Mobile system grounding, safety (long) [RE-wrenches]

[Christopher Freitas]

Greetings to all - I have just joined the wrenches list as the 
manufacturer representative for OutBack Power Systems.  I have meet many 
of you in person while I worked at Ananda Power and Trace Engineering 
during the last 10+ years.  

As far the mobile PV system goes, the Trace SW4048 is not specifically 
designed to be a mobile inverter - most of the mobile inverters include 
another relay internally which connects the AC neutral wire on the AC 
output to the grounding system while the system is operating as a 
standalone inverter.  Once AC is applied via a shore cord or back-up 
generator,  the relay opens and the AC output neutral is connected to 
the AC source's AC neutral which should already have a connection to the 
grounding system at the main panel.  

Since the SW does not incorporate this additional relay,  you either 
need to add it or provide an alternate way to make the connection of the 
AC output neutral to the grounding system when the inverter is running,  
but that disconnects this connection BEFORE the shore cord is plugged in 
(or else a GFCI will trip on the AC source).     

This can be done manuallt with a switch or automatically with a  relay,  
but one of my simplist tricks on systems which only use a shore cord and 
do not use the SW's two AC inputs (utility grid and a back-up generator) 
is to simply plug in the AC input cord to a dummy outlet built into the 
storage area of the cord.  The dummy outlet is not connected to anything 
except it has the ground and neutral teminals of the dummy outlet 
connected to the grounding system of the trailer.  This ensures that the 
connection to ground is disconnected when the cord is plugged in.

I wouldn't worry about the ground rod as long as all of the AC outlets 
have a GFCI and the ground terminals are connected to the trailer's 
grounding system.  This is what is done on tens of thousands of RVs 
safely.  Make sure you simply bond all of the components together and to 
the frame of the battery.  I wouldn't add another breaker in the battery 
negative cable - just make sure the battery box is lined with a 
non-metallic lining (foam insulation) and that the batteries are 
strapped down well.  You could provide an additional fuse as close as 
possible to the positive battery terminal - the NEC requires overcurrent 
protection with in 18" of the battery for recreational vehicles.  I have 
put Class-T fuses in heavy heatshrink tubing with wire ties on the ends 
to reduce the chance of sparks causing a problem.  

I hope this answers a few of your questions and gets our minds 
momentarily off the terrible news surrounding us.  

Christopher Freitas
OutBack Power Systems
cfreitas at outbackpower.com  www.outbackpower.com
Tel 360 435 6030  Arlington WA USA


Gary Higbee, Solutions from the Land wrote:
> 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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