grounded or ungrounded

offgrid me
offgrid me Solar Expert Posts: 119 ✭✭
What is the difference between a grounded pv system and an ungrounded/ floating system. What are the limits of each in terms of voltage from panels. Is ungrounded used just for off grid?


  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: grounded or ungrounded

    :confused: Not sure I understand what you mean.
    The only time a system is "ungrounded" is when it's mobile, as in an RV.
    The other possibility is you're thinking of the neutral-ground bond? That can be problematic for MSW inverters. Also you often see portable generators listed as having "floating neutral".
    But even out here in no man's land we pound stakes in the ground and hook up #6 to everything with a lug on it, so to speak.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
    Re: grounded or ungrounded

    Oh boy--You are one of "those guys". :roll::p;)

    There are grounded systems--like your car. Positive wiring with metal of car as return/negative. Car itself is not grounded but floating because of car wheels.

    There are floating power systems. A car battery connected to a radio with a pair of wires is "floating".

    And there are Earth Referenced systems. Simply, take one lead (typically the negative battery lead from a battery system or the AC Neutral Lead for a Grid Powered system) and tie that to an 8' ground rod driven into the soil/bonded to the cold water pipes.

    There are lots of reason to Earth Ground a system and there are some good reason not to earth ground a power system too...

    For small/portable systems, typically the power is not ground referenced. For example, below 5kW gensets have their AC output floating. For ~5kW and larger, the AC output is typically referenced to the generator chassis and expected to be tied to an earth ground.

    Why do we not ground a system... Simply, because a person will not get a shock if one wire is touched. It is actually a very safe way of wiring out a power system. If any one of the power leads is touched or connects to a piece of metal--no current will flow. Very safe if you are working around water.

    Why not to float a power system. If a lead touches a piece of metal (you get a short circuit)--there is nothing to show that a short occurred. And now, since one lead is shorted, the other lead is now "hot" and can cause a shock/spark to other metal.

    Earth Grounding a large power system. This sets one voltage of the system to zero volts. If there is a lightning strike, an AC Power Line cross (like on the utility pole, etc.)--you know the local power lines will never exceed zero volts to rated voltage (120 VAC maximum for a 120/240 VAC north American power system).

    If you had a floating electrical system, then it would be possible to take the AC lines (or DC lines) to unsafe voltages with respect to plumbing/sinks/etc. in the home.

    There are other reasons for ground referencing a power system... Florescent lights start better when ground referenced. And you only need to put breakers/disconnects in the "hot leads" (i.e., Line A/B for AC mains, + lead for DC power). A ground referenced power system does not need double pole breakers/switches in the return lines (since return lines are always near zero volts).

    And some off-the wall reasons for ground referencing power systems. For some solar cells (Sun Power out of Germany), the solar array needs to be positive grounded for proper operation. If negative grounded, the panel builds up excess charge which reduces solar cell output.

    When working with piping, underground utilities. Power Systems are positive grounded so that the water/natural gas pipes are not subjected to corrosion as the "positive electrode". Cathodic protection

    Regarding recommendations for your system:
    • small/portable solar power--leave floating
    • large/fixed installation (home/work)--Ground reference - battery terminal
    • Fixed installation with MSW inverter--Usually cannot create grounded neutral. Most MSW inverters are not isolated and "grounding the neutral" will create a dead short through the inverter.
    • Fixed installation with small TSW inverter--Float or Ground reference Neutral--more or less your choice. If just a few appliances, floating is fine.
    • Fixed installation with large TSW inverter/electrical system--Ground Reference the AC neutral (as always, read inverter manual to make sure it supports grounded AC neutral).
    Note, if you have a large solar array, especially if mounted on a tall structure or roof--There is a natural DC electric field for the earth (around 100 volts per meter--but can be thousands when thunder storms are around).

    Grounding the Battery Bank will prevent the earth's field from "charging" the DC power bus.

    Also, there is grounding the metal frames on the solar panels. Tying to ground rod is required by code.

    Grounding metal structures, plumbing fixtures, etc. for lightning protection and also to prevent exposed metal from becoming "energized" if it is shorted to DC or AC hot power leads.

    What was your question again? :roll:;)

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • offgrid me
    offgrid me Solar Expert Posts: 119 ✭✭
    Re: grounded or ungrounded

    B.B Thanks for your very concise response. Even my girl friend understood it. I got confused because I had read about positively grounded and floating systems and didnt really understand why or when a system would be either.
    Coot of course my panels, racking and every electrical component will be grounded to an 8' rod pounded into the ground you know for safety sake :p