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Extract fans in dwellings

Competency for persons involved in electrical installation work

Equipotential bonding

Installing downlighters safely

Uninterruptible power supplies

NEW 17TH EDITION TO BE LAUNCHED IN 2008

WIRINGMATTERS

Spring 07 Issue 22

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IntroductionBS 7671: 2008 Requirements forElectrical Installations, IEE WiringRegulations 17th Edition is scheduledto be issued on January 1st 2008 and isintended to come into effect 6 monthslater. The 17th Edition will becompletely restructured compared tothe present 16th Edition and includeschanges necessary to maintaintechnical alignment with CENELECharmonisation documents. The newedition will adopt the IEC numberingsystem. In addition, the layout andparts will be completely revised; forexample, many of the chapters will becompletely rewritten. The current Part 6 (special locations) will becomePart 7 to align with IEC. The nextedition of BS 7671 will includeadditional sections on speciallocations not currently included in BS 7671 and the existing special

locations will be revised to align withchanges in CENELEC harmonisationdocuments. This article is based on thedraft for public comment and thereforethe actual requirements of the 17thEdition may change.

What’s new?Part 1 adds requirements to protectagainst voltage disturbances andimplement measures againstelectromagnetic influences.

Part 3 adds requirements for safetyservices, e.g. emergency escape lighting,and fire protection applications. Also,Chapter 36 requires that an assessmentshall be made for each circuit of anyneed for continuity of serviceconsidered necessary during theintended life of the installation.

In the new Chapter 41, the termsprotection against direct contactbecomes basic protection and protection

against indirect contact becomes faultprotection. Socket- outlets up to 20A foruse by ordinary persons require 30mARCD protection and socket-outlets upto 32A for mobile equipment for useoutdoors require 30mA RCDprotection. Note that certainexceptions are permitted – refer toRegulation 411.3.3.

There are new additionalrequirements for the connection oflow voltage generating sets includingSSEGs in Chapter 55.

Section 559 Luminaires and LightingInstallations is a new series ofRegulations concerning lightinginstallations and also includeshighway power supplies and streetfurniture previously in Part 6.

Chapter 56 has been expanded andincludes requirements for emergencyescape lighting and fire protectionapplications.

There are also changes to inspectionand testing. Changes have been made tothe requirements for insulationresistance; when testing SELV andPELV circuits at 250 V, the minimuminsulation resistance is raised to 0.5 M;for systems up to and including 500 V,(including FELV), the minimuminsulation resistance is raised to 1.0 M.

IEE Wiring Matters | Spring 07 | www.theiet.org

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NEW 17TH EDITION TOBE LAUNCHED IN 2008A BRIEF OVERVIEWby Geoff Cronshaw

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Published by IET Publishing & Information Services Michael Faraday House, Six Hills Way, Stevenage, Herts, SG1 2AY, United KingdomTel: +44 (0)1438 313311 Fax: +44 (0)1438 313465

Sales and Project Coordinator L Hall +44 (0)1438 767351 [email protected] | Editor G D Cronshaw +44 (0)1438 [email protected] | Contributing Editors J Ware, M Coles, J Elliott | Design Sable Media SolutionsIEE Wiring Matters is a quarterly publication from the Institution of Engineering and Technology (IET). The IET is not as a body responsible forthe opinions expressed.

©2007: The Institution of Engineering and Technology. All rights reserved. No part of this publication may be reproduced, stored in a retrievalsystem, or transmitted in any form or by any means without the permission in writing of the publisher. Copying of articles is not permittedexcept for personal and internal use. Multiple copying of the content of this publication without permission is always illegal. Web-offsetprinting by Wyndeham Heron, The Bentall Complex, Colchester Road, Heybridge, Maldon, Essex, UK

Co-operating Organisations The Institution of Engineering & Technology acknowledges the contribution made by the followingorganisations in the preparation of this publication: British Electrotechnical & Allied Manufacturers Association Ltd – R Lewington,P D Galbraith, M H Mullins | Department for Communities and Local Government – I Drummond | Electrical Contractors Association – D Locke, S Burchell | City & Guilds of London Institute – H R Lovegrove | Energy Networks Association – D J Start | Electrical Contractors Association of Scotland SELECT – D Millar, N McGuiness | Health & Safety Executive – K Morton | Electrical Safety Council | ERA Technology Limited –M Coates | British Cables Association – C Reed | Scottish Building Standards Agency | DTI – D Tee | CORGI – P Collins | GAMBICA – K Morris.ISSN 1749-978-X

