J. Allen Byrne
Systems Engineering International Inc.
Users have become accustomed to having high-quality power and battery backup on the telecommunications network. The network of the future will demand even more reliability with respect to powering.
The manner in which network powering is being treated today, with many individual islands of responsibility, will not be practical in the future. However, past practices, methods, standards and lessons learned can be applied to tomorrow`s network.
Some of the main concerns of providing adequate network power are:
Utility power
Power quality
Customer premises power
Power supply requirements
DC power systems
AC uninterruptible power supplies
Batteries
Powering techniques.
Utility power
The utility companies do a good job of generating and transporting clean power, but utility power gets degraded down the line, and the situation is going to get worse. Demand is increasing, but capacity is not, and quality is stable, at best. To prepare for worsening power quality, provide power-critical equipment with power conditioning and backup.
Power quality can be defined as the process of powering and grounding power-sensitive electronic equipment in a manner that is adequate for the operation of that equipment. Just as the telephone companies adequately provide seamless backup power to their network, other service providers and end users-designers, engineers and installers outside the telephone-company central office-should extend and continue that tradition in their areas. These people can achieve power quality by applying proper powering techniques, including protection, conditioning and backup.
Customer premises power
Recent studies have shown that up to 80% of all sensitive electronic equipment failures that can be attributed to poor power quality may result from inadequate electrical grounding or wiring on the customer`s premises, or from interactions with other loads on the premises. Another study states that grounding problems account for 70% of all power problems, and this percentage will only increase as lower direct current logic voltages are introduced. A simple approach is needed to eliminate most of these premises-induced power problems.
This is a challenge that equipment manufacturers, system designers and installation contractors must face. Manufacturers must provide simple and adequate installation instructions, and installers must carry them out properly, as well as abide by basic electrical safety standards (see "Minimizing Premises Power Problems," below).
Installers and designers must also be aware of everything else that impacts the power quality of the installation. For example, power aberrations that affect equipment can arise at such points as the alternating current input (power cord), equipment ground, communications port, control wiring, alarm and monitoring wiring, and adjacent wiring.
Standards-compliant equipment manufactured for connection to the network should operate satisfactorily if the input voltage varies from +6% to -13% from nominal. However, utility power is often outside this voltage window. Also, as processing speed increases and logic voltage levels decrease to 3.3 volts and below, power quality becomes more critical.
In the future, protection, conditioning and backup devices will become an integral part of every network, system, device and peripheral. Conditioned power will be available-at a price-from the electrical utilities; they are already getting into this business. Equipment manufacturers will insist on conditioned power to validate their warranties.
If DC powering techniques were to be applied to the complete network, most of the powering problems now being experienced would vanish. These techniques include:
Converting unstable AC to DC power. This provides isolation.
Using DC to power the load and maintain a charge on the battery.
Using the battery to provide DC power to the load when AC power quality is not acceptable. If the load is DC-powered, then inversion from DC to AC is not required.
Electrically protecting everything that has access to, or intrudes upon, the system. In other words, create an electrically fortified zone.
These techniques sound easy, but because they are costly, they have not yet migrated to the unregulated and market-driven segments of the industry. However, proven power-protection techniques, new powering requirements and user demand for reliability will force acceptance of DC powering as a standard.
Uninterruptible power supplies
Uninterruptible power supplies can be divided into three categories-those that work most of the time, those that work some of the time and the remainder.
UPSs have become synonymous with AC systems, although a traditional DC power system is also a UPS. AC UPSs do serve an important purpose because they can protect against power aberrations and provide seamless power to critical loads.
Generally, AC UPS manufacturers have led the way in onboard system monitoring, fault diagnosis and reporting. These capabilities are now being tied into the network as features of simple network management protocol. However, one of the problems with UPSs is that they are not really fault-tolerant, and there is no built-in redundancy. If a component or an assembly fails, the UPS loses its ability to offer complete power protection. Developing a reasonably priced UPS that offers built-in redundancy in the battery-charging and inverter circuit is a major challenge for UPS manufacturers.
