Grounding antennas on trailers

[Brett Glass]

I'm a wireless Internet service provider. I have a job coming up that involves installing radios on several trailers at a large construction site. These trailers are literally staked into the ground, with huge metal stakes, so they won't blow over. Electrical wires run from a meter at the edge of the lot to a breaker panel in each trailer. The ground bus of each panel, and the neutral wire of the electrical feed, are bonded to the heavy metal undercarriage of the trailer. Since I see nothing else driven into the ground I assume that they're relying on the stakes to serve as ground rods.

I'm thinking that this makes the metal undercarriage of each trailer essentially equivalent to the frame of a metal building, and that it therefore can be used as a ground for antennas for radios. Should I bolt a lug to the undercarriage and ground to it? What is the best and safest way to ground the antennas and antenna masts in this situation?

 

[LarryFine]

I'll respond after a moderator assures us it's okay. This forum is not for seeking installation advice by non-electrical-industry personnel.

 

[Brett Glass]

Larry:

I don't understand your unwillingness to comment. I'm an Electrical Engineer with an MSEE from Stanford and a duly licensed low voltage electrical contractor (as is required by the laws of my state for this sort of work). I'm also one of the relatively few wireless Internet service providers who actually endeavors to follow the NEC when installing equipment (even though the NEC doesn't contemplate many of the devices I'm installing). Doing so raises my costs relative to those of my competitors (many of whom are installing the equipment illegally and/or without grounding), but it keeps my customers safe.

In any event, I am licensed to install data networks, telephone systems, alarm systems, and surveillance systems as well as antennas, radios, RF cabling, and all other systems up to 90 volts. I'm also certified to climb and work on radio towers -- which most electricians are not. I specify, design, fabricate, and install raceways, cabling, data networks, etc. daily. I think that all of this more than qualifies me as a member of the electrical industry. Mike Holt suggested via e-mail that I post my Code question in this forum.

 

[Smart $]

See 830.106

Offhand, I'm assuming with all your qualifications you'll be able to determine what to do without any further assistance... but I could be wrong

 

[Brett Glass]

It never hurts to seek ideas and advice regarding best practices from one's peers.

 

[tom baker]

Brett Welcome to the Forum. Thanks for you reply and your interest in complying with the NEC.
I would hazard a guess that 95% of the antennas in the US are incorrectly installed, IE not per the NEC.
For antennas a bonding jumper is installed from the antenna mast to an grounding electrode. If the trailer frame meets the requirements in 810.21( 2002 NEC), then it can be used as the grounding electrode. Chances are you will need to install a ground rod near the antenna mast.
Art 810 sends you to Art 820 when you use coax as the lead in. 820 is similar to 810 for the locations of the bonding jumper and only the size of the bonding jumper varies.
Your objective is to establish a single point ground, in the event of a lighting strike all the conductive metal is elevated above some ground voltage, but since its all bonded together then there is no current flow.
You can even use the trailer frame to create a single point ground.
I install the impulse supressor right where the coax enters the building and then back out with my 2 AWG BCS to the ground rod, the ground rod is bonded to the antenna mast and then to the electrical grounding system.
I use 2 AWG as its less likely to be damaged. If its exposed I run it in 1/2" LTNM conduit to hide it from the wire thieves.

By the way, I would recommend you purchase a copy of Mike Holts Understanding Low Voltage and Limited Energy Systems.....lots of great pictures!

 

[Brett Glass]

Antenna ground

Antenna ground

Tom:

From what I have seen in the field, I will wager that you are correct: 95% of the antennas out there are installed incorrectly. We care about the quality and reliability of our Internet service, though, and so want to do it right.

As for the thickness of the grounding conductor: As I understand it, 10 AWG would meet code (and may be sufficient for short runs) but more is better. But what to ground to?

Article 810.21(F)(1)(a) permits the grounding conductor to be connected to the "building or structure grounding electrode system as covered in 250.50." 250.50, in turn, refers you to 250.52, which allows you to ground to the metal frame of a building or structure. The question, then, is whether the trailer chassis qualifies.

