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Home> Features & Articles> Uninterrupted Power Supply (UPS) Overview

Uninterruptible Power Supply (UPS) Overview
General information about Uninterruptible Power Supplies to help you choose the right equipment.

Front view of a typical UPSWhat is a UPS?
An Uninterruptible Power Supply (UPS) is basically a device that maintains the correct power to your equipment in the event of mains failure or poor quality mains supply.

A UPS is typically used to protect computers and other digitally controlled equipment.

An Uninterruptible Power Supply uses backup batteries and has an automatic controller that ensures continuous supply when the mains power feed is interrupted.

The Uninterruptible Power Supply Unit keeps a computer and its data safe and running what ever happens to the power supply - (be it: lightning, voltage drop, current surge, fuse blow or power cut). It can be programmed to save your current work on a computer and close down after a set number of minutes and so protect the computer and its data.

A UPS should not be confused with a standby generator, which does not provide protection from momentary power interruptions and could itself cause an interruption as it switches into service, even if set for automatic operation. If you are an outside Event Photographer needing to use a generator, it is recommended that you use it with a UPS. It is advisable to purchase the generator and UPS as a matched set rather than as separate items; so check with your supplier.

Many people have heard of UPS systems and assume that they are all the same; so much so that that the term has become generic. In fact there are many different types of UPS implemented in different ways according to the requirements of the devices that they are supplying with power.

UPS Types
Choosing a UPS needs careful consideration if it is to safely fulfil the function required of it. Before they purchase a UPS, users will need to be aware of the various types of UPS available and to have calculated what the power demand of their equipment is likely to be. Having established this they will also need to consider how much time they need from the battery backup to safely shut down all equipment such as computers without loss of data.
There are various implementations of UPS units; some being more efficient than others, but for the non-technical user the types can broadly be split into two main groups: Off-line UPS and On-line UPS. Rear view of a typical UPS

Off-line UPS
Sometimes referred to as a Standby UPS, an Off-line UPS is the cheapest and simplest solution. With this type of UPS your equipment is supplied by the Primary Power Source (mains) direct until an interruption to the mains supply occurs and the system switches to the Secondary Power Source (battery-backup). When choosing a UPS of this type it is important to ensure that the Primary Power Source is electrically surge suppressed and possesses interference filter features to properly protect your equipment.
During normal operation the battery charger unit is using mains power to charge the battery, and the battery and inverter are waiting "on standby" until they are needed. Should the AC power be interrupted, the transfer switch or transformer automatically changes over to the secondary power source (battery) and when mains power is restored, the UPS switches back to the primary source.

Off-line UPS's are typically available in a size range of between 200VA to 1000 VA. (Watts VA ?)

Note: An off-line UPS will not work with a generator as the input power is usually not "clean" enough for the UPS to switch back.

On-line UPS
Often referred to as a True UPS because under normal operation the On-line UPS is always running off the battery, via its inverter, while the mains power runs the battery charger. This arrangement now has the battery / inverter as the Primary Power Source and so makes an On-line UPS impervious to brief power cuts or voltage sags as the Primary Power Source is maintained and not "switched" if the mains supply fails.
Also whatever the quality of the mains supply, an On-line UPS battery acts as a surge suppressor, filtering line noise and providing protection against harmful spikes.
However, should the mains supply or generator fail for any extended length of time, the battery allows enough time for equipment to be shut down safely.
A suppressed and filtered direct mains supply is also included in the event of battery / inverter component failure.

On-line UPS units typically range from 1000VA to 5000 VA. (Watts VA ?)

Typical Device Power Ratings

Device Power
Computer 0.4 KVA
Phones / routers 0.3 KVA
Flood light 1.0 KVA

Useful Terms:

Transfer Time
The time required for the UPS to switch from mains to battery power. For a true online UPS this will be zero. For off-line (standby) units, it will normally be a few milliseconds.

Hold (or holdup) Time
The hold time of a UPS represents the amount of time the power supply can handle having its input cut off before being interrupted. If the transfer time is close to or larger than the hold time, the UPS may not suitable for your needs.

Specifications to be aware of when choosing a UPS:

UPS TypeThe general design of the UPS.
Very important to check this first.
Load RatingThe nominal maximum capacity of the unit in VA (VoltAmperes). Many units will also specify explicitly the W (Watts) rating of the unit; otherwise you need to determine the
UPS's power factor from the manufacturer to properly determine sizing.
Input VoltageNominal and actual allowable range specifications. Make sure you are getting the right model for your local mains supply and/or portable generator.
Nominal Frequency Generally 50 Hz. (Some models though will automatically handle up to 60Hz)..
Input ConnectionThe right type of plug (correctly fused) and power cord; very important to have the correct amperage for larger units.
Output VoltageNominal and actual range specifications will be provided. Should be the same as the nominal input voltage.
Output Waveform TypeWhether the unit produces a sine, square, or modified square output waveform.
Transfer TimeAn important specification: the typical and/or maximum values for the time required for the UPS to switch from mains to battery power. For a true online UPS this will be zero. For standby units it will normally be a few milliseconds.
Filtering Suppression
and Regulation
Details on the hardware within the UPS used to clean up the mains when the unit is running on AC power.
Operating Temperature
Like PC's and other electrical equipment, UPS's generate heat and cannot be run in a room with insufficient cooling.
DimensionsHeight, width and depth. Check to make sure the unit will fit where you want it to go.
WeightSome larger units are very heavy and may require two people to move.
and Alarms
A short listing of the indicator LEDs on the unit, and conditions which trigger alarms.
Control Monitoring
Hardware and Software
A brief description of any included or optional control and monitoring systems, including a specification of the interface types supported by the unit.
Certifications & ApprovalsWhich certification bodies have approved the unit.
Battery TypeThe type of battery and whether it is user-replaceable.
Battery CapacityBattery capacity in Ah.
Typical Battery LifeNumber of years the battery is expected to last for average use.
Typical Run Time at
Full Load
If the unit powers a load with a VA rating equal to its maximum load, the expected number of minutes of run time.
Typical Run Time at
Half Load
If the unit powers a load with a VA rating of half its maximum load, the expected number of minutes of run time.
Typical Recharge TimeHow many hours it takes to fully charge a discharged battery from mains power.
Battery ExpansionWhat sort of expansion features the UPS has; can extra cells be added to upgrade the unit.
Storage Temperature
Pay attention especially to the lower end of the range as batteries can freeze and be ruined if subjected to excessive cold.

See: Standby / Emergency Electrical Generator

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