Technical Bulletin 3

Understanding UPS Specifications

 

The published specifications for small UPS products are represented in slightly different ways by each UPS manufacturer, to highlight advantages and minimize weaknesses, and often a complete specification is not given. Because an UPS user may not understand many of the terms used to specify UPS equipment, there is a risk that a mistake will be made in the selection of a UPS. As a tool to understand these specifications, the sample spec sheet for a single phase UPS will used. Each entry will be addressed separatelyl

Sample Single Phase UPS Specification
UPS Output: Specification
Voltage 120Vac
Regulation ± 3%
Frequency 60Hz nominal, synchronized, ± 0.05% in battery mode
Power Rating 1000VA
Load Power Factor 0.6
Load Crest Factor 3:1
Transfer Time 5 milliseconds
Current 8.3 Amps
Overload Capability 150%
Waveform Sine-wave
Distortion < 3% for non-linear loads
Transient Regulation ± 5% for a 100% step load change
UPS Input  
Voltage 120V
Voltage Range +10%, -15%
Frequency 60Hz
Maximum Current 12.8 Amps
Power Factor 0.9
UPS Battery  
Type Sealed lead-acid, maintenance free
Rating 12V, 7AH
Number of Cells 3
Diagnostics Periodic automatic test
Autonomy Time 10 minutes at full load
Recharge Time 20 times the outage to 90% capacity.
Overall System  
Architecture Fault-tolerant, Double Conversion, On-Line, Modular
Configuration Single-Phase Input and Output
Bypass Automatic operation
Efficiency 85%
Operation Fully Automatic or Manual Operation,
Control Panel/Metering LED Display, Button Controls, No Metering
Diagnostics & Self Test Self test on startup and periodically

 

 

Communications

 
User Interface RS232 port and Novell style contacts
Networks Via an Ethernet or Token Ring adapter
SNMP Via an SNMP adapter
Modem Via a modem adapter
Mechanical  
Input Hardwired
Output Hardwired
Weight 15.8 kg (35 lbs)
Dimensions (LxWxH) 499 x 134 x 226 mm (19.6 x 5.3 x 8.9 inches)
Environmental  
Audible Noise < 50 dBA
Operating Temperature 0° C to 40° C
Storage Temperature - 20° C to + 60° C
Relative Humidity 5 - 95% non-condensing
Altitude 1000m without derating
Safety UL 1778
EMC - Emissions FCC Class A
EMC - Immunity Not Applicable
Surge Suppression IEEE 587 (ANSI C62.41)
CMNR > 60dB
TMNR > 80dB
Packaging No CFC’s, Recyclable
Options  
Battery Packs Battery Modules and special long autonomy solutions
Maintenance Bypass  
Galvanic Isolation  

 

UPS Output Specifications

Voltage The UPS output voltage must match the requirement of the load. In North America this voltage is 120Vac, in most of Europe 230Vac is used and the rest of the world is 220Vac.
Regulation The regulation specification is the maximum expected deviation for the normal output voltage (e.g. 120Vac) that is expected over the entire range of operating conditions (e.g. load, temperature, altitude). For Off-Line systems the regulation spec refers to the battery backup mode. For On-Line systems the spec is normally ± a few percent an is valid for both the normal and battery backup modes.
Frequency The frequency of the UPS output must match the requirements of the load. Switch-mode power supplies have a wide (47 - 63 Hz) range to be able to operate from either 50 or 60Hz. Other loads may require either 50 or 60Hz.
Power Rating To completely specify a UPS, both the Volt-Ampere (VA) and wattage (Watts) rating must be known. See the section on "Understanding Watts, VA and Power Factor" for an in-depth explanation.
Load Power Factor A power factor (p.f.) in the 0.6 to 0.8 range is typical. A p.f. rating of 0.6 to 0.7 is good for a switch-mode power supply. A p.f of 0.8 is typical for older UPS equipment designs used before the widespread used of computers.

 

Load Crest Factor Most loads used in modern equipment, including computers, require a peak current that is much higher than what is required by a simple load such as a light bulb. The UPS must be able to supply this peak current. The Load Crest Factor is the ratio of the peak to the average (RMS) current.
 

Figure 1. Crest Factor Waveform Illustration
 
Transfer Time

In an On-Line system the transfer time refers to a change from the bypass source to the UPS and vise-versa. A typical time is ¼ of a cycle or about 5-6 milliseconds.

