Grounding system for a metal building

Steve961 · December 2012

I'm trying to develop a comprehensive grounding plan for my off grid cabin to deal with lightning and general electrical safety. I've been reading as much as I can on the subject, but there's a lot of conflicting information, especially for my situation. What makes this a bit more difficult is that my cabin is built with steel shipping containers, has a metal roof, and has two separate solar arrays. It’s located in northern Wisconsin, so it’s not a big lightning area, but I’d still like it to be safe.

If I followed all the advice I’ve read, I'd be grounding the containers, metal roof, chimney, panel frames, panel negative, mounts, charge controller negative, battery negative, inverter ground, AC ground, AC neutral, and the generator. I already know if I grounded all of this together it would probably be a disaster.

The cabin itself is built with three shipping containers welded together and attached to a concrete foundation 2.5 feet off the ground - I wish I would have known about Ufer grounding before I built it. The roof is galvanized steel supported by wood framing. A metal chimney from the wood stove extends three feet from the roofs peak. The cabin is in the middle of a small farm field surrounded by a few taller trees. Here’s a pic of it before the roof array was installed in front of the chimney:

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Array #1 has two Unisolar US64 panels fed into a SunSaver SS-10 (PWM). They are mounted to the galvanized roof with a DIY Unistrut mount.

Array #2, purchased but not yet installed, will have a single ET Solar 190 panel fed into a SunSaver MPPT. The wooden ground mount will be about 20 feet from cabin. The panel from this array will be stored inside the cabin when I'm away due to ongoing theft problems.

The power system has a 104 ah (~200 ah planned) 12V battery feeding a Morningstar SureSine inverter. A Honda EU2000i is occasionally used through an Iota automatic transfer switch.

For the metal building itself, I have read that grounding rods should be set at all four corners, or at least two corners diagonally, of the building. These should then be attached to the building with #6 bare copper wire. I was planning to use the ILSCO GBL-4DBT ground lugs on the building unless there is something better. For the metal roof and chimney I have heard differing opinions on whether or not to ground them. I guess I could use more of the ILSCO ground lugs to connect the metal roof and chimney to the shipping containers.

For the roof array, should I even ground it since it’s already mounted to Unistrut and then to my metal roof? If I do ground it, can I just run a copper wire from the frames to the metal of the cabin since the cabin itself will be grounded? Some people seem to think that grounding the panels will only make them a more tempting target for lightning.

For the ground array, do I really need to ground it? It’s only going to be a single panel and at most 5-6 feet high. And if I do, should it have its own ground rod, or should I just use the nearest cabin rod that’s only 20 feet away?

For the DC and AC electrical systems, since they’re so small, I think it would be safe to just float everything. Since my AC neutral is not bonded to ground, I might run the ground from the AC to one of the external ground rods. Any other thoughts here? Should I maybe ground the SureSine inverter like the manual says?

I know this is asking a LOT, but I trust you guys more than anyone else to give me the best answers. I know I couldn’t have gotten as far as I have without you.

Thanks.

Steve

icarus · December 2012

Re: Grounding system for a metal building

Nice cabin! the steel building will provide pretty good lightning protection, as it becomes it's own faraday cage. What was suggested to me is that for a lighting gorund for the building, you should install ground wires at all four corners. As for protecting the PV, that is a crap shoot, as a close strike makes all bets off. My guess is that the PV system needs to be grounded separate from the building ground,but I will leave that to folks with expertise greater than mine.

My guess is that the PV is going to have little or no effect on the attractant effect of lightning, since the building itself is a bigger grounded target.

Tony

BB. · December 2012

Re: Grounding system for a metal building

From what I have read, you want a real lightning rods/cables along the peak of your roof and do not use the metal roofing for grounding... Lots of current and where ever/how ever the metal roof bonds to the metal below (or lightning ground cabling), is going to be less than satisfactory (high current may blow the relatively poor electrical joints apart).

I am sure you have seen this post--I have repeated a few times over the recent weeks:

BB. wrote: »

A couple threads about Lightning:

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.

There are some good papers on how to work with lightning in there...

My suggestions 2-4 ground rods at opposite corners of the building (personally, I would run a 6 awg wire from the main rod to each of the other ground rods--not for lightning control but for AC/DC short circuit current control and you can pop a breaker/fuse if there is a short in your power system--probably over kill, but won't hurt). Use one of those rods (if practical) for your main ground. Run the DC ground back to the rod. Run the AC Neutral bonding (if you have one) back to the same rod. Run safety grounds as normal to the main AC or DC ground blocks (green wire grounds are really for carrying short circuit DC or AC current to trip the protective fuses/breakers... They are really not for lightning ground).

