Ground Rod at Metal Poles a Waste? (8-5-2K)
GROUNDING METAL LIGHT FIXTURE POLES TO EARTH?
Question: Mike I had an engineer tell me that there was a Code change in the 1999 NEC requiring metal poles for parking lot lighting with HID fixtures to be grounded to the earth. He said that a ground rod must be driven at each pole. If this is true I can't find it, please help me out because I have a job to set 17 in the next few days.
Mike Holt's Comment: The previous question continues to bug me that people insist that a ground rod be driven at metal poles that support lighting fixtures. From the mouth of one of my members "A lot of 'black magic, shaman dancing, urban legends, wild-eyed guesses, and whatever someone taught you years ago' are repeated as gospel."
Let me make a few statements to try to clarify this issue:
- The National Electrical Code does NOT require a ground rod at metal poles.
- A ground rod at a pole will NOT clear a line-to-ground fault for systems that operate at less than 600 volts.
- A ground rod at a pole will NOT reduces the touch potential from the metal pole to the earth from a line-to-ground fault.
- A ground rod is NOT required at a metal light fixture pole by the Lightning Protection Institute http://www.lightning.org.
- A ground rod is NOT required at a metal light fixture pole by the Lightning Protection Standard (NFPA 780) http://www.nfpa.org.
- A ground rod at the metal pole does NOT reduce damage to the fixtures, lamps, and the pole wiring, because the lightning traveled through the equipment on the way to the earth.
- A ground rod at a metal pole does NOT protect the concrete foundation that supports a metal pole from lightning damage. If there were true, then electric utilities would never use concrete poles to support overhead wiring.
- A ground rod at the metal pole does NOT protect the circuit wiring and equipment in the building from lightning damage (open back door). If you want to protect the circuit wiring in the building, then surge protection should be installed on the circuit conductors that go outside to the metal poles.
If driving a ground rod at a metal pole is such a good idea (some people are very passionate about this), then why are there no papers or standards recommending this practice? Don't you think that if this was such a good ideas that someone would have proposed it and incorporated it into the NEC, IEEE standard, the Lightning Protection Code, or some other recommended practice?
From: Maddox, Robert
I believe that the grounding of lighting poles is a holdover from old Power Company standards. Wood poles were first used to carry power wires & then higher voltage lines. A lightning protection wire was strung along the tops of the poles to protect the power cables below. That practice continues today. At each pole a ground wire was brought down to a ground rod. Over the years lights were added to these poles, typically wooden poles. As electrical street lighting became more common some poles were used only for lighting, but the practice continued. Also the Power Company ran street lights at 240V-LL & 480V-LL with no neutral or ground, thus as metal poles came into vogue the ground rod & wire were used to ground the metal pole in case the lines came into contact with the metal pole. Once the Power Company started, everyone copied the practice, many times without knowing why. It just seemed like a good idea!
This is exactly what I believe.
From: Dean 0438
I have been an electrical contractor for over 28 years and have had several opportunities to install metal light poles on projects. Nearly without exception the specifications always directed ground rods to be driven for remote light poles. I have questioned this several times and always have been given the same response "it's in the spec." I'm on your side. I don't understand the value as long as an equipment-grounding conductor is installed in the raceway and is properly sized. An additional (supplemental) ground rod is unnecessary.
From: John West
The most common reason for the ground rod is the diversion of the lightning strike. Think about this for a moment. Lightning strikes a metal light pole, the ballast, and bulb may be destroyed, no argument there, but the demise of these parts is not a done deal. Lets look at the potential path for the energy. The pole has low resistance conductors (wires) to a lower point of potential, the building ground. The lightning energy at the metal pole has two paths, the earth, and the circuit wires. Construct a voltage divider and you tell me what the result is for the wire.
So the solution is to drive a 20 or 30 foot rod and be sure they are at 5 ohms or less (tested by fall of potential).
When ground rods are driven at metal poles in a parking lot, I can guarantee you that ground resistance will never be 5 ohms or less and nobody is going to test the ground resistance, unless required by the plans and specification and enforced by the owner. We must remember that lightning is high frequency current flow and at high frequency the impedance of the circuit conductors is very high (XL = 2 x Pie x F x L), that's why lightning arrestor wires are so short.
