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Choosing the right Power UPS
Guide to professional UPS selection

The following steps will guide you through the process of specifying and selecting a
professional UPS for your organization. For a newcomer to the UPS field it is advised to start
with UPS Basics article.

1. Decide the required UPS Size.

The Power Capacity of the required UPS should be the first thing to decide. Connect to the
UPS only consumers which should be protected. The UPS must back up servers,
Communication items, preferably Personal Computers, and Emergency lights. Connect
monitors only if you need to provide service even if power fails. Avoid connecting printers,
copiers, non-critical lights etc, even if the UPS size permits.

Now calculate the total current drawn from the UPS, which is the total sum of the currents of
every consumer connected to it. Multiplying thus derived total current figure, by the local grid
Phase to Neutral voltage, (normally 120V in North America or 230V in Europe) provides the
required VA Rating. Note the total current of a three-phase system is derived by the sum of
all three phases.

The current demand of the consumers can be found in the users manual or read from the
label attached to the instrument. To get the final figure, reserve extra capacity for any
expected additional future Loads. The VA figure so derived is in most cases 30% to 50 %
above the current, which the equipment consumes. You will have to consult the manufacturer
of the equipment or measure the current for a more accurate figure.

The wattage rating is the second figure to consider. It represents the amount of real Power that
the load requires. The Watts to VA ratio, called Power Factor, is dictated only by the nature of
the load. This ratio can theoretically be any number between zero to one, but it is normally
between 0.6 to 0.8. Unless you have the exact figure,Power Factor 0.7 should be OK for
Personal computers and 0.8 for large systems.

A personal computer (or server) including a monitor will normally require no more than 300
VA. Multiplying the VA rating by a power factor of 0.7 renders 210Watt. The common
presentation is 300VA/210W.Thus, any UPS with a VA rating equal to or above 300VA and
Watt rating equal to or above 210W should be sufficient for a single computer.

2.Decide the required Autonomy time (Back-up time)

Autonomy time is the time that batteries are able to back-up operation and feed the load upon
failure of utility power. Load consumption and size of UPS batteries dictate the back-up time.
UPS systems rated below 20-30 KVA come normally with internal batteries. Medium and high
power systems (above 3 to 5 KVA) are frequently designed to allow back-up time expansion
by adding external batteries.

Statistically, the duration of 90% of utility power outages is less than 5 minutes. The amount
of utility grid outages in the Western World is about 10 to 30 per year. If you are not located in
a specific grid troublesome area, you may expect not more than three power failures a year,
lasting beyond 5 minutes.

The main reason to purchase a UPS is to protect data and hardware. Main causes of data
failures happen due to short time phenomena on the utility grid, such as spikes, sags, dips, and
high frequency noise. Uncontrolled, abrupt power failure can harm both software and
hardware. Data in process or not stored on the Hard Disk is totally lost. In some cases, the
Hard Disk may also be entirely destroyed. The harm done to it, if power fails during the writing
period for instance, is like a scratch made by the needle of a phonograph record should you
turn it off while it is playing.

Unless you provide real time service, or your equipment comprises part of a working industrial
process, which can't be interrupted, you might be happy with a system that backs up your loads
for 5 minutes, and then turns off your computer in a graceful manner. In most cases, mains
outage is shorter than 5 minutes, and UPS turn off won�t be needed.
Therefore, 7-to 15 minute's back-up time should suffice for most applications. You should
however make sure that the UPS is supplied with a software suitable for your computer or
network, and that the software exits the system, while storing on computers hard disk all open
files before turning down the computers, the way you would do it manually.

You should also take in account that back up time is strongly related to the size of your load.
UPS systems sense the battery voltage, emitting audible alarm, including load turn off signal to
the computer, to allow orderly shut down, short time before the battery discharges below a
preset voltage level, at which the UPS will turn off. Lower load draws less current from the
batteries, extending the battery time.
APC's type LS 500 UPS for instance with 3.6 minutes backup time at full load, provides 14
minutes back-up time at half load. Thus, over sizing the UPS by 20 to 30%, which good
practice requires doing anyway, will enable you to protect your computer network without
purchasing a system with bigger battery.

In applications which require continuous operation, even during power absence, other and
additional factors should be considered. The battery should be sized according to the required
autonomy time. The size of the UPS may have to be increased because small units do not
allow battery extensions. You may need to increase UPS size also in order to supply the
charging current of an extended external battery, or in order to decrease the charging time
required by big battery bank. You might also consider installation of a backup generator, to
feed the UPS. Cost, availability of space, and reliability, should all be taken in consideration.

3. Decide the type of the UPS you need

UPS's are roughly divided to three main classes. The Off Line UPS is the simplest and the least
expensive, then comes the Line Interactive type, which overcomes the major disadvantages
inherent to the off-line unit, finally the Double Conversion On-Line UPS which provides the best
power protection.

An off-line UPS, also called Stand-by or Back-UP, feeds the load continuously from Mains.
Upon mains outage, the load connections transferred to the inverter fed from the batteries.
Most of the short term spikes surges and high frequency harmonics are decayed by means of
special filters. However, various phenomena of transient nature during mains existence, like
over voltages and deeps can harm protected data. These units are generally equipped with
spike suppressors, aimed to protect the hardware from high voltages on the utility grid.

