The Role of Isolation 

Transformers in Data Center 

UPS Systems 

Contents 

> Executive summary

Revision 0 

by Neil Rasmussen 

White Paper 98 

Introduction 2

The role of transformers 

in UPS systems 

2

Transformer arrangements 

in practical UPS systems 

10

Transformer-based legacy 

UPS designs 

21

Transformer options within 

a UPS product 

24

Conclusion 25

Resources 26

 

 

Click on a section to jump to it 

Most modern UPS systems do not include the internal 

transformers that were present in earlier designs. This 

evolution has increased efficiency while decreasing the 

weight, size, and raw materials consumption of UPS 

systems. In the new transformerless UPS designs, the 

transformers are optional and can be placed in the best 

location to achieve a required purpose. In older 

designs, transformers were typically installed in 

permanent positions where they provided no benefit, 

reduced system efficiency, or were not optimally 

located. 

This paper considers the function of transformers in 

UPS systems, when and how transformers should be 

used, and how the absence of internal transformers in 

newer UPS designs frequently improves data center 

design and performance. 

 white papers are now part of the Schneider Electric white paper library

produced by Schneider Electric’s Data Center Science Center 

DCSC@Schneider-Electric.com 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 2

Every data center power system includes transformers. Isolation transformers have 

historically had a number of different roles in the power architecture of data centers: 

• Voltage stepdown from medium-voltage mains supply to the utilization voltage 

• Within a UPS, to act as an integral part of the power conversion circuits 

• To create a local ground-bonded neutral 

• Within power distribution units or a UPS, to reduce harmonic currents 

• To provide taps to accommodate abnormally high or low mains voltage 

• To eliminate ground loops with multiple generators or mains sources 

• Within power distribution units, for stepdown from the data center distribution voltage of 

480V or 600V to 208V (North America only) 

• Within power distribution units, to provide additional utilization voltages (e.g., 120V in 

North America and 100V in Japan) 

UPS systems have historically had one or more permanently installed internal isolation 

transformers to provide one or more of the above functions, depending on the design of the 

data center power system. Newer UPS systems do not require power transformers as part of 

their circuits, improving efficiency and reducing weight, size, and cost. Instead, transformers 

are added to a transformerless UPS as needed to achieve a desired function. This paper will 

show why, in most data center power system designs, transformerless UPSs provide the 

same function and performance as older UPSs with internal transformers. Nevertheless, 

there are many cases where a transformer is required and must be either internal to the UPS 

or added externally. Even older UPS systems with internal transformers require additional 

external transformers in many cases. Most importantly, this paper explains why in almost all 

cases where a transformer is needed, the transformerless UPS design is superior because it 

allows the transformer to be installed in a more optimal part of the power path. 

First, this paper explains why older UPSs have transformers and the circumstances under 

which transformerless UPSs behave differently. Second, different transformer arrangements 

will be reviewed and the preferred arrangements will be described. This paper focuses on 

the use of transformers specifically in relation to UPS systems. For a more general analysis 

of grounding in the data center, refer to White Paper 97, Specifying Appropriate Data Center 

Earthing Methods. 

It is widely believed that internal transformers were originally provided in UPS systems in 

order to provide galvanic isolation between the UPS input and output. This is false. The 

reason internal transformers were used in early UPS systems is because they were 

inherently required due to the power inverter technology used in the UPS design. 

The first UPS products were developed over 40 years ago and used a ground-referenced 

battery system. The grounded electronics and battery configuration required these systems 

to have two transformers for isolation from the mains – one on the input rectifier, and one in 

the output inverter. Later improvements in these designs, where the battery bus was moved 

to the neutral wire or electrically floated, eliminated one of the transformers, typically 

removing the rectifier transformer. Taking advantage of high-voltage, high-speed power 

semiconductors that did not exist 15 years ago, most recent UPS designs use newer designs 

that eliminate both the input and output transformer. 

Introduction 

The role of 

transformers 

in UPS 

systems 

 In almost all cases 

where a transformer is 

needed, the transformerless 

UPS design is superior 

because it allows the 

transformer to be installed 

in a more optimal part 

of the power path 

“

”

Specifying Appropriate Data 

Center Earthing 

Related resource 

White Paper 97 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 3

In some data center power system designs, no transformers are needed in conjunction with 

the UPS; but there are many situations where transformers must, or are advised to be, 

installed in combination with the UPS. This paper will explain that there are 92 different 

possible combinations of transformers used with a single UPS system, with many more 

combinations possible with parallel or other redundancy configurations. It is not possible to 

consider the use of transformers in a UPS system without understanding the important 

differences between the different options. 

The configuration of a UPS system falls into three basic categories, differentiated by the 

presence of a static bypass and how it is connected. These three categories are commonly 

called single mains, dual mains, and single mains without bypass. They are 

schematically illustrated in Figure 1. 

In the single mains configuration, one mains connection supplies both the bypass and UPS 

module which are connected together at the UPS. This is the most common arrangement, 

and is the only arrangement supported in many small UPS systems. It is found in most 

smaller data center installations and also found in many large data center installations. The 

main benefits of this system are the simplicity and low cost of installation and the fact that 

many complexities relating to circulating currents and grounding are eliminated. The 

downside of this system is that the actual mains supply system cannot be taken down for 

maintenance without disrupting power to the critical load, although some of these 

disadvantages can be overcome with wraparound breakers on the mains. 

The dual mains configuration is required when the bypass is fed from a second mains that is 

different from the mains feeding the UPS rectifier input. The difference in the mains can 

range from minor (e.g., they are fed from different breakers on the same panel) to major (e.g. 

they come from completely independent sources with different ground systems and even 

different voltages). There are a variety of data center redundancy architectures that specify 

this type of configuration (see White Paper 75, Comparing UPS System Design 

Configurations). Another reason for the dual mains configuration is to allow either of the two 

mains to be taken down for maintenance while providing power to the critical load. Note that 

this configuration can be used, but is not required, when a generator is used, since the 

generator is typically connected to the mains bus upstream of the UPS with an automatic 

transfer switch (ATS) so that it can provide power to other loads, such as chillers, in addition 

to the UPS. The dual mains configuration is required in some data center architectures, and 

is chosen by preference in many larger data centers in order to allow for concurrent 

Figure 1 

Three basic configurations of 

mains and bypass for a UPS 

system 

Bypass

Mains

Output

Bypass

DUAL MAINS

Mains 2

Mains 1 Output

SINGLE MAINS 

WITHOUT BYPASS Mains Output

SINGLE MAINS

UPS 

module

UPS 

module

UPS 

module

Comparing UPS System 

Design Configurations 

Related resource 

White Paper 75 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 4

maintenance and/or to slightly improve the overall system reliability by preventing the wiring 

and breaker upstream of the UPS from being a single failure point for the power system. 

The final configuration, single mains without bypass, is mainly used in environments where 

the mains power quality is considered to be extremely poor, to the point where it has been 

determined that it is not desired to ever power the critical load from the mains via a bypass. 

This can occur in industrial situations, shipboard, or on small islands, where the mains 

frequency (50 or 60Hz) is not the same as the IT load frequency, or in stressed electrical 

grids in developing nations. In some countries (the United States, for example) this is an 

extremely uncommon approach, but in others (India, for example) it is quite common and may 

be the majority of installs in some regions. 

Note: A complete analysis comparing the three configurations above would involve tradeoffs 

in price, complexity, reliability, maintainability, and power quality; such an analysis is beyond 

the scope of this paper. This paper will focus on how to use transformers in each 

configuration. 

The three configurations described above can include one or more transformers in the power 

path. Figure 2 shows the possible locations of transformers in the three UPS configurations. 