A case history involving a facility of the US Postal Service in Lake Mary, Florida is a good example. Before the installation of "lightning protection" and a comprehensive surge protection system the USPS suffered substantial damage to parking lot lights (bulbs, ballast, contactors). In addition, the energy entering the building destroyed additional equipment (long list). The fix was to include "lighting protection" on the poles, surge protection on the poles, surge protection on the contactors, surge protection on key electrical panels. The results after two years (2 Florida summers), minimal damage to the exterior lighting systems, loss of a few surge protection devices on the lighting system, NO interior damages.
If the installation requires "surge protection on the poles, surge protection on the contactors, surge protection on key electrical panels", then I have no problem with the installation of ground rods at the poles. At least someone is thinking and trying to solve a problem, so why not go all the way and drive those ground rods. But driving ground rods at metal poles in the areas of the country where lightning damage to the interior wiring is not an issue is useless.
From: Cross Addicted
My company does installation of cell sites in the Philadelphia area and they have millions of dollars in equipment that needs to be protected from lightning because they naturally are usually the highest points in the area. We install Halos and ground rings in and around the shelters and yes, a lot of times these items are successful in protecting the equipment. Sometimes they aren't. In my opinion, one ground rod and a piece of #6 wire is NOT going to tell a lightning hit where to go. If it chooses to go that path, good, but I don't think that it is all that attractive for the lightning to be bottlenecked in a piece of #6 wire connected to a ground rod. All in all, it's probably not that effective.
I know of a case where a light pole (grounded with a ground rod) was struck by lightning. Not only were the lamps and ballast's burned up, but the lightning also traveled back through the conductors to a 10 pole lighting contactor and it toasted poles No. 2, 3, and 4. Then it melted the wire back about 6 to 8 inches from the termination, but it didn't stop there! It continued back to the load center destroying the circuit breaker serving that lighting contactor, as well as the circuit breakers above, below, and across, completely blowing them into pieces. Then it melting the "C" phase bus blowing 2 - 200a, 600v fuses! Again let me remind you, the metal poles in the parking lot were bonded to ground rods!
So, in conclusion ... lightning will do what lightning does best ... whatever it wants too, ground rods, or no ground rods.
We had an instance at a Sam's Club where lightning struck a 45' steel lighting pole on a raised concrete base that did not have ground rods. The lightning fault traversed 440' through the conductors and ate up 3 eight-pole contactors. This was an expensive repair for the customer. Lately I have seen projects specified with surge suppression within the poles and with ground rods.
I have no problem with ground rods at the pole if the circuit to the pole is protected with surge suppression, but a ground rod at the pole without surge protection is a waste of time and money.
From: Adams, Lynn
I have seen damage to the interior equipment from lightning strikes to the light standards. This occurred at a Sam's club in Pensacola, FL over a period of two years. I replaced a dozen or more Square D power-link remote control breakers, control panels, and equipment. These were rated 480 volt, two poles, and included a third pole space that contains a DC motor to open and close the contacts when directed by the control panel. While anecdotal, these failures were directly related to thunderstorms in the area.
If lightning strikes the pole directly it gives a CHANCE to let the lightning go into the ground there and not be conducted back to the building by feeder conductors (lightning strike, not 'surge' or other vague induced voltage).
I agree that the connection to ground is not required by the Code and the rod at the pole will do nothing to protect the fixture or ballast. Also, I feel the steel in the pole base is as good a ground as a rod driven beside it. Tie the steel to the anchor bolts and the pole to the anchor bolts and the grounding conductor will run with the circuit. The pole is grounded as well as you will get.
I agree that a lot of "black magic, shaman dancing, urban legends, wild-eyed guesses, and whatever someone taught you years ago" are repeated as gospel.
From: Harden, Jeff
I work for the Oklahoma Transportation Authority and take care of the roadway lighting on three different turnpikes. There are 15 interchanges with at least 20 poles and 3 with about 70 poles. Most are supplied with 480 volt, 250-watt high-pressure sodium lighting, and the rest with 120 volt, 250-watt high-pressure sodium. All the poles I have mentioned have ground rods.
I have seen what I thought was lightning damage, like the 5-amp fuse and connector completely ruined and wiring burnt half way up the pole. I can't say for a fact that it was lightning but it was after a lightning and thunderstorm went through that area. I haven't seen any pole or base damage. The area I work is Northeast Oklahoma from Tulsa northeast to Joplin, Missouri and from Tulsa east to Arkansas.
