Question about possible induced currents from large overhead transmission lines

[dr_ed@swbell.net]

I was wondering whether anyone could give me some guidance. I am planning to build a 600 sq. ft ground mounted solar array on my property. The best available site extends at least partially into a utility easement near some huge overhead transmission lines.

Are there issues of induced currents in panels mounted below high voltage transmission lines? I'd appreciate input from anyone knowledgeable in this area. There doesn't seem to be much information on the net about it.

I have to submit a plan to the utility before I build anything in the easement, and I'd like to be able to address this sort of question. I expect them to be resistant to my project in any case...so I'm trying to arm myself with the correct information ahead of time.

Thanks in advance

Eddie

[Sunking]

Goggle EMI. The big issue for you is a fairly significant ground electrode system in the event when and if the high voltage lines fall on your array. Otherwise no concerns. But if the lines fall on your array, even with a significant GES would likely blow up all you equipment and set it on fire. Basically 13.2 Kv to 750 Kv would be applied to all your conductors for the duration of the fault until the utility line switches off from a ground fault. They are set to about 1 to 2 seconds to burn the fault clear like a tree limb or big bird with large wing span.

They will likely make you sign a Wavier to release them from any damage or injury, and inspect the system before they allow you to proceed. You are going to need professional help.

[dr_ed@swbell.net]

Thank you!

I appreciate the excellent advice! Understood on the professional help part. I have lived with those lines over my land for nearly twenty years...would guess the likelihood of a line falling is fairly infinitesimal. I'd be happy to sign a waiver. I just wasn't sure the function of the panels would be unaffected. You cleared my confusion up nicely.

[Sunking]

I have lived with those lines over my land for nearly twenty years...

Well then the threat to you is the family Jewels.

EMI effects are debatable with respect to health issues. Not going there. As for electrical it can cause problems when signals are involved like a computer, telephone, or video display. But for DC Sources of power, Not even a remote problem. FWIW I bet you know what a Power & Protection Engineer is working for a Telco. Been doing it for 33 years for all the big players like ATT. SW Bell does not exist anymore. Land line is DOA.

[dr_ed@swbell.net]

More questions Sunking..if you have time.

I was thinking about the grounding issue. I live in an area with solid limestone rock just below the surface...sometimes it IS the surface. I assume the power company drills deep holes with a rock drill to place ground rods for the transmission poles. Is that correct?. Or is the pole itself the ground? I have a rock drill that will drill to about 4 ft. Sounds like I will need to have someone drill holes for the grounds. Is this the case, you think? They would need to be more than 4 ft, right? I doubt I could get the utility to do it. I guess I could ask.

I was planning on a custom welded steel fixed mount. Maybe eight uprights set in the ground (in concrete). Could you tell me how many ground rods I'd be likely to need? Would it be one for each pipe set in the ground? Can you give me an idea of exactly what kind of ground it would take to work in this situation? The plans I was following only called for one small ground rod for the array, built under normal circumstances.

I looked closely and I'm not going to be directly under the line. I have about 30 ft of clearance from a point directly below the transmission line. That does fall within the "no-build" safety zone, I think. It's close. I'm going to present it as a temporary structure..because they do permit things like fences and gates, which they refer to as temporary.

I can see that I need to present a really good set of plans if I have a chance at getting them to let me do it at all.

I hope I'm not overstepping my welcome by asking so many questions. I really want to understand this.

[Sunking]

I assume the power company drills deep holes with a rock drill to place ground rods for the transmission poles. Is that correct?. Or is the pole itself the ground?

Depends on the structure type. But there are generally no ground rods used. A wooden pole type structure has what is called a Pole Butt Ground. It is a #4 conductor that runs from the top of the pole to the bottom and coiled up. So when the pole is sit in the ground the pole is on top of it pressing into the dirt.

Steel structures use what is called a Concrete Encased (CCE) ground electrode where by the structural reinforcement bar is bonded together, and bonded to the anchor bolts. Look up Herbet Ufer Ground if you want to know more about it. Herbert Ufer was a Dow Chemical Chemist during WW-II and developed the Bomb Magazine Bunkder used in the dessert to prevent static discharges. Th econcrete block has massive surface area and concrete is a very good conductor.

KEEP in mind a utility uses a ground for a completely different reason than low voltage applications.

I was planning on a custom welded steel fixed mount. Maybe eight uprights set in the ground (in concrete). Could you tell me how many ground rods I'd be likely to need?

My guess is two total if you design the concrete piers as a CCE. That is where you need professional help

I can see that I need to present a really good set of plans if I have a chance at getting them to let me do it at all.

I hope I'm not overstepping my welcome by asking so many questions. I really want to understand this.

You bet you will need plans by someone like myself (PE) who can Stamp the Drawings.

FWIW you are asking excellent questions for above and beyond what most ask because they do not even know what to ask.

[dr_ed@swbell.net]

CCE ground

I read a little about this...let me see if I have the gist of it.

