The Requirement Of Uninterruptible Power Supplies Engineering Essay


Within Healthcare establishments the continuity of electrical supplies is an integral function of the service that they provide and quite literally can be a matter of life and death. Statistics show that Utilities Companies within the UK achieved on average a supply availability of 99.98% in the year 1995/96.This does not include very short duration interruptions. This statistic may sound impressive but in a hospital environment it can still prove disastrous as it equates to 97 minutes per year downtime which is not ideal in a service which should run 24/7.

The Electrical Design - A Good Practice Guide (1997) stated that;

"Modern medicine relies heavily on electronic monitoring, and failure of the supply or equipment may result in the loss of life. In a recent case in Honduras fourteen intensive care patients died during a power failure caused by rodent damage to the electrical installation."

Here the electrical infrastructure did not have the necessary resilience and was wholly reliant on a main source of supply which failed. In addition if the efficiency is less than 100% then financial losses can be huge.

Hospital Electrical System Background

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All Healthcare premises require a primary source of supply which is provided by a distribution network operator (DNO). The supply voltages to site vary depending upon each facility. Health Technical Memorandum 06/01 Electrical services supply and distribution (2006) Part A states that voltages will be one of the following;

11KV Large Acute Hospital, typical floor area greater than 8500m2

11KV/400 V Medium sized acute hospital, typical floor area 5500 m2 to 8500m2

400 V TP & N General/community hospitals, health centres, large off site clinics,

Off-site administrative buildings, stores and decontamination facilities.

230 V SP & N GP and Dental practices, small off- site clinics.

In addition resilience is built into the system in the form of secondary and tertiary electrical supplies. Secondary electrical supplies can be one of the following;

Standby Generator

CHP system

Wind Turbine

Photovoltaic System.

Tertiary supplies can include;

UPS (Rotary or Silent)

UPS (Static)

Battery packs

The built in resilience is expressed in terms of "N + 1", which equates to the normal total requirement plus one resilient unit and is an integral feature of the system irrespective of whether there is a dual power supply to the site or a localised UPS supplying at the point of use.

Within the clinical environment a risk orientated approach is adopted, with "cause and affect analysis" which is used to determine the systems that are treated as critical, essential and non essential. Furthermore clinical and non clinical risk categories are established dependent upon business objectives.

Figure 1- Electrical Failure risks evaluation to clinical categories (Health Technical Memorandum 06-01: Electrical services supply and distribution Part A Design Considerations, 2006)

Critical areas are classified as areas which affect the ability of the NHS Trust to operate and these will be supported by a UPS. In general all clinical areas are essential and will have 100% load provision. Non essential areas are mainly offices and administration areas where a power loss will not greatly affect the service.

Figure 2 - Electrical infrastructure generic flow diagram (Health Technical Memorandum 06-01: Electrical services supply and distribution Part A Design Considerations, 2006)

When determining what part of the electrical infrastructure should incorporate a UPS as a tertiary supply, consideration must be taken of the ratio between non essential and essential loading. The more demand on the essential supply then the more demand on the standby system. This is particularly important within the Healthcare sector as the demand for electrical supplies are currently growing between 3-6% each year. This increase is due to changes in medical technology and practices and directly affects risk management and ownership within the hospital environment. In accordance with HTM 06/01 should provide the following within a hospital environment;

A no break supply capable of sustaining the load for the required endurance period.

The UPS should change over within 0.5 seconds.

Harmonic distortion and UPS output sinusoidal waveform should be within tolerance and specification.

Battery autonomy should be achieved as the UPS should be operated at a load greater than 50% battery duty.

UPS Overview

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UPS are devices which maintain an emergency supply of power to a load in the event of a normal power supply failure. Tolerances for the mains supply into the UPS for the voltage are 10% and 5% for the frequency. This is then conditioned to give typically 1% variation for voltage and frequency. UPS work by providing power from a separate source when the original supply has failed. There are two classifications of UPS, rotary and static. A rotary UPS uses a motor generator to provide uninterruptable power and a static UPS as the name suggests has no moving parts and uses power semi conductors. In essence the choice of power protection solutions are determined by the source power quality.

Essential life saving equipment within hospitals is dependant upon several factors including voltage and frequency variations. In some cases the medical equipment can be the cause of electrical pollution because of their intense and distorted high currents. Dynamic loading can be caused simply by the very nature of the medical equipment which will have power quality repercussions throughout the entire site. Therefore UPS systems not only provide backup systems but are vital in providing power quality management.

