Can you connect two UPS in series?

Can two UPS be connected in series

We had one of those unusual applications through last week. A customer had a load of 1000W that he needed to run for an hour. Simples – using our UPS calculator we can get one of our rack/tower VFI3000B units with 2 battery packs and problem solved. Or not. Problem is there is a maximum weight limit of 40kg and a dual battery solution weighed in at 120kg. At that load level, our KR3000RM-LI Lithium unit -weighing in at only 16kg – will deliver over 30minutes, hence the question: “Can you connect two UPS in series?”

Having given this some thought I was struggling to come up with reasons why not. Sure, there may be better ways, for example additional battery packs, but this isn’t an option on the Lithium units. A quick Google search came up with the “Top4 reasons you should never… 1” which intrigued. However those folk were arguing about using non-sinewave units and I couldn’t agree more with that. Our Lithium units are online double conversion, true sinewave with power factor corrected inputs. The output of these units is going to be a lot better than from the mains, and the input current waveform is a smooth as you can get (baby’s posterior and that) so there should be no reason at all not to do this.

Having brainstormed this for some time with the engineers we could only come up with 4 reasons that you need to consider. That is, aside from the fact the UPS’ must be sinewave output and power factor corrected inputs2.

can you connect two ups in series

1. It’s inefficient.

Can’t argue against that. UPS 1 is not only powering the load it is powering UPS2.

For a 1000W load with say 10% losses, UPS2 consumes 1100W and UPS1 will now be compounded eg. 1210W. An extra 110W in losses as opposed to a single unit with a battery pack.

As for runtime, as the load that UPS1 has to deal with on battery is higher than UPS2 it won’t last as long. Taking good quality online double conversion lead acid systems with 6 internal batteries. One UPS will give 13minutes runtime at 1000W. Add to this an additional battery pack and you will get 32minutes runtime.

In a series arrangement, the load on the first UPS will rise to (roughly) 1100W. This has now reduced the runtime of UPS1 to 11minutes. UPS2 will remain at 13minutes giving 24minutes runtime. Not as good as adding the battery pack.

However, in our case adding a battery pack wasn’t an option, so we are able to knock this objection away.

2. No wrap around bypass

In all our online double conversion units, there is a built-in bypass (this can be enabled or disabled in most models). What this does is bypass the internal UPS circuitry if there is an overload or a fault. If there is an overload, both UPS’ would detect this immediately and revert to bypass. Due to this there may be the possibility of lost load. This is because reverting to bypass is not instantaneous and there is a momentary drop in power during this transition. The UPS cannot revert to battery operation when in bypass and if UPS2 has no input then the output is also lost.

Good point well made Henry. However this is a fault tolerance issue and only occurs in the event of a fault. As such it can be noted and a decision made to introduce additional fault avoidance measures or to live with it.

3. Compounded Earth Leakage

The earth leakage on UPS systems (except for our isolated VFI-TX range) is cumulative with the load. That means if you have a load with 1mA leakage, with the UPS of 1mA as well you now have 2mA. Add to that another UPS then all of a sudden you have 3mA. This is a problem for pluggable devices as the limit for such is 3mA. However, the safety standards only require you to take some additional measures, such as supplementary bonding, or by opting for industrial connections.

So another factor to consider but not a deal-breaker.

4. More expensive

True, two UPS (of the same ilk) will be more expensive than a UPS with a battery pack (Even with additional batteries in most cases).

But this is fairly obvious.

Conclusion: Can you connect two UPS in series?

Absolutely. Given the caveat that the UPS’ should be pure sinewave and power factor corrected input there is no reason at all why you can’t.

In our application this was the only solution. Getting the weight down to 40kg for a 1000W load for 1 hour just isn’t possible with lead acid battery packs. Connecting two KR3000RM-Li units in series as above worked a treat. In our tests UPS1 went onto battery and UPS2 was unaffected. UPS1 eventually ran out of battery steam after 29 minutes and UPS2 went onto battery. The only thing disconcerting was the multitude of alarms going off – UPS1 emits a constant tone to warn you its just died and UPS2 starts its backup warning alarm. Having a smaller load UPS2 lasted 34minutes, thus giving the one hours autonomy that we required and weighing in at only 32kg!