Off-line systems are used mainly for low power installations, small offices, personal home
computers and other less critical application, where eliminating 85% to 90% of computer
malfunctions caused by power failure, is satisfactory. In such applications, this low cost
solution, is a good value for your money, it decreases tenfold the probability of a harm to data
and hardware from utility Grid. The inverter output voltage in this class is usually non-
sinusoidal, frequently called semi-sinusoidal. Personal computers will probably work nicely
with such waveform. You should however check with your vendor before using other types
of loads.

Professional installations generally require better protection, Here comes the Line Interactive
UPS, called sometimes Interactive UPS, or On Line Interactive UPS. These systems are
also based on stand-by principle, working directly from mains, switching to battery (via the
inverter) upon mains disappearance. Here however, additional circuits filter and correct output
voltage, keeping it within allowed tolerance band, thus providing conditioned power to the
critical load. This UPS will probably have a sinusoidal type inverter, therefore, special
precautions as regards to type of load might not be required.
Line Interactive UPS units come generally in the range of 500 to 5000 watts, offering a popular
solution for servers, personal computers hubs and other loads in this power range. These units
are used mainly for short backup times, sufficient to provide an orderly shut down.

Like the Off-Line unit, also here the stand by behavior, is in most cases the major drawback of
the Interactive UPS. Practically, UPS operation is verified upon mains failure which is the most
critical moment.Upon mains disappearance, the load is connected to the inverter and batteries.
Any dormant malfunction, such as loose internal connection or weak batteries might be
revealed at the very moment when it is called to action.
Neither off-line nor Interactive UPS systems are able to correct frequency. The output
frequency of any stand-by unit is in normal operation the same as the input utility grid. These
units should therefore be avoided on sites equipped with small back-up generators, exhibiting
generally poor frequency regulation.

The Interactive UPS is a professional solution for business application where the main concern
is to provide software protection from about 90% to 95% of utility grid disturbances. These
units may be utilized in relatively small networks, comprising 10-12 computers. They are
applied in such locations as lawyer offices, accountants, sales agencies, engineering offices etc.
Some prefer to back up with Interactive UPS less critical equipment in the company, such as
personal computers connected to LAN network, whereas the severs are backed up by an
ON -LINE UPS.

An On-Line UPS, also called Double Conversion UPS, or True On-Line UPS, is the ultimate
solution for all applications from one, or two kilowatts up to Megawatt sized consumers. Here,
the load is constantly fed from the Inverter, providing conditioned, stabilized sinusoidal voltage.
The utility line in these systems forms a backup source in case of UPS malfunction. The
Transfer switch will automatically transfer the load to mains in case of overload or UPS failure.

The On-Line unit comprises two converting stages. The first stage converts the incoming ac
power to DC, thus creating a dc-buss, which is fed either from mains or from storage battery.
A second stage converts the DC power back to conditioned ac in order to feed the critical load.
Filters on the DC buss and fast regulating circuits in the converters practically isolate the load
from any abnormal utility behavior.
In normal operation, load energy comes from mains, via the Rectifier and the Inverter. Upon
mains outage the battery supplies the energy, which the load requires (by the dc/ac Inverter).
Should the mains voltage return before the battery was fully discharged, the Rectifier will feed
the load through the Inverter and start a battery recharge regime to compensate for the lost
charge.
Otherwise, the UPS will turn off when battery becomes fully discharged. The UPS will resume
automatically normal operation upon mains restoration. Also here, the Rectifier will feed the
load via the Inverter, recharging the battery.
The Output voltage of an On-Line UPS is generally stabilized within one percent tolerance.
Output frequency is locked to input if it is within preset tolerance band, otherwise a free
running crystal controlled clock will dictate the output.

The merits of Double Conversion ON -Line system make it the preferred choice for business
and industrial applications. It is the best solution in spite of the added system and electricity
costs.

It offers the best power protection, covering all types of mains disturbances

There is no size limit. Standard ON-Line UPS's are available to backup any

installation

With the right system, no practical limit exists on the available back-up time.

Many systems allow power extension to satisfy the needs of a growing enterprise

Units can be connected in redundant configuration to increase reliability or in parallel to
enhance output power.

In addition, this is the best choice, considering such issues as modularity, ability to
work from generator, input power factor correction, hot swapping, maintenance, fault
clearing, supervising, and communicating.

4. Estimate the required Reliability

You can rely on the UPS to back-up your system and protect it from most power grid
problems. You should however consider Reliability issues if you provide real time service.
Unfortunately,though the calculations are very simple, some basic mathematical knowledge
might be required, if you want to make the calculations yourself.

Reliability and Availability are the basic functions for measuring UPS non-failure probability.
You should be acquainted also with some additional interrelated terms, which describe different
Reliability aspects.

MTBF (Mean Time Between Failures), represents the average time (generally in hours)
between expected failures. The UPS manufacturer should supply the MTBF Figure (Either by
means of UPS data sheet or when requested.).

