Off Grid Grounding

AGB

Hi,
Sorry if this topic has already been discussed. I need help understanding grounding in my Off Grid System.
I have all my AC side grounded and no problem there. The problem is the DC side. The panels and the DC surge arrest are about grounded to a separate grounding rod.
I question is, Should I ground the Battery and the charge controller to the same grounding rod as the AC or to the panels?


System Setup
Nine 310W panels. Three in series and all three sets in parallel.
Breaker for each series set and all combined through a bus bar and to another breaker.
Citel DS240S-130DC Surge protection device connected in the combiner box with grounding.
Midnite Surge arrest for the AC side
Schneider MPPT 60-150 charge controller
Schneider SW4048 inverter
48V L16 US Battery

Estragon

As a general proposition, the controller etc. could go to the same ground point as the AC, and the two grounds rods should be tied together.  Always check manuals for specific grounding instructions though.

mcnutt13579

Never, never any "independent" ground points.

All ground rods and metal required to be grounded must always be bonded together with substantial wire.  Usually #8 to #4 copper.  If they are in the same location then they all tie to the same ground rod/pipe/rebar system.

Otherwise the active conductors (hots, neutrals) between these independent grounds will be asked to carry lightning currents or other surge currents.  Better to have a ground/bond wire to do that.

NANOcontrol

The simple explanation is in an ungrounded PV system, both positive and negative legs have to be fused. Just grounding one side of the battery is easier. Must have a single ground connection point. Additional ground rods may be connected at each building structure.

BB.

I would suggest that 6 AWG be the minimum size cable/wire for grounding interconnects and such.

From some earlier European research (that I have not since been able to fine)--I think much of it was looking at the lightning ground on old German churches (as I recall), and they found "fused" (burned open) 8 AWG and smaller cable from lightning hits, and no fused 6 AWG and heavier cables.

Note that "real lightning" ground protection is a complex subject that requires lots of reading/research (and use of experts for larger installations).
NEC (US national electric code) mostly aBddresses 60 Hz grounding for safety. Lightning is actually a low frequency radio current (with lots of voltage and current)--The physics of "radio frequency energy" is quite different from DC or 60 Hz AC.

-Bill

Graham Parkinson

Another off grid grounding question: 

How to ground the propane tank connected via a 50 foot copper pipe buried inside a poly pipe conduit in a trench that will be backfilled?   I have a wire run in the trench to the tank, but am not sure of where to route the wire from the tank.

Our electrician told us that we should ground the propane tank legs to the AC panel ground. Our panel ground goes to a standard AC ground plate on the other side of the house via the typical large bare Cu ground wire.   

Since the tank is bolted to concrete foundations set in the ground it is "slightly grounded" already.  I gather that it probably won't create too much of a problem to have these two routes to earth and there isn't much that could be done to isolate the tank from the earth.

The only other option would be to connect the tank to the AC ground plate directly with another wire run to the plate.

LIkely, the object of grounding the tank to the panel ground or ground plate is so that the propane piping does not carry all of the induced current and currents resulting from differences in ground potential in the event of a nearby strike.  Since we have have plastic water pipes there is no other interconnection between plumbing or gas pipes and the electrical ground.  Our propane appliances are all connected via copper pipe.  The gas kitchen range has no electrical ground as it is a battery powered Unique spark ignition range.

Does anyone know the proper way to do this?

Thanks!

Dave Angelini

Why use copper for propane???

 BB. said:

Note that "real lightning" ground protection is a complex subject that requires lots of reading/research (and use of experts for larger installations).


-Bill

Add prayer as even the professional lightning folks will tell you that. Good advice that will only go so far.

Graham Parkinson

Copper is pretty much standard around here for propane, but there are some that use poly jacketed thinwall SS or even polymer lines (must be protected).  Some jurisdictions worry about the Sulphur in propane reacting with copper and require internally tinned propane lines.

Probably best to run tank ground to panel ground which goes to earth plate as that keeps area between copper pipe and ground wire small in my case, reducing induced potentials from circulating currents.

My understanding though is that generally you want to have the straightest, shortest path to ground to have the lowest inductance and best chance of current following grounding conductor as the leading edge of the strike is a high frequency component that reacts strongly to conductor self inductance.  Larger flatter conductors are better than small wires as this component of the current doesn't have much surface area to travel on in a small wire.

Dave Angelini

All I have ever seen  (many of the US and Canadian states) is poly direct burial gas line and never have had to be concerned with grounding a propane tank. Sand is poured over it and below. If under a road it must be protected with metal.

I would be more concerned with an unneeded ground bringing the lightning into your ground system.

Just adding another way to get in trouble offgrid.

mcnutt13579

Graham Parkinson said:

Another off grid grounding question: 

How to ground the propane tank connected via a 50 foot copper pipe buried inside a poly pipe conduit in a trench that will be backfilled?   I have a wire run in the trench to the tank, but am not sure of where to route the wire from the tank.

Underground propane lines and the tank itself are generally INSULATED from the pipe inside the house.

With an insulated union where the pipe comes out the side of the house and goes to the ground.

NFPA 58 6.9.3.16 Underground metallic piping, tubing, or both that
convey LP-Gas from a gas storage container shall be provided
with dielectric fittings installed above ground and outdoors at
the building to electrically isolate it from the aboveground
portion of the fixed piping system that enters a building.

mike95490

The metallic gas line is supposed to be insulated so electrolysis does not occur.  Lightning will jump right over any dielectric fitting.

Dave Angelini

So why not just use poly and avoid all of these problems.

Graham Parkinson

Thanks all for the useful comments - Since the copper pipe is already installed in the trench and helping keep us warm in the winter, will make sure my plumber brings up a dielectric bushing on his next trip in.  We have very little in the way of lightning risk in our dark damp part of the world.