Lithium Battery UPS System


1This is from out of Barrington, RI, USA. Don’t know much about them except they come top in the Google search when searching for an answer to this “Can you connect two UPS in series” question. They say:

  1. You will void the factory warranty of both UPS systems
  2. You will not increase the capacity of your power protection         
  3. You will not increase run time
  4. You will jeopardize the proper operation of the data center EPO

It is true that you won’t get any additional power handling capacity in this configuration, but why should a manufacturer declare your warranty invalid for connecting to a superior power source, or having a power factor corrected load connected? I’ve just shown you can increase runtime and for properly configured EPO ports on both UPS why would data centre EPO be compromised?

2Power Factor Corrected input means the current waveform is in phase with the voltage waveform and in addition implies the current waveform is sinusoidal. Such a load is ideal for all sinewave UPS systems. Non PFC corrected UPS may take current in chunks, rendering the power factor outside the rating of many UPS. This is why I’ve included this as a requirement when daisy-chaining UPS.

Residual Current Devices (RCD) and Isolated UPS Systems

Consumer Unit

Did you know BS7671:2018 Requirements for Electrical Installations, a.k.a. The IET Wiring Regulations 18th Edition states that any socket outlet under 32A must be protected by a Residual Current Device (RCD)?

BS7671:2018 Section 411.3.3 states that RCDs are necessary on all socket outlets under 32A

Section 4.11.3 is the Requirements for fault protection. Subclause entitled “Additional requirements for socket outlets and for the supply of mobile equipment for use outdoors” states:

In AC systems, additional protection by means of an RCD with a rated residual operating current not exceeding 30mA shall be provided for:

(i) socket-outlets with a rated current not exceeding 32A


In other words any socket outlet that you plug anything into (basically anything powered from a 13A outlet, or up to 8KVA Systems on Commandos) must have an RCD protecting that circuit. There are exceptions to this, dwellings excepted, but only following a documented risk assessment which would have to clearly state why an RCD would not be necessary.

Purpose of RCDs.

An RCD works differently to a miniature circuit breaker (MCB) or fuse. An MCB renders devices safe in the event of an overload, or short circuit to earth. They are rated in Amps, generally in stages from 1-32A. RCDs work by tripping on an earth leakage fault typically of 30mA. This is a fault current of up to 1000 times smaller than the MCB! RCDs are useful as certain hazards can exist in the event of a fault that will not trip an MCB. Typically this involves applications that are, or may, come into contact with water.

Earth leakage is a small current that stems from phase conductors to earth. This causes an imbalance between live and neutral and it is this imbalance that RCDs detect. If the earth leakage is high enough on an appliance due to a fault or water contact then the equipment chassis can deliver a dangerous “touch current” if a user touches it. The RCD is there to protect against this scenario. If your application has water involved, then it is very difficult for a risk assessment to justify the omission of an RCD from the electrical infrastructure unless other safety measures are taken.

Isolation Transformer

An isolation transformer, by its very nature will stop RCDs from tripping – even in the event of an earth fault. See Isolation Transformers – what you need to know for further reference on this. However this isn’t a problem. In fact, the isolation transformer can make the installation more safe than with the RCD alone. Even a device with a fault can be touched by a user without any hazard occurring. Unless – and I can’t stress this point enough – the isolation transformer has the output Neutral and Earth bonded!

N-E bonds are not there for safety, but rather for noise rejection performance by establishing a zero volt neutral-earth voltage. Isolation transformers in conjunction with UPS Systems provide a very resilient power protection solution. However, in order to ensure the system is safe, then you should not bond the N-E. Our isolated UPS systems leave the system floating, providing true isolation and an inherently safe electrical environment. If you use a N-E bonded system and no risk assessment has been carried out to determine that no RCD is necessary then this contravenes the requirements of BS7671:2018.

Decision Flowchart

BS7671 RCD UPS Selection Flowchart

Start by asking if there is a documented Risk Assessment as to why there is no need for an RCD on a socket outlet. If there is, then you’re good to go and any UPS is good for this scenario. You can use isolated (floating or N-E bonded) or non-isolated depending upon your requirements.

