Clean Reliable Power
by Lloyd Borrett
Technical Cornucopia, August 1989
It's quite simple really, our computers run on
electricity. Without it they are worthless. But how many of
us have ever taken the time to think through the
implications of that one simple fact.
Power isn't always clean or reliable
If you work around computers for long enough you learn to
appreciate the damage that can be caused by power
fluctuations. To help avoid data loss and circuitry damage,
some users install such protective devices as surge
protectors, line conditioners and other filtering devices.
Unfortunately, doing so may provide a false sense of
security. While these units offer protection against voltage
spikes and surges, they fail to address the most common type
of power disturbance — loss of power.
Alternating current power line sags — better known as
brownouts — typically account for over 80 percent of all
power disturbances. Blackouts cause an additional 5 percent
of power problems. What this means is that power protection
solutions, while important to system integrity, are
irrelevant for over 90 percent of electrical problems.
Realisation of this has led to a dramatic expansion in
the development and use of backup power systems (BPS).
Essentially there are two types of backup power systems —
the Un-interruptible Power Source (UPS) and the Standby
Power Source (SPS). The difference is that a UPS always
supplies its output power from its storage bank of
The SPS supplies its output power from the mains supply
and switches to the storage bank if the incoming power
begins to deviate from the accepted voltage range. Provided
the SPS switches over fast enough to keep your equipment
running it can be considered to be the same as a UPS.
Typically, a SPS costs less than a UPS.
A good backup power system combines the benefits of surge
suppressors with the added protection of a redundant power
source. During normal operation, a good quality BPS filters
and conditions incoming power. A BPS supplies AC power
during brownouts or blackouts and keeps a system up and
running during power outages.
In most cases, power interruptions are momentary and
users continue working as if nothing has happened. When the
power is off for an extended period and the battery power is
at risk of drainage, the BPS alerts users in ample time for
them to save their work and close down the system.
Networked computer systems are, not surprisingly, much
more susceptible than single-user systems to data loss due
to power failures. In a typical LAN, a single file server is
normally used to maintain files on a hard disk. If a power
failure occurs when the server is updating a disk directory
or a file-allocation table, the results can give the network
In addition, many network operating systems hold the hard
disk directory in random-access memory, a somewhat volatile
state, and only periodically update the hard disk itself. If
a directory on the hard disk had not been updated at the
time of the power failure, it may be impossible to recover
whatever data was still in RAM.
Potential for costly losses
Lost network information can obviously be costly. Large
network databases contain everything from customer accounts
to financial data, as well as irreplaceable creative work.
Damage to this mass storage can mean lost business, billing
errors or scheduling delays.
And consider the intangible losses from damaged data:
lost productivity, energy wasted on data re-entry, dips in
morale, possible lost confidence in the computer system and
worse yet, in the system managers! It can even adversely
affect a company's image among its customers.
LAN managers are thus increasingly turning to backup
power systems to protect against data loss. But while a BPS
on the file server can eliminate some of the risks, problems
still remain. A BPS alone, for instance, cannot shut down a
system by itself or safely warn all network connections of
an impending shutdown.
A safe system shutdown depends on the presence of a
system supervisor at the time of the power failure and
relies on the following sequence of events taking place.
First, the supervisor must realise a power failure has
occurred and notify the workstations to give network users
adequate time to save their files and log off the system.
Then the network server must be physically shut down.
But these are ideal conditions, and rarely met in the
real world. What happens, for instance, when a server is in
operation around the clock without the benefit of a trained
supervisor? Or when remote workstations are not made aware
of a power disturbance at the file server?
To address these questions and provide for orderly system
shutdown, some network and BPS vendors have made several
advancements in both hardware and software. Some BPS
manufacturers, recognizing the need to interface directly
with the machines they support, have added external signals
that each user's equipment can read. The signals,
communicated through standard serial or parallel ports,
indicate when a power disturbance has occurred (i.e. when
the BPS is providing power to the system without mains
support) and when the BPS' battery reserve is dangerously
Because the BPS is already capable of supplying power to
the file server during power fluctuations and outages, all
that is required is a foolproof system to make the processor
aware of the power problem. In general, this strategy
involves the CPU reading the BPS status at periodic
When this status is normal, indicating acceptable mains
power, nothing is done. When the server receives a signal
indicating a failed power line, however, it should warn the
user that the system is operating on battery power. This is
typically done via a broadcast message to remote
If mains power does not resume within an acceptable
amount of time (depending on the battery's capacity and
system load), users should be advised and a system shutdown
should be automatically executed.
A new awareness
The importance of automatic data protection in the event
of power loss has increased as businesses with less
experienced personnel depend more on computers. This new
awareness has led to an increase in the number of
applications programs and operating systems, including
3Com's 3Plus, designed to monitor the status of a BPS.
Systems using 3Com's 3S/401 or 3S/402 servers, for
instance, can monitor the BPS via a direct cable link
between a compatible BPS and the asynchronous serial port
(no boards or other hardware are necessary). The BPS
monitoring function, automatically activated when the system
is powered up, runs as a background process invisible to
users during normal operation.
Then, when a server detects power failure, a new sequence
of events takes place, without human intervention. The
monitoring system broadcasts a status display on the file
server and the workstations are notified that the system is
running on BPS power. After a user-specified period of time
(generally 10 to 20 minutes), the operating system initiates
the shutdown routine, writing all data in memory to disk and
shutting itself down.
In this way, users at remote workstations — which may not
be subject to power fluctuations affecting the server — will
have adequate time to store their data before the system
A supervisor need not be present during the process, so
that a shock to the computer system need not be a shock to
the network administrator.
Saturday, 15 October 2011