From Electrical Safeguards Aid Telecommunications End-users: Because of an Unprecedented Combination of Events Occurring Almost Simultaneously, a New Need is Rapidly Growing to Provide the End-user of Telecommunications Equipment the Protection Really Needed, by Russ Gundrum, Telecommunication Products + Technology
published in October 1983.
“A subscriber loop revolution is already occurring because of active competition in the industry, its forthcoming deregulation, and the amount of electronic equipment that is becoming available. No longer will the business or residential subscriber own just a simple telephone set that is connected to a plain old telephone service (POTS) line. Due to this unprecedented combination of events occurring almost simultaneously, a new need is rapidly growing to provide the end user of this equipment the protection really needed. This is creating confusion over what constitutes the best protection from all standpoints and who is responsible for providing it. This feature provides an update on the latest protection technology that is available, explains why it may be necessary in some locations, and suggests why the end user will probably own it.” (Remember, this was written right before 1/1/84 when Ma Bell was broken up by the US government!)
“There are three external influences that can act on a piece of telecom equipment and cause a problem:
1. Disturbances such as electrostatic voltages, electromagnetic pulses (EMP) and radio interference (RFI) that act directly on the equipment;
2. Disturbances such as noise, voltage fluctuations and complete outages that occur directly on the AC supply line (if one is required);
3. Disturbances such as noise and AC voltages due to power lines or lightning discharges that are induced onto the telecommunication’s line.”
“To adequately protect against disturbances that act directly on the equipment usually involves shielding and elaborate grounding schemes of the metallic case housing the equipment. Certain design modifications, including filtering schemes, may also be required. These are usually the responsibility of the manufacturer and persons that install and maintain the equipment.”
“Disturbances acting directly on the AC supply line are a serious problem today for computers and telecommunications equipment, since most employ electronic designs that make them sensitive to electrical impulses. For this reason, they require a pure, sinusoidal and constant AC power source in order to operate properly. Expecting the power utility to supply this 100% of the time is hopeful, but completely impractical. Power line noise transients, lightning, voltage fluctuations and entire outages (blackouts) do occur and the equipment should be designed to tolerate this ‘real world’ environment.”
“Even if the equipment is UL (Underwriter’s Laboratories) approved, it doesn’t mean that this expensive protection has been provided. Thus, the user is faced with several alternative actions, which must be considered. It is possible to select relatively inexpensive ($35-$50) and simple gas discharge arrestors which are spark-gap devices that clamp or limit the voltage to a certain pre-determined level or graduate to the more sophisticated and expensive devices that may employ the gas discharge tube along with solid-state components, such as metal oxide varistors (MOVs) or zener diodes. A key selling point made for these protective devices is their response time to fast, high-energy transients that operate in the nanosecond range or nearly instantaneously. They claim advantages over the even more expensive type of power conditioners or voltage regulators which use filters and isolation transformers to provide the protective mechanism. Of course some situations demand the ultimate in isolation, an uninterruptable power supply (UPS). This system can provide complete protection against practically every type of power disturbance, including blackouts, but can be expensive.”
“Like the AC supply line disturbance situation, there are similar problems and solutions with induced AC voltages and noise transients acting on the telecommunication line. In the United States, the National Electrical Code requires telephone utilities to provide protective devices on lines appearing at the customer’s premise that are exposed to more than 300 volts rms AC (430 volts peak), for safety reasons. Thus, the ‘primary protector’ that is provided by the telephone company is designed to generally start operating around this voltage level.”
“These devices have historically been spark gap discharge arrestors of either gas tube or carbon block design. However, they also suffer from time-delay limitations and only operate when the voltage reaches the protector’s design rating, which may be well beyond what the equipment can withstand. Also, the unwanted voltage is usually short circuited to ground. This may temporarily remove the circuit from service and usually disconnects the talking parties! Another problem with gas tube protectors has been persistent continuous voltages that they could not handle and that could even ‘noise’ up other circuits when one was in such a ‘glow-mode’ operation.”
