From Get “RID” of Glitches: With Customer-Owned Equipment and Use of Telephone Lines for Data, it is More Important than ever to Eliminate Noise and Disturbances, by Russ Gundrum, Telephone Engineer & Management
published on October 15, 1985.
“Is the problem in the manufacturer’s equipment, in the interconnect company’s wiring and installation, in the telephone company’s local network, or in the long distance carrier’s facility?”
“Due to increased sensitivities of newer electronic devices being utilized in the telecom business, very low levels of longitudinal AC can make the equipment fail or give sporadic operational malfunctions and ‘glitches’ in transmission. Higher level surges or transients from power lines or lightning, can even damage the equipment before the telco-provided ‘primary’ protector operates. Whether this spark-gap type protector is a carbon block or a gas tube, the device usually doesn’t operate until it sees more than 300 volts—as required by the National Electrical Code.”
“This 300-volt level of ‘brute force’ protection really has nothing to do with response time although many manufacturers are fond of promoting the speed of operation with solid-state type protectors or diverters, such as Metal Oxide Varistors (MOVs), diodes, etc. This 300-volt design criteria is mainly one of electrical safety and is aimed at preventing basic telephone instruments from being ‘blown off the wall’ or burning up.”
“As the author pointed out in a recent series of articles on power protection (TE&M Dec. 15, 1984; Feb. 1, 1985 and March 15, 1985), one does not have to resort to alternative means of services that are totally immune from these disturbances, such as fiber optics. Nor does the situation have to become so bad that the end user feels he has to unplug his equipment from the line!”
“SNC Manufacturing Co., Oshkosh, WI has recently introduced a new product, the RID, aimed at solving many of these problems. Although the product’s name stands for Radio Interference Damper, the device is really a very broadband filter that can help ‘get rid of’ low frequency (60 Hz) steady-state or surge induced AC currents and voice frequency harmonics (noise). The RID is simply composed of one of SNC’s long standing products, the 35 or 70 volt noise choke, that is specially packaged in a small black box with 8 position modular jacks for easy connection of personal computer data cords (RJ-45) or standard RJ-11 telephone plugs for the public switched network.”
“The two models both come equipped with a standard 3-prong AC electrical plug for easy mounting in an unused AC outlet near the telecom equipment to be protected. Not only does this provide a convenient way for the consumer to mount the unit (user friendly!), but the AC outlet green wire ground provides the necessary ground path to assure the RID’s maximum effectiveness. The parallel prongs are dummies used only for mounting support. This inconspicuous mounting also accomplishes a critical function by placing the filter as close as possible to the affected terminal equipment—especially if RFI is being picked up on the unshielded inside wiring and if the station set is the ‘demodulator’ of the radio signal.”
“While the SNC noise choke that is built into the RID has been successfully utilized by telcos over the last ten years, it must be realized that these applications were typically at the central office or at a PBX/key equipment location where there was a low impedance longitudinal path-to-ground for the induced AC current to flow. Normally this condition does not exist with ordinary telephone sets at 60 Hz and its voice-frequency harmonics, so the device is usually not effective when placed at the subscriber end of the circuit. However, the RID does provide a capacitive path-to-ground at the RFI frequencies, and hence, is very effective at the higher frequency end of the interference spectrum.”
“Of course, if there is a low longitudinal impedance path-to-ground at the subscriber end, such assome data modems, alarm circuits and cordless telephone base stations, then the RID can be very effective in reducing low-frequency induced currents up to its rating of 35 or 70 volts. The heavy duty RID model (70M) recently has been found to be effective in allowing a large personal computer manufacturer’s data modems to work properly when hooked up to local phone lines. A major insurance company based in Illinois has purchased more than 1200 RIDs to be provided to their agents around the country when they experience such problems.”
“As mentioned in the series of power protection articles, if the customer-provided equipment has a high impedance path-to-ground where the RID would not be effective, then the Single Noise Interference Xterminator (SNIX) would have to be used. The SNIX is an AC voltage and current limiting device for single line applications having up to 30 volts of induced AC. The SNIX-Heavy Duty will handle 50 volts. For multi-pair applications and for lines having up to 350 volts AC, the Telecommunication Interference Filter (TIF) can be utilized.”