If there is not a risk assessment in place then we need to check if there is an RCD fitted. If not, or unknown, then in order to provide the safest environment, the solution is a truly floating isolated UPS. Granted, if no RCD is in place, fitting any UPS does not make the situation less safe, it’s just that a floating isolated UPS does make it safe.

If there is an RCD fitted, and no risk assessment has been carried out, then you must not use any NE bonded system. This removes the safety aspect of the RCD.


According to the 2018 Wiring regulations there needs to be an RCD fitted on any sub 32A circuit. This will cause power to be removed if earth leakage of over 30mA is detected. Any standard UPS will not interfere with the operation of the RCD, however an isolated UPS will prevent the RCD from operating.

However, a floating isolated system, where Neutral and Earth are not connected provides a safe electrical environment. In situations where an RCD should be installed, for example there is water required by the application, and the electrical infrastructure is unknown (for example older installations to which RCD was not a mandatory requirement), floating isolated UPS provide the ideal solution.

An isolated UPS that is floating renders RCDs ineffective but provides enhanced safety by removing any touch current hazard.

On the other hand, a N-E bonded UPS system not only negates an RCD but does not make safe any scenario to which the RCD was required to protect against. There’s a reason for section of BS7671 and this situation violates it.

An isolated UPS with a Neutral and Earth Bond renders RCDs ineffective and does not protect against hazards for which the RCD is intended.

Regen Battery -To Pack Or Not To Pack

AG1500S ReGen Battery

“Do I need a battery pack to go with my ReGenerator?” is a commonly asked question. Well the answer depends upon what you’re trying to achieve. The ReGen battery allows the AG1500S to continue to provide power in the event of a mains power outage. In some circumstances this is essential and in others not so. First you need to understand how a ReGenerator operates.

Under The Hood


The essence of the AG1500S is based on the very latest in Uninterruptible Power Supply (UPS) technology. More exactly it is based on what is termed online double conversion or VFI technology.

Schema diagram for AG1500S ReGen

The incoming AC power feeds a charger which charges and keeps the batteries maintained (if they are fitted). It also generates the DC Bus via an AC Boost Circuit. In between the AC Boost Circuit and the source is a “Backfeed Relay”. We’ll learn more about this later.

The DC bus is a high voltage positive and negative internal power supply rail that the inverter uses to create the AC output power waveform.

The microprocessor controls everything (including the inverter but this isn’t shown for clarity). It constantly monitors the incoming AC power waveform and should it find the power to go out of tolerance it will instantly transfer power from the AC Boost, to the DC Boost and open the back feed relay. Power is then drawn from the battery via the DC Boost circuit. The net result is that the inverter sees no change of any note in the DC Bus and continues to provide power as normal. There is no break, or change as far as the output power is concerned.

What if I don’t have the ReGen Battery Fitted?

In the event of an out of tolerance input power (note out of tolerance means too high, too low, or non-existent voltage, or frequency) the ReGen will switch over from AC source to DC source (the battery). It will do this extremely quickly and will not revert back to AC Power until several seconds after the source is back in tolerance.

If no battery is connected, then the ReGen will detect a low battery condition and simply switch off. Since low battery is classed as an alarm, you may hear the ReGen emit a “dying” tone.

There are power problems which are known as a micro-outages or voltage sags. These are very momentary dips or breaks in power that may not be overly noticeable, but the AG1500S will detect them and transfer to battery to prevent any loss of power. Since the unit will not revert back to AC power until several seconds after the AC power source is restored, the unit will detect low battery and switch off. In such instances it looks like the ReGen has simply switched off for no reason. But there is a reason, and that reason is that there is a power anomaly that the ReGen is designed to prevent getting through to your load. It won’t allow any break or variance from the pure waveform it is intended to deliver.

When the ReGen Battery is fitted, the unit will seamlessly transfer from AC power to the DC power and continue to power your load without any interruption at all. If it isn’t fitted, then the ReGen will switch off when the power is out of tolerance, regardless of how short that power anomaly may be.

Backfeed Relay

You might wonder why I wanted to mention the backfeed relay. This component is in the circuit primarily for safety. What is does is physically disconnect the input power conductors from the unit. This is to prevent the possibility of the output power appearing on the pins of a mains plug should you pull it from the socket – ouch!

