From Power Protection: The Use of Secondary Protectors can help Suppress Noise-to-Ground and Fault-induced Voltages. Part 2, by Russ Gundrum, Telephone Engineer & Management
published on February 1, 1985.
“As electrical power demands increased and loads were further away from the substation or generation point, telco facilities experienced not only greater levels of 60 Hz induction, but increased harmonic influence as well. More rectifiers and nonlinear harmonic producing loads were used, along with the utilities’ application of power factor correction capacitor banks. Depending upon the source of harmonics and the location of a grounded capacitor bank, resonant conditions could be set up that could magnify a high power influence condition over a wide area of telco facilities.”
“The report had earlier mentioned that there was little advantage to buried cable from the standpoint of interference mitigation. It also stated that ‘neutralizing transformers can be very effective. These devices have the advantages of providing mitigation of the line-to-ground voltage of a communication circuit, as well as the line-to-line noise. Thus, these devices help to reduce personnel shock hazards as well as mitigate noise. Both open wire and cable circuits can be helped by the proper use of this equipment.”
“Finally, the EPRI report stated that the replacement of open wire or cable circuits for power line co-sitings with railroads that exceed about 10 miles, installation of a microwave system becomes economically viable. ‘Here, the primary considerations may no longer be AC interference, since cable/carrier can be designed to work satisfactorily in a power line environment.’ It also recommended that the replacement of existing facilities with fiber optics ‘ should probably be approached cautiously, since the technology is advancing, costs are decreasing and experience is limited. Fiber optics is certainly not a mature technology such as microwave.”
“One application that is usually a sure bet for fiber optics is in an extremely hostile electrical environment such as power generating plants or substations. Even in this situation, properly designed and maintained wireline facilities can still be economically utilized today. SNC Manufacturing Co. builds a number of high voltage neutralizing (HVNTs), isolation and drainage transformers for these special applications. HVNTs can cost as little as $300 per pair. However, it is recognized that the size and weight of these units, the engineering calculations required to specify the right voltage rating, plus the careful installation and maintenance procedures that are necessary to insure successful operation, all contribute to making their application possibly less attractive when compared to alternative approaches.”
“For instance, one could install a 23 GHz short-haul microwave system, a short fiber optic cable link or even a shorter (six inches) air-gap device (SNC’s Lyte Lynx or Positron) that would totally solve the problems associated with Ground Potential Rise (GPR) in these environments. (Used also at cell-phone towers) However, the power fault that caused this GPR situation in the first place, can induce sufficient AC voltage on the telco’s wireline facility leading into this alternative’s interface point to damage the electronics on the central office side or cause a signaling malfunction to occur. This longitudinal induction could also cause other problems such as cable pair insulation breakdown, carrier repeater or central office equipment failure or protector operation. Many power company telecom circuits are critical and must not fail. The power company needs to know what fault caused this induction and/or GPR, and needs to do something about it at the same time.” (Otherwise you could have a Northeast blackout type scenario!)
“How susceptible is any manufacturer’s equipment, not only to AC voltages or currents from normal, steady-state power line induction conditions, but under abnormal, transient or surge induced conditions resulting from power faults, many of which are initiated by lightning? The concerns should be more than just on equipment damages or malfunctions resulting in false circuit operations (such as burglar and fire alarms), because today’s requirements for noise-free data transmission, sensitivities to impulses and steady-state noise is very important. Therefore, while GPR problems associated with power stations can be solved by new state-of-the-art equipment, the longitudinal induction problems affecting this equipment can still remain. These can all be easily, quickly and economically solved with a multi-pair Induction Neutralizing Transformer (INT). Introduced by SNC in 1969, standard off-the-shelf units are available for handling up to 600 volts rms (without operating a protector) for as little as $15 a pair. SNC has manufactured special INTs up to a 1200 volt rating. The practical design limit for individual, dry-type units is about 5 Kv. Standard INTs can also be placed in tandem (in series) with one another to double or triple, etc. the overall voltage suppression capability.”
“The beauty of the INT application in this special situation is that it does not have to be as heavy, bulky and expensive as an HVNT unit, because one no longer needs to take the GPR into account! The INT can also be located anywhere in the outside plant cable (or even the central office) and still be effective. The field or remote ground of the INT’s exciting pair (which may or may not be required, depending upon the INT’s location) could be located, as required, anywhere outside the zone of influence (300 volt point) surrounding the ground mat of the power station. This field ground could be in the form of a specially designed drainage reactor called a Transformer Exciting Network (TEN) placed on a working line serving the substation equipment. This is a useful, cost-effective technique when a dedicated hard wire exciting pair is not available.”
“Locating INTs from a maximum noise reduction standpoint is a lot like maximizing your return from a real estate investment. Generally the three most important things to remember are location, location and location! For instance, it can make a big difference where not only the INT is placed in the circuit, but where its exciting pair grounds are located and even the number of INTs that are employed. This information is covered in the abc of the Telephone, Volume 14 on Power Line Interference: Problems and Solutions that was written in 1982 by this author.”
“The above technique is similar to cable loading. Someone once decided that mitigating the effects of capacitance of a shielded cable pair was worth the cost of load coils (adding metallic inductance). Placing the coils at specific intervals offsets additional transmission losses and the frequency-response degradation of voice-frequency signals over long facilities.”