A patient's quality of life is linked to the correct function of medical equipment as it can;

Minimise the need to repeat examinations and hence reduce patients stress.

Prevent accidents caused by improper use of medical devices.

Moreover the whole patient experience can be improved by the correct management of power quality control, and by providing clear and concise instructions in the event of an abnormal power supply.

Technical Aspect.

The power quality of the supply can be dangerous to critical loads within hospitals. The quality of the mains supply is measured in terms of it s waveform, frequency and voltage. There are a number of problems which can be associated with power supplies:

Figure 2 - Power Supply problems (

Sags - These are short duration voltage reductions which are below the nominal mains power supply level. This has the effect of drawing more current which in turn can lead to an increase in component stress and cause heat build up. Within the hospital environment sags are caused by switching air conditioners, machinery, or other types of high in rush loads on and off.

Surges - They are characterised by short duration voltage increases above the nominal supply voltage level. This has the effect of triggering the operation of built in protection devices which are in built within voltage sensitive equipment causing systems to crash. Higher voltage surges can lead to increased component wear and tear and degradation over time. This condition is difficult to spot initially and ends up with component failure.

Brown outs- These are defined as long term reductions in mains supply voltages which can be caused by the heavy demand from air conditioners which are typically used in hospital environments.

Spikes and Transients- Within a hospital spikes and transients can be common due to the wide range of equipment present on site. They are caused by high energy bursts which are super imposed onto the normal mains power supply. The energy spike can cause data corruption, processor damage and system memory loss and typical causes include fluorescent lights, inductive motor loads, fridges and freezers.

Electrical noise- This is high frequency electrical noise which can be classified as either common or normal, the former being present between the supply lines and earth and the latter between phase and neutral. Cable and switch gear faults and electronic equipment such as radio transmitters are often typical sources.

Harmonics- These represent voltage/ current waveforms, the frequency of which are multiples of the fundamental. Harmonics present a growing problem not only within the Health sector engineering world but also the private sector. It can cause mains power supply distortion, over heating of circuits and supply transformers leading to switch gear problems and nuisance tripping of breakers. Harmonic pollution is dealt with under the Engineering recommendation G5/4-1published by the Energy Networks Association. In a hospital environment typical causes of harmonics include.

Adjustable speed drives.

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Variable frequency drives.

Electronic welding equipment.



Transformers and generators.

Medical imaging equipment.

Lighting controls and dimmers.

Copiers and scanners.

All these sources of harmonic distortion have increased within hospitals with new technology. Adjustable speed drives and variable frequency drives have been fitted within hospitals to improve their "green credentials," yet can produce issues within themselves. In order to address harmonics, the Electricity Council Chief Engineers Conference Engineering Recommendation G.5/3 provides the guidance on the limits specified for harmonic currents. These limits are set for currents which may be fed into the mains supply by consumers. The following steps need to be taken to meet the acceptable limit of 5% for Total Harmonic Distortion (THD) when supplying non linear loads;

Connect the UPS to the mains electricity supply via a separate transformer.

Add tuned filter circuits in parallel with the rectifier unit in the UPS. This will deal with the problematic harmonics, namely the 3rd and 5th to give typical harmonic levels of between 2 and 3%.

Figure 3 - Harmonic distortion (Schneider Electrical, Cahier Technique no.152, Harmonic disturbances and their treatment)

Frequency variations - These are caused by secondary back-up systems, namely poorly maintained standby generators which can upset critical load operation.

Blackouts - Uncontrolled complete mains power supply failure which can last for milliseconds or for several hours. The result of this can be to crash lock or re-boot hardware in data or voice processing networks within hospitals and directly affect life saving equipment within theatres, intensive care units and high dependency units.

Power Quality

The power quality is an essential aspect within healthcare facilities and UPS installations are selected to overcome power issues inherent within the mains power supply.

Figure 4 - UPS power supply regulation (

There are typically 3 types of UPS which will provide power protection.

Standby/offline - Not a practical solution within hospitals as it is a low price solution which provides minimum protection. It operates with the Utility power provided during normal operation but any voltage or frequency changes are not regulated and so passed onto the load. In addition the UPS is only available up to 5KVA, but efficiency is high therefore less energy is wasted in terms of heat. However when integrity of supply is paramount in hospitals there is a short break on mains failure and no transient protection.

