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Frequently Asked Questions

Induced A/C Rural Hum and Copper Line Noise Solutions

1. What is Ground Potential Rise (GPR)?

At a power substation:
When a power system fault to ground occurs in the power network, some of the fault current flows back to its source (the power substation). When this current reaches the substation ground grid there is a potential difference between the ground grid and remote earth. When a metallic communications conductor (such as a copper phone wire) enters the substation it is vulnerable to the stress of this GPR voltage.

A difference of potential (60Hz) between the two ends of a conductor that is exposed to a power fault or lightning.


2. What is AC induction ?

AC induction is the measured 60Hz voltage between earth and either the tip conductor or the ring conductor. These two readings should be the same. This is normally measured with a Fluke-type VOM (Voltohmeter) and can be measured either flat-weighted or C-Msg Weighted.


3. What is Power Influence?

Power Influence (also referred to as longitudinal induction) is the harmonic voltage (non 60Hz) that exists between earth and either the tip or the ring conductor at the end of the inductive exposure. It is normally measured with some type of noise measuring meter, i.e. Wilcom T136; Wilcom T132B or EZ; Triplett Models 2, 4, 5, or 7.


4. What is an exciting pair?

An exciting pair is the pair in an Induction Neutralizing Transformer (INT) that is grounded at both ends of the exposure that you intend to neutralize. It is necessary to ground one of the pairs that are going through the INT (a spare pair that is not in service) in order to "excite" the INT. If a spare pair is not available an SNC TEN can be used to excite the INT.


5. I reduced my power influence (PI) using the SNC SNIX. Why does the circuit noise (CN) stay the same?

Several reasons are possible:
a. You forgot to connect one of the yellow wires on the SNIX (either one) to ground, or:

b. The point at which the PI was converted to CN took place ahead of where the SNIX is installed on the telephone pair. The SNIX must be relocated and placed ahead of where this conversion to CN takes place. Once the PI is converted to CN there is no known method of "deconverting" the noise.


6. I don't have any test equipment other than a "butt-set." Is there something I can use to help me with my noise problems?

Yes, you can use an SNC Li'l Zapper or SNC  HumZapper.


7. I've hear the term "circuit balance" used. What does it mean?

Circuit balance, sometimes referred to as longitudinal balance, is the capability of the copper phone pairs to mitigate (reduce) the effects of unwanted longitudinal energy (energy coupled from the power lines that run parallel to the copper phone pairs). Balance is calculated by subtracting circuit noise (CN) from power influence (PI). (PI-CN=Circuit Balance)


8. What is the balance figure I should expect to see on an acceptable copper pair?

On a loaded copper pair you should expect a balance reading of 60 to 70. On non-loaded pairs the reading should be 70 or higher.


9. How much power influence (PI) is acceptable?

Most telephone companies are shooting for 80 dBrnc. The meter faces on today's noise measuring sets will indicate 80 dBrnc to be the demarcation between "acceptable" (green) and "marginal" (yellow). Ninety dBrnc or higher is considered unacceptable.


10. Can I get an accurate balance measurement if my power influence (PI) is less than 70 dBrnc?

Probably not, because disturbing energy is needed to locate various types of cable problems that cause unacceptable balance readings. This usually requires PI readings in excess of 70 dBrnc. There is a test equipment manufacturer (Tempo Research) that has devised a test set that applies a strong (100dbrnc) disturbing tone to the circuit under test.


11. What are the symptoms of power influence (PI) problems?

False rings or signals.
Equipment shutdown or damage.
Noisy subscriber lines or noisy trunks (PBX).
Dialing errors (mutilated digits).
Facility is "Hot" (electrically speaking) to the touch.
Protectors or fuses operate more often than necessary.
Insufficient loop current.The induction subtracts from the loop current. This results in a situation that appears to be a "loss" of loop current from an equipment standpoint.
Time-of-day or seasonal problems.


12. What are the symptoms of power influence (PI) problems to carrier systems?

Analog carrier systems do not function properly.
Analog carrier circuits are noisy
Impulse noise on data circuits.
Mysterious failures of different time durations.