“The RID, SNIX and TIF products are all shown mounted on a product demonstration panel in Figure 1. This demo unit is used in SNC’s many seminars on power line interference problems and solutions that more than 2,000 telco personnel have attended. The before and after results demonstrated by the panel not only show the effectiveness of the product’s performance from a steady-state induction and noise standpoint, but with the dual-trace oscilloscope, their instantaneous response to transient/surge conditions can be shown. It also demonstrates that all these ‘secondary’ protective devices allow standard 48 volt DC battery and 105 volt ringing functions, while providing uninterrupted circuit operation.”
“While the sensitivity of any manufacturer’s telecom equipment to AC induced voltages/currents is not always known, and these conditions fluctuate to various levels during different times of the day and season, it is often hard to predict when customers can experience problems. Data has been published by Bellcore (now Telcordia), on how much induction can be expected on Bell operating company lines. Document #TR-EOP-000001, dated June 1984, on ‘Lightning and 60 Hz Disturbances at the Bell Operating Company Network Interface’, states that an urban loop can have an average of 10 mA of continuous AC current flowing to ground, while suburban/rural loops can have 17-18 mA on the average, and 10% of all loops can have over 30 mA!”
“What is surprising about this data, which was based on a 1980 study of loops, is that 5 mA of leakage current is recognized in the power industry as an electrical safety hazard! The National Electrical Code even requires AC 110 volt circuits that are in bathrooms or outside a house to be equipped with Ground Fault Interrupters (GFIs), which are designed to trip the circuit breaker when more than 5 mA flows to ground. Since more homeowners and businesses are doing their own inside wiring and making a direct, hard wire connection to the telephone network, the problem of induction may be far more serious as a safety hazard than that of someone’s PC data modem not working properly!”
“All of this information is especially interesting in light of another Bellcore document #TA TSY-000057, dated August 1984, on ‘Functional Criteria for Digital Loop Carrier (DLC) Systems.’ This report outlines for manufacturers the specifications to which they should design their equipment. On the subscriber side of the remote terminal, a longitudinal current of at least 17 mA must be tolerated, and on the carrier side of the central office, the unit must be able to work with at least 100 mA of induced AC!”
“The section on electrical safety has some strict requirements, however. For instance, there are three categories for personnel and customer safety considerations. Class A1 category ‘should be inaccessible or have restricted access for contact by customers and may be exposed for contact by personnel and trained personnel.’ Class A1 limits the amount of AC current allowable to 0.15 mA peak. The Class A2 category ‘should be inaccessible for contact by customers, should be inaccessible or have restricted access by personnel, and may be exposed for contact by trained personnel.’ Class A2 limits the amount of allowable AC current to 1.0 mA peak. The Class A3 category ‘should be inaccessible for contact by customers and personnel.’ Class A3 limits the amount of allowable AC current to 3.0 mA peak.”
“Thus, the requirements on DLC equipment manufacturers are greater than that generally allowed for customer-owned and UL-approved appliances for connection to power lines. This compares to more than three times the average current level on BOC urban telephone lines and nearly six times on suburban/rural lines! Does this suggest that telcos should be responsible for getting the AC on their lines down to an acceptable level?”
“Regardless of whether the customer-provided telecom equipment has been adequately protected against these potential interference problems with a RID, SNIX or TIF, the telco may find that they still have to treat their lines in order to allow their new digital central office equipment to work properly. As mentioned in Alan Stewart’s ‘Tech Trends’ column in the June 1, 1985 issue of TE&M, many new digital exchanges, such as AT&T’s 5ESS switch, have incorporated a line circuit that is radically different than anything seen before. In order to prevent problems due to longitudinal induced AC currents, the central office battery is now being ‘floated’, instead of grounding the positive terminal as it traditionally has been done. Of course, now that the equipment has a high impedance path-to-ground, it becomes sensitive to induced AC voltages! It’s the same old story—if one thing doesn’t get you, the other one will! And when a path-to-ground is needed for such circuit applications as ground-start or DID trunks, induced currents from the induced voltages still can cause a problem.”
“Have you ever wondered why the switch manufacturers are so insistent on telcos ‘cleaning up’ their OSP facilities before cut-over to the new C.O., especially when the older switch is still working just fine? Why should the latest, most sophisticated telecom equipment be so finicky and sensitive to the normal conditions found in the OSP? A major reason, as far as wet cable is concerned, is shown in Figure 2. If the C.O. end has a high impedance to ground (open circuit condition) due to the battery being floated, then any low impedance path-to-ground at the far end (such as wet cable) will cause induced voltages to appear higher at the switch, which might cause it to deny service.”