Backfeed Relay

In normal operation the relay is closed, connecting Live and Neutral to the ReGen circuits. Should the AC power fail the unit opens the relay contacts from both the live and neutral conductors. This isolates the output from the input.

Some say, this is great for audio applications! But, sorry there is a but, it requires running the unit for prolonged period on battery. There are two issues regarding this. Firstly, if the power consumption is quite high, the length of time the batteries would last is quite reduced. For example if you had a fairly low powered system of say, 50W , you could run this for over 6 hours! However a 500W system would last just over half an hour. Perhaps not long enough!

Secondly, if you were planning on doing this regularly, then you should note that lead acid batteries as used in the ReGen Battery only have a finite number of charge and discharge cycles of 200-300, which means you could be replacing the battery pack each year. If the batteries are not fully discharged, then you will get a lot longer – typically between 3 and 5 years.


The AG1500S will work without the ReGen Battery pack. It will provide a pure stable output power waveform at all times. However, if the power should fail even momentarily the AG1500 switches off.

So if you need the in-built UPS capability of the AG1500S, or want to run your system truly isolated from the “real world”, then you should consider the ReGen Battery. Available on our online store.

BS8418:2015 UPS System Selector

BS8418 UPS Applications

Many moons ago we blogged about BS8418:2010 (Installation and remote monitoring of detector-activated CCTV systems, Code of Practice) and the requirements for UPS Systems. That standard stated:

Unless the mains power supply is supplemented with a stand-by generator, an uninterruptible power supply (UPS) must be able to power the CCTV control equipment and communications devices for a minimum of 4 hours after mains power failure. Where the mains power is supplemented by a stand-by generator, the UPS needs to be capable of providing stand-by power for a minimum of 30 minutes after mains power failure (for example if the stand-by generator does not start).

The 2015 revision relaxed this somewhat, allowing for a documented threat assessment and risk analysis to determine whether a UPS is required or not. That said, it is difficult to state how any threats or risks are mitigated against a loss of power without a UPS, so the requirement for UPS Systems is likely still to remain in BS8418:2015 installations.

If a UPS is used as the “alternative power source” then this has been changed from a 4 hour requirement to a 30minute requirement when supporting control equipment and data transmission devices. However the standby power capability for the detectors and semi-wired detectors remains at 4hours.

Find a UPS Solution

Enter in your load power and how long you need the UPS to provide backup power for. The UPS Selector will identify any UPS that meet your requirements.

You can filter the selection based upon required features, by clicking the checkbox. Many models are available to by online from our webstore but contact us using the form below for specific requirements or for other products not available to purchase online.


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Quality control of the electricity supply

Sine Wave Scope Trace

This is a guest post by Werner Karau of Antares Power,  Power Inspired Distributor in Germany.

In this edition we consider the quality control of electricity supply.

May we invite you to a small thought experiment?

How do you answer this question: “Would you drive your car without fuel filter?”

“No, of course not“, you would probably reply. jerry-can

Gasoline or diesel fuel is needed to operate the engine. Okay, this is no breakthrough insight. Oil companies provide the quality of fuel in accordance with European standards. After delivery to the petrol station network they are no longer responsible for contaminants until use in your engine. The fuel is filtered before use by the engine of your car. The filters are regularly renewed so that your engine always receives clean and suitable fuel.

Without this filter foreign material can enter the circulation and cause malfunctions in the form of motor standstill, dropouts, reduced power, also accelerated wear, or in extreme cases even destruction and major engine damage.

It is self-evident to have these filters and to look after them.

Now we apply this model on the power, the fuel in form of electrical energy that powers your systems. Electrical energy is an unusual commodity. It is continuously needed, cannot be stored in significant quantities and is beyond quality control before use.

Before installation of complex systems it is sometimes quoted which sockets are to be used with which voltages, frequencies and circuit breakers. That was it mostly. Rarely references to preferred power grid systems such as the TN-S system with better EMC characteristics.

Practically all your systems are “somehow” connected to the existing power grid. An examination of the suitability for the operation of your systems will not take place. cable jumble

Basically, your system matches the high-performance engine of a luxury car, which is being used without a fuel filter. It has been developed with all of your expertise, continuously improved and manufactured under the highest quality standards to be now driven with dirty energy.

