Shielded Wire

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STHOMAS

STHOMAS

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North County San Diego
This is regarding fire alarm/voice notification and fire department communications systems. While it is common knowledge to utilize shielded wire for speaker circuits, and SLC (Signaling Line Circuits) when the SLC are within the same conduit as other (low voltage) circuits to eliminate crosstalk and other interference, the question has come up with how to properly install shielded wire.

First, there are two types of shielded wire. One includes a foil shield around the twisted pair fire alarm cable, and one includes this foil wrap as well as a non-jacketed drain wire intertwined with the rest of the bundle.

I was taught many years ago that in order for the shield to be effective it must be continuous from the first installed device through all installed devices. Additionally, when using shielded wire with a drain, the drain wire must be grounded appropriately per the equipment mfr instructions, typically on one end only, (preferably in the main control panel). Improperly terminating shielded wire results in either creating an antenna and inviting interference, or receiving no benefit from the wire considering the extra money paid for that wire.

Is there a document that explains in general these installation practices with regards to low volt systems, be it fire alarm, paging, two-way communication systems?

Thanks,
 
I would like to see this also. Although I personally don't think shielded wire makes much of a difference.
 
Is there a document that explains in general these installation practices with regards to low volt systems, be it fire alarm, paging, two-way communication systems?
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I haven't seen one in particular because each manufacturer spec's different wire types for their devices.

Usually I ground the end at the can (usual directions) but have had some that said don't - so go figure...

And I've had some that said don't use shielded, just twisted pair.

I agree their should be some uniformity but I always follow the manufacturers recommendation just for my liability reasons (whether I like it or not);)

Jim
 
Twisted wire has very high immunity to noise as the noise is introduced equally into each conductor.
For shielded wire, the foil type gives better shielding.
I install a lot of control wiring, the shield is only bonded at one end, at the power supply. Some instruments are self powered and the signal cable goes to a unit for signal processing, the shield is carried thru but not bonded.
 
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