System earthing (bonding) the GF Protector

cupcakecupcake Solar Expert Posts: 254 ✭✭✭
Hi pals,


Imma bout to bond the DC negative to the DC grounds at the 'one' location.

I gotta negative shunt in the Dc shutooff box.

My question is can I tie into the SMALL screws on the shunt? OR must I tie into the BIG leads on the shunt?

shunt500.jpg



Remember, I'm bonding the Batt Negative to eqipment ground through a 1/2 amp breaker.

Thanks all for your response...


--cakecakecake












~1.5Kw PV in parallel
Morningstar MPPT-60 controllers (2) in parallel
3 Trojan tr-1275's in parallel 450ah total
Samlex 2,000 watt 12-volt inverter hardwired


Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    Big Lugs. Remember your ground will be rated for 100-200 amps or more. And 6 AWG cable will "fuse" around 600 Amps.

    If you are grounding the battery bank through a 1/2 amp breaker--That is for a DC Ground Fault breaker assembly. A current >~1/2 amp through the safety ground will trip the main breaker and turn off the solar array (or similar application).

    I am not a "fan" of DC GFI done this way--But that is another story.

    The small screws are called Kelvin Contacts and are use to connect a precision voltmeter (i.e., Battery Monitor) to measure the small voltage drop across that big precision power resistor. You really do not want any current (other than the voltmeter) going through these connections.

    Four-terminal sensing

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cupcakecupcake Solar Expert Posts: 254 ✭✭✭
    BB. wrote: »
    Big Lugs. Remember your ground will be rated for 100-200 amps or more. And 6 AWG cable will "fuse" around 600 Amps.

    If you are grounding the battery bank through a 1/2 amp breaker--That is for a DC Ground Fault breaker assembly. A current >~1/2 amp through the safety ground will trip the main breaker and turn off the solar array (or similar application).

    I am not a "fan" of DC GFI done this way--But that is another story.

    The small screws are called Kelvin Contacts and are use to connect a precision voltmeter (i.e., Battery Monitor) to measure the small voltage drop across that big precision power resistor. You really do not want any current (other than the voltmeter) going through these connections.

    Four-terminal sensing

    -Bill





    Thanks Bill,



    The wire going from the 1/2 amp breaker to the DC negative on the shunt is only a skinny 14guage wire, should I just loop it under the over the lockwasher of the shunt? Or shuld I use a 14awg ring terminal to make the connection?

    Also why are you not a fan of this DC GFI 1/2 breaker scheme I got goin on... I thought that was the 'proper' NEC requirement/way to go. Elaborate kindly.

    Thanks

    -- Sweet sweetness














    ~1.5Kw PV in parallel
    Morningstar MPPT-60 controllers (2) in parallel
    3 Trojan tr-1275's in parallel 450ah total
    Samlex 2,000 watt 12-volt inverter hardwired


  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    It would probably be better to use a 14 awg ring lug if you can... Remember you want the large cables + 14 AWG cables to both make good electrical connections.

    Of course, you can put the ground connection at the negative ground bus (if you have one). It does not have to be on the shunt.

    The DC GFI--Here is a link you can read:

    http://midnitesolar.com/Forum/index....ic=142.0;nowap

    -Bill

    PS: There are "better" (or at least different) true arc fault detection devices out there. Midnite has them in some of their products. MorningStar has an Arc Fault breaker setup.

    This actually "Listen" for the electrical noise of an arc fault in progress. However, they have been know to false trip too (both for DC and the AC versions used in house breaker panels). A less than perfect solution in some cases.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cupcakecupcake Solar Expert Posts: 254 ✭✭✭
    Bill...

    I read your link..


    Still confused...


    What's the harm?

    + to earth fault at panels trips the 1/2 amp breaker cutting off array power.

    In plain English, what am I not understanding here??
    ~1.5Kw PV in parallel
    Morningstar MPPT-60 controllers (2) in parallel
    3 Trojan tr-1275's in parallel 450ah total
    Samlex 2,000 watt 12-volt inverter hardwired


  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Broadly, the GFI breaks the negative to earth bond, leaving the DC side unbonded, and one conductor shorted to ground. For whatever reason you dont address the first fault, a subsequent second fault may cause a fire. Such array fires have actually occured.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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