20 feet away for the second array... I probably would just drive another ground rod (or a buried ground plate if you cannot easily drive a rod) next to the array and bring a 6 awg wire from the frame work down to the rod. If you want to cover AC/DC shorts, then run a 6 awg from that remote ground rod to the local main ground rod (carry shorting current--not for lightning).

Remember that lightning is an "RF" (radio frequency) event and does not follow the rules of DC/60 Hz AC current flow.

Use only one ground point for your "master" AC/DC grounding... if you you drive two ground stakes 20' apart--You can pick up the voltage gradient of a near by strike and get hundreds to thousands of volts between the ground stakes as the lightning energy dissipates.

-Bill "not a lightning expert--just some suggestions" B.

niel · December 2012

Re: Grounding system for a metal building

let's look at this in a different way. you still want to divert lightning away from the building even though as a metal building it will act as a faraday cage. the standard lightning rods on the roof should take the impact of a strike and divert it to the ground via ground wires. it may be good to use insulated wire here as you don't want any dissimilar metals causing galvanic corrosion. the isolating of the lightning rods from any metal on the roof or building may also be warranted. being the ground wire would straddle the top of the roof it stands to reason one may want to have 2 ground wires to 2 ground rods and those rods would have to be tied underground with at least #6 bare copper wire, but i think you can get away with 1 ground rod.

as to the building you can ground it too but with a separate wire going to that ground rod or rods at the base of the building with a connection that won't present galvanic corrosion as steel to copper presents a problem. this separate grounding would facilitate a path to ground should the lightning hit the building too and not just your rods sticking up in the air and would be needed to complete the requirements that would allow the metal building be a good faraday cage as any emp or even a full strike would have a path to ground and could minimize damages by providing the path away from the building.

in conclusion, you can opt for the lightning rods to be tied together with a single ground wire with insulation to a ground rod. the pv ground shall also have a separate ground wire, with insulation if you can, to also make a bee line to that same ground rod. any and all electrical inside the home shall be tied to this ground rod in a similar fashion as all electrical systems need grounded. finally, tie the building to this same ground rod at its base and that should be in the area of the ground rod as you don't want the ground wires to be unnecessarily long or with unnecessary turns. that means don't let a woman tell you that you can't put the ground rod in its best location because it will ruin the looks of her flowers. you can, as an option, add another rod to the other side of the house if you want to, but this must be tied together underground as i already spelled out. in your case i would not put the ground rod or rods at the corners of the building as the lightning rods will reside in the middle of the roof and is the highest point of the building itself. that means the best place(s) would be in the middle of the building sides directly beneath the peak area of the roof so the ground wires need not be any longer than necessary and not need to have any bends in them. btw, as far as any bends go they should be large and gradual as a sharp turn could literally cause lightning to needlessly jump elsewhere.

tony,
your circumstance is different being your place is not 100% metal enclosed and is why i told you all 4 corners so as to minimize and stray energy from passing through the living space.

Steve961 · December 2012

Re: Grounding system for a metal building

Thanks for the help so far, this is good information. I've drawn up a grounding diagram to try and illustrate what I think everyone is saying. Let me know if it's close. Thanks.

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BB. · December 2012

Re: Grounding system for a metal building

I think so...Run the ground cables on top of the roof--So they stand the chance of getting hit first.

-Bill

niel · December 2012

Re: Grounding system for a metal building

that wasn't what i had in mind. if you ran the lightning rod ground wire to the left of the drawing and straight down the middle of the house to the ground. you would put a ground rod there at the bottom of the side middle of the house and not at the house corners. as the ground wire transitions from the roof to the ground have it loop widely so as to not make a 90 degree angle, but more of a gradual bend like a c cut in half using the upper part of the c. you won't need the ground for the lightning rods on the other (right) side of the drawing, but should you elect to do so that rod will have to be electrically tied to the other ground rod with bare #6 copper wire.

now that you'd have the lightning rods grounded you should now run a #6 copper wire from the pv frames to ground going in a similar manner as the lightning rods going over the side transitioning in a half c and then down to that same ground rod.

all of the electrical grounds inside the place should also be tied with a single ground wire to the same ground rod.

the trailer itself will have a ground wire that will no doubt be fairly short (a ft or 2) and go from the middle bottom of the left side of the place and this too shall go to the ground rod.

in total that would be 4 connections to the same ground rod.