Special installations or locationsThe next edition of BS 7671 will includeadditional sections on special locationsnot currently included as follows: Marinas (Section 709) Exhibitions, shows and stands

(Section 711) Floor and ceiling heating systems

(Section 753) Mobile or transportable units

(Section 717) Fairgrounds, amusement parks and

circuses (Section 740) Photovoltaic power systems

(Section 712).

Special locations are areas ofincreased shock risk, for example:

Marinas. There are particular risksassociated with electrical installationsin marinas. The environment of amarina or yachting harbour is harsh forelectrical equipment. The water, saltand movement of structures acceleratedeterioration of the installation. Thepresence of salt water, dissimilar metalsand a potential for leakage currentsincreases the rate of corrosion. Thereare also increased electric shock risksassociated with a wet environment byreduction in body resistance and contactwith earth potential.

Exhibitions. There are particularrisks associated with exhibitions,shows and stands. These arise from:

1. The temporary nature of theinstallation

2. Lack of permanent structures3. Severe mechanical stresses4. Access to the general public.

Changes to the Existing Requirementsfor Special LocationsThe current special locations containedin the IEE Wiring Regulations will berevised to align with the latest IEC andCENELEC standards.

For example, the requirements forlocations containing a bath or a showerunit will require 30mA RCD protectionon all circuits in a bathroom/showerroom. Zone 3 is no longer defined. Socketoutlets other than SELV and shaverunits are allowed 3 metres horizontallybeyond the boundary of zone 1.Supplementary equipotential bonding isno longer required providing mainequipotential bonding is installed inaccordance with Chapter 41.

The requirements for swimmingpools now include fountains and thezones have changed from A, B, and Cto 0, 1, and 2.

In agricultural and horticulturalpremises and construction sites thereduced disconnection times and 25volt equation no longer appear. TheUK has retained the use of reducedlow voltage supplies for constructionsites which will continue to be arequirement in the 17th Edition.

In caravan/camping parks each

socket outlet must now beindividually protected withovercurrent and RCD protection.

Changes to AppendicesAppropriate changes have been made to the existing Appendices 1 to 7. Inaddition the following new Appendicesare now included:Appendix 8 Current-carrying capacityand voltage drop for busbar trunkingand powertrack systemsAppendix 9 Definitions – multiplesource, d.c. and other systemsAppendix 10 Protection of conductorsin parallel against overcurrentAppendix 11 Effect of harmoniccurrents on balanced three-phasesystemsAppendix 12 Voltage drop inconsumers’ installationsAppendix 13 Methods for measuringthe insulation resistance/impedanceof floors and walls to Earth or to theprotective conductor systemAppendix 14 Measurement of faultloop impedance, consideration of theincrease of the resistance of conductorswith increase of temperature.

Further informationImportant: This article is onlyintended as a brief overview and onlygives a small number of the changes.For further information on the 17thEdition please refer to the IETwebsite: www.theiet.org/DPC


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Kitchens and bathrooms now are often fitted withextract fans either to meet the requirements of theBuilding Regulations or at the owner’s request. Suchfans will need to be replaced or repaired during thelifetime of the installation. The IET Helpline oftenreceives enquiries relating to two aspects of thesefans; the isolation and switching requirements ofBS 7671 and the ventilation requirements of theBuilding Regulations.

In this article we will explain briefly the concepts ofisolation and switching and then discuss thesefunctions in relation to extract fans and in anupcoming article we will cover the ventilationrequirements placed by Part F of the BuildingRegulations.

Isolation and switching. The term isolation and switching, as used in BS 7671:2001 Requirements for Electrical Installations (TheIEE Wiring Regulations) refers to four distinctfunctions: Isolation, Switching off for mechanical maintenance, Emergency switching, and Functional switching.

The concepts of isolation and switching aresummarized in Table 1.