Line Interactive - This type of UPS provides medium range protection and basic voltage regulation before passing through the load. During the UPS voltage changes the line interactive UPS uses the battery for regulation of supply. This means that the UPS will sacrifice standby battery life for regulation of output voltage supply but it will not regulate frequency. Like the Offline UPS this model has a short break on mains failure and poor transient protection, not necessarily recommended for supporting critical services within a hospital where equipment within theatres can be frequency sensitive.

On line UPS - This type of UPS is ideal for the critical services and devices utilised within the hospital environment as it is capable of dealing with power fluctuations. It will protect against all types of power problems and continuously uses the inverter to provide 100% new clean and regulated AC power for the load. Within the health care sector on line UPS are essential for equipment support.

Power Problem


Line Interactive














Electrical Noise








Frequency Variations




Mains Failures




Figure 5 - Summary of Protection for differing types of UPS

UPS within Hospitals

UPS performs many functions within a hospital namely;

Provides emergency power during a mains failure

Conditions the electrical supply

Allows time to save medical files and records by providing a controlled shutdown in the event of a failure.

Allows secondary equipment, such as standby generators time to start up and also synchronise in the restoration of power supplies.

Provides for business continuity, healthcare facilities require protection against data loss and by implementation of a UPS strategy it ensures a continued clinical service. This is required in server rooms, security stations and within telecommunication networks.

Supports patient monitoring equipment such as respiratory equipment, incubators within Neo-natal.

Is required within theatres, intensive care units and high dependency units supporting other systems such as Isolated Power Supplies (IPS). These systems are able to sustain power to sub circuits during and following a first earth fault on the system by maintaining an isolated floating power supply.

Supports diagnostic equipment, hospital information systems, laboratory management systems and Clinical laboratory analysers.

Supports X-Ray machines, Magnetic Resonance Imagers (MRI) and nuclear medicine devices. Quite often these sources can promote electrical pollution but are inexorably linked to the patient's quality of life and hence are essential in promoting the NHS Trust's aim of providing and ensuring a patient's quality of life.

Prevents loss of communication and financial loss.



System reliability is an important feature within the hospital environment due to the sensitive and critical areas where UPS are fitted. The reliability is defined in terms of Mean Time Between Failure (MTBF) and the mains electricity supply will have a direct impact upon this. Engineers and users alike need to have confidence in their systems as there are often no second chances within Clinical areas. Selection criteria for optimal UPS reliability will include;

System Complexity - In general the simpler the installation then the more reliable.

Standard and Custom Design - Standard UPS designs are proven and have recognised benefits whereas custom built solutions require further analysis.

Ease of Operation - It goes without saying that the easier the system is to operate then it will eliminate unforced errors from operators.

Environmental Conditions - It is important that manufacturer's instructions are adhered too; failure to comply can be pernicious and damage components.

System Maintainability - A UPS system must be designed to ease maintenance by maximising availability and thus achieve reliability. This means that the UPS needs to be user friendly for maintainers/engineers and this can be achieved by implementation of features such as bypass switches and replaceable modules. In addition the UPS should be able to be fully isolated.


Manufacturers claim different levels of efficiency, depending upon how the UPS is integrated into the Electrical infrastructure and also for commercial reasons. Different types of UPS will have different efficiencies for example Static UPS are more efficient than Rotary units. This is due to the fact that Rotary UPS have standby losses which are associated with the machines controls, flywheels and associated equipment and represent the energy to either keep a motor running or a flywheel spinning.

Health Technical Memorandum 06/01 Part A states that;

"A UPS will radiate about 3% to 8% of its input power."

The standard for a UPS is that it is measured for heat output by the manufacturers and these results are then recorded on the manufacturer's data sheet for that particular model. This heat is representative of waste energy and depending on the size of the unit or site where there may be many backup systems fitted it can amount to a significant amount of waste.

Comparison of Static and Rotary UPS White Paper 92 states that;

"Data centre efficiency is expressed as the ratio of total data centre input power to IT load power."

This is applicable to hospitals where the IT infrastructure can be large.

Figure 6 - UPS Efficiency

Hulse. J. ( ), 13 October 2010. Re: UPS Efficiency curve. Email to A. Price (

Efficiency is referred to as Power Usage Effectiveness and the higher this value then the lower the efficiency consequently a perfect efficiency would be a value of 1.

In addition to the heat losses from the UPS as the efficiency is not 100% the excess heat may not be recoverable by means such as "free Cooling" and depending upon location and ventilation this excess heat may need to be removed. The removal of the heat would require additional energy in the form of air conditioning increasing the energy profile and impacting upon the UPS efficiency.