Battery charging and battery maintenance remain problems, occasioning much finger-pointing in both directions. Battery manufacturers say that batteries are not charged properly and are often maintained at too high a temperature. UPS manufacturers counter that the batteries do not perform to specifications. There has to be some compromise in this area-perhaps better products, as well as better charging techniques.
Whatever the issues, though, one thing is becoming clear: AC UPSs will have to be applied to all power-sensitive equipment. The understanding and application of these devices will be a basic requirement of the network designer and system installer.
Batteries
All DC power systems and UPSs have three basic problems-battery charging, battery application and battery life.
Valve-regulated lead-acid batteries are the Achilles heel of the power backup industry. In fact, if it were not for having to operate with batteries, most DC supplies and UPSs would work well as power-conditioning devices: They just would not have any reserve power.
Telephone central offices and major computer sites use flooded lead-acid batteries, which work well in a protected environment. However, to meet the need for a more user-friendly battery, manufacturers produced the valve-regulated lead-acid battery, which has caused some major problems for users. These batteries have been advertised as sealed and maintenance-free cells, but they are neither. They have pressure-relief valves that can vent, and the larger ones require maintenance. The American National Standards Institute T1 committee is coming out with a standard that may recommend a maintenance cycle for the valve-regulated lead-acid cell that equals or exceeds that of the flooded cell.
Battery monitoring will become increasingly important, but much work needs to be done in this area. There are several battery monitors on the market, but they are either too expensive or do not do the job adequately. An attractively priced monitor that will collect all necessary battery data and take corrective action is required.
In the future, battery and battery-cell manufacturers will become more application-specific, engineering their products for specific equipment or purposes. Warranty and life expectancy will be governed by application and use. Downsizing and distributed power systems will mean that batteries for new modular power systems must be truly maintenance-free. They will have a much shorter life, and may be treated as a disposable commodity.
Powering techniques
There is a revolution going on in powering network equipment. Among the trends are:
When fiber is introduced into the loop, power cannot be transmitted over the medium, so it must be locally supplied.
With such telecommunications platforms as integrated services digital network, sufficient power for instruments cannot be transported over copper wires from the private branch exchange, so it has to be supplied locally.
The blurring between telecommunications and data communications has led to equipment with differing power requirements mounted in the same closet or rack.
Distributed private branch exchange nodes and reduced DC power needs have brought about a proliferation of low-power, modular systems.
The need to get DC power near to loads will further increase the requirement for low-power, modular systems.
Lack of craft skills will require easy-to-install, low-maintenance, reparable systems with hot-swap capabilities.
Management and monitoring systems will become standardized, simple to use and inexpensive. The power sources will intelligently interact with the loads.
Minimizing Premises Power Problems
Some basic rules that help minimize internally generated premises power problems are:
Ensure an adequate ground is established for the electrical service entrance (see National Electrical Code-1993, Article 250).
Do not install any device that cannot be wired in accordance with the NEC.
Ground everything that should be grounded, including equipment cabinets, raceways, conduits, cable ladder, DC systems (in accordance with ANSI T1.311-1991) and UPS outputs (where required by NEC 250-26).
Never use an isolated or separate grounding electrode. Always use building (common) ground. Ground everything to the same point.
Never rely on conduit for a grounding connection. If a grounding conductor is run through conduit, ground the conductor to the conduit.
Always use a full-size grounding conductor. Treat the ground as a current-carrying conductor, because sooner or later, it probably will be.
Most isolated grounding systems end up not being isolated.
Never buy inexpensive equipment.
If you are using a large quantity of switchmode power supplies, use only those that are power-factor-corrected.
Never connect sensitive equipment to electrical panelboards that have highly inductive or cyclical loads connected to them- for example, copiers, laser printers, motors and air-handling units.
Insist on a tidy installation, but be aware of problems caused for the sake of tidiness-for instance, electromagnetic interference because of coiled wires.
Only allow authorized equipment to be connected to a power-conditioned source.
J. Allen Byrne is executive vice president at System Engineering International Inc., Frederick, MD.