Excessive current on Telco MGN connection?

[westflgator]

Let me start by saying I am not an engineer. I am a telecommunications technicain, I do have to work with the power company engineers and techs to resolve problems occasionally. I have attached a drawing of the premise where my problem exist. The setting is, this is an office building with 50 offices, all having computers, and all florescent lighting throughout the building, with 3 phase service. Problem being we have 3 to 7 amps of current flowing on the Telco ground, and it changes very rapidly up and down from second to second. We turned off all of their circuits one at a time, and no one ciruit seemed to be causing a problem, it was more cumulative. I suspected a bad connection in the neutral due to the rapid change on the Telco ground as well as their neutral in the panel that mostly has 110 circuits feeding out of it, the reading on the neutral in that panel is 11-22 amps also rapidly changing up and down. Their electricians came out and said that the load in that panel was not very well balanced, and this is why the return current is high. My question is shouldn't their neutral handle that current, and why is it jumping up and down so rapidly. The power company has tested and done some preventive maintenance on the neutral, and they say it is good up to the point of entrance. Problem for me is I am at the mercy of the power company and our customer's electricians. Any suggestions on how to resolve this issue would be much appreciated.

 

[westflgator]

I am not sure why my attachment won't open.

 

[westflgator]

Here is the detail of the problem

 

[waross]

With your type of load you can expect neutral currents. If a communication grounding expert has done the grounding you may expect part of the neutral current to return via the improper ground connections.
Another possibility is that some of the neutral current is returning to the utility transformer via the ground. If possible, measure the voltage drop on the neutral and see if changes in the neutral voltage drop correlate with changes in the current over the ground conductor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Around here (residential) the phone ground has no direct connection to the power system ground. A separate ground rod is used.

Important question, How is your facility grounded?

Your drawing has no sign of grounding.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[westflgator]

This is in an electrical equipment room, we are required to attach to the MGN via #6 solid copper. Outside at the Pedestal we are also attached to the vertical MGN on the power pole. Our new practice is to connect to the MGN and drive our own ground rod at the ped and attach to it as well. I have the same reading going up the vertical on the power pole outside, leading me to believe we have a good path back the their neutral.

 

[resqcapt19]

Is there a grounding path in the 200 pair cable? If so that path is in parallel with the service grounded conductor because you have bonded to it two locations. This is assuming that main disconnect where you have made the interior bonding connection is the service disconnect.
Don

 

[westflgator]

thanks for the responses, yes this is a sheilded 200 pair calbe. It is standard practice, that when ever possible to connect to the power company MGN to remain at the same potential as them, and to help mitigate noise that is induced into our cable. If we have a separate ground field of our own it makes us more vulnerable to noise due to changes in power exposure, and the fact that they normally have better grounding than we do. In this senario we normally would find very little current on our ground at the BT (Building Terminal), even in our rural routes with long exposures we normally find less than 2 amps flowing on our sheild in any given section. We are connected to their neutral in the disc panel. Is there a better place to connect to the MGN that would reduce current flow on our ground?

 

[resqcapt19]

Is there a better place to connect to the MGN that would reduce current flow on our ground?

Where you connect it won't make any difference. All of the grounding conductors at the building will be bonded back to the grounded conductor (neutral). There is no way to install this with a bond at both the utility pole and the building and not have your shield in parallel with the utility. What is the total current on the service grounded conductor as compared to the current on your cable? It is possible there is a poor connection of some type on the service grounded conductor forcing more current on to your cable?. If there is no problem with the service grounded conductor, would it be possible to add some impedance in your system to limit the current flow.
Don

 

[alehman]

Re-iterating some of the other responses with slightly different words:

Your cable is in parallel with the combination of utility distribution system ground (since you are connected at the pole) and building grounding system. It's not clear if the BT end is connected to a power system NEUTRAL or GROUND.

Waross has a good point:

If a communication grounding expert has done the grounding you may expect part of the neutral current to return via the improper ground connections.

This is a very common error of communications installers.

You said the BT end is connected to the MGN? By MGN, do you mean power system NEUTRAL? This would be a bad idea for several reasons:
1. It is most likely a violation of electrical code because,
2. It creates a safety hazard
3. As you are experiencing, the cable shield will share the neutral current which may disrupt the communications.

Power system neutrals are intended to carry current in normal operation. If you connect a cable in parallel, it will try to share the neutral current. The safety hazard is that it may become energized due to abnormal conditions such as short circuit on the power system and it may disrupt the power system protective functions.


If the cable is connected to a power system GROUND at the BT end, you have a similar set of possible problems:

The power system grounds will almost always have some amount of stray current which would be shared in your 200pr in this arrangement - not a good idea. If there is a short circuit on the power system, the short circuit current return on the power ground. If your cable is in parallel with the ground, some of the short circuit current will flow in your cable, possible destroying it and causing a fire.

I suggest you stop worrying about the power system neutral current and install a proper communications grounding riser (with a single point interconnection to the building power ground at the point of entrance). The cable shield should be connected to that grounding riser near where it enters the building.

 

[waross]

Florescent lamps generate harmonics.
Computer power supples generate harmonics.
Triplen harmonics add on the neutral rather than canceling.
I am surprised that you have so little neutral current flowing.
It sounds like two problems.
1> A telco ground path in parallel with the neutral.
2> Either a fast cycling intermittent single phase load (Maybe a fact of life rather than a problem),
or
a poor connection in one of the parallel conductors.
Can you measure both the neutral current and the ground current simultaneously to see if there is a fluctuating single phase load (both currents track together) or a bad connection shifting the neutral current from the neutral to the ground conductor (neutral current goes up when the ground current goes down and vice versa).

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[westflgator]

Thanks for the info alehman, You have given me some things to check. When our cable comes up in an equipment room we make our customers provide us with a #6 MGN it is supposed to be connected to a good power company ground. In this case it is coming out of the disc panel. I can't see how or where it connected inside the panel. I will see if I can arrange to meet their electrician out there to open the panel and see how they have it connected. If they have it connected to the neutral, I will get them to disconnect it and run us a seperate ground back to their ground at the meter on the outside of the building. Am I understanding you correctly?

Thanks for your help as well resqcapt19, the only place I was able to access the neutral was in the 2 breaker panels shown in the drawing to the right of the disc panel. The panel on the left has very little current flowing on that neutral, but it is mostly 220 ciruits with a balanced load, the other panel has a very rapidly changing 11-22 amps constantly going up and down, just like the current on our ground. That is what lead me to believe they had a bad connection on their neutral somewhere. The power company says they are good up to the point of entrance. I'm not sure I am qualified to answer the question about adding impedance to our ground. Thanks again for your help, unfortunatley our telco noise mitgation gurus haven't been much help so far.

 

[westflgator]

waross, I will take those readings and let you know what I find. Thanks

 

[alehman]

Another thing the electrician should be considering is the possibility of improper/accidental neutral to ground bonds. These are common and cause stray current on grounds, conduits and building steel (with harmonic distortion if present). That can play havoc with communication systems that rely on power system grounds.