The basic configurations shown in Figure 2 – various combinations of UPS modules, 

transformers, and bypass – are referred to in this paper as “UPS systems”. Likewise, “UPS 

module” refers to the power converters and energy storage component of the UPS system 

that together provide uninterruptible power. A “UPS product” refers to a device in an 

enclosure that contains at least a UPS module, but may also contain various transformers 

and bypass devices. Because the UPS products sold in the market vary greatly in what is 

included in the “product” (bypass and transformer options), this paper will primarily use the 

more specific terms “UPS system” and “UPS module”. 

UPS products are often described as either “transformer-based” or “transformerless”. This 

distinction refers to the presence or absence of an inverter transformer in the UPS product 

enclosure. The various transformers shown in Figure 2 other than the inverter transformer 

Figure 2 

Possible location of isolation 

transformers in the three basic 

UPS system configurations 

Bypass

Mains

transformer Output

transformer

Inverter

transformer

Bypass

Mains rectifier

transformer

Output

transformer

Inverter

transformer

Mains bypass 

transformer

Mains 

transformer

Output

transformer

Inverter

transformer

UPS 

module

UPS 

module

UPS 

module

DUAL MAINS

SINGLE MAINS 

WITHOUT BYPASS

SINGLE MAINS
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 5

are all options that are either optional within a UPS product or can be installed externally to 

the UPS enclosure. This issue is very important to the following discussion and must be 

further clarified: the distinction between a transformerless UPS product and a transformerbased UPS product is the presence of the inverter transformer. All of the other transformers 

that might be used in a UPS system are optional and can be used with either a transformerbased or a transformerless UPS product. 

Note that in any installation there are upstream transformers providing power to the UPS 

system and other loads. The mains, bypass, and rectifier transformers in the diagrams in 

Figure 2 represent transformers that are specifically dedicated to the UPS system; they are 

separate from the transformers that step the voltage down from medium voltage. 

In each of these three UPS system configurations any combination of transformers may be 

present – from none to all. For the single mains configuration, there are 8 possible 

transformer arrangements; for dual mains there are 16 arrangements, and for single mains 

without bypass there are 8 arrangements, for a total of 32 possible arrangements. 

Furthermore, the mains transformers and output transformers can be located either locally or 

remotely from the UPS, which affects the grounding system. This adds an additional 60 

variations, for a total of 92 ways transformers can be installed with a single UPS. Virtually all 

of the 92 transformer installation variations have been used in real installations. However, 

not all transformer arrangements are logical, and there are a few that offer a superior 

combination of performance, economy, and efficiency. To understand when the use of a 

transformer is required or why various transformer locations exist for the three UPS 

configurations, we first must consider the effect of transformers on the neutral and ground 

wiring. 

Characteristics of transformers 

There are different types of transformers, but in this paper a transformer refers to a “deltawye” configuration, which is the type used in almost all UPS applications. Delta-wye 

transformers have a number of characteristics, both good and bad, that impact their use in 

UPS systems: 

1. Isolation of the output neutral from the source 

2. Voltage change (for example, 480V to 208V) 

3. Impedance that limits fault current or acts as a noise filter 

4. Blocking the 3rd, 9th, 15th, and other multiples-of-three harmonic currents 

5. Weight, cost, consumption of natural materials, and taking up space 

6. Electrical losses and contribution to data center inefficiency 

The first four are benefits, and the last two are undesirable penalties. Because the last two 

penalties are severe, transformers should only be used when the beneficial characteristics 

are useful to the mission of the data center. 

Voltage change This is necessary in applications where the mains voltage is not the same 

as the voltage used by the IT equipment. This is a common condition in North America 

where the mains voltage is 480 or 600V in larger data centers. In most of the world, the 

400/230V three-phase mains voltage is the same voltage used by the IT load equipment, so 

this function is not required. 

Impedance This is generally secondary and unimportant in the modern data center. Most 

designs do not require additional impedance, and if it were required it would be more 

effective to create it with a power inductor (sometimes called a “choke”), which is smaller, 

lighter, and more efficient than a transformer. 

Good 

characteristics 

Bad 

characteristics

 Virtually all of the 92 

transformer arrangements 

have been used in real 

installations. However, not all 

arrangements are logical, and 

there are a few that offer a 

superior combination of performance, economy, 

and efficiency. 

“

”
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 6

Blocking harmonics This was historically a useful function to prevent the harmonic 

currents created by the UPS from affecting the mains, and to prevent IT-load harmonic 

currents from affecting the mains via the UPS bypass. However, two major changes have 

changed this situation: both the modern UPS and modern IT loads are “power factor 

corrected”, which means their harmonic current generation has been dramatically reduced to 

the point where no additional filtering is necessary. Therefore, the use of transformers to 

reduce harmonic currents is no longer a necessary function in the modern data center. This 

subject is discussed in detail in White Paper 26, Hazards of Harmonics and Neutral 

Overloads. 

The first three beneficial characteristics therefore have limited or obsolete value, which 

leaves the fourth characteristic – isolation of the neutral from the source – which is by far the 

most important. 

Isolation of neutral from the source This is the most important characteristic of a 

transformer – it is this characteristic that causes transformers to be useful, necessary, or 

even legally mandated under certain conditions. The remainder of this paper will focus on 

this attribute of transformers and how it impacts the use of UPSs in data centers. Because 

isolation of the neutral is the key to determining the role of transformers, we must understand 

this function. 

Transformer isolation 

Transformers are typically represented by the overlapping double-circle symbol shown in 

Figure 2, which will be used to represent transformers in this paper. However, this symbol is 

a simplification of the actual wiring diagram of a transformer, which is shown in Figure 3 and 

is important to understand for the remainder of this paper. 

The primary, or input, is on the left and three power phases are applied to the transformer 

windings (green lines) that are connected in the “delta” configuration as shown by the 

triangular shape. 

The secondary, or output, is connected in the “wye” (Y-shaped) configuration and consists of 

three power phases and a centerpoint, or neutral, connection1

 . There is no electrical 

connection between the input and output; the power is transferred through magnetic fields 

 

1

 The example shown is a “delta-wye” transformer. Although other types such as delta-delta, wye-delta, 

or wye-wye are possible, the delta-wye transformer shown has special advantages and is used almost 

exclusively in data center applications. The technical reasons for this can be found in many textbooks 

on power systems and are not discussed here. 

 Isolation of the neutral 

from the source is the most 

important characteristic of a 

transformer – it is this 

characteristic that causes 

transformers to be useful, 

necessary, or even 

 legally mandated. 

“

”

Figure 3 

Wiring diagram showing input 

and output connections to a 

power isolation (“delta-wye”) 

transformer 

Phase A

Phase B

Phase C

Phase A

Phase B

Phase C

PRIMARY 

(input)

SECONDARY 

(output)

NO NEUTRAL 

on input side No electrical connection

Magnetic fields only

NEUTRAL 

created from 

A, B, C

Hazards of Harmonics and 

Neutral Overloads 

Related resource 

White Paper 26 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 7

between the input and output. What is important to note is that there is no neutral connection

on the input. Even if the supply circuit has a neutral connection, it is not used with a deltawye transformer. The transformer “makes” a new neutral on the output – a new neutral that 

has no electrical connection to any neutral on the input. In fact, the whole output circuit is at 

an indeterminate voltage with respect to the input or ground, which is referred to as “floating”. 

Since IT load equipment is grounded, it is never appropriate to supply floating power at an 

indeterminate voltage because this could cause insulation failure and other hazards. 

Therefore, the new neutral on the output of the transformer is connected to ground in virtually 

all data center applications. 

