Grounded Wye to Grounded Wye Transformers used to take SMA 480V to 208V.

slurry bowlslurry bowl Registered Users Posts: 33 ✭✭
Complicated topic when it comes to grounding/bonding. 

What does anyone think of this? Can anyone direct me to who is an expert on the topic? 

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,923 admin
    This is way out of my wheel house... You really need an experienced power engineer to do the design/wet stamped drawings for the permits/inspectors/etc... (as I understand). My comments are as much for my understanding as addressing your questions. NEC has not always addressed "safety grounds" and "lightning grounds" in a consistent manner (from my understandings).

    If I am wrong in my understanding--Please let me know.

    My (simple) questions are around the grounding... In general, we only want N+G bonding in one location.

    On the left side of the drawing, do you have two N+G bondings (inside the inverter, and at the buck transformer?).

    On the right side, you have N+G bonding at the fused disco, at Meter box, and at the Main service...

    Excuse me if I misunderstood your drawing... But to be sure, do not want Neutral and Safety Ground wiring in parallel at any path in the AC circuitry.

    This "rule" does get a bit violated when working with (for example) utility drops... Where there is (typically?) a N+G bond at the pole transformer, and another set of N+G bonds at the 2-5 homes sharing the same pole transform (N+G bonds in each home's main panel). These multiple N+G bonds are made to separate ground rods at each location--There is not a parallel copper run of ground wires between each ground rod at each location...

    Which brings me to the two GEC 250.66 ground connections... You have a ground rod for the "left side" of the system. And a separate (isolated) ground rod for the right side of the system.

    From my (non-power engineer) understanding... I would be looking at "lifting" the N+G bond in the Fused Utility Disco and connecting the ground rods together and common grounding cable between all electrical boxes (left and right side of drawing). And only one N+G bond in the Utility Service Disconnect.

    If this was in a "lightning prone" area--Then possibly adding ground rod(s) at each major electrical box (i.e., equipment room, out at the solar inverter/racking/etc.)--Especially on the left "solar side" of the system (allow path for lightning to earth as short as possible to earth).

    For me, safety ground is there in case something else goes wrong. In the above drawing, say there was a open neutral between Fused PV Disco and Meter box (open neutral may not flag any errors on solar side--And will probably not affect 3 phase power transfer--So possibly a "silent" failure). Now say there is a short from Lx to box chassis inside the Solar Disco or buck transformer. There is no "ground shunt path" for the Lx short to return to the Utility transformer/OCD (off top of drawing). The only current flow (assuming all boxes are not mounted on common metal rails) is through the two ground rods--Which can be upwards of 25 Ohms to earth--I.e., not near low enough resistance to trip the utility transformer OCD protection.

    Again, sorry for the view from the Kiddy side of power.

    Regarding recommendations for a Power Engineer... All I can suggest is to find other installations in your region and see who was the P.E. on those installations and check references. Not all P.E.s have the experience/knowledge for large/distributed/alternative power systems.

    Good luck,
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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