The Internet Encyclopedia (Volume 3)

P1: JDV

Michael WL040/Bidgolio-Vol I WL040-Sample.cls June 19, 2003 16:10 Char Count= 0

PREVENTIVEMEASURES 75

is not extreme peaks and valleys in the alternating cur-

rent (AC) sine wave, but modulation of that wave. Their

goal is to restore the optimal sine shape. Power purity

can also be fostered by adding circuits rather than filters.

The most important precaution is to keep large machinery

off any circuit powering computing equipment. If possi-

ble, it is preferable to have each computer on a separate

circuit.

The dangers of static electricity can be reduced by in-

hibiting its buildup, providing ways for it to dissipate

gradually (rather than discharge suddenly), or insulating

vulnerable items. Antistatic techniques include the fol-

lowing:

1. keeping the relative humidity from dropping too low
   (below 40%);


2. avoiding the use of carpets and upholstery with syn-
   thetic fibers, or spraying them with antistatic sprays;


3. using antistatic tiles or carpets on floors;


4. not wearing synthetic clothing and shoes with soles
   prone to generating charges;


5. using anionizer(which sends both positive and nega-
   tive ions into the air as a neutralizing influence); and


6. keeping computers away from metal surfaces or cover-
   ing metal surfaces with dissipative mats or coverings.

When installing electronic circuitry, technicians

should ground themselves. A variety of conductive “gar-

ments” can be worn, including bracelets and straps for

wrists and ankles, gloves, finger cots, and smocks.

Uninterruptible Power Supplies (UPS)

Although anuninterruptible power supply,by definition,

counteracts a loss of power, it typically provides surge

protection as well. This is accomplished by means of sep-

arate input and output circuits. The input circuit induces

current in the output circuit. A UPS may also incorpo-

rate noise filtering. UPS systems fall into three categories.

Anonlinesystem separates the input and output with a

buffer, a battery that is constantly in use and (almost)

constantly being charged. This is analogous to a water

tank providing consistent water pressure, regardless of

whether water is being added to it. This is the origi-

nal and most reliable design for a UPS. In the strictest

sense, this is the only truly uninterruptible power sup-

ply; its transfer time (defined below) is zero millisec-

onds. Anofflinesystem sends the primary current straight

through in normal circumstances, but transfers to backup

power if its detection circuit recognizes a problem with

the primary power. The problem might be a complete

drop in primary power, but it might also be a spike, a

surge, asag(drop in voltage), or electrical noise. Aline

interactivesystem is similar to an offline system, but its

output waveform will be a sine wave (as is the input wave-

form) rather than a square or step wave. Aside from its

basic type, the most important characteristics of a UPS

are its

1.capacity—how much of a load it can support (measured

in volt-amps or watts);

2.voltage—the electromotive force with which the cur-

rent is flowing (measured in volts);

3.efficiency—the ratio of output current to input current

(expressed as a percentage);

4.backup time—the duration during which it can provide

peak current (a few minutes to several hours);

5.transfer time—the time from the drop in primary power

until the battery takes over (measured in milliseconds);

6.battery life span—how long it is rated to perform as

advertised;

7. battery type—a small Ni-MH (nickel metal hydride)
   battery support of an individual machine, whereas
   lead-acid batteries for an entire facility may require a
   room of their own; and


8. output waveform—sine, square, or step (also known as
   a modified sine) wave.

A final consideration is the intended load:resistive(as

a lamp),capacitive(as a computer), orinductive(as a mo-

tor). Because of the high starting current of an inductive

load, the components of an offline UPS (with its square

or step wave output) would be severely damaged. Actu-

ally, an inductive load will still have a similar but less se-

vere effect on other types of UPS systems (with sine wave

output).

Large battery systems may generate hydrogen gas, pose

a fire hazard, or leak acid. Even a sealed, maintenance-

free battery must be used correctly. It should never be fully

discharged, it should always being recharged immediately

after usage, and it should be tested periodically.

Some UPS systems feature scalability, redundancy, and

interface software, which can

1. indicate the present condition of the battery and the
   main power source;


2. alert users when backup power is in operation, so that
   they can shut down normally; or


3. actually initiate a controlled shutdown of equipment
   prior to exhaustion of backup power.

A UPS should come with a warranty for equipment

connected to the UPS; the value of any lost data is typically

not covered.

When limited resources do now allow for all equipment

to be on a UPS, the process of deciding which equipment is

most critical and therefore most deserving of guaranteed

power continuity should consider two questions. First, if

power is lost, will appropriate personnel still receive auto-

mated notification of this event? Second, is the continued

functioning of one piece of equipment moot if another

component loses power?

The existence of any UPS becomes moot whenever

someone accidentally flips the wrong switch. The low-

cost, low-tech deterrent is switch covers, available in stock

and custom sizes.

There are occasions (e.g., fires and floods) when power

must be cut to all equipment except emergency lighting

and fire detection and suppression systems (which should

have self-contained power sources). This includes discon-

necting a UPS from its load. Any intentional disruption of