Steve961 · December 2012

Re: Grounding system for a metal building

Niel:

I did some more research and I'm finding recommendations that each lightning rod should have two paths to ground and that the wires run down along the roof edge. They also don't recommend that the two grounds be connected, but do keep in mind that they would be for me anyways since they would be connected to my building shell which is all steel. Here's a pic from another site:

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I'm also a bit worried connecting my DC battery negative to the same ground that is connected to my building metal. This would cause all of the exposed steel, both interior and exterior, to have a path to DC negative. Might that be a bad thing?

Anyway, here's an updated diagram that shows these changes. Let me know what you think. Thanks.

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BB. · December 2012

Re: Grounding system for a metal building

Yes, you want all of the "big metal" to be tide together to one master ground point. You want any DC or AC power (assuming you have ground referenced the DC and AC power return leads to the master ground rod) to have a path back to the power source.

You don't want to (some how) energize the building relative ground (say your battery bank positive lead shorts to the building steel). Now your building is + 24 volts to "earth"... You could have somebody walking in wet grass lean against the building and get a "bite" from the 24 volt + steel of the building. Obviously if it was 120 VAC "hot" to earth, you could have much worse. Or, if you have some unintentional connection between steel and ground--you could start a fire.

The idea is that every possible shorting to the major steel inside the building (sinks, cold water pipes, or in your case structural steel) should be tied to safety ground. It the battery bank or AC inverter attempt to energize a piece of metal (breaker box, steel structure, a mixer falling into the metal sink, etc.)--There would be a path back through the earth/safety ground, and back to the power source (battery negative or Inverter AC ground bonded neutral) to complete the circuit and pop a breaker/fuse.

The idea of the "diagonal grounding" is with Lightning current--it is really "radio frequency" energy--Which tends to travel near the outside surface of the wire (called skin effect). If you had two cables in parallel only a few feet apart--The current would try to leave the cables to the far corners of the metal shell of the building (in a wood stick home, it probably would try for metal piping and electrical runs).

Skin effect - Wikipedia

-Bill

niel · December 2012

Re: Grounding system for a metal building

steve,
i totally disagree with the extra footage to get to ground on the house corners. if you wish to have 2 ground rods then fine, but they should be interconnected underground. you only need 1 ground rod imho and not at the house corner. ground wires need to be as short and direct as is possible without 90 degree bends. in your diagram you are asking lightning to turn 2 bends and the first one on the roof isn't going straight down so the lightning is unlikely to follow the paths you've laid out for it.

you do understand that when tying 2 or more ground rods together underground with bare heavy copper wire that it creates 1 extra large ground and, is not only very acceptable, i'd say it would be preferred. i won't get into your particular needs with internal grounds for electrical equipment in the house on this thread, but there should only be 1 grounding point. if you use 1 rod it is that rod and should have all ground wires going to it as directly as is possible without sharp 90 degree turns. to expand the ground itself is what 2 rods with bare copper wire inter-tying them under ground would do. the inter-tie is necessary to prevent ground loops and voltage differentials from occurring on the differing ground rods.

i am giving you good advice here and it is up to you if you ignore it and what you will do as far as how you ultimately do it, but i'm telling you that what you propose is wrong to do. i think i'd go with no ground rather than what you proposed as your proposition negates the grounding to a point anyway.

Steve961 · December 2012

Re: Grounding system for a metal building

Niel & Bill:

If I can get by with just a single ground rod for my cabin, I will be thrilled. It will save me money and a bunch of time I'd rather be doing something else. Having the ground run off the side rather than down the roof edge will also be a much easier install for me. This is why I come here for the good answers. Searching the net can give too much information sometimes and lead one astray.

Thanks for all your help.

Steve

basewindow · December 2012

Re: Grounding system for a metal building

At the risk of sounding like a broken record, I also have a grounding question in relation to system and lightning.

I too have a steel cabin/shed, however I'm a slightly different situation, and my concern is that I have steel supports inside my shed.

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At the moment I have a single ground rod for my system attached to the Battery negative. (It's all 12v.)

My PV fames are bonded to the roof (steel) by bare copper.

I was under the belief that the steel shed/cabin would act as a faraday cage in the event of a lightening strike. However my concerns were/are :

1- The steel supports inside. Would they also become electrified in a lightening strike?
2- As this entire structure is on a concrete slab, is that going to provide a useful ground at all?

Any ideas?

inetdog · December 2012

Re: Grounding system for a metal building

basewindow wrote: »

.

I was under the belief that the steel shed/cabin would act as a faraday cage in the event of a lightening strike. However my concerns were/are :

1- The steel supports inside. Would they also become electrified in a lightening strike?
2- As this entire structure is on a concrete slab, is that going to provide a useful ground at all?

Any ideas?

