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 TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
c. Isolation transformers. Isolation transformers
age suppressors, filters, etc. are used to protect the
are more expensive power conditioners. They pro-
sensitive electronic equipment. These types of
vide two functions. One is the ability to change to a
equipment are called power conditioners.
a. Transient suppressors. Transient suppressors
new voltage level and/or to compensate for high or
low site voltage. For example, by using 480V input
are very low cost devices available for microcomput-
ers in the form of outlet strips similar to extension
up to the point of use and then transforming to
120V or 208Y/120V, the switchgear and wiring can
cords with multiple receptacles. They usually con-
tain metal oxide varistors (MOVS) and sometimes
be reduced in size and the effect of line drop re-
silicon avalanche diodes (SADS). These are typically
duced. If the voltage at the point of use is too low
due to line drop, it can be manually boosted in steps
disc shaped devices connected between the power
lines and, sometimes, from line to ground. They
by connecting to different taps on the transformer
windings. The second function of the isolation
absorb energy from transients which exceed their
transformer is to provide for the ground reference
threshold (typically 100 percent above normal peak
right at the point of use. This eliminates the prob-
voltage). Because of their small size and low cost
lem of common mode noise induced through "ground
compared with the equipment they serve and the
loops" or multiple current paths in the ground cir-
cost of determining if such transients exist at a
cuit upstream of the established reference ground
given installation, many people provide this protec-
point.
tion as insurance. This type of transient suppres-
d. Voltage regulators. Most of the voltage prob-
sors can be provided for a nominal cost and most of
lems except outages can be handled by the addition
the more expensive power conditioners such as line
of voltage regulators equipped with transient sup-
voltage regulators, static switches and UPS systems
pressors. Several solid-state techniques have been
have these devices built in. They can even be added
developed in recent years to replace the older, slow
to a distribution panelboard, if not included else-
acting electro-mechanical type. One type of fast re-
where. Another form of transient suppressor, a
sponse regulator is the phase modulating type. It
surge arrester, is intended to lower the transient
usually utilizes thyristor (SCR) control of buck and
energy level to that which can be handled by down-
boost transformers in combination with filters to
stream power conditioners, such as MOVS or filters.
provide stable sinusoidal output even with non-
They typically use gas discharge tubes which are
linear loads typical of computer systems. This is
slower acting than MOVS, but can absorb more en-
done in a smooth continuous manner, but at great
ergy. To be effective, however, they too must be
speed, eliminating the steps inherent in the tap
coordinated with upstream surge arresters having
changer. Heavy loads can be delivered for start-up
greater energy absorbing capability. Usually, this is
inrush typical of computer central processors or disc
done at each point of voltage transformation back to
drive motors while maintaining full voltage.
the incoming line and is best coordinated with the
electric utility. Packaged transient suppressor sys-
an electric motor driving an AC generator so that
tems combining the devices described above are
the load is electrically isolated from the power line.
available which, when properly installed, will limit
Motor generators are used widely as a source of 400
expected surges as defined by the IEEE Standard
Hz power for large computer central processors re-
C62.41.
quiring this frequency. Because the frequency toler-
b. Filters. Line filters are used to reduce electro-
ance of the computers is wide, a simple induction
magnetic interference (EMI) and/or radio frequency
motor can be used to drive a brushless synchronous
interference (RFI) to acceptable levels. Generally
generator (alternator). The speed changes with load
small and low in cost, they, too, are usually built
and input voltage variations hold output frequency
into sensitive equipment and the more expensive
well within tolerance and constant voltage is main-
power conditioner equipment. The simplest form of
tained by automatic voltage regulators controlling
filter, a low pass filter, is designed to pass 60 Hz
the generator's field excitation.
voltage but to block the very high frequencies or
f. Uninterruptible power supplies (UPS). For con-
steep wavefront transients. They are not effective
tinuous operation of computer or other sensitive
for frequencies near 60 Hz, such as low order har-
systems when line voltage is interrupted, the only
monics, but become effective in the Khz range. Fil-
solution is a UPS. A properly designed UPS can
ters can be connected line to line or line to neutral
provide computer quality power under essentially
for rejection of normal mode noise. They can also be
all normal and abnormal utility power conditions
connected line to ground for common mode noise
during outages for extended period of time depend-
rejection. Some of the better transient suppressor
ing on battery capacity. This bridges most power
outlet strips also contain these falters.
outages and permits orderly shutdown for longer
7-16
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