13. What do you mean by coupling?

Coupling is the mechanism whereby residual energy on the power lines is transferred to the paralleling phone lines.


14. What is residual energy?

It is the uncancelled energy from the power lines that exists between the phase wires, the neutral, and the ground return path. This uncancelled energy is what "couples" onto the copper phone lines. It is sometimes referred to as "earth return currents" and it is constantly varying in amplitude.


15. You talk about induced AC. What factors go into its make-up?

Voltage induced measured in volts. (Vind)
The mutual impedance of the earth in ohm/Kft or ohms/Km. (Zm).
The magnitude of the induced (ground return) current in amperes. (Igr).
The distance (length) that the power system and communications system parallel one another in Kft or Km.
(L). Shield factor of the cable. (This is a non-factor at 60Hz). (Sf).


Note: After you do your calculations you should measure the voltage because there are so many factors that can affect the readings. Example: earth resistivity that is a part of Zm.


16. What is Mutual Impedance?

Mutual impedance (Zm) is the resistive component of Ohm's law formula, applied to the AC induction between the power facility and the communications facility. It is expressed in ohms/Kft.


17. How do I keep AC voltage off of my cable shield?

Open it at every splice. Just kidding! Actually you want AC current (also known as shield current) on your shield. This AC current will help to control the effects of harmonic noise. Sorry, this doesn't apply to 60Hz, because the shield does not work effectively (about 3-5%) at 60Hz.


18. What will reduce the unwanted 60Hz voltages on my cable facility?

An Induction Neutralizing Transformer (INT).


19. How many INTs do I need to place in my cable to control induced 60Hz voltage?

The number of INTs needed depends upon the amount of induced voltage and the limits for induced voltage set by your company. Usually only one INT will do the job.


20. Where do I place the INT to control 60 Hz voltage in my cable facility ?

The INT could be placed either in the central office, at the center-of-exposure (point where the induced voltage is 50% of the total induced voltage), or at the customer's location. There is really no set answer to this question because it depends on what you're using the INT for and your particular situation. However, there are test procedures and rules to follow that will help you determine the best location for an INT for the problem you're up against. If the INT cures your problem, you have placed it correctly no matter what someone else tells you. You can also call us at SNC and we will work with you on your testing and installation. (Our toll-free technical support number is 800-558- 3325 or 920-231-7370.)


21. Where do I place an INT to control harmonic noise on my facilities?

The placement of an INT to control circuit noise requires testing. While testing is "BORING, TEDIOUS and TIME CONSUMING," it is critical to success in finding the proper location for an INT.

Assuming all the initial legwork has been done (grounding and bonding, etc.) and that an INT will be needed, a good way to start is to employ an SNC HumZapper. Insert the HumZapper into the circuit ahead of the conversion point in the facility (unbalance), if known.  Remember, the conversion point is where the power influence (PI) is converted to circuit noise (CN).

b.   Use the HumZapper to determine where the conversion point is.

c.  In cases where there is no definitive conversion point, but the problem is high PI, a starting point could be somewhere near the point where the power influence is 70 to 80 dBrnc.

If all of this confuses you call SNC for assistance.


22. How many INTs do I need to place in my cable to control harmonic noise?

This is a complex question. Please call SNC at 800-558-3325 or 920-231-7370 and we'll work with you in coming up with an economical solution for your particular case of trouble.


23. I placed an INT in my cable and I still have my noise problem. Why?

There are several possibilities.

No excitation pair or exciting pair is open.
A ground (faulty pair or pairs) somewhere near the INT.
All the pairs are not going through the INT in the same direction (binder group reversed, etc.).
Installed in the wrong location.
Trying to mitigate multiple exposures in the same INT.
DC saturation of the INT (one-way DC in the INT).


24. Do I need an exciting pair to make an INT work ?

Yes, if you are trying to control 60 Hz voltage. In the case of harmonic frequency noise you may or may not need a TEN. If there is enough longitudinal current available for the INT to self- excite then a TEN may not be needed.

