Gate Intercom Grounding

[orrinc]

We are trying to write a specification for grounding and surge protection for Telephone Entry Systems at a remote gate (those boxes you drive up to and select a resident, call, and the resident can open the gate from theor phone).

Typically, from the Entry System, there are underground conduits to the Gate Motors- one as a source of AC power, and one low-voltage conduit for the relays, and another conduit to the main building, which contains the telephone line, and often a data connection for additional gates, etc.

All of this is usually well over 100 feet from the Main Service Entrance, and many of the systems are in the Southeast, where there is much lightning activity and high ground resistance.

The question is whether it is best to:
1. Install a ground rod next to the Entry Panel
2. Ground the Entry Panel to the local electrical service ground with a heavy ground wire
3. Do both, and bond the ground rod to the local electrical service ground.

 

[bennie]

Re: Gate Intercom Grounding

There is many of these systems in operation. I would check some that have been in use for a good length of time. Find out the history. You may save a lot of money and work.

 

[orrinc]

Re: Gate Intercom Grounding

Thanks for the advice, Benny, but that's not the point! We have many systems out there, some are working fine and some aren't. I am looking for a better solution than trial and error, and one that is in compliance with Code requirements.

 

[bennie]

Re: Gate Intercom Grounding

This appears to be a design problem instead of a code issue. What is/are the specific malfunction(s)?

 

[dereckbc]

Re: Gate Intercom Grounding

I have designed several of these system that incorporate, a card scanner, intercom, and CCTV. Best approach I have found to be useful in Miami is to use optical signaling between the gate and control panel. I assume you are using all metallic signal methods, and I have been successful by doing the following:

1. Install a TVSS device at the panel providing AC power. If the AC panel is the main panel I would suggest installing a class ?C? type TVSS, otherwise use a class ?B?. Then supplement by adding a class ?A? device at the motor. This will give sufficient protection of the AC power.

2. Install surge arrestors on all metallic control circuits where they enter the facility and at the intercom panel at the gate. Install a supplemental ground rod at the intercom panel to terminate the surge arrestors and the EGC from the AC service. Increasing the size of the EGC from the AC service panel will not buy you anything because the impedance is to high at lightning frequencies no matter what size it is. What is important is to make sure the supplemental ground rod is bonded via EGC and any metallic conduit used.

Like Benny said it is a design issue, not code. So what ever design you use should comply with code. Hope this helps. Dereck

 

[orrinc]

Re: Gate Intercom Grounding

The question was whether bonding the EGC to a ground rod at the Intercom panel, and using that point as the Common Grounding Point would be a Code violation?

Or if there might be a better way to protect the electronics that was acceptable by the Code . . .

We do use fiber on many systems, especially for video and data connections, but using it for a telephone line is rather costly.

The AC power for the Intercom usually comes from one of the Gate motor junction boxes. It is a dedicated circuit, but it is usually over 100 feet from the main panel.

 

[dereckbc]

Re: Gate Intercom Grounding

Orrinc, The question was whether bonding the EGC to a ground rod at the Intercom panel, and using that point as the Common Grounding Point would be a Code violation?

No it is not a violation see 250.54

 

[orrinc]

Re: Gate Intercom Grounding

Thank you, Dereck!

BHTW- have you found a cost-effective way to replace the telephone line with fiber?

 

[dereckbc]

Re: Gate Intercom Grounding

Orrinc, no I have not found a way to replace the intercom line with fiber on gate controllers. I wished some alarm manufacture made a optical modem that would transport video, data, and a telephome line for gate controllers. The technology certainly exists, as it is done on a daily basis in my buisness.

Just adding a ground rod at the gate controller will not do much except bleed off some of the current flowing on the EGC and metallic conduit. But the ground rod will help discharge lightning in the protection devices installed at the controller itself. Again some energy will be left on the control and power lines going back to the building, but that is where you can another protection device where it enters to further reduce energy levels to tolerable levels.

[ May 21, 2003, 01:53 PM: Message edited by: dereckbc ]