IsolationThe definition of isolation, in BS 7671 (The IEEWiring Regulations) is: ‘A function intended to cut offfor reasons of safety the supply from all, or a discretesection, of the installation by separating theinstallation or section from every source of electricalenergy’. Isolation is provided to permit an electrically-competent person to work safely on all or part of anelectrical installation. Once electrical equipment hasbeen securely isolated from the source of energy andany electrical energy has been discharged, a skilled orinstructed person should be able to safely access partsthat are normally live, or may become live, withoutthe risk of danger from electric shock, electric arcingor explosion or from electrically-powered equipmentand machines.

The corresponding definition in the Electricity atWork Regulations (EWR) adds the specificrequirement that the isolation is to be secure. This

EXTRACT FANS IN DWELLINGSby John Ware

Table 1: Summary of the four functions of Isolation and Switching

Table 2: Principal requirements in BS 7671 and the EWR applicable to isolation in dwellings

Isolation and switching: The four functions

Isolation:Purpose: To enable electrical work to be carried out safely on the isolated circuit or

equipmentFor: An electrically-skilled or supervised person.

Switching off for mechanical maintenance: Purpose: To enable non-electrical work to be carried out safely on the switched

circuit or equipmentFor: Non electrically-skilled persons

Functional switching:Purpose: To enable proper functioning and control of electrical equipmentFor: The user of the installation

Emergency switching:Purpose: To cut off rapidly electrical energy to remove an unexpected hazardFor: Anyone

Summary of requirement Regulation(s) in BS 7671 and the EWR which refer

Every circuit to be capable of being isolated 461-01-01

Neutral conductor to be capable of being isolated 461-01-01(with certain permitted exceptions1) 460-01-04

Group of circuits may be isolated by a common means 461-01-01

A means of isolation to be provided as close to the 460-01-02origin of an installation as possible 476-01-01

Means of isolation to be non-automatic 460-01-01

Means of isolation to be suitably positioned or durably marked 461-01-05

Means of isolation not to be inadvertently or unintentionally energized 461-01-02

Means of isolation may need to be secured in the open position 476-02-02

Requirements where there is more than one device to be operated 461-01-03514-11-01

Circuits containing stored energy may need to be discharged2 461-01-04

Means of isolation to be secure EWR (Regulation 12)

means the isolated equipment cannot be re-energised accidentallyor inadvertently.

The procedure of ‘Isolation’ includes (a) correct identificationof the equipment and circuit to be worked on, (b) disconnection,(c) securing the means of disconnection, (d) posting notices and

1 A neutral conductor falls within the definition of a live conductor. With permitted exceptions for TN-S andTN-C-S systems, the neutral conductor is required to be able to be isolated from the correspondingsupply conductor. A means of isolation of the neutral conductor in a TN-S or TN-C-S system is notgenerally required where that conductor can reliably be regarded as being at Earth potential. The neutralconductor (PEN or N) for supplies provided in accordance with the ESQCR 2002 is considered to beconnected with Earth by a suitably low resistance. For a TT system, the neutral conductor as well as thephase conductor are required to be provided with a means of isolation.

2 Note that motor circuits may include a capacitor such as a start capacitor which would store energy. Thestored energy has to be discharged before work can be undertaken on the circuit.


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(e) proving dead. In some casesadditional precautions will also beneeded. Please refer to the HSEpublication: Electricity at Work: SafeWorking Practices.

Requirements applicable to isolationare given in BS 7671: 2001Requirements for ElectricalInstallations and Regulation 12 of theEWR and these requirements aresummarized in Table 2.Isolation of an extract fanLet us take the case of an extract fan ina dwelling and assume the installationis part of a single-phase TT system. Theissue of isolation will arise during thelifetime of the installation when anelectrician is called in to repair orreplace the extract fan. The electricianwill need to isolate the fan from thesource of energy in order to work safelyon it. As the installation is part of a TTsystem, both the phase and neutralconductors will need to be isolated.The main switch in the consumer unitIn many dwellings the main switch ofthe consumer unit can form a perfectlyadequate means of isolation. In such acase, the electrician will switch off themain switch, take precautions that itwill not be inadvertently orunintentionally switched back on, andthen having verified all the conductors

