Communication Markets Division © 3M 2007. All Rights Reserved. Telecom Visit for Noise Mitigation Improve Network Performance with 3M™ Solutions

Communication Markets Division

© 3M 2007. All Rights Reserved.

Telecom Visit for Noise Mitigation

Improve Network Performance with 3M™ Solutions

2© 3M 2007. All Rights Reserved.


Overview

While visiting a Telco in Wisconsin, I was able to support the diagnostic review of a noisy circuit. A 965AMS was used to gather the data

Two formulas are used in this presentation and both indicate shielding issues.

Suggestion - Steps should be taken to identify bonding and grounding issues.



965 AMS Readings of Circuit

75 dBrnC

33 dBrnC

108 dBrnC

First Test indicates a good balanced pair. The high PI indicates shield, grounding or a power problem.

Second test takes a flat filter PI reading and subtracts the PI reading measured with a C message filter. 33dBmC suggests a power quality problem.

80 dBrnC

33 dBrnC

113 dBrnC 113

-108

= 5

≥30 dBrn Suggest a Power Quality Problem

≥20 dBrn but ≤ 30 dBrn Bonding, Grounding and Power Quality Problem

≤20 dBrn Bonding and Grounding Problem



Corrective Action

Create a Map or use a paper copy of the print Visit closures, splices, cabinets and all other

accessible cable opening to: Verify all shields are bonded together Verify grounds are connected

Take readings at each location: AC Voltage PI and CN VBSA (Follow and chart strongest harmonics)



Bonding and Grounding Recommendations

Inspect Bonding and Grounding. Bond all plant “CO” side and “Field” side together. Ground strategically to reduce noise. Verify grounding conditions. Use 6awg for anything greater then 5 lines. Solid is preferred when buried. Use 10 awg for home NI grounding under 5 lines. Stay away from 12 awg for grounding. Use 4460-D or 4462 Shield Bond Connectors for cable sheaths.



Bonding and Grounding

When possible, bond telephone ground rods to power ground rods or use power ground rods. (This provides a return path for induced current)

Bond all shields (Exchange to Distribution at XB)

• Plant Engineers may decide to not ground based on other considerations: i.e.: Dairy Farms, long aerial drop wires fed from buried plant, etc…

• Plant Engineers may add more grounding based on power company distribution.



Open or Corroded Shields•Use the Opens By Ratio Formula.



Voice Band Spectrum Analyzer

1

2

3

Press or to start

measuring on an inactive pair.

You can also dial quiet

termination to get readings.



Voice Band Spectrum Analyzer

Toggles between and .



Selects to measure PI and CN.

Selects for 0 to 2560 or 0 to 8 kHz.



Readings From Telco

All readings taken had similar harmonic influence.



Looking at the Harmonics

180

300

420

540Peak harmonics to keep note of.

While the test formulas point to shielding issues it is good to track harmonics. These readings suggest potential problems with transformers and /or capacitor banks.



Harmonic Signature Typical Diagnosis

60 Hz Badly out-of-balance three-phase power lines, or single- or two-phase power lines, or poor neutral conductivity. Poor balance does NOT create 540 Hz problems but it can amplify the 9th harmonic.

180 Hz Leading power factor in the power system or a saturated transformer.300 Hz is symptom of poor balance in the power system often noted in conjunction with high 60 Hz interference

300 Hz with 420 Hz and even harmonics

May be caused by 6-pulse rectifiers or an impedance irregularity in the power distribution system

300 Hz and/or 420 Hz together with 660 Hz and/or 780 Hz signals

Often the result of more capacitance on some phases than others in the power distribution system. This condition may be caused by a blown fuse at a capacitor bank or capacitor banks installed on single or two phase power lines.

420 Hz and/or 300 Hz with 660 Hz and/or 780Hz

are of Often the result of more capacitance on one or more phases than on the other phase conductors in the power distribution system. This may be caused by a blown fuse at a capacitor bank or capacitors installed on single- or two-phase circuits or buried power distribution lines.

540 Hz The result is power system circuit resonance. This may be made much worse by oversized shunt capacitor banks or buried power lines. A circuit is considered resonant when any one harmonic, generally 540 Hz, is 6 dB higher than the next highest harmonic (very common condition)

660 Hz with 780 Hz with even harmonics

The result of 12-pulse rectifiers

660 Hz and/or 300 Hz with 420 Hz and/or 780 Hz

Often the result of an impedance disparity. This can be a result of more capacitance on one or more phases than on the other phase conductors in the power distribution system. This may be caused, in turn, by a defective capacitor bank or capacitors installed on single- or two-phase circuits or buried power distribution lines.

780 Hz and/or 300 Hz with 420 Hz and/or 660 Hz

Often the result of more capacitance on one or more phases than on the other phase conductors in the power distribution system. This condition may be caused by a blown fuse at a capacitor bank or capacitors installed on single- or two-phase circuits or buried power distribution lines.

900 Hz Common when the area being served has high impedance ground connections combined with numerous buried power distribution lines. This could also be caused by transformers wired in an open “Y” configuration.

1020 Hz Common if the area being served has high impedance ground connections combined with numerous buried power distribution lines.

Documents

Communication Markets Division © 3M 2007. All Rights Reserved. Telecom Visit for Noise Mitigation Improve Network Performance with 3M™ Solutions