EMF interference

[hbiss]

Scratching my head here. Customer is a drug store. About a year ago a transformer blew someplace in the area which caused damage to quite a few businesses. For this customer we had to replace their small PBX and they had to replace their automated refill system. All was fine for about 6 months when customer complained that they couldn't use any credit card terminals. They use a line share device which simply takes two phone lines and connects them to four credit card terminals as the lines are needed. This has always worked well since no more than two terminals are ever in use at any one time but now it seemed that all were in use at once. Investigation revealed that one credit card terminal was removed (unplugged from the jack) and none of the other three were in use yet the LEDs on the line share device indicated that all four were in use. What was strange was that disconnecting the run that went to the jack for the terminal that was removed returned everything to normal. I suspected a damaged cable but resistance between conductors and to ground of the 20' of CAT3 wire were normal. 20 feet of wire with nothing at the other end. All wiring is visible.

Now the latest problem is hum in the phones and the automated refill system which is connected off the analog PBX. Swapped out the PBX to no avail. Right off the PBX= no hum. Add 30' of wire=hum. This system and wiring has been in place with no problems since 2001.

I'm suspecting an electrical problem and I haven't looked at the service yet. Am I right in thinking a neutral problem someplace, either the customer or the utility?

-Hal

 

[al hildenbrand]

As a process of elimination, examine the grounding electrode system (GES) and its terminations. Check that the Telco system interface is bonded to the GES. After that, test for voltage swings on load unbalance conditions, and take current readings on the GES and grounded service conductor.

 

[The Spunkster]

Please let us know what you find out.

 

[GoldDigger]

Most common cause of hum is incorrect wiring of jacks and cable which ends up using two wires as a signal pair which are not actually a twisted pair in the CAT5 cable or other cabling system.
The other common cause of both hum and noise is an unbalanced load from one of the two wires to ground which can lead to a voltage difference at the far end.

 

[hbiss]

Most common cause of hum is incorrect wiring of jacks and cable which ends up using two wires as a signal pair which are not actually a twisted pair in the CAT5 cable or other cabling system.
The other common cause of both hum and noise is an unbalanced load from one of the two wires to ground which can lead to a voltage difference at the far end.

Not the case here. Remember, not only did I wire this place, it's been there since 2001 without a problem. We can take as a given that there is nothing wrong with the communications wiring or equipment. Something has recently changed to cause a high EMF to be generated.

-Hal

 

[cadpoint]

Why don't you go in off hours and lift the demarc, fox and hound or ring out all the wires with some of your favorite equipment? Funny that you turned off some equipment and things came back, I'd leave it off if your lift the demarc or even isolate the house side.

 

[hbiss]

Telephone service is provided by the cable company through an EMTA located within a foot or so of the PBX. Yes, I did disconnect the CO lines from the PBX. Hell, I even took a new PBX and only connected one of the extensions going to the IVR system to it. Hum is still there even with a butt set and the IVR disconnected from the jack.

Funny that you turned off some equipment and things came back

That wasn't even part of the phone system. What was disconnected was just a run of wire that had nothing connected at the other end. It just went to a jack under a sales counter. At that time I wasn't chasing a hum and chalked it up to the line share device starting to crap out. So I didn't suspect EMF.

-Hal