elevator phone problems 1

[macphoneguy]

to make a long story short, we had 6 elevators that they where complaining about low call Volume i.e. Hard to hear, hard to be heard, and I went though the phone line 12-24 ways taking voltage measurements, Current Measurements even checking the DB on the given lines all where within "Normal" parameters of the phone company,(even called the 1k service line) So finally meet with one of the elevator service company and checked it with the power off in the cab, again everything very Normal, turning power back on everything went to heck as to call volume, Had a osciloscope and when the elevator service guy saw the dramatic differance in line noise, ended up grounding the shield on both end and things where remarkably better,
I know the NEC talks about elevators, but is there anything that specifically talks about emergency phones in elevators ?

 

[GHTROUT]

Inneresting problem. I assume you tried a variety of phones, eliminating the impedance or shielding of a few brands...even though it seems pointless given your tests.

The only thing I was always warned about was running anything through the elevator shaft...I know sounds obvious...but not to all 18 year olds.

It would be interesting if you had a CAT5 tester/certifing box - better yet, one with some TDR capability? - find where any fault existed ? You tested with a scopt...shoot, it sounds like you proved Otis was a slacker... Why do those guys always take the stairs?



~

http://www.GHTROUT.com/

 

[doktor]

We faced the same problems some years ago.
The solutions was to pull a shielded twisted pair cable to each elevator and use ferite cores in both ends of the cable (at the PBX and in the elevator).
The PBX was also carefully grounded.
The MDF was grounded.The shielded twisted pairs cables were also connected from the PBX line card to the MDF.
Hereafter no noise was observed. The "audio level" was also adjusted in the PBX.
Cross talk was not observed anymore.

/// doktor

 

[SYQUEST]

This is a good example of why the SHIELD must be BONDED/GROUNDED when using shielded cable. I have seen it time and again. People install shielded cable and cutoff the bond/ground leads.

Then you have giant antennas, EMI/RFI just loves this stuff!!

macphoneguy:

You never mentioned if you checked the pair with both ends open for shorts/grounds... Results??

....JIM....

 

[doktor]

Just to make it clear!
The shield of the twisted pair cable was led ALL the way from the elevator phone to the line card of the PBX, and was connected to the PBX grounding point.
Hereafter we had a happe customer and the problem was solved.

///doktor

 

[macphoneguy]

to answer some of the questions, other Brands of phone Worked "marginally" better, I know there is some antenna's nearby but what their wattage is I'm not sure, orginally the phone Cable did not have the Shield connected to anything, no shorts to the Shield, or ground that was noticable by things line a VOM or the fluke Microscanner pro

again AC off in car Good signal, AC on in Car bad signal so the "Moral" of the story when checking phone problems in Elevator car check that the Shield is Grounded. in this case it wasn't and that effected the phone line
The elevator people usually have a Traveler cable that follows the car the lenght of it's path, the Traveler cable is actually a Bundle of AC and Controlling wires and in my Case somewhere along the lenght the Phone Cable got too close to the AC power Cable and with it not Grounded, Wala a little thing called Bleed though/RFI

 

[GHTROUT]

OK, here's one. On a length of shielded cable, say from building to building, do you bond both ends (of the length of cable's shield) to ground? Or do you bond only on end and the other end s shield remains loose?

Some will know, some will assume, and some may get the clue: "there is no 'potential' for error when done correctly” And of course, some will debate it with unquestionable facts. However, each has arguments that cannot be discounted,




~

http://www.GHTROUT.com/

 

[pbxn]

Well from a ex telco installer view I would say both ends. The common practice especially with buried cable was to bond in at both ends, but though both ends were bonded one side was bonded at grounding post, the other main cable feed primary ground.

 

[doktor]

We only use to connect the one end of the shield to the ground. Otherwise you may obatin ground loops.
///doktor

 

[macphoneguy]

since the elevator car is retalively close to the equipment room,(usually 10-15 feet usually) I ended up grounding both Ends, as a telco person, if the cable in question would have been to a seperate building that might have a seperate "ground" then I might have done something different as most elevator phone cable are 18 AWG single pair straneded, & Shielded as lest we forget there is AC in traveler cable and the RF from it can do Werid things, and the elevator maintenance people did the grounding of the Shield on the other 6 elevators and the problems with volume mysteriously Vanished. ;-)

 

[ISDNman]

On the interbuilding run it MUST be bonded on both ends. This is a personnel safety issue.

Within a building "audio" guys always just bond one end to prevent "ground loops".

 

[johnpoole]

ground loops are a problem IF the ground between two points in a building have standing voltage between them.. the audio people don't want to provide the only common ground between those two points.. i've had simular problems with copper between buildings on a large site.. ran a common number 6 between buildings and cleared the emf.. we had cable sheath ground but that was adding noise.. when we tied the number 6 down it almost welded itself to the brass lug...

more grounds are not always the solution, better grounds, somewhere besides through my switch

john poole
bellsouth business
columbia,sc

 

[ISDNman]

John,

you are correct, but personnel safety outweights performance issues. If grounding both ends causes a poblem on the interbuilding run then other action is needed.

As you state, the answer was to bond the buildings, not simply leave it floating on one end where anyone that came into contact with it could have that welding current flow through them!

cheers