Some argue that all this is only the problem of the user. We cannot answer this question, however, but see the results in practice.

– Do you know the quality of the electricity at the installation site?

– Or does your customer, which is the user, know the state?

If your system has operating faults or disruptions, which appear suddenly and are inexplicable, components have a premature wear / failure or the unit is even destroyed, the user first seeks for the fault in the system itself.

Noisy Scope Trace Your “workshop team” will be sent to fix the error. If the cause is not to be assigned to an application error, components need to be replaced during the warranty period, so the costs can’t be billed to the user neither. When in doubt, ex gratia payments are to be made. Often the costs are agreed in the context of a maintenance contract.

The power companies cannot be held responsible for it, they guarantee for the security of supply to the delivery point up to a certain degree but not for the quality – especially since most interferences are “home-made” through the user.

Your system meets all the current standards and guidelines for the CE mark. Here, the electromagnetic compatibility, the emission and immunity has been checked. However, these tests will be held under standardized conditions in the test laboratory, real conditions cannot be simulated. Other devices of any manufacturer can be found in the immediate vicinity at each installation site. All are connected to one and the same power grid and some of them are connected via data cable.

Each unit carries the CE mark. It is simply not possible to examine any of these combinations in terms of interference in EMC laboratory. Influences are predetermined. It does not affect every single device. A heating plate is insensitive when compared to systems with sensors that measure electron volt range. The more powerful, precise and highly technical installations are, the higher the clock speed of data processing digital technology (CPU, memory), the lower the flanks of the logical ones and zeros, the more sensitive they react to the slightest disturbance of the power supply.

At this point it is difficult to find a connection with the supply voltage. Some systems are unaffected, others do not tolerate interference. Often service technicians quote our customers: “But the other device always works…“.

Most everything results in a situation, which is not satisfying for everybody involved.

The user receives repairs or services. Why should he make an effort in causal research?

There are almost always the service managers and service technicians of our OEM customers, who request our services.  power quality monitoring

An analysis of power quality creates clarity. We offer you this service for free. The process is very simple. The installation and start-up of our measuring instruments can be handled by anyone. The data acquisition is fully automatic and after an agreed recording time you will send us back the devices for evaluation. You will receive a comprehensive report. Whether before installation or for supportive investigation of an incident.

We often hear from our customers that our analysis has been the basis for a lasting solution to the problem. It is also the basis for deciding whether the user solves the problem or the manufacturer of the systems. For us, the analysis allows the selection of a suitable system to ensure power quality. Everyone involved benefits from it.

Power Quality ReportsUse our service to reduce the number of ex gratia payments or warranty claims in your workshop in order to create more capacity for your customers who come to billable inspections. Less unnecessary service calls, caused by electricity allows more efficiency and increase profit from maintenance contracts.

We have top manufacturers of industrial systems as partners for our solutions aside. When selecting we focus on meeting the industry-specific specifications, e.g. suitability for extreme dynamic loads, overload capability, robustness, etc. The straightforward way to our partners enables us in addition to the direct support also being able to offer an excellent price-performance ratio.

We follow this principle with suitable, compact, appropriately sized and cost-effective solutions, at the point where the power is needed to prepare optimally suitable power quality for your system.

Have a pleasant trip” Yours Antares Power Solutions Team!

Sine Wave Scope Trace

Natural Disaster Survival Kit

natural disaster survival kit

Severe weather conditions and natural disasters are more of a threat in certain geographical areas than others. You need to be prepared if you live in a high risk area. It is recommended that you have an Emergency Supply Kit ready for when severe weather strikes.
A separate kit for home, work and for your vehicle is a good idea

Priority items are:


1. Water and food

tinned food You should have sufficient supplies of water and long-life foods. Stock 4 litres of water per person per day with supplies for three days for evacuation. You should hold 2 week worth of supplies at home and sufficient amount of food for the same period of time. Don’t forget to have the appropriate supply of pet food and of course water is also essential.



2. Important medications

You should have at least one week’s supply of prescription medications and common meditations such as pain killers, antiacids etc. Your survival kit should also include a first-aid kit.