of the fan circuit are dead, be able towork in safety on the fan. Thedifficulty that can arise when usingthe main switch as the means ofisolation is that it is often remote fromthe extract fan and there is a very realrisk of someone inadvertentlyswitching the supply back on. Theelectrician is therefore required totake adequate precautions against thishappening by means such as securingthe main switch with a padlock orlocking the cupboard or door wherethe consumer unit is located andputting notices.A circuit-breaker in the consumer unit In the case of an installation formingpart of a TN system where theneutral conductor can be regarded asbeing reliably connected with Earth,it may not be necessary to disconnectboth conductors and safe isolationmay be able to be achieved byisolating the phase conductor bymeans such as switching off andlocking off the appropriate circuit-breaker providing the device issuitable for such use. Note thatputting a bit of insulation tape over aswitched-off circuit-breaker is notsufficient. Once again, part of theisolation procedure is to verify thatall conductors are indeed ‘dead’.

Local isolatorOften a local isolator, such as a pullcord switch, a switched fusedconnection unit or a switched deviceinstalled outside the bathroom butnext to the door leading in to thebathroom is provided. Providing theswitch meets the requirements ofisolation, the advantages of using sucha switch are:

1. The electrician may decide it is safeto isolate the extract fan at this switchleaving the main switch at theconsumer unit on thereby permittingoperation of other electricalequipment in the dwelling, forexample, the lighting circuits and thesocket-outlet circuits.

2. Providing the isolator is local to theextract fan the electrician may decidethere is minimal risk of it beinginadvertently or unintentionallyswitched back on while he is workingon the extract fan because he is next toit and can stop anyone interfering withit. Securing the means of isolation isthereby simplified.

Once again, the conductors that are to be worked on have to be provedto be dead.

Figure 1: Insertion of a local isolator in the lighting and extract fan circuit for a windowlessbathroom in dwellings

Figure 2: Three-pole isolator which can belocked off. Courtesy of MK Limited


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Mechanical maintenanceAn extract fan should be maintainedperiodically and such maintenancenormally includes cleaning as the fanwill almost certainly get dirty. Inorder for such maintenance to beperformed safely, a means ofswitching off for mechanicalmaintenance is to be provided(Regulation 462-01-01 refers). Themeans of switching off is notnecessarily intended to provideprotection against electric shock buthas to reliably stop the fan turning(Regulation 462-01-03 refers).Switching off for mechanicalmaintenance, in this case, is to enablenon-electrical maintenance to beperformed safely without the risk ofinjury from mechanical movement.The demands of safety are such thatthe same means of preventingunintentional or inadvertent reclosureof the switch must be provided as forthe function of isolation. The meansprovided for the function of isolationwill, in almost all cases, be sufficientto meet the requirements forswitching off for mechanicalmaintenance.

Functional switchingFunctional switching is an operationintended to switch ‘on’ or ‘off’ or vary thesupply of electrical energy to all or partof an installation for normal operatingpurposes. The purpose of functionalswitching is to enable current-usingequipment, such as an extract fan, to becontrolled for normal operatingpurposes. The control may be manual,such as a simple fan on/off switch orautomatic such as a timer circuit orvariable such as a speed control.

Emergency switchingThe provision of emergency switchingrequires the identification andassessment of reasonably foreseeabledangers. Regulation 463-01-01 refers. Anextract fan in a bathroom or kitchen isunlikely to cause danger such that anemergency switch will be required.

Windowless bathrooms. Windowless bathrooms and toilets haveto be fitted with a fan that often includesa timer circuit to ensure the fancontinues to run for a period after thelight has been switched off. Figure 1illustrates a typical configuration for acircuit. The timer unit is often part ofthe fan assembly. A two pole isolatorinserted at X-X in the circuit wouldeffectively isolate the fan and timer unitbut would also isolate the bathroom lightwhich would result in inconvenience

for the electrician who would then needto provide a light source.

A three-pole isolator inserted at Y-Y inthe circuit would be preferable as itwould permit the light to be leftenergized whilst the fan or timer unitwas being worked on.

