Lightning Grounds, exiting roof

VicVic Solar Expert Posts: 3,145 ✭✭✭✭
This is new construction, solar roof-top off-grid install.

Am using UniRac SolarMount fixed racks, and will be using 6 AWG Solid array grounding conductor.

How do the pros make the transition from the bare conductor to PVC conduit to penetrate the roof? My plan is to enter PVC Conduit, perhaps with a cord grip, into a PVC LB, Through an Oatey flashing on the roof deck, into a flexible PVC transition inside the soffit, which will be connected to 1/2" PVC conduit that runs vertically, attached to the wall of the building. This conduit will terminate at grade level, and GC attached to the ground rod(s), and into a trench around the perimeter of the structure. Am reluctant to note that the GC is really a U-shape from one ground rod, up around the array connections and exits the roof either to the same ground rod or to a second one, but this will bring up the inevitable GROUNDING discussion!

The eave overhang at the lower edge of the roof, where this ground conductor/rod is located is about 32 inches, so simply running the grounding conductor around the edge of the roof and down to the vertical conduit seems kind of bush-league, and there will be a metal rain gutter along that edge as well, possibly creating additional problems in a Lightning event.

I do hate making the 60 degree bend from the roof (30 degree slope) surface into the conduit, but that seems to go with the territory -- difficult to make that a gradual bend, as the LB will be close to the roof deck, and the same would apply to dressing the GC around the lower roof edge.

Comments greatly appreciated. Do "know" that technically, this should really be done with continuous copper strap, but am NOT going there.
Thanks, Vic
Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.

Comments

  • FrxddyFrxddy Solar Expert Posts: 113 ✭✭✭✭
    Re: Lightning Grounds, exiting roof

    I'll be interested to hear what "the pros" have to say. I have always been of the understanding that lightning does not like going around corners. I was taught that any wire that you think might carry lightning should be no less than an 18 inch radius.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Lightning Grounds, exiting roof
    Frxddy wrote: »
    I'll be interested to hear what "the pros" have to say. I have always been of the understanding that lightning does not like going around corners. I was taught that any wire that you think might carry lightning should be no less than an 18 inch radius.


    The stray Voltage that permeates the air in an area experiences lightning strikes does not like going through 90 degree bends; it tends to take a short-cut jump across which defeats the purpose of the protection circuit.

    Lightning goes wherever it wants to, however it wants to.

    Usual way to transition from PV wires to conduit is via weather-tight fittings. Upright, wire sag to drop water off, approach from below, usual suspects, don't trust the roofing got with big vats of tar, et cetera.

    Others may have additional information and alternatives worthy of exploring. Wait for it. :D
  • VicVic Solar Expert Posts: 3,145 ✭✭✭✭
    Re: Lightning Grounds, exiting roof

    Yes, am sensitive to the bending of a GC used for Lightning. It is interesting to note that on Baoadcast towers that the GC, usually strap, tho, is formed aroung the concrete tower foundation. The bend radius is ususlly less than one half inch. That ground is attached to the extensive radial system, via another 90 degree bend.

    I am resistant to having the cord grip oriented vertically, but that is not a huge deal, even if the grip leaks, tho.

    Making gentle bends would create a trip hazard, so would like very much to avoid that.

    Coot, yes, the fields surronding a lightning strike are HUGE. Many discharges are hundreds of thousands of amps, with a rise times in the sub-nanosecond range. Huge di/dt, and EVERYTHING has inductance which impedes current flow, thus allowing a large voltage gradiant on ANY conductor.

    And so on. Was uncertain of what you were referring to regarding the tar. The roof will be ashpalt shingles.

    OK, Thanks, am still looking for info. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Lightning Grounds, exiting roof

    Vic;

    There are people who think the solution to sealing out water is to slather lots of goop on - buckets of tar. :roll:
    I'm of the mindset that if you can prevent water intrusion mechanically you're further ahead. Thus having the wires go up into a weather tight seal to enter the conduit is preferable to horizontal or going down. Enough sag on the lines to let the water drip off and keep it from migrating upwards and you're good for years.

    Of course you have to adapt the principles to the application.
  • VicVic Solar Expert Posts: 3,145 ✭✭✭✭
    Re: Lightning Grounds, exiting roof

    Coot, OH,

    Yes have seen the Weather Heads under panels with drip-loops, YES, good practice. BUT will be running the PV in metal pipe, for all that conduit provides, plus it is the EMI/RFI shield. So near each + and -- PV string conductor, there be a cord grip into a transition box. These are hidden under PV panels, but will be exposed to wind-blown rain, and condensation. Am sure that the conduit will get some moisture inside.

    The PV string conductors will transition into the power room, via LB/LBTs, etc.

    Yea, slathering tar and goo on the transtions does not seem to work very well. My roofer does use an adheasive backed elastomeric stuff, called something like "flash-band" on the metal cone type pipe jacks, it seems to work.

    OK Thanks again, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,137 ✭✭✭✭✭
    Re: Lightning Grounds, exiting roof

    Do not use the roof for solar! Avoids all the problems. The best thing from my experience in the high country is to have a convenient 40 foot Pine on the Northeast side of the array. Works very well. The only thing better is 2 pines;)
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Lightning Grounds, exiting roof
    Do not use the roof for solar! Avoids all the problems. The best thing from my experience in the high country is to have a convenient 40 foot Pine on the Northeast side of the array. Works very well. The only thing better is 2 pines;)

    Of course there may not be any yard to put panels in.
    Sometimes the roof is the only piece of large real estate available. :roll:

    And I sure wouldn't mount thousands of dollars worth of panels on trees. Not just because they're all dead around here either.
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,137 ✭✭✭✭✭
    Re: Lightning Grounds, exiting roof
    Of course there may not be any yard to put panels in.
    Sometimes the roof is the only piece of large real estate available. :roll:

    And I sure wouldn't mount thousands of dollars worth of panels on trees. Not just because they're all dead around here either.