0. A Faraday cage will not work as well when there are insulated wires running from a point outside the cage to a point on the inside.
1. The steel supports would either be raised to the same potential as the outside, but without a voltage gradient from one point to another to deliver a shock, or pulled down toward the ground potential the same as the outside, again with not much of a voltage gradient. Make sure that there is a good electrical bond everywhere steel meets steel.
2. If you can connect your ground system to 20 foot or more linear or mesh grid of rebar in the concrete pad, you will have a very excellent ground, better than a driven ground rod.
If you cannot connect to the rebar, just having the building on the concrete and the structural members bolted to anchors in the concrete will help some, but not nearly as much.
(A concrete ground electrode, using a ground conductor inside the concrete is called a Ufer ground if you want to look up more information.)

niel · December 2012

Re: Grounding system for a metal building

technically those inner steel supports could carry a strike towards ground and negates the faraday cage. part of my reasoning for steve with having the lightning rods insulated from the steel structure and go straight to ground rather than the steel building was to enable bypassing the building in case of a strike. the building itself being metal can still be struck and should still be bonded to the ground rod, but not as a pathway for the lightning rods to ground.

as to the concrete pad, i dislike ufers, but that's only my opinion and matters not if your pad is already laid with no way to connect to the rebar. you could still use a ground rod outside of the pad, but if it is too far from the house then go ahead and drill into the concrete so that you could drive the rod through it to the soil beneath it.

as to a ground screen, they are good to use, but in my opinion not in concrete. if i were given the option to do it before the concrete was laid i'd have buried it below the concrete in contact with the soil. if, however, the ground is rocky then it would be a moot point if you were to encase it in concrete or connect with the rebar. the nec only bowed under pressure from construction companies for an easy and cheap ground for them to utilize and most soils are more conductive than concrete.

basewindow · December 2012

Re: Grounding system for a metal building

Thanks for the info. I hadn't thought of using the rebar and I might have a look at that, I'd have to drill into the concrete which is quite thick as some points.

So what you are saying is I'd basically need three separate grounds each with its own ground rods or rods. 1 for the PV system, 1 for the structure and another for the insulated lightning rod system?

But in reality just stay well away from anything metal in a lightning storm?

Grounding never seems to be a simple issue does it!

Regards,

Cam.

inetdog · December 2012

Re: Grounding system for a metal building

basewindow wrote: »

Thanks for the info. I hadn't thought of using the rebar and I might have a look at that, I'd have to drill into the concrete which is quite thick as some points.

Grounding never seems to be a simple issue does it!

Regards,

Cam.

Even if you drill into the concrete, getting a good ground depends on either finding a very long continuous piece of rebar to attach to (again, per NEC about 20 feet) or else relying on the tie wires at rebar intersections being tight enough to effectively bond the rods together. It might or might not work and you would have to drill down and then test to find out.

Ufer grounds can be very good when planned before the concrete is poured, but very difficult to add in later. Situations where there are ends of rebar sticking out for later tie downs or wall additions are the easiest to retrofit.
When planned, they deal with the rocky or thin soil problem very well, since the concrete has such a large surface area in contact with the soil. If the soil layer is thin enough, though, nothing that connects only to it will provide a good ground. For those situations a well casing may be the best thing available since it has to reach an aquifer.

When you need or want separate ground electrodes for different purposes, it does really get interesting. At that point you are up against the NEC (Wiles) preference for all ground electrodes to be bonded and neil's quite justified concerns about inviting lightning into your house. :-)

niel · December 2012

Re: Grounding system for a metal building

if you can drive a single ground rod and attach everything to it as a single grounding point. you shouldn't need separate grounding rods. i don't want you daisy chaining your ground connections from the lightning rods to the house and then to the ground rod. the pvs, lightning rods, and the house will all have their own ground wire lead going to 1 common ground rod.

how far out do you have to go to get to the soil away from the building? if it's a foot or so then go with it, but several feet out can cause troubles one of which is tripping on the wire.

vtmaps · December 2012

Re: Grounding system for a metal building

inetdog wrote: »

For those situations a well casing may be the best thing available since it has to reach an aquifer.

around here, casing is used only until bedrock is reached. A well might have 20 ft of casing, but be 200 ft to water. --vtMaps

inetdog · December 2012

Re: Grounding system for a metal building

vtmaps wrote: »

around here, casing is used only until bedrock is reached. A well might have 20 ft of casing, but be 200 ft to water. --vtMaps

Here (California) casing is required full depth to avoid bridging multiple subterranean aquifers on your way to the one which has the water quality and recharge rate you need.

Grounding system for a metal building

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