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Posted By: sentrex1 to shield or not to shield - 11/12/13 10:38 PM
normally when i install 70 volt paging mostly Bogen i always use
16x2 shielded cable,
recently i was hired by a competitor to help with a large paging system in a hospital i was surprised that they were not using shielded cable he said that you didn't have to with 70 volt paging.
what is your take on this
thanks
Posted By: Silversam Re: to shield or not to shield - 11/12/13 11:08 PM
Input cabling is always shielded. Output (speaker) cabling is almost NEVER shielded. The shield can act as an antenna and pick up noise.

Sam
Posted By: sentrex1 Re: to shield or not to shield - 11/12/13 11:12 PM
makes sense but i wonder why on Bogen web site they say to use shielded thats why i have been using shielded wouldn't unshielded cable make interference on lets say data wires crossing in ceilings ect???
Posted By: hbiss Re: to shield or not to shield - 11/13/13 04:01 AM
I was taught that you never use shielded cable for speakers or 70 volt lines unless the speakers are on an intercom talk back system.

Where does Bogen say to use shielded?

Quote
... wouldn't unshielded cable make interference on lets say data wires crossing in ceilings ect???

Interference with data is an old wife's tale or at least a product of some CGs fertile imagination. I don't care how or where you run CAT5 it's gonna work fine.

-Hal
Posted By: Mercenary Roadie Re: to shield or not to shield - 11/13/13 06:14 AM
Interference can show up on any 25/70/100v system. I know this because it happened at a School district I use to work at.

Brand new state of the art Fire Alarm system installed along with a Paging system and when they were turned on you could hear the data from the fire alarm system on the paging speakers. The paging system required shielding per the manufacture and the installer ran shielded cable, but didn't connect the shields together. Once the shielding problem was corrected, the data noise went away.

So the end result is do what the manufacture of the equipment requires.

Oh, and never use shielded cable for regular speaker cables as it causes all kinds of problems. You really don't even need to use twisted pair cable. If you are running speaker wire in conduit, just use regular wire the same as you would use for an ac circuit. Otherwise standard 2 conductor cable is just fine at the appropriate gauge.
Posted By: Silversam Re: to shield or not to shield - 11/13/13 11:51 AM
Originally Posted by hbiss
I was taught that you never use shielded cable for speakers or 70 volt lines unless the speakers are on an intercom talk back system.

-Hal

Yup. In that case the speaker (in talkback mode) becomes an input and therefore requires shielded cable.

Sam
Posted By: Rcaman Re: to shield or not to shield - 11/13/13 01:42 PM
Technically, a shielded cable is actually a capacitor. Think about it and it makes sense. You can cause more problems with shielded cable than you can solve. An amplifier is designed to output into a resistive or inductive load, not a capacitor. Eventually, if the conditions are right, the shielded wire could provide an induced emf back to the finals in the amp and wipe out the transistors.

Some systems, however, are designed with a shielded output in mind. I am reminded of the old nurse call systems and Valcom talkback systems. Both were designed to use shielded cable.

Regarding the new addressable fire alarm panels, I would highly recommend using shielded cable and make sure you separate input pairs from the output power pairs. It makes a VERY big problem if you do not.

Rcaman
Posted By: tim10 Re: to shield or not to shield - 11/13/13 09:59 PM
Hate to beat a dead horse, but I agree, do not use shielding on speaker cables unless the manufacturer requires it for special application such as a school intercom talkback type scenario.
Posted By: sentrex1 Re: to shield or not to shield - 11/13/13 11:57 PM
Hal i cant find it now but i will call Bogen a get an answer tomorrow will update
Posted By: hbiss Re: to shield or not to shield - 11/14/13 09:42 PM
Don't go out of your way, just curious.

-Hal
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