3. Documents

important documents
You should have copies of personal documents including birth certificates, passports, medication lists, medical records, insurance and bank policies etc. in a waterproof container ready to be taken with you during evacuation.




4. Personal hygiene items

including soap, tooth brushes and toothpaste, towels, deodorant and other essential items.






5. Electronicsmobile

Your emergency supply kit should also include your mobile phone, chargers, battery backup, battery-powered radio, extra batteries and a contingency power supply solution for operating any medical equipment that is used regularly.











7. Other essentialsflashlight

Blankets, flashlight & spare batteries, multipurpose tools and maps should also be included in your emergency supply kits.





You may also want to include:

  1. Whistle, dust mask, glasses and contact lenses
  2. Sleeping bags for each person
  3. Paper plates, cups, plastic containers
  4. Water cleansing drops, matches
  5. Fire extinguisher
  6. Spare clothes and sturdy shoes
  7. Baby formula, nappies, bottles etc.


When a disaster strikes it is never pleasant but with the emergency supply kit you will feel more comfortable and at ease knowing you are prepared for any scenario.

Top 5 interesting facts about UPS

Facts about Uninterruptible Power Supply

You probably know how Uninterruptible Power Supplies work and what their purpose is, but we have the top 5 interesting facts about UPSs that you probably didn’t know!


UPS facts - UPS inventor

The history of Uninterruptible Power Supplies is somewhat a mystery – nobody can name one person or one date when UPSs systems were invented. There were many theorists and scientists involved in research and studies, however nobody  can claim to be the inventor of UPS.



UPS facts - ups patent
source: google/patents

This rather amusing acronym stands for “Apparatus for Maintaining an Unfailing and Uninterrupted Supply of Electrical Energy”, which is a first ever UPS patented in 1934 by John J. Hanley. Mr. Hanley mentions in the document that the invention is to be used with fire alarms and other safety systems to protect properties and lives. It is amazing how far UPSs have come since then and how the variety of applications widened since 1930s. The AMUUSEE was an ancestor of UPSs as we know them now. We are certainly grateful that UPSs are now called Uninterruptible Power Supply rather than the tongue twister that is used to be back in the day.

You can read the whole patent document at google/patents.


UPS facts - largest ups in the world

The largest Uninterruptible Power Supply system in the world is a 46-megawatt system in Alaska.  The system is called the “Battery Electric Storage System (BESS)” and is located in Fairbanks, Alaska where it powers the entire town! It can provide up to 15 minutes runtime at 26 megawatts, providing enough backup power until the generator comes on. The runtime is achieved by four battery strings, each containing 344 series of connected battery modules. That is one truly amazing UPS.


UPS facts - batteryA UPS is nothing without batteries. Did you know that majority of faults within UPSs are related to batteries? Good batteries matter and so to ensure your UPS provides maximum reliability, you need to organise for appropriate maintenance. Power Inspired Uninterruptible Power Supply systems come with VRLA batteries with 3-5 year design. This time is however approximate and depends upon many factors, including the environment in which the UPS is stored. You can see how to prevent premature battery failure in our previous blog.



UPS facts - TX Series Isolated UPS SystemsPower Inspired’s TX series provides unique safe UPS technology. The units contain an isolation transformer which protects from electric shock and ensures the UPS is isolated and safe. TX series units range between 1KVA – 10KVA, they have a small footprint and the design is highly modern and minimalistic. The larger units (3KVA, 6KVA and 10KVA) also come with wheels for ease of installation. TX series UPS are designed to provide highest degrees of power protection for laboratory, industrial and medical applications. More information about TX series can be found here.

UPS ECO mode – what’s so good about it?

UPS eco mode

All of our online double conversion UPS systems feature a setting option to run the UPS in an ECO mode. What does this mean? And is it beneficial for you? We will try to answer those questions for you.

An ECO mode in UPS systems essentially means that the UPS’s inverter is in a standby mode. It only kicks in if the mains power fails. By enabling ECO mode on your UPS it will basically have the same operation as a regular line interactive UPS.  In a nutshell, by enabling the ECO mode you’ll expose the load to raw utility power.

What’s good about it?

