Changing Technology Makes Telecommunications More Susceptible to Power Quality Disasters
By William Bush
Telecommunication systems have undergone major changes in the past decade, as they have become
more electronic and less electromechanical. Another change has been much greater use of
computer-generated data. These changes mean that telecommunication systems are now more
vulnerable to power quality problems. The electronic circuits are more susceptible to noise pulses than
voice communication, and the noise can completely change the computer data.
This new complexity is seen in todays telecommunication systems, which include satellites that handle packets of data, and telephone central offices that convert voice to computer-like digital signals. Within 10 years, these systems will become even more sophisticated, using fiber optics instead of copper wire as the primary communications medium. Todays analog telephones and fax machines will be replaced by all-digital units. Projecting further into the future, the next century will see widespread use of cellular phones in the office and home, replacing plug-in-the-wall phones. What does this mean to the contingency planner? It means that we will need to learn more about the new telecommunication systems because we will be called upon to deal with more complex telecom recovery problems.
Power quality associated with this new high-speed generation of telecommunication systems is being affected by factors similar to those computer systems, i.e., high-tech systems, used in low-tech environments. In addition, equipment from different vendors is being integrated with varying degrees of performance and reliability. Added to this is the involvement of people with different levels of technical expertise: installers, maintenance technicians, architects, builders, contractors, electricians and telephone companies. When you put all these factors together, telecommunication system interoperability is a hard-to-reach goal.
Telecom failures that can be traced to power quality problems include no line access, call cutoffs, ringing with no answer, wrong numbers and unexplainable malfunctions. Reliability problems can be linked to broken units, those that are sensitive to site location, units not installed to manufacturers instructions, and those sensitive to power quality and the operation of other co-located equipment. Other reliability problems are less obvious, such as time and season-sensitive units, and sensitivity to facilities operation and maintenance routines.
In solving the above problems, customers resort to consultants and others to supplement equipment manufacturer support. This adds to the telecommunications budget, which may then have to cut other functions that affect power quality. For example, redundant power supplies, backup power and protection circuits may be eliminated to stay within the budget.
One approach to achieving equipment interoperabilty is use of standards, for which the telecom industry is noted. Unfortunately, the customer or manufacturer may choose not to follow a recommended standard. Sometimes there is no recommended standard on a particular function. And, the standard may be generic and subject to interpretation.
The essence of standardization is interoperability. This is so important to regional economic growth that EC92 is moving standardization from voluntary to mandatory. In the US, OSHA and some local authorities require mandatory adherence to some voluntary standards. A prime example is the ANSI/NFPA 70 National Electric Code (NEC).
Recommended standards are not meant to infringe on equipment design. Rather, the intent is to standardize on a reasonable site ambient so that multiple manufacturers can co-locate, interconnect and achieve interoperability in a cost effective and reliable manner. This means that communication links may use standardized protocol, but electrical protection is still a function of equipment design and an acceptable site ambient.
Communication system power quality factors are a function of market conditions. The EC92 market will require product safety and immunity verifications for ESD (electrostatic discharge) and EMI (electromagnetic interference). The US market is subject to FCC and product safety requirements, but only in a segmented fashion. For example, where the 1990 NEC applies, equipment should bear a product safety listing mark from an NRTL (nationally recognized testing laboratory). And, FCC Part 15 EMI requirements do not apply to all market segments. Reliability and interoperability are more a function of the users preference of manufacturer than a voluntary standard. Furthermore, mixing different manufacturers equipment at the same site leaves the entire site subject to the immunity ability of the most susceptible equipment.
Equipment noise immunity can be influenced by undue site ambient conditions. Even the NEC says that the equipment may not function as intended when code requirements are applied to minimum safety requirements. This is another reason equipment immunity is so market dependent. Equipment that doesnt work well at a certain site ambient may require changes that lower the noise immunity of co-located equipment.
