Transformerless inverter grounding

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lamagra
lamagra Registered Users Posts: 8 ✭
in PV Installers Forum - NEC, Wiring, Installation #1
I would like to confirm the idea that, for transformerless inverters, the PV grounding system's point of return (i.e. where the neutral and ground are connected) should be in the same panel where the PV system is connected. In other words, connecting the PV grounding system to the existing grounding system, if the PV system is connected to a subpanel for instance, could lead to tripping of RCDs upstream of the inverter, up to the main panel.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,696 admin
    #2
    Yes--With Any GT inverter, you do not tie the Neutral and Earth (Safety) ground anywhere except in the main breaker panel (typically).

    What you do not want are parallel current paths--Some "return" current flows in the White wire (US) and other current flows in Green Wire/conduit fittings. You can get problems with over heating green wires/conduit connections/metal electrical boxes if that happens.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • lamagra
    lamagra Registered Users Posts: 8 ✭
    #3
    Yes but in the case of PV, if the point of neutral-ground bonding is at the main breaker panel while the PV system itself is connected to a subpanel, then unwanted tripping will occur upstream. On the other hand, bonding the PV's neutral with ground at the point of connection (i.e. subpanel) will solve this issue by restricting the tripping to the PV connection. Don't you agree? Please keep in mind that I'm talking specifically about grid-tied unisolated inverters.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,696 admin
    #4
    I am not quite sure I am following your question...

    I believe you are in Lebanon (or nearby)? I do not know how your electrical systems are installed there...

    However, a transformerless vs transformer isolated GT inverter would not change the AC subpanel/mains wiring requirements.

    In many countries, as I understand, the neutral is either "floating" or bonded at the utility distribution transformer. In the US, the neutral is bonded both at the pole transformer and at each home in the Main AC panel (to local home cold water pipe, ground rod, etc.).

    In the US, GFI (ground fault interrupter or residual current breakers) are only installed on "branch circuits" (wires from banel breakers to final wire runs (or in the electrical outlets themselves).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • lamagra
    lamagra Registered Users Posts: 8 ✭
    #5
    The return path of any fault current at the DC side of the inverter will have to pass through the main panel even if the connection is made at the subpanel for instance?
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  • Ethan Brush
    Ethan Brush Solar Expert Posts: 235 ✭✭
    #6
    Lamagra,

    I think you are a bit off base here.  You refer to things like "PV neutral" , "neutral ground bond".  None of these terms are relevant to transformerless inverters.

    Also you CAN connect a TL inverter to a  "sub panel" , in fact larger systems are almost always done that way



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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 7,031 ✭✭✭✭✭✭
    #7
    lamagra said:
    The return path of any fault current at the DC side of the inverter will have to pass through the main panel even if the connection is made at the subpanel for instance?
    You are mixing DC and AC up my friend.
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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  • Ethan Brush
    Ethan Brush Solar Expert Posts: 235 ✭✭
    #8
    lamagra said:
    The return path of any fault current at the DC side of the inverter will have to pass through the main panel even if the connection is made at the subpanel for instance?
    I am not quite sure what the context of your question is, but you may be correct.  Since TL inverters do not have isolation between the two sides,  a DC side fault to ground (ground being properly bonded metal stuff, not actually dirt) will result in AC current flowing from its source (typically the utility transformer) through the inverter, through the fault to the metal stuff, through the equipment grounding/bonding conductors, through the main bonding jumper at the electrical service, to the utility neutral/grounding conductor and back to the transformer.  Not sure if that is what you were getting at.  In practice though, the inverter will sense the fault and shut down very very quickly, before any OCPD would trip.
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 7,031 ✭✭✭✭✭✭
    #9
    They shut down too quickly sometimes. We had to replace a TL because of this.
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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