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Uninterruptible Power Supply (UPS)

Uninterruptible Power Supply (UPS)

What is a UPS

An Uninterruptible Power Supply is a device that sits between a power supply (e.g. a wall outlet) and a device (e.g. a computer) to prevent undesired features of the power source (outages, sags, surges, bad harmonics, etc.) from the supply from adversely affecting the performance of the device.

There are basically three different types of devices that are termed UPS’s.

Standby Power Supply (SPS)

In this type of supply, power is usually derived directly from the power line, until power fails.  After power failure, a battery- powered inverter turns on to continue supplying power.  Batteries are charged, as necessary, when line power is available. This type of supply is sometimes called an "offline" UPS or ‘Battery Backup’.

The quality and effectiveness of this class of devices varies considerably; however, they are generally quite a bit cheaper than a "true" UPS.  The time required for the inverter to come on line, typically called the switchover time, varies by unit.  (Most UPS units claim a transfer time to battery of about 4ms. Note that even if a computer can stay up for 100ms, it doesn't mean that 100ms switchover is okay.  Damage can still be done to a computer or data on it even if it stays up.)  Other features to note in this class of supply is line filtering and / or line conditioners.  Since appliances connected to the supply are basically connected directly from the power line, SPSs provide relatively poor protection from line noise, frequency variations, line spikes, and ‘brownouts’.

Hybrid UPS systems

(or Constant Voltage Transformer.)  When normal operating line power is present, the supply conditions power using a Ferro resonant transformer. This transformer maintains a constant output voltage even with a varying input voltage, and provides good protection against line noise. The transformer also maintains output briefly when a total power outage occurs.

"True" UPS systems

Those supplies that continuously operate from an inverter.  There is no switchover time, and these supplies generally provide the best isolation from power line problems.  The disadvantages to these devices are increased cost, increased power consumption, and increased heat generation.  Despite the fact that the inverter in a "true" UPS is always on, the reliability of such units does not seem to be affected.  In fact, there are more failures in cheaper SPS units.

How does UPS work

UPS -    acts as a power conditioner.  It takes raw commercial power, “cleans” it up and regulates it.  Then it provides this “clean” output power to your protected loads.  When the commercial power drops out, or falls below a predetermined voltage level (known as a “brownout”), the UPS switches to its inverter mode.  In the inverter mode, the batteries provide DC power to the inverter, which then feeds the transformer and provides power to your protected critical loads. 

A UPS performs the following functions:

  • Absorb relatively small power surges.
  • Smooth out noisy power sources.
  • Continue to provide power to equipment during line sags.
  • Provide power for some time after a blackout has occurred.
  • Some UPS or UPS/software combinations also provide the following functions:
  • Automatic shutdown of equipment during long power outages.
  • Monitoring and logging of the status of the power supply.
  • Display the Voltage / Current draw of the equipment.
  • Restart equipment after a long power outage.
  • Display the voltage currently on the line.
  • Provide alarms on certain error conditions.
  • Provide short circuit protection.

In a true UPS, the whole switching operation is transparent to the protected equipment.  The critical load never knows its power source has changed.  (A Standby Power System (SPS) breaks the power during switching which interrupts the operation of the load equipment).  Some UPS’s monitor the frequency and sample the waveform to insure that a minimum output power standard is maintained.  Any time commercial power falls below the preset minimums, the UPS switches to its inverter mode and provides “clean” power to your protected load equipment. 

Generally speaking, UPS’s are used to provide short-term or “ride through” power to allow protected load equipment to keep functioning though commercial power glitches, or short-term outages.  Alarms notify users to end their tasks and shut down their equipment before the UPS runs out of battery power, thus saving valuable information and minimizing or altogether eliminating system recovery times.  Extended runtimes can be achieved through use of larger batteries or multiple battery strings.

Downtime costs vary with each business.  To determine your UPS needs, you need to determine just what downtime costs you.  Ten or fifteen minutes of downtime might mean you do some other productive task until the power comes back.  It may cost you little or nothing, except the inconvenience of having to rearrange your schedule.  On the other hand, downtime may cost you several hundred thousand Rands a day in lost revenues.  In some cases, downtime can even cost you your business.  If, for instance, you are an Internet Service Provider, excessive downtime could put you out of business.  While it might take you anywhere from a few minutes to a few weeks to recover lost data, you might never recover lost customers.  In a manufacturing application, downtime might interrupt a manufacturing process and cause you to scrap an entire production run.

The point is, in this electronic age, you need power protection.  You can prove it to yourself if you take the time to add up the downtime costs to your business. The formula is quite easy to remember:

$$ x downtime = loss
$$ x uptime = profit

The next step in determining your UPS requirement is to determine what your “critical load” is.  What equipment has to be operating to keep you up and running? How long can you keep going if your telephone system goes down?  How about your LAN / WAN?  If you had to do business with a short-term “skeleton crew” of computers, what would you keep running and what could you do without?