The ECO mode has some great advantages, such as efficiency improved from 94%-97% to 98%-99% and lower operation costs (up to 4% reduction on energy use). It may also prolong the lifetime of some of the UPS components due to the decreased operating temperature on these components – the UPS is in a bypass mode hence some components are not in use which may prolong their life span.

What’s bad about it?

However the ECO mode not only has positives, it also comes with some risks: the fact that the UPS must first detect power failure and then turn on the inverter results in a transfer time that in some critical applications cannot be tolerated. Unlike the standard (online) mode of fully featured online UPS systems (our VFI-T, VFI-B and TX series) where the switchover time is nonexistent, this may be an issue for some critical, highly sensitive equipment. The ECO mode can improve UPS efficiency by about 2%-4%, however at the cost of possible downtime due to the switchover time. The switchover time can last anywhere from 1-16 milliseconds during which time your equipment will be exposed to any power problems present on the mains. Some equipment may be okay to cope with the transfer time, however some more complex and critical loads may be unable to tolerate it.

The ECO mode comes with some other risks: Besides the risk of reduced electrical protection which may have a negative impact on reliability and possible operation issues, the UPS in ECO mode will switch to battery at any instance of power problem which may have been easily dealt with without reverting to battery using standard online mode. This may negatively affects the battery’s lifetime and wear.


These are the advantages and disadvantages of using UPS in an ECO mode. The ECO mode can improve efficiency and reduce operation costs of your UPS, however while choosing your default operation mode, you need to bear in mind all the associated risk and decide if your equipment can take on the risk. As an operator you need to thoroughly consider all pros and cons and decide what settings will work best for you.

UPS Classification to EN62040-3

EN62040-3 UPS Classification Curves

EN62040 is the European Harmonised Standard for Uninterruptible Power Systems (UPS). Part 3 (EN62040-3), published in 2011 is the Method of specifying the performance and test requirements. Here we’re looking at UPS Classification which is at section 5.3.4 entitled “Performance Classification”.

UPS Classification

If you take a look around our product lines you will see UPS models beginning with such acronyms like VFI, VIX and VIS. Well, there’s more to them than appears at first glance, as these acronyms let you know at a glance what the UPS technology is. I say that, but it is not technically correct, as the standard specifically states that the classification is performance based and does not exclude any particular technology or topology.

The UPS Classification is of the following nomenclature and we’ll go through each part in turn:


AAA = Input  Dependency Characteristic

This part consists of 2 or 3 letters and describes the relationship between the UPS output and the UPS input during normal operation. Normal operation means mains power is present.

Three classifications are offered, VFD, VI and VFI. They are so designated depending whether or not the output (V)oltage and (F)requency  are (I)ndependent or (D)ependent on the input Voltage and Frequency.

VFD: Voltage & Frequency Dependent

UPS Classification. Voltage Frequency Dependent (VFD) Topology

This topology is also known as “offline”. In normal operation what is at the UPS AC input, is at the UPS AC output. Hence the UPS output is dependent on the input voltage and frequency.

If the mains should go out of tolerance the switch moves from AC input to the Inverter. This topology is the lowest cost of all UPS topologies and is almost certainly to have a square wave inverter. The switch does mean there is a slight break during the switch over.

VI: Voltage Independent (Frequency Dependent)

Voltage Independent UPS Topology

The VI topology is similar to VFD with the exception of the Buck and Boost Autoformer. This device allows the UPS Output to be lowered (“Bucked”) or raised (“Boosted). The advantage of this is to prevent needless battery operation for sustained over or under voltages. To that end, the output voltage is not directly dependent on the input voltage but it is either the same as, or a set percentage higher or lower. So IMHO, saying it is Independent is pushing it a little, but there does need to be a way of differentiating from offline units. The frequency of course is dependent.

Lower cost VI units will have a square wave inverter, much the same as offline units. Higher specified devices will have a sine wave inverter.

VFI: Voltage & Frequency Independent

Voltage Frequency Independent (VFI) Topology

VFI Technology is also known as Online Double Conversion, due to the fact the AC is converted to DC and then from DC to AC. In the much simplified diagram above the batteries and rectifier feed a common DC link. This DC link powers the inverter – which will practically always be a sine wave. The UPS AC Output is fed from the inverter. Thus the output voltage and frequency are truly independent. Note some units will in fact track the input frequency and are therefore not precisely independent, but the main point it is the inverter circuit that is the power source.