Failure Rate (λ = Reciprocal of MTBF) represents the average number of failures during a
particular interval.

Reliability (R (t) = e ^ (- λt)) is defined as the probability of no failure before time t. For
comparison purposes "t" refers normally to one year (8760 hours).

Suppose the MTBF of a UPS is 100,000 hours. The Reliability figure becomes 0.9161
(e ^ (-8760/100,000). We can therefore expect that during one year interval 91.6% of the time
(totaling 334 days) the UPS will operate satisfactory and feed the load with conditioned power,
but during 8.4% of the time (totaling 31 days) the UPS will be down. In case of ON-Line
system the load will be fed during the down time from, the utility grid.

For high mission requirements, this Reliability figure is generally unacceptable. An additional,
Redundant, UPS can be connected to work in parallel with the first unit. Thus, normally both
units share the load. Should one unit fail the second will continue to feed the load. As long as
one system works correctly, the load is supplied with conditioned voltage.
Evaluating the proper formula shows that the Reliability figure will increase to 0.993. Thus
99.3% of the time it will be fed from the UPS. And only 0.7% of the time ( 2.5 days per
annum ) it will depend on the reliability of the utility power. Adding additional redundant units
increases the Reliability to any desired value.

In practice however, no UPS is left incorrect during 31 days. Two Additional Figures are used
to describe the real situation:

MTTR � Mean Time To Repair, expresses in Hours, the total time, which takes to restore the
UPS to normal operation, starting from the moment of its failure until the service personnel
complete their work.

Availability = MTBF/(MTBF + MTTR), represents the degree to which the system is
operational when required.

Assuming that the stand alone UPS in our example will be repaired on average within 24 Hours
(MTTR=24), the Availability becomes 0.99976 (100000/(100000+24)). Meaning that
99.976% of the time the unit will supply conditioned power. The down time now decreases to
2.1 hours per annum. We can further proceed to improve this figure by adding additional
redundant units.

Parallel connection of UPS systems is possible only if units operate at the same frequency, have
same voltage are synchronized, and have an ability to isolate a faulty UPS disconnecting it from
the common output buss.

5. Refine the deal

At these stage you should already know the KVA rating of the UPS you intend to buy, the
type of the UPS (Standby/Line Interactive/On-Line) The autonomy time needed, and the
configuration (Stand alone or Redundant).

Now you can contact a nearby dealer, or try choosing UPS system by yourself or by selecting
a suitable unit on the Web.

Here are some tips to help you while concluding your Power UPS deal:

Increase UPS capacity to allow for future expansion.
Don't buy backup time that you don't need. Longer autonomy time does not mean
better equipment.
In case of Online units make sure you can add external batteries in future, and that you
can increase power by adding parallel units if expected
If you do not see a clear advantage for your application, don't pay more. Higher price
does not necessary mean a better or more reliable unit.
Don't buy any extras if you are not sure that they are needed for your application.

Most can be added in the future whenever required.

Some manufacturers provide 6-figure guarantee to cover damage to your Equipment,
the enormous sums are seldom meaningful, you won�t generally be paid above the value
of your backed up hardware. The main items, such as loss of data, or discontinuity of
service to your clients are in most cases not covered
Make sure that the UPS comes with software which shuts down gracefully and
automatically all backed up computers.
Make sure that the UPS includes surge protectors on input and on its
communication lines.
Make sure that the UPS performs automatic battery tests, warning whenever the
batteries should be replaced. Otherwise you will discover that your computer goes down
when the lights turn off. Don't be tempted to buy manually initiated test. Nobody
remembers to keep the routine.
Avoid falling into a trap of misleading terms. Smart UPS should regulate output Semi-
sinusoidal output is not sinusoidal. On -Line must be double conversion and be able to
regulate output frequency. Note that "Automatic self-test " does not always mean battery test as well
Backup time has no meaning if you don't consider the amount of UPS loading. Partial
load increases dramatically the time the battery backs-up the equipment.
Prefer hot swap-able Batteries. The batteries of some systems are not accessible by the
operator and should be replaced by qualified technician.

When purchasing the UPS, make sure that after sales service is in good hands. Check
who provides the service, how fast is it, and whether it is provided in your premises.

Check Guarantee. Duration, Where and When. It is not uncommon to get several

years guarantee, 7 days per week at your premises.

Check the make of the UPS you are going to buy. Check that manufacturing company

is at least ten years in the UPS business. Get recommendation if it is unknown.

6. Purchase the UPS

The previous steps, provide general information about the system you might need. This
information however does not serve as a substitute to a professional consultation. Unless you
have a professional consultation, we strongly advise to make sure with
the supplier that his equipment works properly with your specific computers
and loads, and that you are covered against any possible malfunctions due
to inadequacy of the offered UPS to your critical hardware.

Numerous manufacturers are probably able to meet your requirements. Most provide well
designed and reliable systems. The top producers are: APC- MGE UPS Systems, GE, Gammatronic, Powerware- Eaton and Liebert- Emerson,

Email or Call Tech Power Group for a detailed consultation

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