The electrican called upon to work onthe timer unit or extract fan has to verifythat the correct circuit has been isolatedand all the conductors are indeed deadbefore he starts work as there is a realrisk of incorrect wiring in such a circuit.

A mains voltage extract fan may be installed in zone 1, 2 or 3 providing the additional requirements listed below are met:

Requirement Zone 11 Zone 2 Zone 3

IP rating At least IPX4 (Regulation 601-06-01) The fan has to be suitable for the conditions (Regulation 512-06-01)

RCD protection 30 mA RCD Note that Manufacturer’s protection is instructions for mains voltage fans required2. may include the recommendation (Regulation that 30 mA protection be provided.601-09-02) Such instructions should be followed.

A SELV extract fan may be installed in zone 1, 2 or 3 providing the requirements listed below are met:

Source The safety source, such as the transformer, may have to be installed outside of the zones(Regulation 601-08)

IP rating At least IPX4 (Regulation 601-06-01) The fan has to be suitable for the conditions (Regulation 512-06-01)

Table 3: Requirements applicable to a fixed extract fan installed in a location containing a bath tub orshower basin

Requirement Zone 1 Zone 2 Zone 3 Outside the Outside the Zones bathroom

Local isolator Isolator operating at mains A local isolator may be installed but it permitted? voltage not permitted. Only SELV has to be suitable for the conditions

switches are permitted (Regulation 512-06-01)(Regulation 601-08-01)

IP rating At least IPX4 Isolator has to be suitable for the (Regulation 601-06-01) conditions (Regulation 512-06-01)

Table 4: Requirements applicable to a local isolator1 for an extract fan installed in or adjacent to alocation containing a bath tub or shower basin

1 Note that plate-type switches are unlikely to have a suitable IP rating and will need to be installed in zone 3 or outside the zones.Often such switches are installed on the outside of the bathroom but next to the door leading into the bathroom. Pull cord switches areunlikely to have a suitable IP rating and while the pull cord itself may enter zone 1 or 2, providing it is an insulating pull cord, the body ofthe switch will have to be installed in zone 3 or outside the zones. Hence, in a bathroom with a ceiling height greater than 2.25 m or 3 mthe body of the pull cord switch may be installed in zone 3 or outside the zones (But see Regulation 512-06-01) and the insulating pullcord allowed to enter zone 1 or zone 2 (See Regulation 601-02-01 and Figures 601A and 601B in BS 7671).

1 Fixed current-using equipment such as an extract fan may be installed in zone 1 if (i) it is suitable for the conditions of that zone,(ii) can reasonably only be located in that zone and (iii) RCD protection is provided (See Regulation 601-09-02(iii)).

2 The Residual Current Device used is required to have a rated residual operating current not exceeding 30 mA in accordance withRegulation 412-06.



Figure 3: Inline extract fan and ducting

Additional requirements applicable to an extract fan installed in a location containing a bath tub or shower basin in a dwellingA fixed extract fan can be installed in a location containing abath tub or shower basin providing the additionalrequirements listed in Table 3 are met.

The requirements given in Table 4 are required to be met fora local isolator installed in or adjacent to a location containinga bath tub or shower basin.

Supplementary bondingWhere a mains voltage fan or its local isolator, if any, isinstalled in zones 1, 2 or 3, local supplementary bonding has tobe provided connecting together the protective conductor ofthe fan circuit and extraneous-conductive-parts in these zones(Regulations 601-04-01 and 601-04-02 refer).

Inline extract fansInline extract fans providing air extraction through ductingare often installed in the loft above a bathroom to providebathroom ventilation as illustrated in Figure 3. An extract fanmounted in the loft above the bathroom is not ‘in thebathroom’ and hence is not subject to the supplementaryrequirements placed by Section 601 of BS 7671. Note that thefan is, of course, subject to the general requirements containedin the other Parts of BS 7671.

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IntroductionThe IET technical advice line oftenreceives calls relating to thecompetency of persons carrying outelectrical installation, maintenance,inspection & testing and similaractivities and what qualifications arerequired to be classed as an“electrician”. The helpline often takesenquiries from persons currentlyworking as electricians who have noformal qualifications who now wish tofind an appropriate qualification, andfrom those who wish to enter theelectrical industry from a wide rangeof backgrounds.