BB = Voltage Waveform Characteristic

This part of the UPS Classification determines if the output waveform is sinusoidal or not on both normal operation and battery operation. Normal operation is the first character and battery operation the second.

The character can be either one of “S”, “X” or “Y”, for linear and non-linear loads. A linear load is a resistive one, such as a heating element. The reference non-linear load is a smoothed bridge rectifier circuit. Note that in the definitions below, the output is considered sine if the Total Harmonic Distortion (THD) is under 8%.

“S”: The output is sine for both linear and non-linear loads.

“X”: The output is sine for linear load, but non-sine for non-linear loads.

“Y”: The output is non-sine for both linear and non-linear loads.

For example, a square wave offline UPS will be designated VFD – SY. The VFD is for Voltage Frequency Dependent, but since the output follows the input, then in normal mode the output will be as per the mains supply, eg a Sine. When on battery though the output is non-sine for both linear and non-linear loads.

Note, the nomenclature we use in our product range follows this only loosely. For example we use VIS and VIX to determine line interactive sine, and line interact non-sine. Just want to avoid any confusion.

CCC = Dynamic Output Performance

This is a 3 digit number.

The first digit refers to the voltage variation due to a change in operation mode e.g. from mains to battery.

The second digit refers to the voltage variation due to a step change in a connected linear load.

The third digit refers to the voltage variation due to a step change in a connected non-linear load.

Each digit is designated 1, 2 or 3 depending upon how the output voltage reacts according to the curves in the graph below.

EN62040-3 UPS Classification Curves

It looks complicated but not as bad as it looks. It also gives a very useful guide for knowing what UPS technology is required for your application. There are 3 curves in the chart with 2 limits, one for overvoltage and one for undervoltage.

Performance Level “1” is required by sensitive critical loads.

Performance Level “2”will be accepted by most critical loads.

Performance Level “3” will be accepted by most general purpose IT loads (eg switch mode power supplies).

For example a high quality online double conversion UPS (such as our VFI Range) will give 111 performance. Our VIX UPS Range range will be 321.


Just as an aside, this is my take on the standard and I’ve deliberately not copied any text verbatim or copied any diagrams from the standard for obvious reasons. You can buy the standard from the good old British Standards Institute: BS EN62040-3:2011. And you’d be better off becoming a member first.

AC ReGenerator Input Vs Output

What a great question received into technical support today:

Can you show the difference between the input and output voltage waveforms on the AC ReGenerator?

We have a bit of bespoke in-house test equipment that we use for looking at the quality of the mains supply. It allows us to view both normal and common mode noise levels, as well as viewing an AC power waveform. We can connect it to the output of the ReGenerator and show you a nice clean output waveform. We can also connect it to the input and show you another apparently clean input and output waveform, or a distorted waveform, or a clean high voltage waveform. The point is, the output of the ReGenerator is the same, regardless of the input waveform. If the input waveform is good, then this isn’t really showing us anything.

So what we have done is to create a secondary piece of test equipment that provides a distorted power waveform to the ReGenerator:

Distorted AC power waveform

This oscilloscope trace shows a pretty messed up AC power waveform, with a high Total Harmonic Distortion. Such a power waveform will cause untold problems with electronic systems, and with any inductive devices such as those containing transformers or motors. Any system requiring a quiet noise floor will be sadly disappointed as the noise levels will be remarkably high on the line to neutral. Any standard filter devices will redirect this to the earth resulting in common mode noise issues as well. Clearly not good for Audio Visual applications!


When we feed this power waveform into an AG500 ReGenerator, we get this at it’s output:

Power Inspired AG500 ReGenerator Output Waveform

See the harmonic distortion is eliminated! Not only protecting the inductive elements but ensuring that noise levels have improved dramatically. Exactly the sort of environment your AV equipment wants to see. This is the main difference between ReGenerators and filters. The ReGenerator recreates a new AC power waveform. A filter can only try to remove what shouldn’t be there.

Read more about the Power Inspired AG Series ReGenerators.

Buy online.