When using fire alarm MC (18-2 twisted shielded pair) to wire an addressable fire alarm system what is the proper method for splicing the "drain" wire when making "T-taps"?

I was told to twist them together in the JB then wrap them with tape to prevent them from touching the metal JB thus preventing ground-faults.

Is the shield coming in contact with a grounded surface the cause of ground-faults?

Is this the correct way or not and why.

Why the heck are you making T-Taps?


Without a doubt.



[This message has been edited by iwire (edited 06-22-2005).]

Why not? That is one of the big advantages of an addressable system. It is listed as a Class B supervised circuit even with T-Taps.
Don

If the drain wire is insulated relative to the MC jacket, then it should remain isolated from the j-box and any raceway. It is typically terminated on the control panel end only. A drain in contact with the j-box when it should be isolated is unrelated to the condition of the communication conductors making contact with the drain or the MC jacket.

Don I am fully aware it is allowed, many things are allowed, that does not mean they are all good ideas. I will not do it, and the engineers in my area will not draw it that way.

The OP did not state it was class B and in my area class B is pretty much history.

In RI for instance even if the panel is only Class B the EOL must be brought around back to the panel. If the panel is Class A capable the system must be wired Class A.

If I recall most of the manufactures only allow one device per T-Tap.

It surprises me you think this is an advantage.

Ron, the panels I work with will indicate ground fault if the shield touches ground.

Siemens for instance is very sensitive to shield grounds.

Bob

Have done several addressable systems, and have had problems that were instantly gone when an untaped shield or drain wire was found in contact with a steel box mounted on steel studs. They were not ground troubles, but various data problems. I usually tape each cable's drain separately before connecting them to the other cables because it is so much easier to take apart and put together the cables later if you have to troubleshoot. I have never seen anything like these supervised systems that makes a man have to do so much unwirenutting and rewirenutting over the course of getting all the bugs out.

T-tapping was always OK with the large well known manufacturer whose product we installed...the system instantly goes into a trouble condition if any device does not report in, no EOL needed. This is only the initiating circuit, however. The audio/visual circuits in the systems we did were always run in supervised loops with no t-taps. They were not addressable.

[This message has been edited by poorboy (edited 06-22-2005).]

Yes - be sure to insulate that drain wire. If "t" tapping just join all the drains together and tape them very good.
You only attach those drains at the panel- any where else and you will have troubles when you turn on the system. I do not like "T" tapping. sticking with straight in and out makes trouble shooting much easier.
Just my 2 cents

I like to use heat shrink tubing on the drain wires. In and out is better than tee
tap in my opinion.

[This message has been edited by plcnewbie (edited 06-22-2005).]

It is true that the big advantage of an addressable system is the ability to T-tap.

There should be no problems because each device (smoke / heat detector and pull stations
) will have thier own unique "address". Each device periodically is verified by the control panel and if the device does not respond the system goes into trouble. So yes, we will be T-tapping but we will keep it to a minimum. The horn/strobes are looped with an E.O.L. resistor.

Also, as to keeping the drain isolated from ground. I was just told "Thats the way we always did it." I hate that answer. I always want to know why. Thanks for all the responses.

Brian

The problem(s) resulting from the Cable's Shield being connected to "Ground" at multiple points, come in the form of "Ground Loops".

Connecting the Drains to a single common "Ground" point, removes the possibility of Loop Currents being shuttled around... provided the Cables are terminated correctly.

As with most Cabling and Electronic stuff, a "COMBUS" - like what you are installing Cables for, should have "Grounding" connections terminated in a "Star" fashion (all bond at the same point).

With a "T" Tap, the tapped loop segment(s) don't bond Shields Directly to the Star Point, instead they bond kind of Vicariously via the "Trunk".

Scott35