A number of typical questionsrelating to competency, qualificationsand training for electricians are givenbelow with appropriate answers.

What is an electrician?The term electrician is generic, has nolegal status and is in no way protected.Anyone may refer to themselves as

being an electrician. As such, the termis no indicator of a person’s level oftraining, technical qualificationsachieved, or the extent of relevant workexperience accumulated - all factorswhich will affect a person’s ability to doelectrical work properly. The ability toperform a particular task properly isdefined as competency. Generallyspeaking, a suitably qualified andcompetent electrician should be able toperform a wide range of installationactivities in domestic, industrial andcommercial installations. Personswishing to employ an electricianshould ask for some evidence of theircompetency. This might includeproduction of certificates obtainedfrom successful completion ofrecognised training at a technical orfurther education college, a JointIndustry Board ElectrotechnicalCertification Scheme card, or proof ofmembership/affiliation with arecognised industry body.

What are the legal requirements? The Electricity at Work Regulations1989 imposes duties on personsinvolved in electrical workcommercially whether employers, theself employed or employees, includingmost trainees.

Regulation 16 (Persons to becompetent to prevent danger andinjury) states:

“No person shall be engaged in anywork activity where technical

knowledge or experience is necessary toprevent danger or, where appropriate,injury, unless he possesses suchknowledge or experience, or is undersuch degree of supervision as may beappropriate having regard to thenature of the work.”

It is stated in the Memorandum ofguidance on the Electricity at WorkRegulations 1989 (HSE: 1989) that “theobject of the regulation is to ensurethat persons are not placed at risk dueto a lack of skills on the part ofthemselves or others in dealing withelectrical equipment”.

It continues: “the scope of ‘technicalknowledge or experience’ may include:(a) adequate knowledge of electricity;(b) adequate experience of electricalwork;(c) adequate understanding of thesystem to be worked on and practicalexperience of that class of system;(d) understanding of the hazardswhich may arise during the work andthe precautions which need to betaken;(e) ability to recognise at all timeswhether it is safe for work tocontinue.”

What qualifications are needed tobecome an electrician?People have come into the electricalinstallation industry from a number ofroutes and may have a number ofdifferent electrically-biasedqualifications.

COMPETENCY FORPERSONS INVOLVEDIN ELECTRICALINSTALLATION WORKby Jon Elliott


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Concentrating on electricalinstallation work, typically personswanting to become electricianscomplete a City & Guilds qualification.Over the years, City & Guilds hasoffered a number of qualificationswhich provided the knowledgeevidence requirements for electricaltrainees:- 236 part “A” and “B” certificates (notavailable since the late seventies)- 2360 part 1 (not available sinceOctober 2006) and part 2 (no longeravailable from October 2008)- 2330 part 1 and 2 certificates (bothavailable from September 2004)

All of the above could, and in thecase of the 2330 can, be studied bypersons not directly employed in theelectrical installation industry.

What subject updating and further development qualifications are available? For persons who have been employedin the electrical industry for sometime and who have no electricalqualifications, or who qualified priorto the advent of the 16th Edition, themost appropriate subject updating or“refresher” course is probably the City& Guilds 2381 “16th Edition”certificate. Those wishing to refreshand/or improve their skills in relationto the inspection, testing andcertification of an installation maychoose to take the City & Guilds 2391Inspection & testing certificate, whichmay also be beneficial to thosewanting to be graded as an ApprovedElectrician on the Joint IndustryBoard (JIB) ElectrotechnicalCertification Scheme, or who wish tobecome a Qualified Supervisor for anNICEIC Approved Contractor or ECAregistered company.

What about persons wishing to become electricians havingqualifications that are not electricalinstallation work specific?Many persons have entered theindustry having successfully

completed electrically biased BTEC(and later Edexcel) National Certificateor diploma courses or otherelectrically related qualifications. Inthe case of qualifications other thanthose provided by City & Guilds listedabove an assessment will have to bemade of their suitability in terms ofproviding the necessary knowledgebase for someone involved in electricalcontracting if they wish to obtain aJIB grading or become affiliated withother electrical organisations.However, qualifications primarilyrelating to electronics and/orcomputing even if at a notionallyhigher level in the NationalQualifications Framework may not besuitable without the addition of someelectrical installation specifictraining/experience.

