FW500 Ground/Bond Issue

I have just been testing the 48V AirX turbine wired into my FW500 box and found that it was energizing the entire box (bad buzz!). It is wired through the appropriate switch and breaker on the positive side, and wired directly to the negative buss. After removing the supplied bonding wire between the negative buss and the ground buss, there is still residual voltage, albeit at a very low rate. Three questions for the forum:
1) Is this common for the FW500?
2) Why is the DC box shipped with the bonding wire between the negative and ground busses?
3) What is the best practice for wiring an AirX into the FW500?


  • BB.BB. Super Moderators, Administrators Posts: 32,656 admin
    Re: FW500 Ground/Bond Issue

    In general, you want to ground metal boxes to Earth Ground and/or Battery ground so that if there is a short from a hot wire, it does not energize your metal boxes, conduit, chassis, etc... If there is a short, the fuse/breaker in the hot lead is supposed open and protect that circuit/wire against any further shorts.

    Generally, in a fixed installation, Earth Ground (ground rod, metal water pipe, etc.) is tied to Battery Negative. And all metal boxes, chassis sheet metal/frames, and even metal water pipes are all tied at one point--the earth ground rod.

    For mobile applications--The metal frame of the car/truck/trailer is generally the "earth ground" equivalent. And many times, the metal chassis is also the return conductor (i.e., car radio, etc.)--However, that is not usually ideal electrically (not all chassis/body connections--such as car doors--are properly electrically connected).

    Not sure what the buzzing is in your application--does the frequency of the buzz change with the RPM of the turbine?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JCOrdJCOrd Registered Users Posts: 6
    Re: FW500 Ground/Bond Issue

    Thanks, BB.
    The "buzz" is actually my attempt at a humourous way of describing the feeling of DC current from the turbine energizing the box. My concern is that when I energize the 1.2 kW array, it may energize the box as well. The bonding wire seems to allow current from the turbine to flow to the box itself, so my question I suppose is one regarding the consequence of leaving the bonding wire off altogether (safety & code).
  • BB.BB. Super Moderators, Administrators Posts: 32,656 admin
    Re: FW500 Ground/Bond Issue

    Assuming that the battery bank negative bus is earth grounded... And that all positive leads go to the battery positive bus and have the appropriate fuse/circuit breaker to protect the positive circuit wiring for shorts (i.e., 15 amp breaker on 14 awg wire leaving the battery bank, etc.).

    Then the green wire/earth bonding is designed to keep all metal objects (electrical boxes, inverter cases, plumbing fixtures, battery bank return leads (and for some systems, the AC Neutral wire at or near zero volts with respect to ground too--but that is not a simple discussion either).

    If there is a short circuit from a "red" battery lead to a metal box/device chassis, that current will be shunted to the common DC ground point and then back to the battery bank (completing the DC circuit). The wiring (earth bonding) should be heavy enough to draw enough current to trip the breaker/fuse at the battery bank positive bus (i.e., that breaker on the 14 awg wiring going to some LED wiring).

    The reason for grounding the battery negative bus to "earth ground" is to ensure that the negative leads never go much above zero volts WRT to ground either (that way, if properly wired, the return leads do not need their own fuses/breakers since nothing will happen if a return wire is accidentally grounded to earth).

    So, normally, a 6 awg wire is used as the minimum gauge DC grounding/bonding wire... It is usually heavy enough to safely trip any breaker/fuse in the circuit.

    For example, using this random wire chart, the fusing current of a 6 awg wire is around 668 Amps.

    If you have heavy currents (say a very large high wattage inverter on a 12 volt battery bank) with 300+ amp fuses--you may want to look at making that green/chassis bonding wire a heavier gauge.

    So--the idea is that any metal and neutral leads in your installation should be "perfectly safe to touch"--either with your fingers or other grounded metal objects.

    You will see that NEC talks about a DC GFI system--that is not the same as a GFI breaker/outlet like used in a home near a sink or outside that can open a Hot to Earth short (such as your body) with only 5 milliamps (0.005 amps) of current (should not stop your heart).

    The DC GFI system is a 0.5-1 amp fuse (approximately) between the Negative Ground bus/common point and earth ground. If there is a DC Red/Hot to Earth Fault, it will pop/trip the fuse/breaker and (is supposed to) turn off the solar array or solar charge controller.

    I have HUGE issues with that circuit in earth ground referenced DC power systems--but that is (probably) for another discussion.

    So--if you are getting a "buzz" from your metal box--The lowest voltage I have every felt electrical flow through my fingers was on a 24 volt battery system. So--My guess is that either the box was energized to >24 Volts, or you where (for example) sitting on a piece of metal or energized electrical cable and the box was grounded (actually, my 24 volt shock was sitting on ground metal rails and handling some 24 volt wiring--and I felt some "stinging" in my legs through damp jeans).

    So--Pick a point that is "zero volts" (typically a ground rod or metal cold water pipe in ground)--and measure you various points in the battery shed/area. See if you find any points significantly above ground that you did not expect.

    Note--depending on your meter type--you may need to measure both AC and DC voltages.

    Also--a quick way to find out if you have a "series" hidden short some where energizing some metal or wiring--Use a 12/24/120 VAC light bulb to put some load on the circuit. Typical DMM's have 10,000 to 1,000,000 Ohms of resistance--and they can "detect" voltages that are not really there (very small leakage currents). So using a simple filament lamp to "ground" the "possible hot" metal will safely show you if the metal has serious energy behind it or not (i.e., still reads XX volts on DMM when lamp is clamped across the test points).

    Does this help?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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