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Posted By: GOROSSI46 Fire alarm/Nurse call wiring? - 07/31/05 04:51 PM
I tried to ask the licensed guy I work with this question but the idiot couldn't answer it for me. But what is that silver insulation stuff in the 18/2 used for and also what is that silver ground wire used for if you don't use it for for nurse call applications? Does anyone have a good website they could direct me to that explains lv wiring? I'm a 1st year apprentice so please don't bash me.
Posted By: luckyshadow Re: Fire alarm/Nurse call wiring? - 07/31/05 05:08 PM
I don't think anyone would bash you if you are asking serious questions and not bashing others . please feel free to post your questions here, there are a lot of smart people here that are willing to answer.Just remember when calling some one an idiot - you never know who is reading your post.
What you are describing is called shielded cable. That silver foil is the shield that helps keep the wiring from picking up unwanted signals. The silver wire is called a drain wire. On most electronic equipment like fire alarm systems and Security systems you must join the drains, insulate,and isolate them from any ground source. They are connected at one point in the system ,usally the panel.
Posted By: GOROSSI46 Re: Fire alarm/Nurse call wiring? - 07/31/05 05:17 PM
Thanks for the reply. So is that shielding used for anything or can it be cut right off when you strip the wire? I'm not trying to bash but I think its pretty sad a licensed electrician I work with couldn't answer that for me, especially when I have to point stuff out to him on prints. One more question. If your doing nurse call do you splice those drain wires and cap them or just cut them off completely?

[This message has been edited by GOROSSI46 (edited 07-31-2005).]
Posted By: rad74ss Re: Fire alarm/Nurse call wiring? - 07/31/05 05:45 PM
The silver film is an outer jacket to protect against Radio Frequency and ElectroMagnetic Fields.

The only time the outer jacket comes into play is if you are using an RF/EMI backshell (which is rare).

With those backshells the jacket is squeezed between two internal components of the shell. The jacket is thus bonded to the shell and any errant waves are bled off through the shell to ground. This is mainly used on power cables and is used to keep the waves produced by the wires in the cable from going out into the atmosphere and slapping around your low voltage signal.

The drain wire is there to bleed off any induced voltage etc. in your 18/2 to ground. This wire is left open on one end and grounded at the main control panel. If both ends of this wire are grounded you have an antenna. Since these signal cables to not generate enough powerful waves to require backshells the jacket can go away. Some people do not worry about induced problems on cables and don't use either, but I always use the drain.

Unless of course you have a backshell. You can look those up on the websites of Cannon ITT and Amphenol if you want to see what they are like.

Posted By: luckyshadow Re: Fire alarm/Nurse call wiring? - 08/01/05 12:33 AM
I remove the foil shield then I twist the drains together (from all the cables) then wrap that around all the cables , then tape them to insulate them. I make a habit of this regardless if the drain is being used or not. This way when the drain is being used it is second nature to do so. I never cut them off .The other electrician most likely has never dealt with or was not taught about shielded cable. The best thing to do when faced with a question is to do what your doing, ASK IT , and Research it.
Posted By: the High Wirey Re: Fire alarm/Nurse call wiring? - 08/01/05 04:26 AM
Rad74ss and Luckyshadow gave you some good advice. Especially about bashing your lead man. Many electricians and technicians, who should know better, do not know how to handle a drain wire. Improper handling of a drain wire can cause much grief, and the grief is amplified by the size of your system.

I would like to expand a little about using shielded wire (usually two conductor shielded cable, 12 to 22 gage) as is found in audio, control and in instrumentation systems (you will encounter it everywhere) :

1-the silver (bare) wire is known as a drain wire, not a ground wire. The drain wire is usually connected at the panel end, and is usually not connected at the far end of its' run.
2-a two conductor shielded cable in a system may pass through several splice points in running from end to end. At any splice point in the run, the silver (bare) wire must be treated as an ungrounded conductor. Always think of that bare drain wire as a conductor that the factory forgot to insulate. You must do the insulation for the factory at every point in the run where the conductors are spliced or terminated.
3-never allow the silver (bare) drain wire (or the foil) to contact anything throughtout the run, end to end.
4-never bunch (connect more than two) drain wires at any point in the system, including at the panel end. You may encounter a cable with many two conductor (with drain) cables contained under one common jacket. Each will still be built the same as one two conductor cable, and each drain wire must be insulated individually.

Remember: "we don't need no stinkin' ground loops"

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