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Fire Alarm Systems & Radio Interference

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ryant35

Member
Location
Cypress, CA
We are proposing a new fire alarm/sprinkler monitoring system in an area in Southern California where we have had issues with radio interference from an AM radio station antenna in the area.
The building we are working in is only a few hundred feet from the towers.

We are planning on running a Class B SLC loop down the side of the building and using addressable modules to connect to waterflow & tamper switches along one side of the building.

Has anyone had any experience with avoiding this type of interference? Does shielded wire work or does it create an antenna on the SLC circuit?
 
gadfly56

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
ryant35 said:
We are proposing a new fire alarm/sprinkler monitoring system in an area in Southern California where we have had issues with radio interference from an AM radio station antenna in the area.
The building we are working in is only a few hundred feet from the towers.

We are planning on running a Class B SLC loop down the side of the building and using addressable modules to connect to waterflow & tamper switches along one side of the building.

Has anyone had any experience with avoiding this type of interference? Does shielded wire work or does it create an antenna on the SLC circuit?
Click to expand...

Shielded wire will help reduce noise on the SLC. However, the real issue may be what happens at the device itself? Some years ago Siemens had a batch of addressable pull stations that could be activated if you keyed a radio within a few feet of them.
 
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GrayHair

Senior Member
Location
Nashville, TN
Commercial Broadcast RF

Commercial Broadcast RF

I agree with gadfly56 on the shielded wire, not only for the SLC loop, but to devices as well. Solder and insulate the drains at splices and transponders.

First and foremost, talk to the Tech Support Group of the equipment manufacturer. You'll need the station's frequency, emission designator, effective radiated power and distance to the radiator (there will be more than one if the station operates directional at any time). Wall and roof-deck material might also be good to know.

The radio station engineer should also be a good source of technical info for the station and aware of any proposed changes in the transmitter/antenna. As I think about it, that's probably the first person you should talk to. There may also be certain cable lengths you'll want to avoid; the station engineer can tell you.

The transponders may need to be mounted in metal cabinets well attached to a solid ground; however, doing that brings longer conductors to the devices and conductors are potential antennas.

Some systems can use fiber optic modules for networking panels, but if they're available for the SLC loop, they would help keep RF out of the panel. If they are available for the SLC loop, you can use remote output modules to activate the horns/strobes (no horn/strobes connected directly to the panel). Yes, there are some extremes in my suggestions, but solving RF problems can be a nightmare.

Here is a YouTube link showing what RF from a fairly high power AM transmitter can do. The idiot touching the plants to the antenna is getting RF burns through his rubber gloves. Not to mention the tissue heating from the non-ionizing radiation :dunce:. The fence around the base of that tower isn't just to keep people from climbing the tower.

Regards and good luck!
 
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