PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

27.3 BPS Characteristics

Here
are the most important characteristics of a BPS:

The
VA rating of a BPS specifies the maximum power
the UPS can supply, and is determined by the capacity of the
inverter. VA rating is the product of nominal AC output voltage and
the maximum amperage rating of the inverter. For example,
Barbara's 120V APC Back-UPS Pro 650 can supply about
5.4A (650VA/120V). Connecting a load greater than the amperage rating
of the inverter overloads the inverter and soon destroys it unless
the BPS has current-limiting circuitry. Watts equal VA only for 100%
resistive loads (e.g., a light bulb). If the load includes capacitive
or inductive components, as do PC power supplies, the draw in VA is
equal to Wattage divided by the Power Factor
(PF) of the load. Most PC power supplies have
Power Factors of 0.65 to 0.7. For example, Robert's
APC Smart-UPS 1000 is rated at 1000VA but only 670 Watts, which means
that APC assumes a PF of 0.67 when rating wattage for this unit.

The run time
of a BPS is determined by many factors, including battery type and
condition, Amp-hour capacity, and state of charge; ambient
temperature; inverter efficiency; and percentage load. Of those,
percentage load is most variable. The number of Amp-hours a battery
can supply depends on how many amps you draw from it, which means the
relationship between load and run time is not linear. For example,
our APC Back-UPS 600 can supply 600VA for five minutes, but can
supply 300VA (half the load) for 22 minutes (4.4 times longer).
Doubling load cuts run time by much more than half; halving load
extends run time by much more than twice.

Many people believe VA rating and run time are somehow related. There is no such relationship, except that units with larger VA ratings typically also have a larger battery, which provides longer run time for a given load, both because the battery itself is larger and because the unit is supplying fewer amps than its rated maximum. It is, however, quite possible to build a BPS with a very high VA rating and a tiny battery or vice versa.

Utility AC
voltage is nominally a pure sine waveform, which is what power
supplies and other equipment are designed to use. The output waveform
generated by BPSs varies. In order of increasing desirability (and
price), output waveforms include: square wave,
sawtooth wave, and modified square
wave (often somewhat deceptively called near
sine wave, stepped approximation to sine
wave, modified sine wave, or
stepped sine wavemarketers are desperate
to get the word "sine" in there,
especially for units that don't deserve it). The
cheapest units generate square wave output, which is essentially
bipolar DC voltage with near zero rise-time and fall-time, which
allows it to masquerade as AC. Midrange units normally provide
pseudo-sine wave output, which may be anything from a very close
approximation to a sine wave to something not much better than an
unmodified square wave. The output waveform is determined by the
inverter. The inverter is the most expensive component of a BPS.
Better invertersthose that generate a sine wave or a close
approximationare more expensive, so the quality of the output
waveform generally correlates closely to unit price. Astonishingly,
we once saw specifications for a no-name BPS that listed output
waveform as "pure square wave,"
presumably intending to confound buyers with
"pure" (a Good Thing) and
"square wave" (a Bad Thing).

We have heard reports of fires caused by connecting a surge suppressor between the BPS and the PC. Although we have not been able to verify the reports, it makes sense that feeding square wave power to a surge suppressor designed to accept sine wave input could cause it to overheat. On the other hand, there is nothing wrong with using a surge suppressor between the BPS and the wall receptacle. In fact, we recommend it, both to provide increased protection against spikes reaching the PC and to protect the BPS itself.

Although it sounds trivial, battery
replacement method is one of the most important characteristics of a
BPS. Batteries must be replaced periodically, perhaps as often as
annually if you have frequent long power outages. Better units have
user-replaceable batteries. Lesser units must be returned to the
factory for servicing. It's both much less expensive
and much more convenient to be able to replace batteries yourself.

Before you buy a replacement battery from the UPS maker, check Graybar, W. W. Grainger, and similar industrial supply vendors. You may be able to find identical replacement batteries for half or less the price charged by the UPS maker.

