Induced AC voltage

[cferguson]

I am an electrician for a DOT and an experiencing what I feel is an induced voltage on some of our CU #18 AWG communication cables. Some of these are installed on utility pole lines 12" below the distribution neutral, but the majority are underground or a combination of the two. The underground plant is 2" or 2 1/2" galvanized, and the individual conduit runs are no more than 300' between concrete pull boxes.Many of these cable runs are 5000'+, between traffic intersections.
The normal procedure is to bond the steel inner jacket of the communication cable at one end of the run, normally the "feed" end. None of the conduit runs are bonded together, nor is the conduit bonded to the service panel.
There is normaly between 10v and 20v on the jacket, measured to the service neutrial. There has been times I have drawn an arc while removing the jacket bond to perform a test.

Any advice will be helpful in trying to quiet the noise on these communication cables.

 

[hbiss]

Re: Induced AC voltage

I don't think this is an induced voltage, particularly in the underground runs. What I believe you are seeing is the voltage difference between two places on the utility neutral. You have one end of your shield bonded and are measuring a voltage from shield to that neutral 2000' away. No voltage drop on that shield (actually just a 2000' extension of your test lead) but all kinds of stuff on that neutral.

-Hal

 

[charlie]

Re: Induced AC voltage

Part (maybe all) of your problem may go away if you follow the NESC.

Rule 215C3 Multiple Messengers on the Same Structure (pole)
Communication cable messengers exposed to power contacts, power induction, or lightning, shall be bonded at intervals specified in Rule 92C.

We install lightning arresters and ground rods every 1200 feet in order to meet Rule 92C which requires four grounds per mile. All of the various communication companies attach to the pole down ground at all of the grounding electrode installations. I highly recommend you do the same with your cable.

 

[dereckbc]

Re: Induced AC voltage

Multi-ground your shield as Charlie suggested.

 

[newt]

Re: Induced AC voltage

You may be picking up return current if the primary voltage is wye.You have a paralell path for return current one is earth the other conductor.If your conduit is steel you have a good path for ground current.

 

[highkvoltage]

Re: Induced AC voltage

cf. Are you sure that you are connecting the shield to the proper terminal strip. The reason I am asking this is you would have a service disconnect and you would feed the controller from the load side of your disconnect. In some traffic control cabinets you now have a neutral bar (isolated) and a equipment grounding bar. If you hook the shields up to the neutral bar you are correct you would draw an ark. Make sure that they are landed on the right place. Also make sure that the neutral is isolated in the cabinet. Check your power supply that feeds you electronics. Good luck.