Yet Another PV Panel Grounding Question
BillD
Registered Users Posts: 4 ✭
This one is a little different as it involves two disparate panel locations.
I'm upgrading an off-grid system from 1 to 5 kW. Most of the new panels (three strings) have to go on the roof due to rough terrain (no place for ground mounting). There is one existing ground mount tracker array (An old WatSun with the tracking disabled) which I intend to put the other two strings on.
For the roof strings I'll do a exterior down rod to the house GE from the racking and a EGC (EBC) in conduit with the PV wiring to the roof combiner. The combiner box ground will not be tied to the racking at the roof. Trying to keep the lightning outside the house.
The issue involves the old tracker array which is about 100 feet from the house GE. It has its own ground rod from a prior installation. My thought is to ignore that ground rod and run what is essentially a down rod to the house GE from the array frame. (the array is actually a little higher elevation than the roof racks). Then in similar fashion I'll run a EGC (EBC) to the combiner at the array which would be isolated from the down rod, as is the case with the roof.
Here's the question. The tracker pipe mast is anchored in a big chunk of concrete with rebar which goes down maybe 6'. It probably comprises a poor grounding electrode (Watsun instructions say it is not a good grounding electrode and to sink a proper ground bar). It is impossible to isolate the array frame from the mast and anchor so there will be some grounding from the mast and anchor. So, is it be better to ignore the tracker ground bar and just tie the array frame to the house GE or to tie the tracker array to its local ground bar and then tie that to the house GE? My thought is that if there is a nearby lightning strike the ground voltage differential on the tie to the house GE would be less from the mast and anchor than from the ground bar. Thoughts?
Thanks in advance.
Comments
Before I try to get into specifics, just a general suggestion.
Lightning grounds, they should be short runs of 6 AWG wiring on the outside of walls to a ground rod at the base. No sharp turns, but nice rounded turns instead (lighting tends to "jump" from wiring at sharp turns). You want multiple lightning grounds (one at the base of each structure to be protected). Lighting will not go more than a few 10's of feet down a 6 AWG cable before trying to find a different low impedance path. Lightning is a "radio frequency" current (very low frequency RF). So the "impedance" (inductance/capacitance & resistance) that affects the path lightning will follow--Not just resistance.
For safety grounding, you want to tie all structures and power sources/electrical boxes/metal conduit together with (typically) a 6 AWG minimum cable... This is so that any short from "hot" to ground will not energize a structure, but will find a low resistance current path back to the "source" (main panel, fuses/breakers, etc.) so that energized structure will trip the breaker/fuse/etc. That generally means that each ground rod should be connected back to the main panel/ground via the 6 AWG cable (solid 6 AWG if exposed/direct burial, stranded OK if protected in conduit). For example you don't want a rack mount or light pole to become hot (solar panels, AC power to outlet/yard flood light/etc.) where somebody could grab the metal and get electrocuted through wet grass/puddle/water pipe/properly grounded metal electrical box. The minimum resistance of a ground rod to earth is 25 Ohms. A short to ground of 120 VAC is not usually enough to trip a breaker (I=V/R=120v/25ohms=4.8 amps of current). That is why the AC safety ground wire should be "stitched" to all outside structures/boxes/well heads/etc.
"The combiner box ground will not be tied to the racking at the roof."
My question is what do you mean by "combiner box ground"? Do you mean the metal box will not be tied to a local ground rod? Or do you mean the negative DC solar terminal--Negative ground--will not be tied to the ground rod?
In general, you want to have grounds (chassis/box/structure) and power cables referenced to local ground rod. Because you cannot directly tie hot and DC power cables to grounds, usually some sort of surge suppressor is used instead:
https://www.solar-electric.com/search/?q=surge+supressor (suggest looking at Midnite brand both AC and DC voltage units available--Not a spark gap, but MOV based).
So, for your solar power, you would tie the combiner box sheet metal to structure+ground rod cable... And use a Surge Suppressor between the + and - solar power cables and local ground rod. Allows DC power to be "isolated" but high voltage lightning energy to be sent directly to local ground rod. And, there should be a 6 AWG cable from local ground rod back to the earth+safety ground connection back at the home (ground rod next to exterior of walls--As you say, don't want to bring lightning energy into the home).
By itself, a concrete foundation and rebar can be an "acceptable grounding" system. UFER:
https://www.ecmag.com/section/codes-standards/what-ufer-ground-concrete-encased-grounding-electrodes
For lightning, there are lots of advantages to multi-point grounding (ground rod(s), UFER, metal cold water pipe, well casing, etc.)--You want to give the lightning the best path to local earth ground. Plus a 6 AWG back to the home main ground rod+AC safety ground connections (for DC and AC power/short protection).
With a direct lightning strike--Probably anything subjected to a direct strike will need to be replaced. As you say, keeping it from entering the home is the best protection for safety.
