911 center radio station on phone line?

[ericsherman37]

We went out to a local 911 call center today. They wanted us to run a new circuit for some equipment they're going to install soon. When we got there, the lady asked us to check something else out too. Apparently they were having problems with a local radio station coming in over their phone line - so loud, in fact, that they were having trouble hearing callers on the phone. They had the teleco out, and they installed some filters - these helped dampen the radio station's volume, but they told her they thought it was a grounding issue. So she asked us to check it out.

All the grounding looked fine. The main grounding electrode was a ufer; the clamp was accessible and we tightened it down. The telephone system had a couple ground rods of its own, but they were adequately bonded to the main power grounding electrode conductors. The neutrals weren't rebonded anywhere past the service disconnect. We didn't see anything wrong.

On our way back to the shop, it finally occurred to me that perhaps the "on-hold" music somehow got tied into the regular line. Now keep in mind that I know about as much about phone systems as I do about astrophysics (not much) so if anyone has other ideas/theories then I'd love to hear them.

 

[480sparky]

Unfortunately, sheilded phone lines should have been installed to begin with.

One thing to look for as you pull up on a new job site is nearby towers.

 

[ericsherman37]

Well the building also houses the local police station - they've got all sorts of antennae and stuff up on the roof.

 

[Brian Oshman]

Could be a certain wire run matching the 1/4 wavelength of the radio frequency and the signal is just being induced into the system. Is there a way to temporarily disconnect the phone line from the system and check the system itself and then check just the phone line? I know that may be a tall order due to it being an emergency station but if you could set up an auxiliary phone or have it trunked to another phone so they could still receive calls.

Could try to lengthen the line run to change the "tuning" of the wire length and it may not pick up the frequency any longer.

Could also go through the components and disconnect things one at a time to see if it stops at any point.

Unfortunately, RF is tough to track down and with the amount of cabling involved it will be tough to narrow it down. Could also try to run a couple of temporary lines to the equipment to see if it stops and then just abandon the existing and run new.

 

[tom baker]

I had this problem on phone lines for a SCADA system in a building next to a AM radio tower. A small capacitor was installed from tip to ground, ring to ground. Have you tried contacting the radio engineer?
Last resort would be the FCC, or tell the engineer that...

 

[K8MHZ]

I had this problem on phone lines for a SCADA system in a building next to a AM radio tower. A small capacitor was installed from tip to ground, ring to ground. Have you tried contacting the radio engineer?

Last resort would be the FCC, or tell the engineer that...

All telephone systems are required by the FCC to be able to accept any interference subjected to them.

It is the responsibility of the manufacturer and installer of the TX system to assure that the above requirement is met.

The state's public service commission is often a good resource to get the telco off their butts and fix the problem.

Unless you are familiar with RFI, the PSC and the FCC you should not attempt to tackle this job.

 

[dereckbc]

This is a fairly common problem. It takes two things for this to occur:

A AM radio station. It will not be the police or any other FM radio because it cannot be demodulated
The radio station broadcast tower will be close by.

However this is not the Sparkies problem to fix, it is the telephone equipment vendors problem to fix because they have to isolate which line or lines the signal is entering, and then take appropriate action by installing filters or reconfigure the grounding scheme.

One note to keep in your mind. You said the PBX had its own two ground rods, plus bonded to the service ground to meet code. If push comes to shove try removing the connection from the ground rods at the PBX, not at the rods and see what happens. My guess is that is the problem as I have seen it happen a few times What could be happening is the AM signal is entering or flowing through the loop created between the two ground points and being demodulated in the PBX. Telephone equipment needs to be single point grounded from a single ended power source. They have created a loop between the service ground and external ground rods. It would be the first thing I would look at.

 

[wireguru]

I wouldnt TOUCH anything to do with the phone lines in a 911 center. Especially one who's operators are so incompetant they
A: do not have a service contract and on call tech available for their telecommunicaiton systems;
B: actually put up with a radio station interfering with 911 calls, and
Cno offence) asked the guy putting in an outlet for the copier if he could fix their 911 lines!

I would install a non gfi receptacle over a bathtub so the HO could use their toaster to heat their morning bagel while in the tub before I would touch 911 phone lines......

 

[K8MHZ]

On our way back to the shop, it finally occurred to me that perhaps the "on-hold" music somehow got tied into the regular line.

I have experience in county 911 centers. From backwoods to state of the art.

I have NEVER encountered a 911 center (PSAP to be exact) with music on hold.

Also, FM stations will indeed cause interference with wireline devices.

I have many years of experience in telecommunicatons. Grounding is NOT a method to reduce interference. Shielding is. Filtering just masks the problem.

The TX system should have no problem with RF signals if manufactured correctly. Think about it, how to radio stations talk on the phone?

As stated, this is NOT a problem for sparkies and could constitute a HUGE liability issue for anyone working on the system.

 

[ericsherman37]

Could try to lengthen the line run to change the "tuning" of the wire length and it may not pick up the frequency any longer.

They're getting a bunch of new phone equipment installed in a few days, maybe that will take care of the problem.

Have you tried contacting the radio engineer?
Last resort would be the FCC, or tell the engineer that...

Haven't contacted them. They simulcast on an AM as well as an FM station - I'm not sure which one the phone line is picking up.

Unless you are familiar with RFI, the PSC and the FCC you should not attempt to tackle this job.

We're not Our shop has a licensed and quite knowledgeable low voltage technician and we sluffed the job over to him.

Cno offence) asked the guy putting in an outlet for the copier if he could fix their 911 lines!

None taken, I wouldn't trust me to touch ANY phone line. But to some people, wire is wire. I'm sure the telco guys told the lady "you need to have an electrician look at this."

As stated, this is NOT a problem for sparkies and could constitute a HUGE liability issue for anyone working on the system.

Agreed. Hence the sluffing.

Thanks for all the comments and contributions, everyone... sounds like a tricky problem.

 

[jeremysterling]

I would install a non gfi receptacle over a bathtub so the HO could use their toaster to heat their morning bagel while in the tub before I would touch 911 phone lines......

Well the building also houses the local police station - they've got all sorts of antennae and stuff up on the roof.

But do they have a bagel toaster?

 

[wireguru]

They're getting a bunch of new phone equipment installed in a few days, maybe that will take care of the problem.

if it doesnt take care of the problem, at least it will give them someone to blame :roll:

 

[hillbilly1]

This may be slightly off topic, but may be relevant. Their is a shopping center in Atlanta that has a television trnsmission tower in the parking lot. No MC cable is allowed to be used there, and all (EMT) conduits must be spot welded, along with connections to boxes. The inspector said that a loose connection would reflect the towers transmissions back to it effectivly shutting down the transmissions.

 

[hbiss]

The inspector said that a loose connection would reflect the towers transmissions back to it effectivly shutting down the transmissions.

That inspector has a screw loose. It's what happens when sparkies get involved with something they have no business doing.

But to some people, wire is wire.

And your responsibility is to explain that isn't the case and say no.

I'm sure the telco guys told the lady "you need to have an electrician look at this."

I seriously doubt that. They will tell the customer to have their phone system vendor look into it. And that is EXACTLY who should be handling this, not some wiz kid sparkie LV guy from your company. Your boss just can't stand turning away a buck, can he?

-Hal