In an Off-Line system the transfer time refers to the change from the utility source to the inverter when there is a power outage and is about 5 to 10 milliseconds.

Current The output current refers to the maximum current available at the normal output voltage.
Overload Capability If more load is added to a UPS and the new total is more than the rating, the UPS must be able to continue to function. At typical Overload specification is 1.25% for 10 minutes or 150% for 1 minute.
Waveform Not all UPS products have a sine-wave output and most modern loads do not require a perfect sine-wave source. Lower cost UPS equipment will have a quasi square-wave or a step sine-wave output waveform.

 

UPS Output Specifications

Distortion If the UPS has a sine-wave output a distortion figure indicates how good the wave shape is. A typical distortion value is 3% for linear loads and 5% for non-linear load.
Transient Regulation When a load is added or removed the UPS inverter has to adjust. The maximum amount the output voltage deviates from the nominal value indicates the transient regulation. If the voltage deviates too much, some loads will fail.

UPS Input Specifications

Voltage The input voltage specification is the nominal voltage the UPS expects to see on its input.
Voltage Range All UPS products are designed to operate over a range of input voltages. A typical range is +10% to -15% or 102Vac to 132Vac for North America or 196Vac to 253Vac for Europe.
Frequency Modern UPS equipment will operate on either 50 or 60Hz as long as the voltage is within the specified limits. Some UPS equipment is designed to operate only on 50 or 60Hz.
Maximum Current The maximum current occurs when the UPS is fully loaded and the input voltage is at the minimum allowed for normal operation (usually about -15%).
Power Factor The UPS input is a load on the utility power source. The UPS input power factor (p.f.) varies with how much load is on the output of the UPS and in modern UPS equipment is 0.9 or better. In large UPS equipment (e.g. 25kVA) the input power factor could be a very important part of the overall system design. In small UPS equipment, power factor is usually not a concern unless many units are being installed.

 

 

To simplify power calculations in an AC system, a technical term called the power factor (p.f.) is used. The power factor is a number between 0.0 and 1.0 representing the fraction of the total power delivered by the AC source which is consumed by the load to do work. Mathematically, power factor is defined as:

abbreviated as p.f.

UPS Battery Specifications

Type The most common battery used in UPS equipment is a sealed, lead-acid, maintenance free type. Ni-Cad (Nickel-Cadmium) batteries are a more expensive option. Large UPS systems often use wet, lead-acid storage batteries.
Rating Batteries are rated by specifying an open circuit DC voltage and a Ampere-Hour (AH) rating.

UPS Battery Specifications

Number of Cells Battery cells are normally connected in series to form a string. Often battery strings are connected in parallel.
Diagnostics The battery is the weakest link in a UPS system. Automatic checking and diagnostics of battery problems is now common in UPS products. Testing must not put the load in jeopardy.

 

No. of Modules

2

3

4

5

6

7

8

9

10

11

12
Load1

Battery Autonomy Time Estimate (Minutes)
400 VA

18.5

30

43

52

66

70

73

77

81

84

91
800 VA  

11

15

21

28

35

42

49

53

56

63
1200 VA    

10

12

15

19

24

27

33

38

42
1600 VA      

9

11

13

15

19

21

24

29
2000 VA        

9

10

12

14

15

18

21
2400 VA          

9

10

11

12

14

15
2800 VA            

9

10

10.5

11

12
3200 VA              

8

9

10

11
3600 VA                

8

9

10
4000 VA                  

8

9
4400 VA                    

8

Sample Battery Autonomy Chart

 

Autonomy TimeThe autonomy time is specified at full load so normally more time is actually available because a UPS is normally not loaded to 100% of capacity.
Recharge Time After a power failure, the battery will have to be recharged. Typically it takes 20 times the discharge time to recharge a battery to 90% of its capacity.

Overall System Specifications

Architecture The architecture of the UPS specifies if it is fault-tolerant, redundant, single or double conversion, etc. Some common architectures are Off-Line, Line-Interactive, True On-Line and Fault-tolerant.
Configuration Configuration specifies the number of input and output phases. Low power UPS equipment will be one phase input and output. Larger equipment will have three phases for both the input and output. In the mid range, three phase input with a single phase output is used.
Bypass A bypass specification is given for True On-Line UPS products. The bypass circuitry operates automatically when needed. In larger UPS equipment it is normally possible to manually override the automatic operation. Both relay and static bypass circuits are common and offer ¼ cycle (4-6 mS) transfer times, which is adequate for most applications.