Using Surge Suppressors at the Combiner box (the array location), and another suppressor at the house wall where the array cables enter the home.
Grounding the Battery DC negative bus (6 AWG minimum) to the local (outside wall) ground rod (sometimes "buried ground plates" are an option (cold water pipe/propane pipe.etc, tied together).
And AC surge suppressor(s) at the AC main panel. And main panel safety ground/neutral bond (normally single point neutral/AC White wire tied to green wire ground in main panel--And run 6 AWG minimum from panel safety ground to house ground rod).
As a very rough answer--Turns out that AC inverter output stage is more typical lighting induced failure than the DC side of off grid power systems (from WindSun, the original founder of our forum here and one of the owners of NAWS--Our host). So installing surge suppressors on the AC panel (and/or AC inverter AC output) is a good idea. All of the AC wiring in the home is susceptible to lightning too.
Keeping lightning from hitting your system is good (lightning rods). One former poster here would run the last 10 feet from the array to the home with outdoor cordage... And unplug the array from his home if lightning was expected (at least 10 feet between array cable and home). Obviously, you do not unplug in the middle of a lightning storm.
The issue of Neutral+Earth Ground bonding (and grounding in general) is not a simple answer... For example, many larger power sources (more than 3,500 Watt output AC inverters and Gensets) may default from the factory to have AC Neutral+Frame ground connection made internally to inverter/genset. Multipoint grounding of AC neutral+safety ground is not desired. The "parallel" connection(s) of green+white wire means that your safety ground is carrying part of the AC load current (not good). If you have ground fault breakers at the inverter or genset, this will trip the GFI breakers.
However, in systems with multiple buildings (such as homes served by a pole transformer) or remote well shack/shop/etc... Sometimes (for lightning control) having neutral+ground rod bonds at the remote facilities is standard (and local surge suppressors a good idea). For example, you have 5 homes and 6 neutral+earth bonds (one at the pole transformer, and one at each home). The current will go mostly through the neutral connection, with little current through the earth itself (remember upwards of 25 Ohms from ground rod to earth vs a faction of an Ohm for electrical wiring).
This post has links to multiple lightning FAQs and posts:
https://forum.solar-electric.com/discussion/comment/84242#Comment_84242
I hope this helps. This is a complex subject and lots of "it depends" answers. Questions?
-Bill
I'm using #4 for the down rod. Depending on the standard you consult, the down rod gauge for lightning protection ranges from #6 to #2. The Iron Ridge grounding clamps only go to #4, which seems a reasonable compromise. Good tip on avoiding tight radius bends on the down rods from the roof and the array. I had not seen this mentioned in other posts. They will both be run on the exterior of the building, away from any conductors, like EMT or pipe..
For anybody who's interested, this approach stems from this post:
https://www.solarpaneltalk.com/forum...d-gec-required
and several others which indicate that a rooftop array is really an expensive Lighting Rod (Air Terminal) and should be treated as such, with a down rod connection to a ground bar. Using the system Grounding Electrode provides lightning protection and also satisfies code. As is pointed out in the video in the linked post, NEC 690.47(d) which specifies an independent grounding electrode for roof arrays is wrong and dangerous.
Remember I try to give reasons why A/B/C... I am not an expert by any means. Just telling you my understand of the issues. Code/Others may have differing opinions and requirements (NEC was really not great for lightning control in decades past... Have not look recently to know how much, if any, has changed).
Lightning wants the most direct path to the earth. If you home rod is close and the wiring is very short run, then it should be OK.
What you want is all the stuff in the home to be at the same voltage ideally (think Faraday Cage--Inside the cage, all surfaces are at the same voltage--Inside a Faraday Cage, you cannot tell if the cage is at zero volts or a million volts.
Of course, if you put two probes in the ground a 100' apart--A lightning strike will produce a voltage gradient. And there is nothing you can really do about that.
Where there has been issues with voltage gradients (besides lightning)--A power line short to the pole/tower (buried cables, etc.). This creates a gradient around the tower... And you get an animal such as a cow--With their legs "at the corners" of a cow, that can direct stray current right through the cow's heart and kill it.
https://stopthesethings.com/2019/05/03/french-farmers-re-volt-cattle-killed-by-stray-current-latest-victims-of-wind-power-onslaught/
https://www.progressivedairy.com/topics/management/stray-voltage-affects-cow-behavior-milk-production
Lightning will not travel more than 10-20 feet or so down a 6 AWG cable before it finds a different path to ground (AC impedance vs DC resistance).
Tying the two ground rods (and grounding systems together) between the home and the array (or other outbuildings) is to prevent voltage gradient issues for DC (solar/battery power), and AC (60/50 Hz house power).
I have added some links below for further light reading:
-Bill