When an isolation transformer has a grounded neutral, its output circuit is often referred to as 

a “separately derived source”. Grounding the output neutral can be achieved by directly 

connecting the neutral to the nearest grounded metal (equipment enclosures, ground rods, 

water pipes2

 or ground wire known to be grounded), or it can be connected to an existing 

neutral wire known to be grounded, or it can be connected to ground through a grounding 

resistor (this is only used on high power busses and not on final distribution to the IT loads). 

All three of these techniques are used in data centers and will be referred to in the next 

sections. 

Considering the above isolation properties of a transformer, we can now describe the key 

beneficial – and sometimes necessary – functions resulting from isolation: 

• Changing different mains grounding systems to the system required by data center IT 

equipment 

• Creating a new neutral connection when the mains neutral has serious power quality 

problems or the neutral is subject to disconnection when upstream 4-pole circuit 

breakers are used (as required in some countries) 

• Combining two sources without the need to connect their neutral wires together 

• Preventing circulating currents that could cause Residual Current Detectors (RCD)or 

other safety systems to activate unnecessarily 

Since these functions must be understood in order to understand how and why transformers 

are used and where to apply them, each function will be briefly explained. 

The first function of changing a mains grounding system in a data center to the grounding 

system required by IT equipment is clearly an essential function. IT equipment in a data 

center is always operated from a TN-S grounding system (see sidebar “Ground System 

Types” for description of grounding nomenclature). In some cases, the mains provides a TNS system so no change is required. Generally, TT or IT grounding systems require 

conversion to TN-S by use of a transformer before they can be utilized by IT equipment. This 

grounding conversion can occur before the UPS or after the UPS. In some countries such as 

the USA and Great Britain, TN-C grounding systems are common and are converted to TN-S 

without a transformer. 

The second function, of creating a new neutral when the mains neutral has serious power 

quality problem, is used when the provided mains neutral is shared with other customers, is 

generated a distance from the data center, or is deemed unreliable to the point where it might 

either be interrupted or become disconnected from earth. In developed countries and in most 

large new buildings, the TN-S neutral source is within the customer premises and typically 

close to the data center. In this case the quality of the neutral would be considered excellent 

and the second function is redundant. But in other situations the neutral to ground bond may 

be outdoors, possibly distant, shared, and part of a degraded or overloaded distribution 

 

2

 The use of water pipes to establish system grounding is not permitted in all countries; consult local 

regulations. 

> Ground system types 

TN-S Terra neutral 

separate 

The supply neutral is a separate 

wire from the ground wire and is 

connected to the ground at a 

single point. Both the neutral 

and the ground are distributed 

to the loads. All neutral and 

ground wires lead to the central 

bonding point 

TN-C Terra neutral 

common 

The supply neutral is grounded 

at the supply. A single wire 

provides both the neutral and 

ground function 

TT Terra terra 

The supply neutral is grounded. 

A local ground is created near 

the load to provide chassis 

grounding 

IT Insulated terra 

The supply neutral floats with 

respect to ground. A local 

ground is created near the load 

to provide chassis grounding 

IT-HRG Isolated terra with 

high-resistance ground 

The supply neutral is a separate 

wire from the ground wire and is 

connected to ground at a single 

point through a large power 

resistor. Phase to ground faults 

are current limited by the 

ground resistor. Protective 

controls are required to warn 

operators or trip breakers when 

faults are detected 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 8

system. Under these conditions the neutral may have significant offset or noise voltage with 

respect to ground, or worse it might lose its ground connection or become interrupted. This 

problem is made worse in tropical climates where it can be difficult to maintain low 

impedance metal-to-metal bonds over time. If a mains source under a loss of neutral 

condition were passed directly to the IT equipment, massive equipment failures could occur 

due to the higher voltage. These problems are common in developing countries and are the 

reason why a data center power system design often requires additional power transformers 

when deployed in emerging markets. 

The third function, combining sources without the need to connect the neutrals, is a function 

that is unique to emergency power systems that have backup sources, such as commonly 

used in a data center. A data center may be fed from a combination of multiple mains 

services and generators that are combined with switches to assure power continuity to the 

critical load. The bypass path within a UPS is itself an alternate power path from the UPS 

module that is essentially “combined” at the output of the UPS. Whenever two sources are 

combined through a switching arrangement, a situation may arise where there are two input 

neutral connections and a single output neutral connection. This leads to the problem of how 

to connect a single output neutral given two input neutrals as shown in Figure 4. 

Since switching between neutrals supplying an IT load creates a momentary open-neutral 

condition which can be hazardous or destructive, the neutral to the critical load should never 

be switched. This means if two alternate sources are combined in the UPS, they must have 

their neutral wires permanently connected to each other. However, connecting the input 

neutrals together to the output neutral can create circulating currents between the input 

neutrals as shown in Figure 5. 

While these circulating currents are a minor nuisance when the bypass and rectifier come 

from the same source, this can be a hazard if the two input neutrals come from different 

sources. The connection of two independently derived neutral sources together is universally 

not permitted by law. The insertion of a transformer in series with one of the two sources 

solves this problem. Whenever a dual mains UPS is supplied by separately derived neutral 

sources, a transformer is required. Note that some dual mains systems have the two inputs 

supplied by sources with a commonly derived neutral and these don’t require a transformer. 

Figure 4 

Problem: How to create a 

single output neutral, given two 

input source neutrals? 

Figure 5 

Circulating current created by 

connecting two separate input 

neutral wires to create a single 

output neutral 

Bypass neutral

Output

Rectifier neutral

If neutrals are connected here,

 currents circulate between input neutrals

Bypass

UPS 

module

Bypass neutral

Output

Rectifier neutral

Which neutral is 

output connected to?

Output neutral

Bypass

UPS 

module

? ?

?
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 9

The fourth function, preventing circulating currents that could cause RCDs or other safety 

systems to activate unnecessarily, is also related to the situation where sources are 

combined such as in a dual mains configuration. Circulating currents between neutrals 

always occur when separately derived neutrals are connected to each other, but as pointed 

out in the previous paragraph, this is not permitted by law so a problem should not be 

created. But circulating currents can also occur even when a UPS is supplied from two inputs 

that are derived from the same neutral. Therefore, in any system where both a rectifier and 

bypass neutral connection are provided to the UPS, any RCD protection on the supply 

circuits will activate unnecessarily. An isolation transformer located in either the rectifier 

supply, the UPS module output, or the bypass is needed to prevent RCD activation. 

At first it seems that if the rectifier input neutral connection could be omitted, the circulating 

current problem should be solved. In fact, all UPS systems designed for dual mains are 

designed to operate without a rectifier neutral connection; the UPS input rectifiers draw power 

between the input phases and do not require a neutral connection to function. As long as the 

rectifier source is known to be grounded, the rectifier neutral need not be provided. Since 

there is no longer any rectifier neutral connection, it seems that circulating current should not 

be possible. Unfortunately, although this it is widely believed that the absence of a neutral 

connection on the rectifier eliminates circulating currents, it is not true. 

Figure 6 shows the dual mains UPS system configuration with the rectifier supply neutral not 

connected. The red line shows the flow of circulating current which still exists but flows 

through the UPS module instead of the rectifier neutral. Any UPS inverter module that has an 

output neutral will inject current onto the output neutral bus in excess of any neutral current 

required by the load. This “excess” neutral current is a side-effect of inverter operation and is 

caused by reactive loads, non-linear loads, and imbalances in the load currents. This 

“excess” neutral current is not consumed by the IT loads and returns to the mains via the 

bypass neutral. This current may appear small under normal conditions but can become 

large under various loads or mains voltage imbalances. If the bypass mains supply includes 

RCD protection (mandated in some conditions in some countries), then these protective 

devices will sense this neutral current as an unexpected current and misinterpret it as a 

ground fault, possibly shutting down the system. This leads to a very important principle of 

data center power system design: In DUAL MAINS systems with RCD protection, there 

must ALWAYS be a transformer present somewhere in one of the mains paths. 

