Why fire SLC alarm cable shall not be shielded? Aply same for NAC?

[DavidCR]

Blessings, and a hello to everybody.
I've seen in wiring manuals, and cabling guides, from fire alarm panels of main manufacturers (Notifier for example) that SLC fire cable for inteligent panels should not be shielded, and if shielded, the shield shall be broken or isolated from devices ground.

Does somebody know why?

In my case, my only concern is that I'm afraid the cable may pass near a normal power wire. Is there a real negative effect from a motor 120-240AC wire on the SLC for example?
I've asked..., some say the cable lenght may be reduced a little by impedance, others say to use shielded if possible, but i'm not happy with the explanations.

I've also read that the only shielded cable recomended is for voice cable from amplifiers to speakers. is this true?

An explanation or contact with information on the topic will be apprecciated.

 

[DM2]

The reason it doesn't need to be shielded is that there is enough noise immunity built into the system, such that if you following the wiring requirements in the manual, along with NFPA 70, Article 760, there is not need.

the shield shall be broken or isolated from devices ground]

I'm not sure where this comes from, however if you're going to use "shielded" cable, it should only be terminated at the beginning of the circuit. The reason for this is the different ground potentials at multiple locations and the ground loop that would be created.

cable recomended is for voice cable from amplifiers to speakers

I'm not sure if your saying that you only need to use shielded on speaker circuits, or if you should use the same type of shielded cable used for speaker circuits. If it's the former rather than the later, speaker circuits tend to put out a lot of noise. In either case Notifier publishes a document for the type of cables to use for various circuits. Follow the recommendations in that guide.



Regards,
DM

"Real world Knowledge isn't dropped from a parachute in the sky but rather acquired in tiny increments from a variety of sources including panic and curiosity."