Sample UPS Loss Calculation

To support a 2000KW IT load, this UPS running at 95% efficiency must draw 2105KW from the utility.

2105 KW which is utility power required for the load, x 0.05, which is the efficiency loss, =105KW loss.

105KW x 8,760 hrs = 919,800KW/hrs losses per year.

(Bouley D. and Christin JF. (1992). Comparison of Static and Rotary UPS White Paper 92)

The European Code of Conduct on Energy Efficiency and Quality of AC Uninterruptible Power Supplies is a document to which major UPS manufacturers within Europe voluntarily sign up too. It outlines efficiency levels for specific sizes of UPS from 10-800KVA, loading from 25% to 100%. This forms the benchmark for efficiency and design. It is important to note that a UPS will not always be loaded up to 100% and in a hospital the loading is very much determined by the clinical turn over.

Energy Profile

NHS Trust's in accordance with mandatory targets have a legal requirement to reduce their energy consumption. This is difficult to achieve due to a number of factors. New technology and Clinical functions put pressure on current systems and departments. At the present time UPS technology is such that it is not 100% efficient and contributes to a Trust's ever increasing energy profile. Energy benchmarking, sub metering, improved technology, better system configurations and design along with better training and education can improve the situation. But better long term planning within the NHS is required to alleviate the pressure to achieve energy and Carbon Dioxide reduction targets.

The S. Giovanni-Addolorata Healthcare complex in Rome has a central-lised main UPS system supported by individual units which support critical end-use areas. It utilises Energy management strategies through load management to build a picture of energy wastage on site. The overall philosophy being;

"You can't save what you don't measure."

(A. Prudenzi, V. Caracciolo, A. Silvestri) (n.d) Man 4 Identification of Electric Load Typical Patterns in hospital for Supporting Energy Management Strategies

Albeit UPS will be a very small proportion of the energy wasted upon site it is still an area in which electrical load profiling needs to be taken. Within the hospital environment a UPS will not be loaded fully all of the time which can add to energy wastage.


The maintenance of UPS installations will be dictated quite often by Manufacturers instructions. But the level of Maintenance whether it is Planned Preventative Maintenance, breakdown maintenance or servicing is determined by a number of conditions. The environmental conditions have a large impact upon system performance; CIBSE Guide K recommends design ambient between 15 and 25Ëšc for location of UPS.

Environmental Conditions for UPS installations

The technology used, location, position and the load application within a hospital will affect the UPS output.

Considerations for UPS location are;

The environment should be clean, dust free and preferably air conditioned. The smaller the room then with regard to dust control positive pressure for the ventilation may be incorporated.

Not to be located in sunlight

Not located in areas subject to vibration

Not positioned directly below pipe work with the potential risk of leaks.

Air conditioning should be fitted where UPS installations are fitted in hospitals by the coast to take into account the prospective high saline content.

UPS positioning should allow for easy access for maintenance.

Should be treated as any other sensitive electronic equipment.

Temperature and Ventilation considerations are;

Ventilation grills should not be obstructed

The space around the unit should comply with design specification.

There should be adequate ventilation in small areas to allow for heat to be dissipated.

Air conditioning environments should be provided for larger UPS multi-module systems.

Although UPS installations are designed to operate up to 40Ëšc the optimum performance will be between 20-22Ëšc due to battery life expectancy

Figure 7 - Battery service life

Hulse. J. ( ), 13 October 2010. Re: UPS Battery Life curve. Email to A. Price (

Battery life deteriorates dramatically above 25Ëšc.

Figure 8 - Expected Service life v Floating voltage range

Hulse. J. ( ), 13 October 2010. Re: UPS Battery expected service Life curve. Email to A. Price (


The humidity levels should not exceed 90%.

These recommendations should be taken into consideration when implementing UPS within hospitals to maximise performance. To optimise UPS performance then the following considerations need to be adhered to and noted;

The input power and tolerances of supply for both mains and secondary site supplies such as main site generators, quality and short circuit level.

The type of load to which the UPS will supply and power factor.

The output power supply details. This is particularly important due to the sensitivity of certain medical/IT equipment.

The requirements of the UPS equipment. This includes all switching arrangements and autonomy.

Environmental conditions as discussed.

Site conditions and limitations.

New Technologies

- Discuss latest machines on the market

High Power factor correction