Amps? Possibly Ohms? etc.). A typical Digital Meter will not draw enough current to see any spark.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Jim PJim P Registered Users Posts: 5
    Re: Why do I get a spark when the DC is off?

    There is absolutely no way to separate the wires. They were all put in place before the drywall was up and the fan is at least 50 feet from the box. No idea where the problem point would be and I cannot think of removing drywall over all that distance.

    I'm going to check for AC when I turn the DC back on but have not had time yet. I'll post back when I can do that. May not be today.

    Thanks.
  • BB.BB. Super Moderators, Administrators Posts: 30,505 admin
    Re: Why do I get a spark when the DC is off?

    I am not as concerned about "cross coupling" between AC and DC wiring--Although, it would have been a nice idea to keep the parallel AC/DC runs separated.

    When looking for stray voltage/current... Use a 120 VAC filament bulb and a pair of alligator clips to "load down" the wiring... Then measure the voltage and current. It will separate the "coupling" from the dead shorts.

    With DVM's--It is pretty easy to get mislead by DVM readings where they will read "full voltage" with only mAmps or uAmps of current in the wires.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • boBboB Solar Expert Posts: 975 ✭✭✭✭
    Re: Why do I get a spark when the DC is off?

    Try it again. Does it spark "repeatedly" ?? A "BIG" spark or a little spark
    or what ? How can a (hand-held volt) meter make something spark ?

    Doesn't make any sense. What kind of meter is it ? Try another meter, maybe ?
  • Jim PJim P Registered Users Posts: 5
    Re: Why do I get a spark when the DC is off?
    boB wrote: »
    Try it again. Does it spark "repeatedly" ?? A "BIG" spark or a little spark
    or what ? How can a (hand-held volt) meter make something spark ?

    Doesn't make any sense. What kind of meter is it ? Try another meter, maybe ?

    Maybe spark is the wrong word. Maybe arc is better. Anyway, the wires are covered so I toughed the ends of the two cables from the meter to the ends of the two wires and saw (and heard) a flash. It is the only meter I have but it has proven reliable for several years.
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,183 ✭✭✭✭✭
    Re: Why do I get a spark when the DC is off?
    boB wrote: »
    Try it again. Does it spark "repeatedly" ?? A "BIG" spark or a little spark
    or what ? How can a (hand-held volt) meter make something spark ?

    Doesn't make any sense. What kind of meter is it ? Try another meter, maybe ?

    Well it could if it was set for current reading... There is part of a Fluke meter fuse at work in the wall! Of coarse I did not do this...
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: Why do I get a spark when the DC is off?

    You should have multipliable DC disconnects, the charge controllers, the solar panels and any secondary loads like your fan require individual DC breakers or fuses

    Sounds like the secondary loads are hard wired to the battery bank which could be a fire hazard

    A picture and or schematic of your system would be needed to give you useful information
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