From: Halberstadt, Alan
I'm with you on the pole grounding issue. I did a Wal-Mart store here a few years ago and the poles all had ground rods, cad-welded connections, AND an equipment ground. Several times I had service calls due to lightning damaging the energy management controls for the panel serving the outside lights. There was no damage to the breakers, however, only to the low- voltage controls hooked up to the Sq. D motor-operated 480V breakers. Seems to me that the ground rods did absolutely no good there. Keep up the good work!
From: Toomey, Ken
I had an experience in a petroleum facility where a metal light pole got hit by lightning, which resulted in the destruction of the underground supply wiring for a distance of at least 100 ft. from the pole. The fixture was undamaged. At this facility there is also an underground grounding ring, which is used to dissipate any static electricity that could result in ignition of petroleum vapors. All metal structures and large equipment were connected to this ring as well as all of the metal light poles in the facility. We saw no other physical damage to the pole other than a little scorch mark where the lightning apparently struck the pole. The concrete base was undamaged.
Equipment grounds pulled with the feed conductors grounded the light poles. Even with the elaborate grounding present at the petroleum facility, the conductors were still cooked beyond recognition. I don't see how adding a ground rod at the pole would be of much help.
From: Rodriguez, Luis
My two cents - I have witnessed the removal of ground rods used as part of an existing lightning protection system that had been hit by lightning (Tampa Bay area- Central Florida). One such ground rod was covered with a glazed coating. It turned out this coating was glass. The deduction made was that the energy dissipated from a lightning strike was so large on the small surface of the rod that the heat generated fused the (sand?) material to the rod.
Mike's Comment: Great, we install a ground rod then it becomes useless.
From: Mark Flesner, Musco Products, www.musco.com
Mike, we recently received some information with regard to problems in Florida as they relate to concrete pole bases where lightning has exploded chunks of concrete from the pole. We feel that installing a ground rod for lightning protection is at least a good practice.
Mike's Comment:I reviewed the pictures, it was just a small chunk of concrete about 5 inches in diameter and no more than 2 inches deep, which did not cause any structural damage to the concrete bases.
We (MUSCO) feel that a ground (designed to meet the lightning protection criteria of NFPA 780) should be installed at each pole because it will be the path for lightning if lightning strikes the pole. If the ground system is no adequately sized for lightning current (which it is not), this could create an unsafe condition. First, if lightning hit a pole with an undersized or poorly designed ground system the equipment grounding conductor to the pole could be destroyed, hereby disrupting the grounding system. Secondly, the lighting could cause a condition where the equipment on the metal pole might be damaged, thereby creating a fault. The two conditions could result in a dangerous condition where the fault energizing the metal pole (when the equipment grounding conductor is no longer operational).
Mike Comment:Makes sense to me, but I have never hear (neither has MUSCO) seen this condition occur.
We at MUSCO products understand that the NEC and Lightning Protection Code does not address this subject, but we recommend the following for our metal poles which vary in length from 40' to 120' tall:
- Provided each metal pole with at least one copper-clad steel ground rod of not less that 5/8"
in diameter and not less than 10' in length. An 8' ground rod is acceptable, provided it extends
vertically into the earth at least 10'.
- The ground rod should be bonded to the metal pole by a copper down conductor sized no smaller
than No. 2 if the metal pole is 75' or less above grade. If the metal pole is than 75' above grade,
the conductor should not be less than No. 2/0.
- All metal components of the steel pole should be bonded to the pole.
- All bonding conductors should maintain a downward path to the earth, free from "U" or "V" (down and back up) bends. No bend should exceed 90 degrees nor should it have a radius of less than 8".
Mike Holt's Comment:
I understand that MUSCO would like a ground rod installed at their poles in accordance with their specifications, I just wish that there were a standard recommending this practice. Plus, what's the good on using a No. 2/0 wire ground wire on an 8 or 10 foot ground rod?
Mike, I understand that there is no Code requirement or recommended standard that metal standards for light fixtures be grounded to the earth, but there are two factors that should be considered:
1. Protection of building wiring and equipment. An electrical system designed to protect the internal wiring and equipment of a building from lightning entering the back-door (through the outside conductors to the metal poles) should include:
- Grounding the metal standards to earth
- Surge protection on the poles
- Surge protection on the contactors
- Surge protection on key electrical panels
2. Protection of the structural supports of the lighting fixture standards.
What better ground can you get from the anchor bolts, any reinforcing rod, and all of that concrete? For those of you who are trying to get out of installing an equipment grounding conductor, forget it. An equipment grounding conductor (wire or metal raceway) must be bonded to the metal pole, to clear line-to-ground faults.