“The end user’s objective is to receive only pure DC voltages, the signaling frequencies and the message information that is being transmitted over the telephone company’s facilities. This is impossible to achieve, particularly on long loops serving rural-type environments, since they will have AC induced voltages on them of a varying nature (depending on the power loads or under storm conditions). If, after spending a lot of money to solve these low-voltage surge problems, all the protection problems still have not been solved, more money will have to be spent to correct them. And, of course, the telecom equipment experiencing these problems is FCC registered! Does this sound similar to the power company situation and the UL-approved equipment that was experiencing problems?”
“Many people may feel that if their equipment meets the requirements of the Federal Communications Commission (FCC) Part 68—this ruling defines standards for the design, manufacture and installation of equipment that connects directly to the public switched telephone network (PSTN)—their equipment should never experience any induction or voltage surge problem. However, as pointed out in a September, 1980 FCC ruling, there are no requirements in Part 68 to protect terminal equipment from voltage or current damages originating on the telephone company’s side of the demarcation point. Part 68, therefore, was written primarily to ‘prevent harm to the network,’ not vice versa. So any ‘secondary’ protective devices that are used on customer owned and maintained equipment does not have to be FCC approved and registered.”
“An interesting element in all of this, of course, is who pays for this additional protective device?”
“Not only is there big business in repair and return of equipment, but this is a competitive terminal equipment market. It costs money to design the equipment with the right type of protective devices for all situations. If many subscriber loops don’t have these types of problems, then why should all equipment be so designed? Also, Part 68.214 requires recertification if changes are made to registered equipment, which many manufacturers are obviously reluctant to do.”
“Most people think that phone wires are safe to work on, and the majority of them are. But Bell Telephone Laboratories performed a random survey of noisy loops in 1971 and found that 10% of all telephone lines could have up to 115 volts of steady-state induced AC voltage and 10 mA of current flow. Most people would consider these lines too ‘hot’ to work on!”
“Special protective devices were developed long ago by the telephone company to use on their open wire lines and for lines entering power substations and generating plants. Today, that same protection technology has been redesigned and improved to conform to the stringent technical performance criteria of the modern day telecommunications plant. For the past 13 years, telephone, railroad and power utility communication and signaling circuits around the world have used this equipment, which is produced by the SNC Manufacturing Company in Oshkosh, Wisconsin. More than 500,000 lines are in service today because of this equipment.” (Remember, this was written in October 1983)
“SNC has recently packaged its equipment with standard 25 pair connectors for easy connection into the public switched telephone network. Known as the Telecommunication Interference Filter (TIF), it will be installed by the customer, between the telephone company’s demarcation point or jack and the customer’s equipment that he wishes to protect. The TIF unit is offered in standard sizes designed for treating up to 6, 12, 18, 25, 50 and 100 pairs. The TIF is a multipair longitudinal choke, commonly referred to as the Induction Neutralizing Transformer (INT). Each TIF has the first pair wired to a Transformer Exciting Network (TEN), which is a specially designed drainage reactor tuned to 60 Hz.”
“The TIF units can also provide up to 30 dBrnc (decibels with reference noise C-message weighted) reductions in power influence (harmonic noise) levels that could be acting on any equipment unbalance to cause a circuit noise problem. Additional circuit noise reductions can be obtained because the TIFs can ‘mask’ or improve the longitudinal balance performance characteristics of the equipment it is used with. Typically the TIF’s balance is greater than 70 dB through the voice-band frequencies. Last, but not least—especially for data circuits—the TIFs can provide substantial reductions in impulse noise levels that can act on the terminal equipment to cause errors and require retransmission due to garbled, uninterpretable data. All of this may sound too good to be true for costs as little as $23 per circuit. Furthermore, the equipment has the advantages of simplicity, no power requirements (a passive device), no maintenance, continuous operation, can pass DC, is extremely reliable and easy to install.” (Also, no time-delay response and no clamping the circuit to ground to cause a circuit outage.)