Removing the rectifier neutral connection is NOT sufficient to prevent circulating currents. 

It should be clear at this point that the dual mains configuration has, by far, the most complex 

issues relating to grounding and transformer use. Many mistakes are made in the application 

of transformers and the appropriate grounding of dual mains systems, mistakes which often 

result in intermittent problems and unexpected downtime. These problems are simplified in 

single mains systems (with or without bypass). Often a dual mains system is chosen without 

consideration of these complexities, and a single mains system might have been a better 

choice because there are fewer things that can go wrong in the design and the installation. 

Figure 6 

Circulating current can still 

exist even if the rectifier input 

neutral is not connected 

 In Dual Mains systems 

with RCD protection, there 

must ALWAYS be a transformer in one of the mains 

paths. Removing the 

rectifier neutral connection 

is NOT sufficient to prevent 

circulating currents 

“

”

Bypass neutral

Output

Rectifier neutral

(not connected) Currents generated by UPS neutral still 

cause circulating current on bypass neutral

Bypass

UPS 

module
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 10

The single mains configuration can be a reliable and cost-effective choice because the 

theoretical reliability advantages of a dual mains configuration are not always realized in 

practice. This is why the single mains configuration is often used even in very large ultrahigh availability data centers, especially when redundancy is achieved by dual path or N+1 

UPS configurations. 

Paralleling and other redundancy configurations 

The previous descriptions of transformer applications in the three basic UPS configurations 

assume that the UPS system includes a single UPS module. However, it is common for data 

center power systems to be comprised of a number of UPS systems, which in turn can be 

comprised of a number of UPS modules, which can all be arranged in many different 

configurations to achieve redundancy. The various commonly used approaches for obtaining 

redundancy are described in White Paper 75, Comparing UPS System Design 

Configurations. In addition to the redundancy function, UPS systems are often paralleled as 

a means to increase power capacity. 

The discussions of grounding and transformer use in this paper apply without modification to 

parallel UPS systems comprised of multiple parallel UPS modules, where a single bypass is 

used. Example of such a UPS systems with internal UPS module paralleling would be the 

APC Symmetra. However, there are configurations of data center systems that include 

parallel or cascade configurations of complete UPS systems, introducing many more possible 

points of system grounding and possible transformer locations. Most of the principles of 

applying transformers described in this paper still apply to these systems, but such systems 

often create special challenges for grounding and for residual current detection. 

Given the basic understanding of the application of transformers as provided in this paper, it 

is possible to determine the optimal approach to using transformers in specific redundancy or 

paralleling situations. However, a complete discussion of transformer options for the 

numerous paralleling and redundancy configurations is beyond the scope of this paper. 

We will now consider each of the three basic UPS configurations and show the preferred 

transformer arrangements based on the situation. 

As stated earlier in this paper, 92 different arrangements of transformers are possible with a 

single UPS, even before redundancy configurations are considered. Instead of attempting to 

describe and compare each of these arrangements, this paper will describe the functions of 

the transformers in the three previously identified generic configurations of single mains, dual 

mains, and single mains without bypass. The functions and benefits of the different 

transformers will be described within each of these three configurations. From this, the 

preferred arrangements of transformers will be deduced and described. 

Transformer options for the “single mains” configuration 

The single mains system is the most common UPS configuration, used in over 50% of all 

UPS installations. In smaller systems (below 100kW), it is even more dominant, making up 

approximately 90% of all installations. The advantage of this system is that it is relatively 

simple to design and install, yet it provides a bypass that gives a variety of redundancy and 

maintenance benefits. Small UPS systems below 10 kW typically only have a single input 

and must be used in the single mains configuration. UPS systems over 10 kW usually 

provide for dual mains input but are converted to single mains by simply connecting the two 

mains inputs together. The majority of UPS systems shipped with dual mains input 

connections actually end up configured for single mains input. 

Transformer 

arrangements 

in practical UPS 

systems 

Comparing UPS System 

Design Configurations 

Related resource 

White Paper 75 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 11

In the single mains configuration, transformers can be located in three positions as shown in 

Figure 7. Any combination of these transformers, including none or all three, might exist in a 

UPS installation. 

While Figure 7 accurately represents the flow of power, it does not completely describe the 

grounding system because the physical location of the transformers must also be considered. 

The inverter transformer, if present, is always integral to the UPS, but the mains and output 

transformers can be either located at the UPS or remotely located. Table 1 describes the 

function of each transformer, when it is needed, and the issues related to its physical location 

(local vs. remote). 

Transformer 

location

Functions 

 provided When needed Local / remote issues

Mains

•Change mains voltage 

•Change grounding system to 

TN-S 

•Isolate UPS and the output 

from poor quality mains 

neutral 

•Mains voltage must be changed to 

match the UPS voltage 

•Mains grounding system not 

compatible with the load AND there is 

no output transformer 

•If transformer is local to the UPS, a high 

quality TN-S grounding system is 

created at the UPS output 

•Locating this transformer remotely from 

the UPS requires that the TN-S 

grounding system is created at the 

transformer location, which is not ideal 

Inverter

•This transformer provides no 

functions in the single mains 

configuration, but may be 

present due to the design of 

the UPS 

•Never needed in this configuration 

•On older UPSs, this transformer may be 

integral to the UPS design and cannot 

be removed, but it is not providing any 

useful function 

Output

•Change the UPS output 

voltage 

•Change the grounding 

system to TN-S, if the UPS is 

not using TN-S 

•Improve the quality of the TNS grounding system if the 

input transformer is not 

present or is remotely located 

•UPS voltage must be changed to 

match the IT load voltage 

•UPS grounding system is not 

compatible with the IT load AND there 

is no input transformer 

•Input grounding system is compatible 

with the IT load but the quality of the 

neutral circuit is poor OR the input 

transformer, if present, is remotely 

located 

•If the IT loads are far from the UPS, 

locating the transformer remotely 

(closer to the IT loads) has the 

advantage of creating the TN-S ground 

near the IT load, improving the quality of 

the grounding system 

Figure 7 

Possible transformer locations 

in the “single mains” UPS 

configuration 

Table 1 

Characteristics of the three 

SINGLE MAINS transformer locations 

Bypass

Mains

transformer Output

transformer

Inverter

transformer

UPS 

module

SINGLE MAINS

Transformer locations
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 12

The following observations can be made from a study of the functions of the transformers in 

the single mains UPS configuration: 

• If voltages must be changed, transformers must be installed. For example, if the mains 

voltage is 600 V, the UPS voltage is 480 V, and the IT load voltage is 208 V, then both 

input and output transformers are required. This is a fundamental requirement and is 

the first issue to be considered in determining where to put a transformer. 

• If the mains and/or UPS ground system are not TN-S, then transformers must be 

installed. For example, if the mains grounding system is TT, and UPS grounding 

system is IT-HRG, then both input and output transformers must be installed. This is a 

fundamental requirement, but some aspects of this problem are under the control of the 

system designer. For example, a designer could ensure that a high quality TN-S 

grounding system is provided at the data center, so no ground system changes are 

required. 

• The inverter transformer provides no function or benefit to the single mains UPS 

system. Since the presence of the inverter transformer defines the difference between 

a transformer-based UPS and a transformerless UPS, it follows that there is no 

advantage to using a transformer-based UPS in the single mains configuration. 