I would think that it should, because it consists of large steel I-beams, is bonded to the grounding bus bar in the panelboard, and is staked into the ground with more than a dozen steel stakes (which must create a better ground than your typical system of ground rods). Also, the trailer's metal roof is bonded to it with strips of sheet metal, which would suggest that it's meant to serve as a ground for that as well. It's not clear from Article 550 whether it counts as the metal frame of a building, though.

By the way, Internet radios don't use the sort of cabling or surge suppression contemplated in Chapter 8. They don't have a coaxial cable running into the building; the radio is inside the antenna enclosure and Cat5 cable runs inside. But I do place the surge suppressor/power injector at the point of entry, which I locate as close to an electrical outlet as possible. (The grounding lead of the surge suppressor is meant to connect to the center screw of an electrical outlet.)

 

[tom baker]

Brett you've got the idea, I can suggest you follow the NEC as close a possible.
With the power over IP radios, the applications are ahead of the NEC. But follow the NEC in Art 800 as 725 covers LANs, but 800 covers phone with LAN applications.

 

[petersonra]

Tom:

From what I have seen in the field, I will wager that you are correct: 95% of the antennas out there are installed incorrectly. We care about the quality and reliability of our Internet service, though, and so want to do it right.

What makes you think that grounding it will make it one whit more reliable?

As for the thickness of the grounding conductor: As I understand it, 10 AWG would meet code (and may be sufficient for short runs) but more is better. But what to ground to?

Why is bigger better? There is ideally no current flow, so size should not be a factor at all.

Article 810.21(F)(1)(a) permits the grounding conductor to be connected to the "building or structure grounding electrode system as covered in 250.50." 250.50, in turn, refers you to 250.52, which allows you to ground to the metal frame of a building or structure. The question, then, is whether the trailer chassis qualifies.

I would think that it should, because it consists of large steel I-beams, is bonded to the grounding bus bar in the panelboard, and is staked into the ground with more than a dozen steel stakes (which must create a better ground than your typical system of ground rods). Also, the trailer's metal roof is bonded to it with strips of sheet metal, which would suggest that it's meant to serve as a ground for that as well. It's not clear from Article 550 whether it counts as the metal frame of a building, though.

If they are all bonded together already, take your pick. You are making an assumption that more connections into the earth make for a "better" ground and that somehow this is desirable. I am not sure there is any evidence that is indeed the case.

By the way, Internet radios don't use the sort of cabling or surge suppression contemplated in Chapter 8. They don't have a coaxial cable running into the building; the radio is inside the antenna enclosure and Cat5 cable runs inside. But I do place the surge suppressor/power injector at the point of entry, which I locate as close to an electrical outlet as possible. (The grounding lead of the surge suppressor is meant to connect to the center screw of an electrical outlet.)

Again, I think you are making an unwarranted assumption that the connection to ground is especially important in protecting your equipment. This is common thinking but it is not like a storm sewer line where all the undesirable water is channeled off to the river. The only time any electrons get channeled to earth is if there is a current path there from a source.

 

[dereckbc]

I'm thinking that this makes the metal undercarriage of each trailer essentially equivalent to the frame of a metal building, and that it therefore can be used as a ground for antennas for radios. Should I bolt a lug to the undercarriage and ground to it? What is the best and safest way to ground the antennas and antenna masts in this situation?

I specialize in radio and protective grounding systems and might have an idea for you.

Try to orient the antennas straight above the point where the ground buss is bonded to the frame. This will allow you a straight and short as possible bonding run, and form a single point ground.

In addition I would install an ADU (antenna discharge unit) right on the frame adjacent to bond, then run the lead in from there on the underside. This will further inhance the single point ground.

By following these tips, if a lightning or an internal fault were to occur, the electrical system, radio equipment, etc would rise and fall at the same potential created by the single point ground. The further away the bonds points are away from each other, allows potential diiferences to occur during faults like lightning or power faults.

 

[tom baker]

Dereck:
Thats exactly how I install my antennas.
I use a 21 ft water pipe, sch 40. Bond a 2 AWG to the bottom of the mast, then to a ground rod, and then to the building ground. The impulse supressor is mounted just inside the building on an isolator, from it a 2AWG runs out side and down to the ground rod. The 2" mast is hinged and I have winch I can bolt on to lower it to the ground. The coax runs down inside the water pipe. Very clean install. I am limited to 20ft above grade by the FCC.