Douglas Krantz - Technical Writer - Describing How It Works
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When Should I use Shielded Cable in a Fire Alarm System?

When Should I use Shielded Cable in a Fire Alarm System?


Greetings Douglas,

In general, which fire alarm/notification devices require shielded cables? My understanding is that many do not, other than voice annunciation devices.

Looking over several go-bys, (oil field on the North Slope) pretty much all the cabling wired out to the devices are shielded. A fire protection vendor is advising otherwise.

Thank you, C

All fire alarm systems are package-deals; all fire alarm systems are tested and certified as being reliable by third-party testing laboratories like UL, FM, ULC, CE, CCC, etc. This testing included the type of wire being used between the control panel and the devices.

Shielding affects the ability to send and receive signals in two major ways.
  • For higher speed data-rates, like what is used in the oil field and other industries, the signals are "Transmitted"; the signals travel along the wire pathway. The shield on the wire reduces extraneous electrical noise so it doesn't interfere with the data as it travels along the pathway.

  • For low-speed data-rates, like what is used in fire alarm systems, the signals are "Turned-On and Turned-Off" to all the devices at once. The capacitive effects of the added shield can interfere with the data being sent, corrupting the data.

To keep out the extraneous electrical noise, some of the older fire alarm systems required shielded wire; to keep capacitive effects from corrupting the data, most of the newer fire alarm systems cannot use shielded wire. Because of the extremely high reliability requirements for life-safety fire alarm systems, even the type of wire is included in the package-deal for the third-party testing and certification.

Somewhere in their printed literature, the fire alarm manufacturer will indicate which type of wire has been certified to be used. A phone call to the manufacturer's technical support people can also provide the answers to the question of which type of wire should be used. Asking the fire alarm installation company is also a good resource for answers.

Douglas Krantz

facpdoug@douglaskrantz.com
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