Rooftop WiFi access point grounding

[rblake]

Hi all, sorry for the long post but I want to give you all the relevant details.

I am working with a local ISP to install an outdoor wireless internetaccess point. I have some questions about grounding and lighting protection. We are in an area that gets a few severe thunderstorms every year.

All the electrical service and telephone / data lines enter the building (an older wood frame house) in the same area, just above ground level on the north wall. The electric meter, service panel and telephone network interfaces are all bonded to either the conduit that enters the electric meter, or to the conduit that enters the building from the service panel. The service panel is apparently connected to one or more ground rods (can't see, but a large bare wire goes through a small PVC conduit to the ground) and to the water pipe on the other side of the basement.

The access point electronics and antenna are mounted on a tripod located atop the center of the roof. The antenna is on a 10' steel mast. The access point is inside a fiberglass NEMA 4 enclosure. The antenna lead goes through a PolyPhaser filter protector mounted on the enclosure. Power and data will come in over CAT 5 cable.

There is a swamp cooler nearby, with power run through metal conduit and a sheet metal cone and flashing over a hole in the roof. The conduit goes through the cone unbroken.

I would like to connect the tripod base, coax ground and a CAT 5 surge protector to the ground lug on the NEMA enclosure. Then run the ground wire and the CAT5 cable through flexible plastic conduit to the existing sheet metal cone under the swamp cooler. Inside the attic I would install another CAT 5 surge protector, and run the ground wire through the attic to the wall with the service entrance, then bond it to the service entrance conduit.

My concerns are for equipment safety and performance, as well as code compliance. I have read section 820 of the NEC, but I'm not sure that I understand all the implications.

Is this plan reasonable, safe and in code? If not, what do you suggest.

Thanks in advance for your help.
Reed.

 

[hurk27]

Re: Rooftop WiFi access point grounding

The antenna tripod and mast should be bonded to the main service electrode if this can't be done within 25' then install another ground rod then bond this rod back to the main service grounding electrode system with at least a #6awg. One point I should make is lightning will do damage to equipment with even the best of grounding systems installed. But the NEC is more concerned about the protection of people not equipment. so I would avoid bringing lightning into a building to get to the grounding electrode system as it can jump off any where and cause fires and other damage to other electrical systems. Try to keep your entrance wireing to one entrance point. And then do all your bonding at this point on the outside of the building to the service electrode. this keeps the grounding/bonding to a single point.
Maybe Dereck will jump in here.
Here is a good web site to get in some good reading on lightning protection:
The Lightning Saftey Institute
And:
The Lightning protection Institute

 

[dereckbc]

Re: Rooftop WiFi access point grounding

RBLAKE, the only thing I can add to what Wayne has already said is consider mounting the antenna directly above the electrical service entrance, run all data and power cables straight down to service electrode. This will facilitate terminating Polyhaser discharge unit, DC power protector, etc at the point of entrance and allow a single point ground that all the services use. This method will afford the highest degree of safety, protection, and performance of the equipment.

I am not familar with the RF power output of WiFi transmitters, but I suspect it is in the 1/4 watt range. So the added distance from the center of the roof to edge should not make any difference in signal performance at the height you mentioned.

 

[rblake]

Re: Rooftop WiFi access point grounding

Originally posted by dereckbc:

I am not familar with the RF power output of WiFi transmitters, but I suspect it is in the 1/4 watt range. So the added distance from the center of the roof to edge should not make any difference in signal performance at the height you mentioned.

This particular unit runs about 30 mW. My reason for putting the antenna in the middle of the roof was to provide space for guy wires if I need to use a much taller mast.

The distance from the middle of the roof to the edge is less than 20'. The service entrance conduit could be reached with about 25' of wire. Is this verging on too much ground wire (i.e. should I move the antenna), or is it acceptable. If it is acceptable, can I run the ground wire through the attic, or does it need to go along the top of the roof?
Thanks again,
Reed.

 

[hurk27]

Re: Rooftop WiFi access point grounding

can I run the ground wire through the attic, or does it need to go along the top of the roof?

I is never a good Idea to ever run down leads into a building as I said before lightning can jump off this lead anywhere and cause a fire or damage other equipment inside of the building. It is always best to run these leads straight as possible and short as possible. But keep in mind one thing Lightning protection is not an easy task and requires a great amount of understanding. This is why most states require a licensee to install it. I would say if your that close to the service, and you run at least a #6 down conductor. and keep the entrance of all your cables to one point, It would be about the best you could do with out a big investment.

 

[dereckbc]

Re: Rooftop WiFi access point grounding

RBLAKE, it is permitted by code, but not good pratice. I suggest mounting your mast near the edge leaving enough room for guys, but leaving short as possible ground wire run.