The
length of warranty is a reasonably good indicator of the quality of
the unit. Better units have a two-year parts and labor warranty,
although the battery is usually excluded. Lesser units often carry a
one-year warranty, and we have seen many of them fail not long past
that time. The cheapest units may carry only a 90-day warranty.

Inexpensive BPSs may provide few or no
configuration options. They may, for example, be permanently set to
transfer to battery if the input voltage drops below 102VAC or rises
above 130VAC. Better BPSs offer flexible options for setting such
things as transfer voltage thresholds, warning type (audible, visual,
email and/or pager notification, etc.), delay before warning, warning
duration, and so on.

Inexpensive units provide few status indicators, typically only an
LED that illuminates when the unit is operating on battery. Better
units provide detailed LED or LCD status displays to indicate such
things as load percentage, battery charge status, overload
conditions, and battery replacement required.

All units include some form of overload
protection. Less-expensive units often use a fuse, and may need to be
returned to the factory if that fuse blows. Better units use a
circuit breaker that can be reset by pressing a button.

Most units include two types of receptacle, often differentiated by
color. The first sort are backed up by the battery; the second sort
are surge-protected only, and are useful for connecting items (such
as laser printers) that you want surge-protected but do not want to
run from the UPS. Also note that units vary greatly in how many
receptacles they provide and how convenient they are to use.
Inexpensive units mount a few receptacles on the back panel. Better
units provide additional receptacles, and arrange themeither
by spacing or by making the receptacle a female connector on a short
extension cordso that connecting a power brick or oversize
plug does not block other receptacles.

There are two aspects to BPS manageability:

All but entry-level BPS units include a
network interface port. By connecting that port to a serial port on
the computerwhich usually requires a nonstandard
cableand running automatic shutdown software supplied with the
OS or the BPS, you can allow the BPS to shut down the computer in an
orderly manner during a power failure before battery power runs out.
If your computer runs unattended, automatic shutdown is a valuable
feature. Some BPS models support automatic shutdown via a USB link.
If yours does, make sure the unit also supports serial connection, or
it will be unusable with Windows NT 4 and other OSs that
don't support USB. Note that if you share one BPS
among computers, you will be able to shut down only one of them
automatically unless you purchase expensive hardware designed to
distribute the automatic shutdown signal to multiple computers.

Simple Network Management Protocol (SNMP)
can be used to centralize monitoring and control of a large network.
In that environment, having SNMP-capable BPSs is important, but in
typical home-office and smallbusiness environments, SNMP support is a
nonissue. Inexpensive BPSs do not support SNMP. Midrange and high-end
SPSs may include it as a standard or optional feature. If SNMP is an
issue for you, make sure that the BPS manufacturer supplies an MIB
that is usable by your management package. If you
don't know what an MIB is, don't
worry about it.

Here are some BPS characteristics that are promoted by marketers but
are largely meaningless:

This item is pertinent only if you use shutdown software, either that
bundled with the OS or that provided by the BPS manufacturer. The
shutdown software provided with a modern OS recognizes most common
BPS models, and can usually be configured to support oddball
requirements from off-brand BPSs. Most people use the automatic
shutdown software bundled with the BPS, as it is usually more
functional and supports specific features of the BPS model. In that
case, the only thing that matters is that it runs on your OS, which
it is likely to do unless you're running something
relatively uncommon such as OS/2. Linux support, formerly rare, is
now common. OS certification should be at most a checklist item.

Typical BPSs have nominal switching times of 2 to 4 ms.
That's best case. Under adverse conditions, such as
an extended period of low-voltage, partially discharged batteries,
and so on, transfer time can be longer. A typical BPS might list
worst-case transfer time of 8 ms, which should be within the
hold-time of any decent power supply, even operating under adverse
conditions. Shorter is obviously better here, but
don't give nominal switching time too much weight.

Most BPS makers include a connected equipment warranty, typically for
$25,000. In theory, if your equipment suffers damage attributable to
a fault in the BPS, the BPS company pays to repair or replace it.
That sounds good, but the truth is that few people ever collect on
such warranties. There are so many exclusions and limitations,
including the fact that the coverage is often subrogated to your home
or business insurance, that such warranties are all sizzle and no
steak.