As stated earlier in this paper, there are 8 possible transformer arrangements in a single 

mains UPS configuration. When remote vs. local location options for the mains and output 

transformers are considered, the number of options grows to 18. While there is not one 

combination that emerges as the universal best solution, some of these variations can be 

eliminated as sub-optimal, and a few emerge as clearly superior. The analysis of all 18 of 

these options is not presented in this paper, but can be derived from the information 

presented in this section. Tables 2a-2c below summarize the preferred transformer options 

for the single mains UPS configuration, with guidance as to where each approach is used. 

SINGLE MAINS 

Local input 

transformer

Application: Small data centers where the mains ground system is poor or not TN-S, or the mains 

voltage is not the same as the IT load voltage 

Features Notes 

Mains ground systems: TN-S, TT, IT-HRG, TN-C

Input voltages: UPS and load voltage must match 

Power range: Up to 1MW, but limited in North America to 

less than 100kVA due to mains and UPS being 480V or 600V 

above this power rating 

Efficiency: High 

IT load neutral bond: Established at the UPS output

•If the input transformer is local to the UPS, then its neutral 

connection is not connected to ground at the transformer, but 

brought to the UPS where neutral is connected to ground at 

the UPS output (preferred method) 

•If the input transformer is remote from the UPS, then the 

neutral may be required to be connected to ground at the 

transformer, and the UPS neutral must be connected to this 

neutral and cannot be connected to ground at the UPS

Table 2a 

SINGLE MAINS preferred transformer option 1: 

Local input transformer 

 There is no advantage 

to using a transformerbased UPS in the single 

“

mains configuration ”
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 13

SINGLE MAINS 

No transformers 

Application: Small data centers with high quality TN-S mains ground system, where mains and 

UPS and IT load voltage are all the same (400/230V, 380/220V, 208/120V, or 200/100V) 

Features Notes 

Mains ground systems: TN-S only

Input voltages: Same as IT load 

Power range: Up to 1MW, but limited in North America to 

less than 100kVA due to mains and UPS being 480V or 600V 

above this power rating 

Efficiency: Highest 

IT load neutral bond: Established at the TN-S source

•The use of a UPS with a built-in inverter transformer offers no 

benefits over a transformerless UPS in this arrangement, 

because the output neutral must be derived from the TN-S 

source due to presence of the bypass

SINGLE MAINS 

Remote output 

transformer(s) 

Application: Large data centers 

Features Notes 

Mains ground systems: TN-S, TT, IT-HRG, TN-C 

Input voltages: Mains and UPS voltage must match 

Power range: Any, but not the most cost effective alternative 

in smaller data centers 

Efficiency: High, but lower if multiple output transformers are 

used 

IT load neutral bond: Established at the output transformer

•The UPS must be rated to operate with the specified mains 

ground system. Not all UPS can operate with an IT-HRG 

ground system, and some UPS require an input neutral 

connection 

•There is no neutral connection between the UPS and the 

output transformer(s) in this system 

•The output transformer may be a single transformer or its 

function may be divided among many PDU transformers

Another option for large data centers is to add an input transformer to the “single mains with 

remote output transformer” arrangement, to allow for use with very poor mains grounding 

systems or a mains supply with an incompatible grounding system, but this would be an 

uncommon situation. 

The single mains configuration has fewer options and is simpler to design for and deploy than 

the dual mains system described in the next section. In practice, this means there are fewer 

Table 2b 

SINGLE MAINS transformer option 2: 

No transformers 

Table 2c 

SINGLE MAINS transformer option 3: 

Remote output transformer(s) 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 14

design and installation mistakes made. This configuration will remain the most common, and 

should be considered for both large and small data centers. 

Transformer options for the “dual mains” configuration 

The dual mains configuration is used in many, but not all, larger installations. Figure 8

shows four options for transformer location in a dual mains system 

There are two core reasons that the dual mains system is used. The dual mains approach 

must be used when the system architecture prescribes that the rectifier and the bypass 

supplies come from different sources – for example, separate utility substations or in highly 

specialized redundancy architectures. This requirement is actually quite uncommon and is 

restricted to architectures designed for extreme redundancy. Note that in most systems with 

alternate supplies, such as diesel generators or secondary utility mains supplies, an 

automated transfer switch is provided upstream of the UPS because there are other loads in 

addition to the UPS – such as cooling plants – that must be backed up. 

The second reason the dual mains design is used is to allow for concurrent maintenance of 

the distribution wiring and breakers feeding the UPS. If the breakers and wiring supplying the 

two mains inputs are separate, either can be shut down without shutting down the critical 

load, which will be powered from the remaining path during the maintenance. While this is a 

common reason for specifying this arrangement, there are alternative ways to allow for 

concurrent maintenance without using this approach, such as a dual power path architecture 

or concurrently maintainable parallel breakers. 

The diagram in Figure 8 accurately represents the flow of power but does not completely 

describe the grounding system, because the physical location of the transformers must also 

be considered. The inverter transformer, if present, is always integral to the UPS, but the 

rectifier, bypass, and output transformers can be either located at the UPS or remotely 

located. Table 3 describes the function of each transformer, when it is needed, and issues 

related to its physical location (local vs. remote). 

Figure 8 

Possible transformer locations 

 for the DUAL MAINS configuration 

Bypass

Mains rectifier

transformer

Output

transformer

Inverter

transformer

Mains bypass 

transformer

UPS 

module

DUAL MAINS

Transformer locations
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 15

Transformer 

location 

Functions 

provided When needed Local / remote considerations 

Bypass 

•Isolates the bypass neutral from 

currents generated by the inverter 

•Changes bypass mains voltage 

•Changes bypass grounding 

system to TN-S 

•Isolates bypass from poor quality 

neutral 

•Protects the UPS from open 

neutral conditions caused by 

upstream transfer switches or 

breakers 

•The bypass voltage must be changed 

to match the UPS voltage 

•The bypass grounding system is not 

compatible with the load AND there is 

no output transformer 

•RCD protection is used on the mains 

supplies AND neither a rectifier or 

local inverter transformer is present 

•4-pole breakers are used on the 

supply to the bypass (open neutral 

can occur) 

•The output neutral of the UPS is always 

the same as the bypass neutral, unless 

there is an output transformer present 

•If this transformer is local to the UPS, 

and either a rectifier or inverter 

transformer is provided, or the rectifier is 

capable of operating without an input 

neutral connection, a high quality TN-S 

grounding system is created at the UPS 

output 

•Locating this transformer remotely from 

the UPS requires that the TN-S 

grounding system is created at the 

transformer location, which is not ideal 

Rectifier •Change rectifier mains voltage 

•Isolates the UPS input from an 

incompatible mains grounding system 

•Mains to UPS voltage change is 

required 

•This transformer does not completely 

protect the UPS from open neutral 

conditions caused by upstream transfer 

switches because the UPS neutral 

normally is derived from the bypass input 

Inverter •Isolates the bypass neutral from 

currents generated by the inverter 

•The bypass is from a separately 

derived source and no rectifier 

transformer is present 

•This transformer provides the same 

neutral isolation as a rectifier transformer, 

but cannot perform a voltage change 

•The load power always flows through this 

transformer, so isolation between mains 

inputs is better provided in the bypass 

path since it increases system efficiency 

Output 

•Change the UPS output voltage 

•Change the grounding system to 

TN-S, if the UPS is not using TN-S 

•Improve the quality of the TN-S 

grounding system if the input 

transformer is not present or is 

remotely located 

•The UPS voltage must be changed to 

match the IT load voltage 

•The UPS grounding system is not 

compatible with the IT load AND there 

is no input transformer 

•The input grounding system is 

compatible with the IT load but the 

quality of the neutral circuit is poor 

OR the input transformer, if present, 

is remotely located 

•If the IT loads are far from the UPS, 

locating the transformer remotely from 

the UPS and closer to the IT loads, has 

the advantage of creating the TN-S 

ground near the IT load, improving the 

quality of the grounding system 

The following observations can be made from a study of the functions of the transformers in 

the dual mains UPS configuration: 

• If voltages must be changed, transformers must be installed. For example, if the mains 

voltage is 600 V, the UPS voltage is 480 V, and the IT load voltage is 208 V, then all 

three – bypass, rectifier, and output transformers – are required. This is a fundamental 

requirement and is the first issue to be considered in determining where to put a 

transformer. 