Since I'm building a metal structure that will be set in concrete footings, the grounds could conceivable just be a #4 copper wire of appropriate length (20 ft is what I read) clamped to the upright, in contact with the earth, and buried under the concrete. I didn't see a depth requirement other than needing to be covered by two inches of concrete.

Does this wire need to be fully extended to be effective? Is it better if all the legs of my structure are connected by clamps to a single "ring" of ground cable encased in the concrete ( this intuitively seems better to me), with a couple of additional "free ends" buried at opposite ends of the structure?

Or are a couple of 20 ft #4 copper wires clamped to the structure (which is itself a continuous conductor anyway, since it's welded steel) at opposite ends and properly extended and buried just as good?

You said that the power company grounds their power poles for a different reason than my reasons for grounding a low voltage installation like an array. I think I understand that, but could you explain that. I guess what I'm asking is...."are there reasons other than the fact that the two systems are vastly different in the amount of current they need to ground out?"

[Sunking]

Since I'm building a metal structure that will be set in concrete footings, the grounds could conceivable just be a #4 copper wire of appropriate length (20 ft is what I read) clamped to the upright, in contact with the earth, and buried under the concrete. I didn't see a depth requirement other than needing to be covered by two inches of concrete.

Does this wire need to be fully extended to be effective? Is it better if all the legs of my structure are connected by clamps to a single "ring" of ground cable encased in the concrete ( this intuitively seems better to me), with a couple of additional "free ends" buried at opposite ends of the structure?

Or are a couple of 20 ft #4 copper wires clamped to the structure (which is itself a continuous conductor anyway, since it's welded steel) at opposite ends and properly extended and buried just as good?

OK everything you are stating sounds like you are picking up from NEC and minimum requirements for residential system. That will not likely fly with the utility. The NEC calls it a Concrete Encased Electrode and stated here:

250.52 Grounding Electrodes.
(A) Electrodes Permitted for Grounding.
(3) Concrete-Encased Electrode. An electrode encased by at least 50 mm (2 in.) of concrete, located horizontally near the bottom or vertically, and within that portion of a concrete foundation or footing that is in direct contact with the earth, consisting of at least 6.0 m (20 ft) of one or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 13 mm (1⁄2 in.) in diameter, or consisting of at least 6.0 m (20 ft) of bare copper conductor not smaller than 4 AWG. Reinforcing bars shall be permitted to be bonded together by the usual steel tie wires or other effective means. Where multiple concrete-encased electrodes are present at a building or structure, it shall be permissible to bond only one into the grounding electrode system.

This is why you need an engineer to help you out with this because that is not likely to satisfy a utility. If it were me it would be a true Ufer Ground where all the reinforcment bar is bonded together along with the anchor bolts set in the concrete, and a 2/0 AWG tag line from the concrete to bond each upright. I would be contacting the utility transmission Engineer, give him the location, and ask for the fault current required for operation. That drives the design.

You said that the power company grounds their power poles for a different reason than my reasons for grounding a low voltage installation like an array. I think I understand that, but could you explain that. I guess what I'm asking is...."are there reasons other than the fact that the two systems are vastly different in the amount of current they need to ground out?"

Simple the NEC which is where residential and commercial code resides forbids dirt or ground to be used as a conductor as the impedance is way too high to be of any use. Utilities operate under NESC rules and earth is used as a conductor.

So here is the deal your home is 240/120 VAC, and most of your branch circuits are 20 amps protected by a breaker in your main panel. If you have a really great ground electrode system at your location you might be able to get it as low as 10 Ohms. A 10 ohm fault to earth on a 120 volt circuit delivers 120 volts / 10 ohms = 12 amps of current. So what does a 20 amp circuit breaker do with 12 amps of current on it? Nothing as that is a normal load current.

The utility transformer that serves your house operates at 13.2 Kv, and a standard 17 Kva transformer is used for a typical 200 amp 240/1120 service. The fuse feeding the primary of the transformer is 2 amps. The utility bonds their transformer to the pole ground and the service grounded circuit conductor of both primary and secondary of the transformer. That impedance will be on the order of 5 ohms or less. So if the 13.2 faults to earth through a 5 ohm earth ground = 13200 volts / 5 ohms = 2640 amps. What happens to a 2 amp fuse with 2640 amps flowing through it?

Go one step further and let's say by an act of God the utility ground fails from corrosion, and now the 13.2 Kv sees your house crappy 1000 Ohm ground rod electrode. 13,200 volts / 1000 Ohm's - 13.2 amps. What happens to a 2 amp fuse with 13.2 amps flowing through it?

To operate most fuses and breakers in 1 full cycle of AC current requires a fault 6 times the protection device rating. So for a 20 amps breaker you want a fault to produce at least 120 amps or 1 ohm of resistance. No residential ground electrode system will ever be able to achieve 1 ohm or less resistance. Most residential system ground electrode systems are 100 ohms or more. That is completely useless at low voltages.

Now here is a comment that will make you ask WTF. Grounded systems are dangerous and unreliable.

[dr_ed@swbell.net]

Several WTF's

Let me guess the answer...then you can tell me if I'm right.