 

 

Overall System Specifications

 

 

Figure 2 True On-Line UPS in Normal Model
 
Figure 2 shows a typical single phase configuration using a double conversion true On-Line configuration. The bypass path is shown.

 

Efficiency In a low power UPS, the efficiency is not big concern. In larger power ratings, a more efficient UPS saves on operating costs. Because Off-Line units only have to trickle charge the battery, the can be 97% efficient or better. On-Line UPS units will be 80 to 90% efficient in low an medium power designs and up to 94% efficient in large (>500kVA) models.
Operation Most currently available UPS equipment is designed to be "plug and play," which means as soon as power is applied, it starts automatically. Older and big UPS equipment requires commissioning and a controlled start up procedure.
Control Panel/Metering All UPS equipment has a control panel to indicate the UPS is operating normally or in a fault condition. Where required metering is available as either a standard or optional feature.
Diagnostics & Self Test Modern UPS equipment will do a self-test each time it is powered up, periodically and when requested to do so. Diagnostic information is stored and available via display and RS-232 communications.

Communications Specifications

User Interface The most common user interface is a set of two contacts that are Novell compatible. More expensive units will have an RS-232 port for use with software that controls, monitors and automatically and safely shuts down a server or workstation.
Networks The two common ways to interface the UPS to a network are 1) via an Ethernet or Token Ring adapter and 2) via special software on a computer that is connected to both the network and the UPS.

Figure 2 Typical Network to UPS Connection
 
SNMP An special adapter is needed to connect the UPS to a network running SNMP (Simple Network Management Protocol).
Modem The RS-232 port can be connected to a model which will allow dialing out and receiving calls to check the UPS status and to control the UPS.

Mechanical Specifications

Input To supply power to the UPS input, a power cord is normally used. If the maximum input current is above 10 Amps, hardwired terminal connections are normally used.
Output The UPS will normally have country specific power outlets or 10 Amp IEC 320 outlets. If the maximum output current is above 10 Amps, hardwired terminal connections are used.
Weight The weight is self explanatory.
Dimensions (LxWxH) The Dimensions are self explanatory.

Environmental Specifications

Audible Noise An audible noise figure of 50dBA or less is acceptable for most office environments. Larger UPS units, designed for installation in equipment rooms might have an audible noise figure of up to 70dBA
Operating Temperature Normally the operating temperature is 0° C to 40° C.
Storage Temperature The normal temperature range for storage is - 20° C to + 60° C. If the battery is internal to the UPS, this storage range may not be appropriate without causing damage to the battery.
Relative Humidity The standard relative humidity specification is up to 95% with out condensation.
Altitude The thinner air at high altitudes is not as effective at cooling electrical equipment. A limit on altitude is specified to guarantee operation at 40° C. If the maximum temperature is limited to less than the 40 degree limit, the UPS can be operated at higher altitudes.
 
   

Figure 3 Sample derating curve for high altitude.
   
Safety The safety standard for UPS equipment in the US is UL 1778. In Europe the equivalent standard is EN 50091-1.

Environmental Specifications

EMC - Emissions All UPS equipment in the US must meet FCC Class A and UPS models designed specifically for office environments must meet FCC Class B. In Europe CISPR 22 Class B is required for office environments and Class A is acceptable for industrial environments.
EMC - Immunity There are no immunity requirements in the US. In Europe immunity is covered by the generic standard, EN50082 and EN50091-2.
Surge Suppression Surge testing to ANSI C62.41 (formerly IEEE 587) is standard in high quality UPS equipment.
CMNR The Common Mode Noise Reduction, (CMNR) capability of a UPS is not something that one should normally be worried about. The CMNR value varies with frequency and cannot be understood unless a graph over a frequency range is given.
TMNR The Transverse Mode Noise Reduction capability of a UPS varies with the architecture used. A True On-Line UPS gives the best TMNR performance. Like CMNR, TMNR varies with frequency.
Packaging In Europe, the packaging of modern UPS equipment must be recyclable. The process of making the UPS, including the shipping box, must not use CFCs.

Options

Battery Packs Small modular UPS equipment will have expansion battery packs to add autonomy time.
Maintenance Bypass A maintenance bypass allows removal of a UPS or repair and test of a UPS while the critical load is operating from the bypass source.
Galvanic Isolation An isolation transformer provides galvanic Isolation and is normally not needed. In special cases where galvanic isolation is required, it can be added as an accessory to the UPS or as a locally purchased item.


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