18/2 shielded conductor

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joeyww12000

joeyww12000

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Chatsworth GA
I am helping on a commercial fire alarm system and had a question on the silver conductor inside 18/2 shielded cable. I assume this conductor is a ground, but at the devices there is nowhere to terminate this conductor so to me it seems useless. What is it for? Ive been told when you make up taps you have to make sure its covered with tape because if it touches anything it will show trouble in the system.....???? Anyone know what Im talking about?
 
It is not a ground it is a 'drain wire' (probably a better name for it but I don't know it) and it should not touch anything but the foil in the cable and at the panel one end it should be 'grounded' once.

If the cable leave the panel goes around the building and comes back to the panel you only ground one of them.
 
thanks jwire, that raises another question you may be able to answer.........We ran out of 18/2 with this drain wire so I pulled a few devices without it. Is this going to show any trouble in the system?
 
First, if shielded wire is used in the first place, technically it should continue to be used through the rest of the circuit. You most likely will not see a trouble unless this is a super high tech system that supervises the shield, very rare!

The purpose of the shield in most cases is to cancel RF interference. This is typically desired for speaker circuits and firefighter phone circuits (two-way communication). Most fire alarm panel mfr's do not recommend shielded wire for the SLC as it reduces the maximum allowed length of wire.

If you added non-shielded wire to a circuit provided with a shield, the intent of the shield goes out the window.

As iwire mentioned the shield is only grounded on one end of the circuit. Too many installers ground this drain wire at all j-boxes and device terminations, this is the wrong method. The shield and drain wire should be intact from the start of the run to the end, protected from touching ground (twist and tape, safe it off) and grounded only at one end, usually at the panel.

Iwire is also correct in stating that some mfrs have a shield terminal at the device but it is rare.

Hope this helps..
 
Very well stated......

Very well stated......

STHOMAS said:
First, if shielded wire is used in the first place, technically it should continue to be used through the rest of the circuit. You most likely will not see a trouble unless this is a super high tech system that supervises the shield, very rare!

The purpose of the shield in most cases is to cancel RF interference. This is typically desired for speaker circuits and firefighter phone circuits (two-way communication).
Click to expand...

Also used for the RS-485 circuits used for communications between panels and RS-232 circuits used when a panel is connected to a 'gateway'.

STHOMAS said:
Most fire alarm panel mfr's do not recommend shielded wire for the SLC as it reduces the maximum allowed length of wire.

If you added non-shielded wire to a circuit provided with a shield, the intent of the shield goes out the window.
Click to expand...

It's worse than out the window. The technical term for an ungrounded length of drain wire? It's called an antennnae.

STHOMAS said:
As iwire mentioned the shield is only grounded on one end of the circuit. Too many installers ground this drain wire at all j-boxes and device terminations, this is the wrong method. The shield and drain wire should be intact from the start of the run to the end, protected from touching ground (twist and tape, safe it off) and grounded only at one end, usually at the panel.

Iwire is also correct in stating that some mfrs have a shield terminal at the device but it is rare.

Hope this helps..
Click to expand...

I like to use heat shrink tubing & single Buchannan 500 insulated screw terminal (Euro-style).
 
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My question would be, is twisted shielded wire required for the fire alarm system you are installing? I know that most Simplex systems, and I think that most Notifier panels do also. If it is something like a Silent Knight they you should be fine as it has no requirements for shielded wire.
 
Most FA manufacturers are now going away from shielded cable for SLC circuits. Here we proved to our local Simplex Tech that their system will indeed run their ID net on THHN without any issues....he was amazed....
 
Shielded vs. not

Shielded vs. not

Scanless and Stallzer:

true that when all is new and right, the shield seems extra. But my experience is that the non-shielded cable (not EMT) is more likely to be susceptable to interference from lightning. Of course, thhn in EMT solves that problem.

I figure that it's often spec'ed for a good reason. Nearly everything around me is in EMT or is shielded these days. I always request the wiring methods be submitted in the permitting process, whether on the riser diagram or in other documents. Most designers will play it safe with shielded.
Mike
 
Weaver Road said:
Scanless and Stallzer:

true that when all is new and right, the shield seems extra. But my experience is that the non-shielded cable (not EMT) is more likely to be susceptable to interference from lightning. Of course, thhn in EMT solves that problem.

I figure that it's often spec'ed for a good reason. Nearly everything around me is in EMT or is shielded these days. I always request the wiring methods be submitted in the permitting process, whether on the riser diagram or in other documents. Most designers will play it safe with shielded.
Mike
Click to expand...

Is your THHN rated for fire applications??
 
iwire said:
As far as I know it does not have to be.
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What I was getting at is per installation instructions such as a Silent knight 5820XL:

5.3.1 Wiring 5815XL in Style 4 (Class B) Configuration
No special wire is required for addressable loops. The wire can be untwisted, unshielded, solid
or stranded as long as it meets the National Electric Code 760-51 requirements for power
limited fire protective signaling cables. Wire distances are computed using copper wire.
 
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