    Just my opinion, I do get one?
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Lightning Grounds, exiting roof
    Just my opinion, I do get one?

    Sure! And if enough people disagree with you we'll make you an Honourary Moderator*. :p


    *Moderator: synonym for "target". :p
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,137 ✭✭✭✭✭
    Re: Lightning Grounds, exiting roof

    No thanks on the high paying job offer;)
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • VicVic Solar Expert Posts: 3,145 ✭✭✭✭
    Re: Lightning Grounds, exiting roof

    Hi Dave,

    Thanks for the meaningful reply. I'd rather track, but this new site is on a ridge-top. Very little flat ground, and the property runs from the center of the ridge top to the north.

    There are a number of trees, which would shade a tracker during much of the morning and afternoon. And, am reserving some of the flat space for towers and antennas. Need to keep the PV array as far away from the antennas as possible. I could scalp the property, but do not want to ... and so on.

    But because this is an exposed ridge, think that some attempt to ground the PV array is in order.

    I may add a tracker at this cabin location, as there are a couple of spots that have a fairly good view of the horizons.

    There are some Digger Pines on the ridge. One had been blown apart by llightning prior to my tenure.

    There are a couple of advantages to roof PV installs, one is that it gets the panels up and away from blowing debris etc, and as in my case it can reduce the shading.

    BUT, if I was doing a tracker, would still want to ground the PVs, and bet it is a bit more challenging getting a good, low Z ground on a moving array.

    Thanks for the help, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • mike95490mike95490 Solar Expert Posts: 9,571 ✭✭✭✭✭
    Re: Lightning Grounds, exiting roof

    I'd put some edison lightning rods on a couple of the nearby tall trees, and use your planned antenna towers as rods. Make your house and panels a "less attractive target".
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
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  • VicVic Solar Expert Posts: 3,145 ✭✭✭✭
    Re: Lightning Grounds, exiting roof

    Hi mike,

    Thanks, that is what I am trying to do ... the towers have good Halo ground systems.

    But, when there is a strike, would like the PV array to have the best path to ground that I can provide.

    No part of it is perfect, but am trying to do the best that I can (am willing to) do.

    The dwelling has a perimeter ground, and UFERs in the footings, but it is not perfect, and, as Coot noted, lightning has a mind of its own, and will do what it chooses, when it desires etc.

    The roof is getting shingles tomorrow, so the array GC penetrations will be located today. Will still have some design flexibility after the shingles are installed.

    OK, Thanks, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Lightning Grounds, exiting roof

    you already pretty much know what i say on the subject matter. the best possible path to ground is directly to the ground and not into the home or dwelling as prescribed by the nec. if that is tied underground to the electrical ground rod as well then the ground lead coming into the house will be a secondary path to ground and ground loops will happen. if this isn't inspected do away with the ground lead going into the place as it should only be outside. if it is inspected then appeal to the inspector because the nec blew this one. if he insists on that lead going into the house then go with it and disconnect it later.

    another thing you are probably aware of is the use of heavy braid. this allows more of the higher frequency content to go to ground, but it's expensive. multiple wires to ground (heavy strands no smaller than #8 in case of breakage) could achieve the same thing, but the nec would disallow this because they can't follow more than 1 wire at a time.:roll:
  • VicVic Solar Expert Posts: 3,145 ✭✭✭✭
    Re: Lightning Grounds, exiting roof

    Hi niel,

    Thanks. Yes, have followed the threads and recall that you had noted this ground connection going into residence as a bad idea.

    I am not running any GC in the conduit with PV conductors ... prob will pull in one, but leave it unconnected to any part of the PVs -- a just in case GC.

    Right now, the AC and DC power system grounds are connected to one UFER, there is no ground rod, but there could be.

    This lightning GC is exclusively for the purpose of grounding the PV array and racks. It uses the UniRac-supplied WEEBs, and the rack grounding connector. As such, the WEEB is a lay-in lug for #6 AWG bare solid GC. Braid would be a lower Z GC, and copper strap lower Z, yet ... but connecting these to the PVs or racks becomes more difficult, and any inspector is expecting WEEBs and a solid GC. There is still the little issue of safety ground for the array, and totally separate PV array ground and power system ground is not the best. Some suggest that the array GC run to the power sys ground rod and terminate there, and the power sys ground is a separate conductor from that rod or a UFER into the dwelling ... but this is only marginally different than what you have noted as entering the residence.

    Of course there are the PV power conductors which will enter the power room, attached to the residence. This power room is a sea Cargo Container, basically a Faraday Cage, which is grounded, as well. All of this stuff is subject to many intreptrations, from many sources, and is highly site/installation dependant.

    Perhaps this will rekindle the entire GROUNDING discussion! OH NO !! TU, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,137 ✭✭✭✭✭
    Re: Lightning Grounds, exiting roof

    You are the one who mentioned tracking ! My point is if in a lightning prone area the last thing one does if there is a choice offgrid, is compromise the dwelling roof and power systems by asoociating them together. Diversify your assets! Good Luck!

    Oh yea, it is not politcally correct to call them diggers, they are now grey pines!
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Lightning Grounds, exiting roof

    vic,
    i'm not sure i'm 100% following you on all of this and is why i didn't chime in earlier. is it that the pvs are too far away from the ufer and you're not sure how to handle it? there should only be 1 ground even if that ground is a ufer, but there isn't any reason why it couldn't be a bigger ufer and unless you are in a very dry area a ground rod driven down by the ufer and attached to the ufer would be a good idea, but i'm not there and i don't fully understanding all of your circumstances let alone what your confusion is about.:confused:
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