What is the National QualificationsFramework?The National QualificationsFramework categorises all accreditedqualifications on a scale ranging fromentry level through to level 8. In thefield of electrical installation worklevels 2, 3 and 4 are of relevance.Broadly speaking, level 2 is “firstcertificate” level, will cover a narrowrange of work activities and in manycases may be awarded as an interimstage on the way to becoming fullyqualified in a particular occupation.Completion of part 1 of the City &Guilds 2360 or 2330 qualifications willresult in the award of a level 2certificate. Level 3 is “craft” level, orthe level required to be competent in aparticular occupation. Completion ofpart 2 of the City & Guilds 2360 or 2330qualifications will result in the awardof a level 3 certificate. Level 4 wouldequate to technician level and as suchfalls beyond the remit of this article.

What is a domestic installer?In 2005 the Building Regulations asapplied to England and Wales weremodified to include requirements fordomestic electrical installations.

Companies or persons carrying outelectrical work in domestic premiseswere required to either notify therelevant Building Control departmentfor the area where notifiable work wasbeing carried out prior to startingwork or to become registered asdomestic installers. A qualifiedelectrician generally met therequirements to become a registereddomestic installer. However, manypersons carrying out work activities indomestic premises requiring a degreeof work on the electrical installation,such as central heating installers andkitchen fitters did not. As such therewas a need for a recognisedqualification to be developed thatwould provide “non electricians” withthe necessary knowledge and skills tobe considered competent for suchwork. The examination board EMTAAwards Ltd (EAL) developed a level 2qualification for domestic electricalinstallers in conjunction with a


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number of interested parties from theelectrical industry. It should beremembered that due to the limitedcourse content of this qualification,successful completion does not meetthe requirements of any electricalorganisation for grading as anelectrician.

What is an Electrical Apprenticeship?When school leavers wish to becomeelectricians the best route into theindustry is via an apprenticeship. Thiswill normally take 3 to 4 years tocomplete. The apprentice will receivepractical “on the job” work experienceand training throughout this time andwill be given opportunities to be moreinvolved in work as time progressesand their skills and abilities develop.During the first three years, theyattend a further education college on aday-release basis (that is, attendingone day per week during college termtime) where the knowledge obtained inthe workplace is reinforced byinstruction and training. They willalso receive key skills training forcommunication, application ofnumber, information technology,working with others, improvinglearning and performance andproblem solving. On completion oftheir studies at college they receive atechnical certificate and take theAchievement Measurement 2 (AM2)timed practical assessment and withthe assistance of their employercompile a site based logbook in orderto obtain their level 3 NationalVocational Qualification (NVQ) orScottish Vocational Qualification(SVQ) as appropriate.

What are the JIB Grading requirements?ElectricianIn order to be registered as anelectrician with the Joint IndustryBoard for the Electrical ContractingIndustry (JIB) a person must: have been a registered apprentice orundergone some equivalent method oftraining and have had practical

training in electrical installationwork, and have obtained an NVQ / SVQ Level 3in electrical installation work (orapproved equivalent such as successfulcompletion of the City & Guilds 2360part 2 certificate and have passed theAchievement Measurement 2 (AM 2),or be able, with the application forGrading and any other relevantsupporting evidence (i.e. the City &Guilds Electricians' Certificate) whichmay be required, to satisfy theGrading Committee of his experienceand suitability), and be 21 years of age (this requirementmay be waived if the applicant hasobtained a pass in the City & Guilds2360 Electrical Installation TheoryPart 2 Course or approved equivalent),in addition to the above Electricians are expected to be ableto carry out electrical installationwork efficiently in accordance withthe National Working Rules for theElectrical Contracting Industry, thecurrent IEE Regulations for ElectricalInstallations, and the ConstructionIndustry Safety Regulations.