Based on present draft standards, the following are power quality-related candidates for standardization:
* Exposure sources for unwanted voltages and currents
* Severity of exposure level
* Protection techniques according to exposure level
- Facility lightning protection system
- Serving AC power surge protection devices
- Facility grounding system, including the grounding electrode system and grounding/bonding distribution
* Cable entrance facility
* Communication circuit overvoltage protectors
* System grounding of power conversion units
- Separately derived sources (AC)
- DC power plants
* Selective location of sensitive electronic equipment
- Locate away from probable lightning influences
- Connectivity to reduce common impedance effects
- Minimum length of grounding-bonding conductors
* Materials and workmanship suitable for equipment life
* ESD immunity levels for equipment
* Facility communication wiring system distribution
This level of site ambient standardization is not likely to change unless equipment is independently powered at each peripheral and interface links are nonmetallic (such as fiber optics). In other words, the site ambient readily influences the equipment powering and grounding scheme, which in turn influences the metallic links.
Table 1 lists some of the potential power quality-related problems associated with telecommunication systems. With much greater sophistication expected in future telecom systems, these factors will become increasingly important. For example, grounding and bonding that is adequate for a system today will require even greater attention when the equipment operates at higher speeds. Also, fiber optic-based systems still have to solve the problem of how best to supply power for fiber optic electronics.
Another consideration is battery backup for building telephone systems and fiber optic systems. Not all installed telephone systems have battery backup provisions, which can be difficult to add if they are not included. If battery backup is desired, it brings with it all the problems associated with batteries, including initial charging, maintenance and monitoring.
|Table 1: Power Quality-Related Disasters Associated With Telecommunications Systems|
Poor Equipment Immunity
* Communication speeds are too high
* Excessive interconnect lengths
* Susceptible interconnect wiring scheme
* Unmatched components
* Inherently noisy components
* Separation of telecom and AC power surge device
* Power quality requirements not stated or vague
* Installation instructions too interpretive
* Too low ESD immunity
* Not tested for product safety
* Too low capacity internal power supplies
* Temperature/pressure cycling upsets circuits
* Poor post-installation support
* No guidelines on OEM add-ons or replacements
Unreasonable site Ambient
* Improper servicing AC system grounding, including service panel bonding and grounding, grounding electrode conductor, missing or improperly sized/routed, not bonded to other electrodes, main bonding jumper missing or improperly sized
* Grounding electrode system: improper bonding of multiple electrodes, missing bonding of multiple electrodes, excessive bonding lengths, improper connections
* Exposed service without surge protection: no secondary AC surge protection devices, false reliance on utility primary protection
* Improper feeder/branch circuit distribution: missing or improper aceg, improper use of non-metallic raceway, non-dedicated shared circuits that cause mutual interference
* Exposed facility without lightning protection
* Facility without proper lightning protection: Class 2 structure wired with Class 1 wiring, does not conform to NFPA 78 LPC, no common bonding with grounding electrodes (not connected to telecom or electrical electrodes), improper bonding of roof-top towers
* Missing or improper bonding of metallic entries: piping, ductwork, cable shields ungrounded, bonding should be minimum length required for telecom over voltage protector
* Missing or improper vertical grounding system: no dedicated extension of grounding system
* Poor power quality: utility problems, facility backup power problems that include no backup power equipment, standby generator voltage frequency/stability, not enough ride-through time, too sensitive--transfers too often, not routinely tested, not matched to type of load (harmonics), improperly wired or grounded.
* Equipment-specific power conditioners: not matched to source and load (linear vs. nonlinear loads), sometimes not needed, impact of scattered locations
* Poor distribution methods: use of extension cords, insecure connectivity, ground prongs removed from AC plug, overloaded outlets, non-NRTL listed power strips, inadequate power strips, grounding not considered
* Poor environmental control: temperature/humidity/dust/pressure specs neglected, which can cause site deterioration
* No definitive ONE facility grounding system: integration of all grounding structures, declaration of principal ground access point, relative ground at each floor, facility power and grounding schematic
Poor Materials and Workmanship
* Facility lifelong materials: conductivity and sufficient dielectrics, corrosion control, accessibility and maintainability, listed material approved for the purpose, installed for facility expansions, designed for circuit functionality, proper withstand specs.
* Workmanship: qualified installation and maintenance, understanding of circuit functionality, labeling of apparatus and wiring runs
This article reprinted with permission from Power Quality Magazine.
William Bush is President of Telecom Reliability Services.
This article adapted from Vol. 5 #1.
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