Lightning protection

[jazzman]

Hello,
The management company of a gated community heard that I owned an earth ground tester and called me to ask if I would check the grounding electrode system at their entrance gates. They have ground rods installed at the service panel and the gate motor enclosures. There is a quality surge protection device installed at the utility meter and low voltage surge suppressors installed at the equipment but the grounding electrode system was measuring a very high ohmic value over 700 ohms. They have contracted me to install new ground rods in an attempt to achieve an ohmic value of 25 or less. My question is this: What am I connecting to the ground rods? Am I grounding the gate motor enclosure or am I connecting to the equipment grounding conductor? Or both? This is not the only community having these problems. I know of two others in my area (Gulf Coast of Florida). Any information or advice would be greatly appreciated. Thanks in advance.
jazzman

 

[FionaZuppa]

i would think you would want to bond as much metal together and then tie that to the ground rods. as example, a metal moving gate that is hinged, then you should bond that using some flexible SS braiding from swinging gate (near pivot point) to fixed metal, and then bond out the fixed metal.

connecting the service gnd to rods does leave possibility of putting line voltage on all the metal if neutral of poco lifts.

 

[starbolin]

... They have ground rods installed at the service panel and the gate motor enclosures....

Is the SE at the gate or are they remote? Are the rods at the equipment enclosures bonded to the GES or are they auxiliary rods? Is there more that just a rod at the service panel GES? What is the raceway type used in the gate branch circuits?

 

[petersonra]

my question would be why do they think 25 Ohms is desirable?

who talked them into that idea?

I am not opposed to trying to get there but if you are starting at 700 Ohms it may be a lot harder than you think to get there and it amy serve no real purpose.

 

[frank.basciano]

Grounding Electrode System issue

Grounding Electrode System issue

Hi jazzman,

The 25 ohm reference no doubt comes from the 2014 NEC 250.53(2) - it shows up in earlier codes as well. However, this section refers to using supplemental grounding electrodes in order to get the resistance to earth equal to or less than 25 ohms. I too live in the Gulf Coast area and getting a 25 ohm resistance to ground is going to be difficult especially in that you are starting with 700 ohms or more. Was the measurement taken during a "dry" time when the earth may have been dry?

Nevertheless, it will likely be necessary to use supplemental grounding electrodes, please refer to Article 250.53 for more information on installing these additional grounding electrodes.

Something to think about, an airplane when it is flying is NOT grounded, yet gets hit with lightning more often than you think and the plane remains functioning.

Bonding is just as important than the grounding electrode system (aka: earth-termination systems). There are numerous references to bonding the NEC, please see 250.194 which directly relates to metal fencing.

I too live on the Gulf Coast of FL, the single electrode systems are typically not adequate but are often installed. If you add electrodes, please make sure that they are all bonded together as well as all metal, like mentioned in an earlier post.

You can find more information on lightning protection systems and grounding/earthing here (starting on pg 58): http://www.dehn-international.com/sites/default/files/uploads/dehn/pdf/lpg-chapters/chapter5.pdf

I hope this helps. Contact me if you need more help.

 

[K8MHZ]

I was told the NEC grounding requirements did not provide protection from lightning. They exist mostly for voltage stabilization and to have a uniform voltage reference.

NFPA 780 is the book on lightning protection.

 

[Pharon]

The 25 ohm reference no doubt comes from the 2014 NEC 250.53(2) - it shows up in earlier codes as well. However, this section refers to using supplemental grounding electrodes in order to get the resistance to earth equal to or less than 25 ohms. I too live in the Gulf Coast area and getting a 25 ohm resistance to ground is going to be difficult especially in that you are starting with 700 ohms or more.

That's not what this section says. It says that you are exempt from driving a second ground rod IF you test the first ground rod to be 25 ohms or less. You could have two ground rods reading 1000 ohms and still be Code compliant.

Also, this does very little, if anything, to protect a structure better from lightning.

 

[Curious nerd]

What grounding can and can't do

What grounding can and can't do

Suppose you built a grossly unaffordable grounding system which was only 0.5 ohms to the rest of the planet. If something in the system were hit by a median lightning strike of 30,000 amps, its potential would rise to 15 kV. A good ground system helps by diverting some current from structures and equipment. If 10,000 amps are flowing through your equipment and 20,000 are going through your downlead and ground field, that's pretty bad, but it could be three times worse. Minimizing ground loops is critical. If all the equipment rises to tens of kilovolts, they'd all better do it equally and at the same time.