Grounding multiple arrays in Grid Tie system

echoth

Hi everyone.  New forum user but longtime solar user and NAWS customer.  Existing system was installed by solar contractors in 1999, no longer in business.  I’m in the middle of installation of a second array installation being added to the existing.

Current System:

3kw system using 48 – 64w Unisolar panels  (48v)
Dual 5500 w Trace inverters connected for 240v located in garage
24kw battery bank at 48v
Array location is 300ft from garage.  Array grounding is at panels and Inverter system ground is tied to that ground

New System:

2.9kw on garage roof using Panasonic HIT240w (4 strings of 3)
157v at 20 amps out of combiner. Each string is properly fused at combiner.
Midnight Classic 200 controller and midnight lightning arrestor
50a breaker in Midnight bigbaby box

New controller will connect to the existing battery bank plus/minus providing additional charging source for total of 5.9kw.  Actually less than that since the aging Unisolar system is < 2.5kw.

 

Question:

Proper grounding. Should I:

1.       Ground the garage array to a separate rod at the garage and ground the new equipment to that rod only.  Is there an issue with grounding potential differential with the separate grounding between the two systems with both connected to the battery bank?

2.       Ground the garage array to a separate rod at the garage and ground the equipment to the existing inverter grounding block.  (Note, there are no more ports on the block bigger than 10awg or maybe 8awg)

3.       Ground the garage array and all new equipment to the existing grounding block in the inverter (Note, there are no more ports on the block bigger than 8/10awg)  Fear of lightning strike at this point traveling through existing system with ground 300ft away but more importantly wiring issue on the block with no more large gauge ports available.  I do have a lightning arrestor at combiner.

4.       Option 1 (separate grounding) but connect the two grounds together through the inverter grounding block and new combiner ground block.  With inverter grounding block port availability can I run multiple 10/8 awg to compensate for lack of single 6 or 4awg wire if they need to be connected.

Thanks for any enlightenment.

Tim Harrington

BB.

Welcome to the forum Tim.

OK, there are (in my humble opinion) two major reasons for grounding. One is to "trip a breaker/fuse" if there is a hot to metal short. And the second is for lightning protection.

For the first reason, all grounds need to be connected together for a common system. Basically, a 6 AWG (minimum) wire that ties each ground rod in the system together. Nominally, a normal ground rod to earth connection is not low enough resistance (can be as high as 25 Ohm resistance between rod and earth) to trip a fuse/breaker if there is a hot to metal short.

The second for lightning--And that is why 6 AWG is highly recommended (neglecting code requirements for the moment). 6 AWG cable is heavy enough to have strength to resist corrosion/phycical damage (connection to ground rod, burial in the earth). And second, 6 AWG is heavy enough that it will not "fuse" if hit by a nominal lightning strike. Thinner wire can be fused by lightning, leaving you "unprotected" if there is a second strike.

For safety bonding (short circuit)--The actual physical path of the ground wire does not matter. But for lightning, the path matters a lot. Basically, the lightning ground cable needs to run directly from the array down to the ground rod at the based of the array, on the outside of the building (and each ground rod should be connected to every other ground rod by the 6 AWG cable). The lightning cable should have wide radius turns as sharp 90 degree angles have "high impedance" and lightning can "avoid" the turn and take a different path to ground. And lightning will only follow your 6 awg cable a few dozen feet--It will not follow a 100's of feet of cable between (for example between your two ground rods 300 feet apart) as the earth itself has lower impedance.

You should have AC and DC surge suppressors between the power cables and earth ground/rod at each end of long cable runs.

http://www.solar-electric.com/installation-parts-and-equipment/midnite/cipr1/suprde.html

For normal installations, you only connect the Battery DC ground to a ground rod at one location--Typically the negative Battery Bus to the nearby ground rod at the edge of the building/battery shed. And your SPDs connected to the same ground rod (if local to the building) or to the array ground rod (if remote array).

The same shed ground rod used for DC grounding should also be used for grounding your AC neutral Bond. All other grounds (i.e., safety ground bonds to hot/cold water pipes, natural/propane gas lines, etc.) should be tied to the same ground local ground rod (if shed is remote to main home, then a 6 AWG cable from shed ground rod to building ground rod).

Some more grounding discussions/information:

http://forum.solar-electric.com/discussion/comment/84242/#Comment_84242

-Bill

echoth

Thanks Bill for quick response.  Am digesting the great info and already knew some of this conceptually but you gave me some other things to consider in the design. Both old and new systems have SPDs.  One of my current issues is hooking up the Midnite Classic controller with built in GFP which defines the DC battery minus to not be connected to ground so the GFP fuse will trip.  My current system has DC and PV negative all grounded to common, as you describe above.