A bit of caution here. If a spare pair in the INT is used to self-excite the INT it must be grounded at both ends of the exposure. This spare pair should be labeled in some way so that someone in the future doesn't come along and put that pair in service - doing so would disable the INT and your problems would return. The beauty of using a TEN is that the TEN will excite the INT without having to use a spare pair (if a spare pair is even available).


25. My cable has no spare pairs to excite an INT, what can I do?

Using a TEN or a SUPER TEN will solve this problem. In cases where the telephone switching machine is an AT&T 5E, it may require two TENs or two SUPER TENs.


26. I installed noise chokes at my customer location and they didn't reduce the noise. What happened?

A choke needs current to make it work. There was probably not enough longitudinal current available at the customer location to excite the choke. The noise was already present at the network interface. This is why chokes are usually installed at the Central Office.


27. Why does a SNIX work when a noise choke won't?

Unlike a standalone choke, a SNIX has both the "in-the-pair" connection (like a choke) as well as a drainage arrangement for longitudinal currents. As a result the SNIX limits both current and voltage at the end-user location.


28. What does the 70 stand for on a 70-type noise choke?

The 70 is the amount of longitudinal 60 Hz voltage that will cause the choke to begin to saturate. In other words, a 70-volt choke is effective up to 70 volts of longitudinal 60 Hz voltage.


29. I have a small key system that has operating problems when the 60Hz voltage on the pairs is between 10 and 20 volts. What can I do to economically solve this problem ?

The placement of a SNIX at the network interface will probably solve the problem. If the voltage is in the 30 to 50 volt range a SUPER SNIX will probably work best.


30. How can I get one of your Noise & Protection Solutions catalog?

Call us at 800-558-3325 or 920-231-7370. Or fax your mailing information to us at 920-231-1090.


31. Do you make protection equipment for substation environments?

Yes. We have provided High Voltage Neutralizing transformers and Isolation Transformers to the utilities and communications industries for more than 20 years. We've also developed a modular protection system (Lyte Lynx®) that incorporates this magnetics approach along with a more contemporary approach that employs electronics and fiber optics. This equipment is discussed in more detail in the High Voltage Protection section of this home page, or you can call us for our most recent literature.


32. What kind of problems do steady-state AC induced voltages cause?

Equipment malfunctions.
Equipment damage.
Electrical safety hazards.


33. My digital carrier regenerators are protected with gas tubes in and out. Why are they continually being damaged during adverse weather conditions?

The reason they become damaged during storms is because the voltage-to-ground is often too low to operate the protectors. The result is that damage occurs to the regenerator's resistors, thyristors or power supply zeners as longitudinal current flows through the regenerator.

For example, on a carrier route with paralleling power for two or three miles, the protectors at each end of the exposure will operate during a power fault. This causes a momentary low longitudinal impedance at each end while reducing the voltage-to-ground at the middle regenerator to a level that is insufficient to fire its protectors. Damaging surge current flows through the circuitry of this regenerator because the voltage-to-ground is not high enough to operate the regenerator’s protectors.

A second possibility is if the protection on only one side were to operate, then "large" (for the electronics) amounts of induced current will flow through the regenerator to the operated protector.


34. How can I protect my digital carrier systems from the effects of induced AC or weather related problems?

A properly placed Digital Induction Neutralizing Transformer (DINT) reduces the magnitude of surge current, saving regenerators and office fuses, which keeps the system operational.


35. What are the symptoms of AC induction in connection with digital carrier systems?

Framing errors.
Momentary or mysterious carrier failures including the spare span line.
Excessive protector operation at regenerator locations and RT's.
Unable to do single-ended testing on the facility.
Excessive operation of line fuses in the Central Office.


36. Why am I experiencing a rise in power influence (PI) during certain times of the day or season?

These "time of day" or "seasonal" occurrences result from peak load or surge conditions on the power system. For instance, increased air conditioning loads in the summer months might cause a rise in PI, as may the startup of a factory that draws a lot of power at certain times of the day to run heavy equipment (such as three phase motors).


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