• If the mains and/or UPS ground system are not TN-S, then transformers must be 

installed. For example, if the mains grounding systems is TT, and UPS grounding 

system is IT-HRG, then all three – bypass, rectifier, and output transformers – must be 

Table 3 

Characteristics of the four 

DUAL MAINS transformer locations 

 In many cases there is 

no advantage to using a 

transformer-based UPS in a 

“

Dual Mains configuration ”
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 16

installed. This is a fundamental requirement, but some aspects of this problem are 

under the control of the system designer. For example, an architect could ensure that 

a high quality TN-S grounding system is provided at the data center, so no ground 

system changes are required. 

• The inverter transformer provides the function of breaking the neutral connection 

between the rectifier and bypass inputs, but this function is also provided by either a 

rectifier or bypass transformer. The inverter transformer function is redundant if either 

a rectifier or bypass transformer is installed for another purpose. Since the presence of 

the inverter transformer defines the difference between a transformer-based UPS and a 

transformerless UPS, it follows that in many cases there is no advantage to using a 

transformer-based UPS in the dual mains configuration.

As stated earlier in this paper, there are 16 possible combinations of transformer 

arrangements in this system. When remote vs. local location options for the mains and 

output transformers are considered, the number of options grows to 56. While there is not 

one combination that emerges as the universal “best solution”, some of these variations can 

be eliminated as sub-optimal, and a few emerge as clearly superior. An analysis of all 56 of 

these options is not covered in this paper, but can be derived from the information presented 

in this section. Tables 4a-4d below summarize the preferred transformer options for the dual 

mains UPS system, with guidance as to where each approach is used. 

DUAL MAINS 

Local bypass transformer 

Remote output transformer(s) 

Application: Large data centers 

Features Notes 

Mains ground systems: TN-S, TT, IT-HRG, TN-C, inverter 

and bypass grounding systems can be independent 

Input voltages: Mains and UPS voltage must match 

Power range: Any, but impractical below 100kW 

Efficiency: High, but lower if multiple output transformers are 

used 

IT load neutral bond: Established at the output transformer

•Equivalent functionality can be provided by moving the bypass 

transformer to either the rectifier or inverter locations but 

efficiency is degraded 

•The UPS must be rated to operate with the specified mains 

ground system. Not all UPS can operate with an IT-HRG 

ground system, and some UPS require an input neutral 

connection 

•There is no neutral connection between the UPS and the 

output transformer(s) in this system. 

•The output transformer may be a single transformer or its 

function may be divided among many PDU transformers

Table 4a 

DUAL MAINS transformer option 1: 

Local bypass transformer and remote transformer(s) 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 17

DUAL MAINS 

Local bypass transformer 

Rectifier transformer 

Remote output transformer(s) 

Application: Large data centers with poor mains ground system, different mains and UPS 

grounding systems, or different mains and UPS voltage

Features Notes 

Mains ground systems: TN-S, TT, IT-HRG, TN-C, inverter, 

bypass, and UPS grounding systems can be independent 

Input voltages: Any combination 

Power range: Any, but impractical below 100kW 

Efficiency: Lowest, and even lower if multiple output 

transformers are used 

IT load neutral bond: Established at the output transformer

•There is no neutral connection between the UPS and the 

output transformer(s) in this system 

•The output transformer may be a single transformer or its 

function may be divided among many PDU transformers 

•A transformer-based UPS with a transformer in the inverter 

location provides no additional functionality and degrades 

efficiency.

DUAL MAINS 

No transformers 

Application: Small data centers with high quality TN-S mains ground system, where mains and 

UPS and IT load voltage are all the same (400/230V, 380/220V, 208/120V, or 200/100V) and RCD 

is not required 

Features Notes 

Mains ground systems: TN-S only, bypass and mains 

common 

Input voltages: Same as IT load 

Power range: Up to 1MW, but limited in North America to < 

100kVA due to mains and UPS being 480V or 600V above 

this power rating 

Efficiency: Highest efficiency 

IT load neutral bond: Established at the TN-S source

•Both the bypass and the rectifier must be TN-S and both 

neutrals must be derived from a common ground connection 

•RCD protection on the bypass and rectifier inputs cannot be 

used in this configuration, because the inverter will inject some 

current into the bypass neutral

Table 4b 

DUAL MAINS transformer option 2: 

Local bypass transformer, rectifier 

transformer, and remote output 

transformer(s) 

Table 4c 

DUAL MAINS transformer option 3: 

No transformers 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 18

DUAL MAINS 

Local bypass 

transformer 

Application: Small data centers with high quality TN-S mains ground system, where mains and 

UPS and IT load voltage are all the same (400/230V, 380/220V, 208/120V, or 200/100V) and RCD 

protection is required or the bypass is separately derived

Features Notes 

Mains ground systems: TN-S on rectifier input, bypass can 

be any type 

Input voltages: Mains, UPS and load voltage must match 

Power range: Up to 1MW, but limited in North America to < 

100kVA due to mains and UPS being 480V or 600V above 

this power rating 

Efficiency: High

IT load neutral bond: Established at the bypass transformer 

•This method is used if the bypass and inverter supplies are 

separately derived (separate neutral bonding connections), or 

RCD protection is used 

•The transformer could have been placed in the rectifier or 

inverter location to achieve RCD compatibility, but this would 

reduce the UPS mode efficiency 

•If the input transformer is remote from the UPS, then the 

neutral may be required to be connected to ground at the 

transformer, and the UPS neutral must be connected to this 

neutral and cannot be connected to ground at the UPS

Note that none of the preferred arrangements have a transformer in the inverter location. 

Most transformer-based UPSs have the transformer in the inverter location, yet this is the 

least useful place to have a transformer. This again reinforces the key finding of this paper: 

that the historical transformer-based UPS is obsolete because its transformer is in the wrong 

location. The better solution is to use a UPS where the location of the transformer (if any are 

needed) can be established in the optimal location during the system design. 

The most important characteristic of the dual mains design is that there are many options and 

the interactions between the options and other equipment such as RCD and circuit breakers 

is quite complex. The successful installation of a dual mains system requires not only a 

careful tradeoff analysis and design, but also requires care during installation. These factors 

are further compounded in designs of paralleled systems or complex redundancy 

configurations. Unfortunately, design and installation errors are much more common in the 

dual mains design than in the single mains designs, reducing system reliability and 

performance. These factors are not always considered when a dual mains design is 

specified. 

Transformer options for “single mains without bypass” 

The single mains without bypass configuration is commonly used when the quality of the 

input mains power or the mains grounding system is poor, degraded, or shared with industrial 

loads. The diagram of the “without bypass” configuration, with the possible transformer 

locations, is shown in Figure 9. 

Table 4d 

DUAL MAINS transformer option 4: 

Local bypass transformer 

Figure 9 

Possible transformer locations in 

the SINGLE MAINS WITHOUT BYPASS 

configuration 

 The historical transformerbased UPS is obsolete because 

its transformer is in the wrong 

“

location ”

Mains 

transformer

Output

transformer

Inverter

transformer

UPS 

module

SINGLE MAINS 

WITHOUT BYPASS

Transformer locations
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 19

When the mains power is of very poor quality or the grounding system is unstable, it can be 

undesirable to ever expose the IT load to the mains via a bypass, so no bypass is provided in 

this system. This configuration is almost exclusively used in developing nations, where poor 

power can be quite common. In developed countries with stable power grids, the bypass is a 

valuable feature and either the single mains or the dual mains systems predominate; the 

“without bypass” configuration is quite rare. 