From: Faber, Neil F.
The fact is that anytime you have a steel pole mounted bonded to the steel in a piece of concrete, you will have created a low-resistance ground connection, much lower than a driven ground rod. So adding a ground rod to a steel pole on a foundation is like adding a Radio Flyer wagon to the back of your Dodge pickup to get more hauling capacity.
I have no reference papers to quote, but I have studied the general question of what makes the best ground and why, and I can back that up with Ohm's law. The problem we face with lightning strikes is safely dissipating the energy into the earth without damaging equipment or killing people.
Ohm's law states that V=R*I (Voltage equals Resistance times Current.) Since a lightning strike is an extreme over-voltage with a certain amount of energy, Mother Nature sets the voltage and current that the lightning imposes on your pole. The only thing that we can hope to influence is the resistance, in this case the resistance from the pole to the earth.
The Ufer ground (NEC concrete encased electrode) is an electrode made from at least 20 feet or more of 1/2" or larger steel encased in 2" or more of concrete [250-50(c)]. So, if our foundation contains steel, our pole is bolted directly to an Ufer ground.
A Ufer Ground is named after an engineer named Herb Ufer who was in charge of Underwriter's Laboratories in Los Angeles from 1927 to 1953. Apparently he was involved in testing grounding electrodes for ammunition storage buildings in Arizona in 1942. Of course, dry sandy soil is virtually the worst (i.e. highest resistance) grounding condition you can have. But the concrete encased electrodes he tested all measured 5 ohms or less. After several sites throughout the Western US were tested with Ufer grounds in the 1960s with very good results, the 1968 NEC recognized the use of concrete encased (Ufer) grounds. (The Ufer ground works so well in tough grounding conditions that some jurisdictions in Arizona require the use of an Ufer ground on all electrical services).
Why does the Ufer ground work so well? Several things enter into that. First of all, realize that concrete has a great capacity to absorb and hold water, and that wet or damp concrete has a low electrical resistance. (How many electricians out there have gotten a jolt while working live wires on a concrete floor?) So our buried foundation is a good conductor. It is going to absorb soil moisture; a metal ground rod will not. But then, as the soil dries out, the soil around the concrete will stay damp longer because the soil moisture that the concrete originally absorbed will slowly migrate back to the now dry soil. Again, a metal rod will not act as a moisture reservoir in this way. It's pretty common knowledge that wet soil is always more conductive than dry soil.
Second, realize that the soil your pole sits in has an electrical resistance that is set by the laws of physics. The soil resistivity can be affected widely by temperature, compactness, moisture content and other parameters. Chemicals and salts can enhance it, but they can also be washed away by water, so you're stuck with the original soil. Soil resistivity is measured in terms of ohm-centimeters, which is the resistance of a cubic centimeter of soil measured between opposite sides. This means that as the surface area goes up, the resistance of your circuit goes down. So when you compare the surface area of a pole foundation to that of a ground rod, the foundation obviously has a much, much larger surface area. Given the fact that both the pole and any rod will be in the same soil, the foundation will have a much smaller resistance to ground than the rod because it has such a larger surface area in contact with the earth.
Finally, since more tightly compacted soil has lower resistance than un-compacted soil, the weight of the foundation tends to keep the soil more compacted. The driven ground rod will do absolutely nothing to keep the surrounding soil compacted.
So, where does this leave us? It leaves us with the fact that attaching a metal pole to a concrete foundation creates a connection to ground that no ordinary ground rod can match. But more importantly, the question of adding a ground rod is moot. Your pole is well grounded whether you like it or not.
The real question becomes how to ensure that the anchor bolts that hold the pole have a metallic path to the bottom of the foundation to make sure the concrete doesn't have to act alone to get the energy into the bottom of the foundation. One way is to make sure the foundation has a re-bar cage going all the way to the bottom of the foundation, which is connected to all four anchor bolts. Another way is, as some utilities I have worked for required in their substation foundations, to have a coil of copper placed in the bottom of the foundation. One end of the coil extends through the concrete foundation to either connect to the anchor bolts or directly to the steel pole.
