Basic Grounding Question

rene26

Hi all,

I have a question regarding the grounding of a battery-based, roof-mounted PV system. There is some debate over the routing and eventual destination of the EGC bonding the panel racking. Being smaller than #6, I understand that it needs to be run in a pipe coming down the side of the building, but then where does it go? Does it have to go directly to the grounding electrode (in this case a ground rod)? Does it have to remain unbroken along it's entire length? Why? Some have advised that we bring it to the DC negative terminal block with the inverter charger, which then in bonded to the grounding electrode via an appropriately sized GEC.

Opinions?

Thanks in advance for your help. Grounding is something that the more you think about it, the more complex it gets ;-)

Rene

BB.

Re: Basic Grounding Question

There is some debate over the routing and eventual destination of the EGC bonding the panel racking.

I like to break it into two discussions--Lightning and "Safety" grounding. The two requirements are actually quite different and what is "safe" for one is actually dangerous for the other.

First--looking at lightning control. The conductor should be connected per code (using NEC/UL/NRTL Listed) using correct clips/hardware, non (mechanically) spliced wire from array/mounting frame work, down the OUTSIDE of the home to a ground rod pretty much driven the 8-10 feet (minimum) into the earth besides the home.

The reasons are--Non mechanically spliced is a requirement by code--They do not want the safety ground to be disconnected accidentally during maintenance later (human factors type argument).

Next, lightning is not "DC" but really a "high frequency" energy wave form (approximately 7.5 kHz maximum) and it flows down wire/cabling following a different set of rules than pure DC (or low frequency AC at 50/60 Hz) would. Basically lightning wants to flow on the "outside" of a conductor--Or if there are several conductive paths, it wants to flow out at the edges/perimeter of the building--If you have two ground runs, the should be at opposite walls/corners of the structure. If you bring a lightning ground down the "middle" of a building (with rest of panel power connections), there is a greater chance of the lightning to "jump" from the metal conductor and attempt to "spread out" towards the building edges/conductors in walls/etc.

Also, lightning can "find another path" if bends are made tighter than (as I recall) 18 inch radius turns--No "square bends" in lightning ground cabling (again, AC wiring does not "care").

Now for safety ground... Usually, safety grounds run with the AC/DC wiring--but for reasons above--if you have lightning in your area--Not a good idea.

To meet the intent behind safety grounding a system (DC or fallen AC wire from utility power), connect a Safety Ground cable from the "lightning ground rod" back to the main AC/DC ground rod back near (typically) the AC main panel. This will help prevent any metal solar frames/mounting framework from becoming energized if there is an AC or DC fault somewhere.

For DC grounding of your backup battery bank (off grid/hybrid inverter systems, etc.)--There is a requirement in the current NEC (national electric code) to install a DC Ground Fault Interrupter setup of some sort. The reasons are (a bit) strained and the actual solution as suggested by code are actually (in my humble opinion) quite dangerous. It involves installing a small 1 amp fuse or circuit breaker between DC battery bus "Return" (typically negative bus) and the building safety ground (typically the "master" ground rod). I believe it is very dangerous to do that--the reasons are in this thread if you want to read more about the issue:

• Ground Fault Protection
• Wind-Sun Forum thread

For more discussions on grounding:

A couple threads about Lightning from a post on the "Working FAQ" thread:

Off Grid Grounding Technique?
Another Question, this time about Lightning

Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

And our host's consolidated FAQ page:

www.windsun.com
Lightning Protection for PV Systems

From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

Towards the end of this thread is a very nice discussion of proper generator grounding.

-Bill

Remember that local building inspectors can have their own requirements for grounding--You should show your plans before implementing in case they disagree (if you will need a building permit for your installation).

-Bill