Mobile system grounding, safety [RE-wrenches]

[Gary Higbee, Solutions from the Land]

Hi Wrenches,

I'd appreciate some feedback on a mobile system I've been working on. The
system is a 48-volt nominal 800-watt trailer-based PV array with Trace PVGFP
(ground-fault protection) and SW4048 inverter. The inverter AC output
utilizes a standard AC sub-panel and both 20 amp output breakers are routed
to ground-fault outlets.

We previously discussed grounding the trailer when stationary, and my
feeling is that I will utilize a short ground rod, which is attached to the
trailer frame (which is attached to the system ground). It seems that we
should drive the ground rod in a couple feet (hopefully avoiding underground
pipes and power lines!), and pour a gallon or so of water over the rod to
create a larger and deeper conductive path.

Though additional views are appreciated regarding grounding the system when
stationary my current question is whether it would be safe to operate AC
devices when the trailer is being transported or if we're parked in a place
where we don't have soil in which to drive a ground rod. This trailer is
part of a demonstration project and the last thing we want to demonstrate is
how high a bystanders hair stands when standing on wet ground and touching
the trailer (should a fault occur).

We've considered dragging a strap on the ground, but it seems that this
wouldn't make a very good ground. It also seems that the ground-fault
outlets should protect against a fault from the system hot to the trailer
frame, but what about an AC system fault where the frame is hot, to someone
not on the trailer (and essentially at earth ground) and touching the
trailer. How do RV's handle this, as they utilize on-board inverters and
such to power AC circuits?

Any comments on the DC end? I was working on the battery box, and in my
sweaty state brushed the frame while tightening the hot end of the battery.
Fifty-something volts speaks with some authority under these conditions. It
seems that the PVGFP should take care of array-to-ground faults, but that
there really isn't anything to take care of a battery-to-frame fault short
of (pun sort of intended) the pyrotechnic display of a direct hot to
negative short. I wonder whether I might want to install a second DC175 in
the negative line, which wouldn't prevent a shock should a sweaty person
come between the battery plus and the frame, but would prevent a major mess
should the battery plus ever touch the frame, through a wrench or whatever
(yes, an insulated wrench will be a permanent part of the battery box).

Comments are appreciated!

Thanks.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     Gary Higbee  (ghigbee at efn.org)
          (541)607-1818 (Eugene)
         (541)902-8544 (Florence)
   Solar, wind, and hydro site analysis,
    design, and installation assistance
         ~Solutions from the Land~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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