You said grounded systems are dangerous and unreliable. I'm guessing that's partially explainable by what you just told me.

The average household breaker won't even trip if a fault occurs...so the ground just goes live and acts as a shock hazard? Possibly indefinitely, since how would I know?

Or (in the case of the CCE ground) is it what you said about earth not being a good enough conductor?...so even with all that carefully engineered bonded rebar and bonded mounting bolts and attached ground cable...the weak link is the earth itself...if my array, for instance, suffered that falling transmission line, the ground would not dissipate the charge and the metal structure would melt down and set anything flammable afire.

Am I on the right track with either of these answers?

If I seem to be obsessed with this whole idea of electrical grounds, it's because its a concept I have trouble really getting my mind around. I've been reading a lot about Faraday cages lately...and the subject of whether they should be grounded...there is controversy among the various "experts" on that...and the best information I found agrees with you....most systems are dangerous and unreliable.

So why DO we provide ground "protection" anyway?

I'm a sailor, and I remember many arguments about whether sailboat masts should be grounded to the keel for lightning protection. The consensus, as I recall, was that a grounded boat was more likely to survive a lightning strike, but that an ungrounded boat was perhaps less likely to get a strike in the first place.

I find the subject confusing. Guess it's no wonder I'm not an engineer. Thanks for the tutorial, Sunking.

[Sunking]

The average household breaker won't even trip if a fault occurs...so the ground just goes live and acts as a shock hazard? Possibly indefinitely, since how would I know?

Here is the disconnect. You are making the mistake even electricians and engineers make. You assume ground means dirt or earth. It has been a source of confusion for a century, and thus why terms in electrical codes have been changed recently. There are too many definitions fo rme to go into and out of scope for a DIY forum. I will not go into any depth because most will misunderstand and come away with very deadly ideas. But for example one term is Equipment Grounding Conductor (EGC or the Green Wire in your AC power cord), is now called an Equipment Bonding Conductor (EBC). It needs absolutely no connection to dirt. Here is an example. One is code compliant and the other is deadly. Can you tell which one? I will not Answer.

Or (in the case of the CCE ground) is it what you said about earth not being a good enough conductor?...so even with all that carefully engineered bonded rebar and bonded mounting bolts and attached ground cable...the weak link is the earth itself...if my array, for instance, suffered that falling transmission line, the ground would not dissipate the charge and the metal structure would melt down and set anything flammable afire.

Am I on the right track with either of these answers?

It would if the conductors and steel are not large enough to dissipate the heat energy imposed on them for the amount of time the current is applied to them for the duration of the fault. There are too many variables to give a direct answer. Assuming the size of the steel structure and bonding conductors can withstand the current, and have a low enough impedance to cause enough current to flow to operate the over current device protection device in 1 second or less things are considered safe.

But lets go to a very plausible scenario. Let's say those overhead lines are 13.2 Kv transmission lines from the sub-station 5 miles away being fed on 2.6 Mva transformer with a 100 amp relay protection mechanism. The 5 mile impedance of the transmission line is 60 Ohm's. You install heavy structure steel framing and use 4/0 copper bonding conductor to earth which is overkill. But your earth impedance is only 500 Ohm's. So with the line impedance of 60 Ohms and your ground electrode impedance is 500 Ohms will only induce 23 amps of fault current on a 100 amp fuse. If that line falls on your equipmet and you are someone else should touch it, you will look like these two fine young men stealing copper to a 13.2Kv transformer at a Walmart.






Any Questions?

[Naptown]

The one on the right is the correct one There can be only one grounding electrode

There is also a lot of confusion between the equipment grounding conductor and the grounding electrode conductor

[inetdog]

The one on the right is the correct one There can be only one grounding electrode

But the point Sunking wanted to make is that in addition to the two-ground setup being against code, it also means that in this example the fault current will be limited to only 6 amps because of the 20 ohm resistance of the two grounds in series. So the circuit breaker or fuse will not blow and a substantial part of the line voltage will appear on the "grounded" metal of the load.

Bad in many ways (fire, electrocution, etc.)

Note also that if there were a (properly wired) Ground Fault Circuit Interrupter, it would trip and protect both the load and the user.

In the right-hand diagram, the equipment grounding conductor will carry the fault current at a much higher amperage and the fuse will blow.
If the two grounds were bonded together, then it would be safer, but still not necessarily code compliant depending on where the two parts of the circuit are located.

[Sunking]

In the left diagram, the equipment grounding conductor will carry the fault current at a much higher amperage and the fuse will blow.

I hope that is a typo because the diagram on the left has no EGC.

[dr_ed@swbell.net]

Wtf

My last response has been deleted. Perhaps by the admin. Not sure why, when I did pick the correct answer...but no big deal.

I repeat my question though. My understanding is that in neither case would the breaker trip. Not enough amperage. In one case 12 amps in the other 6 amps..and since it takes six times the breaker rating..there would have to be 120 amps to trip the breaker. Am I correct?

Thanks to all who responded. I'm learning here.

[russ]

Post deleted - not by me - maybe you failed to use the ''submit reply'' button