An inspection of the diagrams reveals that a single mains configuration with bypass appears 

to become equivalent to the without bypass diagram if the bypass function is disabled. It is 

commonly assumed that a system can be installed with a bypass and then toggled back and 

forth between the “with” and “without” bypass configurations. This mode of installation is 

only viable if either an input or an output transformer is present. The reason is that if the 

bypass is wired, the neutral of the inverter output transformer cannot be connected to ground 

if the bypass neutral is already grounded. If the only transformer present is the inverter 

transformer, and the output is separately derived by connecting the inverter transformer 

neutral to ground, then the bypass must not be wired and cannot be used. This limitation 

applies to mechanical “wrap-around” bypass as well as to automatic “static” bypass. 

The inverter transformer, if present, is always integral to the UPS, but the mains and output 

transformers can be either located at the UPS or remotely located. Table 5 describes the 

function of each transformer, when it is needed, and the issues related to its physical location 

(local vs. remote). 

Transformer 

Location 

Functions 

provided When needed Local vs. remoteissues 

Mains 

•Change mains voltage 

•Change grounding system to 

TN-S

•Isolates UPS AND the output 

from poor quality mains 

neutral 

•The mains voltage must be changed to 

match the UPS voltage 

•The mains grounding system is not 

compatible with the load AND there is no 

output transformer 

•If this transformer is local to the UPS, 

a high quality TN-S grounding system 

is created at the UPS output 

•Locating this transformer remotely 

from the UPS requires that the TN-S 

grounding system is created at the 

transformer location, which is not ideal 

Inverter •Isolates the output from poor 

quality neutral 

•The system output neutral must be 

separated from the input neutral AND there 

is neither a mains nor output transformer 

•The neutral of this transformer can be 

connected to ground because there is 

no bypass, creating a separately 

derived source 

•On older UPS, this transformer may be 

integral to the UPS design and cannot 

be removed 

Output 

•Change the UPS output 

voltage 

•Change the grounding system 

to TN-S, if the UPS is not 

using TN-S 

•Improve the quality of the TNS grounding system if the 

input transformer is not 

present or is remotely located 

•The UPS voltage must be changed to match 

the IT load voltage 

•The UPS grounding system is not 

compatible with the IT load AND there is no 

input transformer 

•The input grounding system is compatible 

with the IT load but the quality of the neutral 

circuit is poor OR the input transformer, if 

present, is remotely located 

•If the IT loads are far from the UPS, 

locating the transformer remotely, 

closer to the IT loads, has the 

advantage of creating the TN-S ground 

near the IT load, improving the quality 

of the grounding system 

Table 5 

Characteristics of the different transformer locations 

for a SINGLE MAINS WITHOUT BYPASS system 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 20

As stated earlier in this paper, there are 8 possible combinations of transformer 

arrangements in this system. When remote vs. local location options for the mains and 

output transformers are considered, the number of options grows to 18. While there is not 

one combination that emerges as the universal “best solution”, some of these variations can 

be eliminated as sub-optimal, and a few emerge as clearly superior. An analysis of all 18 of 

these variations is not presented in this paper, but can be derived from the information 

presented in this section. Table 6a and Table 6b summarize the preferred transformer 

options for the single mains without bypass configuration, with guidance as to where each 

approach is used. 

SINGLE MAINS 

WITHOUT BYPASS 

Local input transformer 

Remote output transformer(s) 

Application: Large data centers with poor mains power quality and poor mains ground system, 

where UPS and IT load voltage are different 

Features Notes 

Mains ground systems: TN-S, TT, IT-HRG, TN-C

Input voltages: Any 

Power range: Any, but impractical for smaller data centers 

Efficiency: High, but lower if multiple output transformers are 

used 

IT load neutral bond: Established at the output transformer

•Above 100 kVA an output transformer is required in North 

America to convert 480 V UPS voltage to the IT load voltage 

•There is no neutral connection between the UPS and the 

output transformer(s) in this system 

•The output transformer may be a single transformer or its 

function may be divided among many PDU transformers

Table 6a 

SINGLE MAINS WITHOUT BYPASS preferred option 1: 

 Local input transformer and remote output transformer(s) 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 21

SINGLE MAINS 

WITHOUT BYPASS 

Local input transformer 

Application: Large or small data centers with poor mains power quality and poor mains ground 

system, where UPS and IT load voltage are the same (400/230V, 380/220V, 208/120V, or 

200/100V) 

Features Notes 

Mains ground systems: TN-S, TT, IT-HRG, TN-C

Input voltages: Any 

Power range: Any, but limited in North America to less than 

100 kVA due to UPS being 480 V or 600 V above this power 

rating 

Efficiency: Highest efficiency 

IT load neutral bond: Established at the UPS output

•If the input transformer is local to the UPS, then its neutral 

connection is not connected to ground at the transformer, but 

brought to the UPS where neutral is connected to ground at 

the UPS output (preferred method) 

•If the input transformer is remote from the UPS, then the 

neutral may be required to be connected to ground at the 

transformer, and the UPS neutral must be connected to this 

neutral and cannot be connected to ground at the UPS

The use of this configuration, single mains without bypass, is common in countries with very 

poor power. However, in most cases where it is used, the single mains configuration with a 

bypass would be a better choice. The reason why many customers choose not to use the 

bypass is because it potentially subjects the load to raw mains power. This is true even with 

transformer-based UPS systems, because they don’t isolate the bypass. This is a problem 

because in the older transformer-based UPSs, the transformer is in the wrong location. 

If the transformer is moved to the input, instead of at the inverter output, then it can protect 

both the UPS and the bypass path (this is described in more detail in the next section). 

A major problem with the single mains without bypass configuration is that the load fault 

current is always limited to the UPS inverter output current, which is typically much lower than 

the mains available short-circuit current. This can cause difficulty clearing breakers 

downstream of the UPS, and can result in a total load drop when a downstream fault does not 

clear, or even if it does not clear quickly. In contrast, in a configuration with a bypass, output 

faults activate the bypass and the mains current is available to help clear output faults. 

When designers and users understand all the options, the use of the single mains without 

bypass configuration can be expected to decline in use. The single mains with bypass, when 

combined with an input transformer, is the same cost, weight, and efficiency but preserves 

the additional option of using the bypass, which can be beneficial. In general, there are 

common historical configurations using transformer-based UPSs that will be replaced by 

newer configurations. This will be described in the next section. 

A transformer-based UPS is defined as a UPS with an integrated transformer in the inverter 

output location. This paper has shown that none of the 8 preferred UPS/transformer 

arrangements have a transformer in the inverter output location. Note that some 

transformer-based UPS systems, as well as transformerless ones, may have additional 

optional transformers. Nevertheless, because older UPS systems have inverter transformers, 

there are arrangements in common use that were originally utilized to take advantage of the 

Table 6b 

SINGLE MAINS WITHOUT BYPASS preferred option 2: 

 Local input transformer 

Transformerbased legacy 

UPS designs 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 22

integral UPS transformer. In this section, we will review these arrangements, explain why 

they are not optimal, and what transformer arrangements should be used instead. 

