Continuous Systems
Availability: 24/7 Power Protection
Strategies For Converged Networks
BY FRED STACK
Voice and data networks today represent the lifeline of most
businesses. Increasing amounts of critical information are being housed on
networks of significant size. Moreover, telecommunications facilities
managers are faced with the dilemma of how to power a variety of equipment
-- often a hybrid mix of information technology and telecommunications
networks -- as reliably and economically as possible. This means devising
power protection plans that encompass both AC- and DC-powered equipment.
A NEW ENVIRONMENT
Two factors have combined to change the power protection needs of
telecommunications applications forever. First, the critical nature of the
applications has increased to where availability in the "high
9s" is not merely a goal, but an expectation. Second, the convergence
of information technology and telecommunications networks has created a
whole new "communications environment," and therefore a new set
of challenges.
Instead of just supporting telephony services across a geographic area,
that same local telephone network may now provide access to the Internet.
Similarly, telephone services can now be provided through cable. Modern
telecommunications facilities are faced with powering a variety of
equipment as reliably and economically as possible. The birth of
co-location facilities provides an example of the growing need for AC and
DC power in a secure, protected space.
MOVING AWAY FROM -48VDC
Traditional telephony applications -- from central office exchanges to
PBX networks -- have standardized around a -48VDC source for a number of
reasons. First, the negative polarity reduces corrosion problems with
underground cables and conduits. The low voltage makes it easy to
implement, as well. Systems using less than 72V generally do not require
licensed electricians, nor are they governed by the NEC. As opposed to
commercial AC, DC power is the "standard" for carrying voice
signals. DC is also considered more reliable because it is more easily
stored than AC.
PROTECTION FOR TODAY
Today's converged communications networks often require facilities
managers to accommodate equipment that requires an AC power source. The AC
power is often derived by way of inverters powered from the -48 VDC
system. An alternative is the use of an AC UPS, which is often more
straightforward and efficient, and less expensive. In comparison to DC
systems, AC UPS uses higher voltage batteries, provides regulated voltage
output, is available in higher power capacities, and allows for longer
power distribution distances.
As availability demands continue to increase, power quality and
environmental factors become much more complex issues. It is important
that the AC UPS provides power as reliably as the associated DC power
system. In many systems, the DC-powered equipment is not fully operational
without the AC-powered equipment and vice versa. It is, therefore,
important to take a total systems approach.
Conventional telecommunications power systems employ inverters powered
from the -48VDC power plant to supply the AC-powered loads. To achieve
high levels of power reliability and availability, four- or eight-hour
battery backup times are typical. This approach is not always appropriate
for facilities where a significant amount of electronic loads require AC
input power. The conventional information technology facility relies on AC
UPS systems with 15 minutes of battery backup time supplemented by
permanently sited standby generator systems. Clearly, a systems approach
to the entire facility power requirements is needed to provide the desired
level of reliability and availability at facilities having both AC and DC
equipment.
A TOTAL SYSTEMS APPROACH
To avoid compromising the availability of load equipment, the power
system needs to be about 10 times more reliable than the load equipment.
Therefore, redundancy in the power system is required. To facilitate
implementation of the power system redundancy, a number of critical
telecommunications and information technology devices are available with
dual input power connections.
Distributed Redundancy
In its basic form, distributed redundancy involves creating two
(redundant) power protection system busses and redundant power
distribution systems. This eliminates as many single points of failure as
possible, all the way up to the load equipment's input terminals. In order
to provide "fault tolerance," some method of allowing the load
equipment to receive power from both power protection busses must be
provided. To protect against fast power system failures, such as circuit
breaker trips or a power system fault, either dual input load equipment or
very fast transfer switches need to be applied between the two independent
power sources. For today's large telecommunications facilities, dual
redundant UPS systems with redundant AC power distribution have been
deployed.
With large, convergent telecommunications facilities, an emerging power
system configuration is the use of small distributed, redundant DC
rectifier systems supplied from large, dual redundant AC UPS systems.
Small, self contained DC rectifier systems along with AC Power
Distribution Units (PDUs) can be located throughout the information
technology data center to supply either AC or DC power to the load
equipment. The "best practices" of large information technology
data centers are merged with the DC power systems of telecom facilities to
optimize reliability. Redundant, standby generators can be used to provide
dependable power in the event of a sustained commercial AC power failure.
This solution ensures that the air conditioning system is powered as well,
since heat buildup within the room can shut down the load in as little as
five minutes.
SUPPORTING THE HYBRID SOLUTION
Specifying and maintaining a high level of power availability absolutely
requires access to a number of support services. Preventative maintenance
is critical to long-term effectiveness of a power protection program.
Contracting an outside firm with power protection-specific experience is
often the best way to ensure optimum performance and reliability.
Make sure your vendor includes regular testing, checking the battery
plant, verifying that the UPS and rectifiers are operating properly, and a
thermographic survey of selected equipment and connections to prevent
failures in the electrical systems. Telecommunications sites, particularly
if they are unmanned or remote, need thorough periodic reviews. These
reviews should be included in an effective power protection program, to
prevent problems or recommend corrective actions.
Fred Stack is vice president, marketing, for Liebert Americas. For
more information, please visit Liebert's
Web site.
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