“Had the early designers of the telephone system also taken a similar approach with lumped longitudinal inductance (INTs) at specific intervals along a long, metallic facility, there would probably be few of the problems that are discussed in this article and the industry would not be in the predicament it finds itself in today.”
“The REA, in its Telecommunications Engineering and Construction Manual (TE&CM) 451.8 dated February 1981, states that problems with noise in central office equipment, subscriber owned equipment, or even test equipment, can be caused by excessive induced longitudinal 60 Hz voltage on the cable conductors. ‘This type of problem must be identified so that appropriate remedial measures can be taken. The normal noise mitigation procedures available to the telephone company, such as improved shielding, better grounding, etc. will not be effective. Since the problem can also be related to power system balance, it should be discussed with the power company representatives. They can determine if the system can be rebalanced to provide long term relief. Power system loads, especially during periods of high demand for air conditioning and heating are unpredictable and can change quite rapidly. Such conditions are beyond the control of the power companies in maintaining good system balance. In areas where these conditions exist, the mitigation can only be accomplished by the telephone company.’” (Quite frankly, this includes a lot of areas, especially where you have single phase power exposures!)
“The REA practice goes on to mention the two ways that are available to reduce the level of induction. The appropriate measure is determined by the number of cable pairs involved and/or the level of induced voltage-to-ground. ‘When there are many circuits involved or where the recorded or anticipated induced voltage-to-ground is 50 volts or higher, a neutralizing transformer should be installed along the cable. The application of neutralizing transformers is discussed in TE&CM Section 451.5. In those cases which occur infrequently, where only a few circuits are involved and the anticipated voltage-to-ground is below 50 volts, longitudinal chokes can be utilized, as discussed in TE&CM Section 451.4.’” (A word of caution here about chokes. These single pair devices are strictly current limiting only. They do not reduce the induced voltage. In fact, they can increase the amount of voltage on a cable pair, and if the induction level is higher than their voltage rating, the units can saturate and generate noise! They also need to be located at the end of the circuit where there is a path-to-ground for longitudinal current to flow, or used in conjunction with a TEN, which makes for an expensive cost per pair mitigative solution. This low impedance path-to-ground condition used to be at the central office end, but now that most offices “float” the central office battery from ground, the chokes are now ineffective. Therefore, there are very few applications for this device in the network.)
“Both the INT and noise choke are built into a convenient carrying case and equipped with plug-in terminals and test leads. This allows a craftsperson or engineer to quickly identify which device will work the best for his particular problem. Known as the SNC HumZapper, this test set is reported to be the only one on the market that can actually measure noise out of a circuit! The noise set shows exactly how much noise reduction was achieved by the HumZapper when the circuit became quiet. It gives the before and after results and an indication of the improvement that can be expected with a particular device at that point in the circuit before permanent equipment is ordered and installed.”
“A number of the interconnects and service-oriented firms that are sprouting up are also utilizing the HumZapper. They realize that there are many situations where they may be losing maintenance contracts or desire to cut expenses by minimizing maintenance calls. They try the equipment in the HumZapper to see if it solves their customer’s problem—especially if they have been unsuccessful working with the telephone company and they feel the problem is in the line. In this typical ‘finger-pointing’ situation, the telco says the problem is in the customer premises equipment and the subscriber or interconnect says it’s in the telco facility. After proving with the HumZapper which equipment works the best, the customer can specify the appropriate mounting and packaging for the number of circuits desired.”
“For instance, if the customer has a multi-line key or PABX equipment, an economical and effective unit for all induction conditions is the SNC Telecommunication Interference Filter (TIF), which is essentially an INT pre-wired to 25 pair connectors for easy connection to the demarcation block. For a customer with only a single line to treat, a Single Noise Interference Xterminator (SNIX) is probably all that’s needed. It is especially useful if radio frequency interference (RFI) problems exist and/or a data modem is being connected to a personal computer. The SNIX is essentially a noise choke with a drainage reactor, similar to the TEN, that can come with either binding post terminals, 30 inch leads or RJ-11 type modular jacks for easy connection.”
“It must be pointed out that not only is the SNIX more expensive on a cost per pair basis than the TIF, it can only suppress up to about 30 volts or 50 volts if the SNIX-Heavy Duty unit is used, while the TIF exceeds 300 volts. The TIF can suppress over 95% of all noise-to-ground (power influence or harmonic voltages) in addition to the low frequency steady-state or fault induced voltages. This is all done instantaneously and on a continuous basis up to the value of the primary protector provided by the telco. The FCC ruled in 1980 that this equipment is essentially like ‘secondary’ protectors, and thus does not have to be FCC registered. However, as opposed to other secondary protectors on the market, the TIF does not clamp the circuit and take the voltage to ground. Therefore, the circuit is unaffected and is still in use.”
“While the application of the TIF or SNIX may definitely result in a significant noise reduction to the customer, this may not always occur since the mitigation equipment is placed at the subscriber end of the circuit. Any circuit unbalances in the telco facility, which is still exposed to the induction, could cause metallic noise that would pass through the TIF or SNIX. Only the telco can solve that problem, possibly by placing similar type mitigation equipment in their cable facility or at the central office end of the circuit. Had this been done in the first place, the customer might not have needed a SNIX or TIF! Isn’t that the big dilemma out there? Who’s responsibility is it to do these things? You know who is ultimately going to pay for this grade of service? The subscriber, one way or another—if he is still using telco facilities, that is! And that, after all, is the bottom line, isn’t it?”