As for damage to the pole foundation, we have to realize that there is simply a tremendous amount of energy involved in a lightning strike. If everything is just right, the object taking that strike can pass all that energy without damage. For all we know, the aggregate used in the concrete could make the difference with regard to damage to the foundation. How wet or dry the concrete is could enter in. Unless someone studies the phenomena more, it's all speculation. Also, there's the unpredictability of lightning. How do you explain lightning hitting a house causing all the lamps in one chandelier to burst, but not hurting anything else in the house?
That, in 1000 words or less (actually 1026), is my take on the question of grounding metal poles. I would like to acknowledge CADWELD for the background information on Herb Ufer and the origins of his wondrous grounding system.
From: Hilliard, John
I am an electrical superintendent for a city located in the Midwest (population of 50,000 people). We investigated an accident by where a child not wearing any shoes was leaning against a metal pole was shocked. The metal equipment grounding conductor to the metal pole had become loose and was not making contact. The metal pole supported an outdoor advertising sign, which had a line-to-ground fault. Tell that young child and his parents that grounds are not needed on metal poles!
Mike Holt's Comment: Driving a ground rod at the metal pole would not have reduced the touch potential and not cleared the fault. With a ground rod, the pole would have remained energized and the touch potential from the pole to the earth would still have been line-to-ground potential (120 or 277 volts). This is the point I'm trying to get across! I have a video demonstrating this.
I believe it can reduce touch potential at the pole. The ground rod (or connection) serves to place the surrounding earth at the same potential as the (spread the voltage drop over a bigger area around the fault), thereby decreasing the potential difference between any two points in the faulted area. If there is a solid connection to the earth, it will have lower impedance than the path through a person standing on the earth.
False, the ground rod does not spread the voltage (earth does not rise to line voltage, well maybe it does for about 2 inches around the ground rod. The step-potential around an energize ground rod is about line voltage (120 - 277 volts). I have a video demonstrating this concept.
From: Slade, Walt
Mike I was dealing with a dairy customer who had metal posts supporting 277V lights in a pen area. He called me with a very unfortunate story, several cows were electrocuted one evening and a photo showed all of them arranged in a circle around the light pole.
The metal light pole was bonded to a ground rod, the metal pole had no low impedance return ground path (equipment ground), and the 277V source was protected with a 15A breaker.
When I got to the site, I connected a voltmeter from the metal light post to a nearby metal fence, then gently tugged on the wires going up the inside of the pole to the light. The hot wire must have slipped out of a wire nut because I heard a sizzle and the voltmeter registered 277 volts from pole to the fence! I then measured 8 amps flowing from metal pole into the earth on the No. 6 wire to the ground rod, not enough to trip the 15A breaker.
I explained the need for a ground return on the circuit conductors to the electrician and showed him some of your articles in Power Quality Assurance magazine about this subject. The answer I got was "You won't find a single parking lot light with a ground return except a local rod." I explained that, some dead cows should be proof enough on the uselessness of the ground rod to clear a fault, but I don't think he understood.
Anyway, there is no debate about this subject. Use a low impedance ground return path (not a ground rod) or risk death to people and livestock.
From: Dean, William C.
Without the ground rod, a lightning strike would have two paths to ground (back on the circuit ground wire and through the pole base) provided it doesn't make additional paths en route. Adding another path should decrease the resistance to ground, and the path of least resistance will get the most current.
Since the Code doesn't require a ground rod, to me it boils down to a cost-benefit analysis/engineering judgment. If the pole is in an area subject to lightning, or is the tallest structure in the area, I would spend the additional money and install a ground rod. You wouldn't have to avoid much damage to recoup the cost of the ground rod. This may be why you can't find much technical analysis on the subject. It may cost more to analyze the specifics of the installation (soil resistivity, base construction, etc) than to just put in the rod.
Mike's Comment: Ah, another great comment "Cost Analysis Judgment". What is the likelihood of lightning striking the pole and it traveling back into the premises wiring? What is the history in the area and industry? How important is down time? Yes, there might be some installations where driving a ground rod might be a good idea, but there needs to be some basis on this decision.
From: Postigo, Raul A.
Effective grounding of metal lamp post is required in the Fifth Edition of the "National Electrical Safety Code". It is most generally accomplished for multiple circuits by connecting the metal standard to the multi-ground circuit neutral and, in addition, by using a driven ground rod at each location."
The "National Electrical Safety Code" is the electric utility code (high voltage) and they utilize a multi-ground circuit system where they use the neutral for grounding. So this does not apply to lighting circuits and equipment at building parking lots.