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 manufacturer’s 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 user’s 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 doesn’t 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
Unreasonable site Ambient
Poor Materials and Workmanship
This article reprinted with permission from Power Quality Magazine.
William Bush is President of Telecom Reliability Services.
This article adapted from Vol. 5 #1.