When you know what equipment you need to protect, you need to size the UPS.  This is usually best left to the industry experts / vendors who have ample experience in calculating UPS requirements for various types of office equipment.

Herewith a basic explanation, with some guidelines for sizing a UPS

The basic building block for sizing a UPS is the VA (Volt Amp) rating of the load equipment. The VA rating is obtained by multiplying the volts x the amps. To determine the VA rating for a piece of equipment, either measure the voltage and amperage with a meter, or take the information off the nameplate.  Manufacturers usually use worst-case scenarios for nameplate information.  If it says 3 amps @ 115V, it’s a safe bet it will never draw more than 345VA. (In reality, it’ll probably draw a whole lot less).  Repeat this step with each piece of equipment that you want to protect.  Make sure you write down the voltage and the amperage for each one. Add up all your figures, and you’ll know approximately what size UPS you need.  If you go strictly on nameplate readings, you’ll have room to spare.  If you base your calculations on measured readings, you may want to add 25% or so to the total to make sure you’ve got it covered.  The only consequence of over sizing your UPS is a higher cost and extra runtime.  If you have any questions about the voltage or amperage of a piece of equipment, the manufacturer’s Tech Support should be able to provide you with the correct information.

There is one important element to your load equipment that you need to be aware of.  Copy machines, laser printers, and any equipment with heaters, pumps or compressors are sudden death to most UPS’s.  They cause output voltage sags that can shut your UPS down in a heartbeat.  Older equipment - CPU's, disks, monitors, etc. - almost universally requires more power than new equipment.  Care must be taken to properly size a UPS that will be protecting any of these types of equipment.

Once you know the voltage and amperage of your load equipment, and the approximate size UPS you need, you still have to determine the amount of runtime you want your system to have.

Extended Runtime Options

For most extended runtime packages, larger, heavier batteries are used in place of standard batteries.  In cases where runtime requirements exceed the capability of the batteries that will physically fit inside the UPS, or in its standard external battery cabinet, additional battery strings and cabinets must be used.  A battery cabinet is an enclosed case where the batteries are stacked on top of each other on shelves. 

Batteries - even sealed batteries – require maintenance.  There is no such thing as “maintenance-free” batteries.  Voltages must be measured, posts must be cleaned, connections must be tightened and load tests must be performed.

One bad battery in a string can drag down all of the batteries in the string.  Thus, when you find a bad battery, replace it immediately - don’t assume the “good” ones will cover for the “bad” one.  It will drag the “good” ones down to its level.  In battery language, it’s called equalizing.

With a healthy battery string, equalizing is beneficial.  It keeps all of the batteries at about the same charge. When one battery is seriously deficient, equalizing can be detrimental because it keeps all of the batteries at a reduced charge.

Although the cost of replacing all UPS batteries every three to five years can be high, the cost of taking the “cheap” route can ultimately be much higher … DOWNTIME.

Battery costs are not excessive with one or two strings of batteries.  Beyond the extended run times provided by two strings of batteries, though, you may want to consider purchasing an AC or DC generator.

The last step in determining your power needs is to identify your existing commercial power source.  What is your building voltage?  Does the service panel have enough circuit-breaker space and ‘ampacity’ to power your UPS? 

Once you have your sizing, runtime and service panel information you are ready for business!

Benefits of UPS to business:

  • Computer functions don't stop because the power fails.
  • Users are not inconvenienced by a computer shutting down.
  • Equipment does not incur the stress of another (hard) power cycle.
  • Data isn't lost because a machine shut down without doing a "sync" or equivalent to flush cached or real time data.



A complete loss of power – a voltage drop below about 80V is often considered to be a blackout since most equipment will not operate below these levels.

Sag or Brownout

A decrease in voltage levels that can last for periods ranging from fractions of a second to hours.  Can be caused by heavy equipment coming on line such as shop tools, elevators, compressors etc.  Also occurs when utility companies deliberately do this to cope with peak load times.


An instantaneous and tremendous increase in voltage often caused by a direct lightning strike on a power line, or when power returns after a blackout.


A substantial increase in voltage lasting a fraction of a second - often caused when high-powered appliances such as air conditioners are switched off.


Electromagnetic Interference and Radio Frequency Interference.  Caused by, among others, lightning, generators, radio transmitters, and industrial equipment. 


Circuitry that converts DC (Direct Current) battery power to AC (Alternating Current) power, required by most computer equipment.

Surge Protector

Circuitry consisting of capacitors and rod-core inductors etc., for suppressing surges and spikes - usually embedded in a power strip.

Line Conditioner            

A transformer that attempts to smooth out fluctuations in input voltage to provide near-uniform output voltage or voltage waveform.