Transformer-based UPSs have the transformer after the output of the inverter and before the 

bypass connection to the UPS output. This is the least useful place that a transformer 

can be located in a UPS for these reasons: 

• It does not protect the UPS input 

• It does not isolate the rectifier ground system from the mains 

• It is always dissipating heat because it is handling the full load power 

• It can’t allow the output neutral to be isolated from the input due to the presence of the 

bypass 

It is important to understand that the transformer exists in this location because it was 

required as part of the UPS inverter circuits, and was not added to solve these other 

problems. It does provide one key function, of isolating the inverter from the output, but this 

function can also be provided by transformers placed in other, and better, locations. Even 

though transformer-based UPSs have the transformer in a sub-optimal location, the 

transformer is part of the circuitry and not optional, so many early data center designs took 

advantage of it. There are two main designs that take advantage of the integral transformer 

in a transformer-based UPS, which are summarized in Table 7a and Table 7b along with 

recommended improved replacement designs. 

 None of the 8 preferred 

UPS/transformer arrangements have a transformer 

in the inverter output 

 location 

“

”
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 23

DUAL MAINS 

Inverter transformer 

Remote output transformer(s) 

Application: Not recommended for new designs. Was commonly used in large data centers

Features Notes 

Mains ground systems: TN-S, TT, IT-HRG, TN-C 

Input voltages: Mains and UPS voltages must match 

Power range: Any, but impractical for smaller data centers 

Efficiency: High, but lower if multiple output transformers are used 

IT load neutral bond: Established at the output transformer

•The UPS rectifier must be rated to operate with the specified 

mains ground system. Not all UPS can operate with and IT-HRG 

ground system, and some UPS require an input neutral 

connection 

•There is no neutral connection between the UPS and the output 

transformer(s) in this system 

•The output transformer may be a single transformer or its function 

may be divided among many PDU transformers

Recommended replacement: 

DUAL MAINS 

Bypass transformer 

Remote output transformer(s) 

Advantages of replacement Notes

Higher efficiency: moving the transformer to the bypass path improves 

efficiency because the load power is not normally creating losses in the 

transformer

•The bypass transformer may be available as an option of the UPS 

•The neutral of the inverter output, if any, is now connected to the 

rectifier input neutral, instead of the bypass input neutral 

•The IT load neutral is still established at the output transformer 

•Compatibility with RCD is preserved

It is important to recognize that in the above situation the legacy inverter transformer was 

providing a function of isolating the inverter from the bypass neutral. In this case it is better 

to locate the transformer in the bypass path for the reasons provided. However, as stated in 

the earlier sections describing the single mains and dual mains recommended systems, not 

all installations require isolation between the inverter and the bypass neutral. In designing a 

new facility, the recommended designs described in the earlier sections should be considered 

first; the examples here are only to show how a few popular legacy designs should be 

improved. 

Table 7a 

Legacy DUAL MAINS configuration 

with inverter transformer 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 24

SINGLE MAINS 

No bypass 

Inverter transformer

Application: Not recommended for new designs. Was used for large or small data centers with poor 

mains power quality and poor mains ground system, where UPS and IT load voltage are the same (400/230V, 

208/120V, or 200/100V). 

Features Notes 

Mains ground systems: TN-S, TT, IT-HRG, TN-C 

Input voltages: Mains, UPS, and IT load voltage must match 

Power range: Any, but limited in North America to < 100kVA due to UPS 

being 480V or 600V above this power rating 

Efficiency: High efficiency 

IT load neutral bond: Established at the UPS output

•The UPS rectifier input must be rated to operate with the 

specified mains ground system 

•Not all UPSs can operate with and IT-HRG ground system, 

and some UPS require an input neutral connection 

•In very poor mains ground system, the rectifier is exposed to 

various hazards

Recommended replacement: 

SINGLE MAINS 

No bypass 

Input transformer

Advantages of replacement Notes

Protection of UPS from mains faults: moving the transformer to the input 

isolates the UPS rectifier from ground or neutral faults on the mains 

Ground system compatibility: The UPS is always operated from the TN-S 

output of the transformer, independent of the mains ground system 

Voltage adjustment: The input transformer can include taps to allow 

adaptation of the UPS to chronic high or low mains voltage 

Voltage conversion: The input transformer can step the mains voltage up or 

down

•Any UPS can be used, without regard for the mains 

grounding system 

•The input transformer may be available as an option of the 

UPS 

•The neutral of the inverter output, if any, is still bonded to 

ground 

•The input transformer neutral is not bonded to ground at the 

transformer but is connected to the UPS output neutral

The above two tables represent the most common ways that transformer-based UPSs were 

used in earlier system designs. There are other variations of these designs, but in each case 

the historic location of the transformer is not optimal, and moving the inverter transformer to 

the input, the bypass, or eliminating it is a better solution. 

This paper has shown that there are a number of preferred designs that include a local input 

transformer that might be located on the bypass, the rectifier, or on the combined mains. To 

simplify design and deployment, UPS manufacturers should offer such input transformers as 

readily available, pre-engineered UPS options. 

Table 7b 

Legacy SINGLE MAINS configuration 

with inverter transformer 

Transformer 

options within 

a UPS product 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 25

Some preferred designs include a remote output transformer. Such transformers are best 

placed within PDU units or at the IT room and are not appropriate options for a UPS system, 

unless needed for a voltage change. 

Many UPS installations include transformers. This is true whether the UPS is transformerbased or transformerless. Transformers have important functions in a UPS installation 

including changing voltage, interfacing between ground systems, eliminating neutral loops, 

and protecting from various types of faults. Although most modern UPSs are 

transformerless, transformers are still often, but not always, necessary in the system design. 

There are many system designs where transformers are needed or even legally required. 

Transformer-based UPS systems almost never have the transformer located in the optimal 

location to achieve design goals. Transformerless UPS systems remove the transformer and 

allow the flexibility of omitting it or moving it to the best location for an intended purpose, 

optimizing the design. 

This paper has identified over 50 different arrangements of transformers that can be used 

with UPS systems. Of these, 10 are identified as the preferred solutions for data center 

designs, the specific choice depending on the preferences and constraints of the design as 

described in this paper. Larger-power UPS systems are found to have different preferred 

transformer arrangements than smaller-power UPS. Some preferred arrangements are 

needed to satisfy severe power conditions and degraded mains grounding. All of the 

preferred arrangements are best satisfied by using transformers in conjunction with a 

transformerless UPS. 

Conclusion 

Neil Rasmussen is a Senior VP of Innovation for Schneider Electric. He establishes the 

technology direction for the world’s largest R&D budget devoted to power, cooling, and rack 

infrastructure for critical networks. 

Neil holds 19 patents related to high-efficiency and high-density data center power and cooling 

infrastructure, and has published over 50 white papers related to power and cooling systems, 

many published in more than 10 languages, most recently with a focus on the improvement of 

energy efficiency. He is an internationally recognized keynote speaker on the subject of highefficiency data centers. Neil is currently working to advance the science of high-efficiency, 

high-density, scalable data center infrastructure solutions and is a principal architect of the APC 

InfraStruXure system. 

Prior to founding APC in 1981, Neil received his bachelors and masters degrees from MIT in 

electrical engineering, where he did his thesis on the analysis of a 200MW power supply for a 

tokamak fusion reactor. From 1979 to 1981 he worked at MIT Lincoln Laboratories on flywheel 

energy storage systems and solar electric power systems. 

About the author 
The Role of Isolation Transformers in Data Center UPS Systems 

Schneider Electric – Data Center Science Center White Paper 98 Rev 0 26

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for Data Centers 

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Selecting an Industry Standard Metric 

for Data Center Efficiency 

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Electrical Efficiency Modeling 

for Data Centers 

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Implementing Energy Efficient 

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Guidance for Calculation of 

Efficiency (PUE) in Data Centers 

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