Approved ElectricianIn order to be registered as anApproved Electrician with the JIB aperson must have met therequirements to be graded as anelectrician above and mustadditionally:

have had two years experience as anelectrician subsequent to thesatisfactory completion of trainingand immediately prior to theapplication for the ApprovedElectrician grade, or be 22 years ofage, whichever is the sooner, and have demonstrated competence andobtained a suitable qualification (suchas the City & Guilds 2391 qualification)in the inspection, testing andcommissioning of installations.

Approved Electricians are expected: to possess the practical, productiveand electrical engineering skills with

adequate technical supervisoryknowledge so as to be able to work ontheir own proficiently and carry outelectrical installation work withoutimmediate supervision in the mostefficient and economical manner be able to set out jobs fromdrawings and specifications andrequisition the necessary installationmaterials be able to accept responsibility forthe proper completion of jobs and, if required, supervise otheroperatives.

ReferencesElectricity at Work Regulations 1989(HMSO: 1989)Memorandum of guidance on theElectricity at Work Regulations 1989(HSE: 1989)Requirements for Grading of ElectricalOperatives. (Joint Industry Board:2006-2007)

Further Information QualificationsIn general- Qualifications and CurriculumAuthority: www.qca.org.ukElectrical installation work- City and Guilds: www.city-and-guilds.co.ukDomestic electrical installer- EMTA Awards Ltd: www.eal.org.uk

ApprenticeshipsIn England and Wales- JTL: www.jtlimited.co.uk- Modern Apprenticeships:www.apprenticeships.org.ukIn Northern Ireland- The Electrical Training Trust:www.ett-ni.orgIn Scotland- The Scottish Electrical CharitableTraining Trust: www.sectt.org.uk- The Scottish Enterprise:www.scottish-enterprise.com/modernapprenticeships

Electrotechnical Certification Scheme- Joint Industry Board: www.jib.org.uk


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IntroductionBS 7671: 2001 (incorporatingAmendments No 1: 2002 and No 2: 2004)has requirements for protectionagainst electric shock, and lists achoice of five basic measures whichshall be used to protect againstindirect contact. Protection by earthedequipotential bonding and automaticdisconnection of supply is the mostcommon measure. Its purpose is thatunder earth fault conditions, voltagesbetween simultaneously accessibleparts are not of such magnitude andduration as to be dangerous.

Main equipotential bondingRegulation 413-02-02 requires mainequipotential bonding to be carriedout. Its importance is oftenunderestimated (see Figure 1). Anearth fault in the current-usingequipment produces a fault current(If) which flows along the circuitprotective conductor and back to thesource. A small proportion of thecurrent may flow through the mainequipotential bonding conductordirectly to earth, and then back to the source.

The potential difference between the

equipment exposed-conductive-partand the simultaneously accessibleextraneous-conductive-part is:

Uf = If R2

Where:If is the fault current

R2 is the resistance of the circuitprotective conductor.

(Ignoring any reactance of the circuitprotective conductor, and any smalleffect of current flowing in the mainequipotential bonding conductor)

The effect of connecting the mainequipotential bonding conductor to theextraneous-conductive-part is tominimise Uf. Without this conductor,the potential difference wouldapproximate to the voltage dropproduced by If along the full length ofthe earth return path, and this couldbe significantly greater than (If R2).Therefore, failure to install allnecessary main equipotential bondingconductors within an installation willcertainly increase the shock riskassociated with indirect contact.

Installation of main equipotentialbonding conductorsIEE Guidance Note 5 recommendsthat main equipotential bondingconductors should be kept as short aspracticable and be routed to minimisethe likelihood of damage ordisturbance to them. The connectionsto gas, water and other servicesentering the premises must be madeas near as practicable to the point ofentry of each service, on theconsumer’s side of any insulatingsection or insert at that point or anymeter. Any substantial extraneous-conductive-part which enters thepremises at a point remote from the main earthing terminal or barmust also be bonded to this terminalor bar.

Extraneous-conductive-parts shouldpreferably be bonded using individualmain equipotential bonding

Equipotential Bondingby Geoff Cronshaw


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conductors. Alternatively, two or moresuch parts may share a mainequipotential bonding conductor, butwhere this arrangement is employedthe conductor should be continuous,i.e. disconnection of the conductorfrom one extraneous-conductive-partmust not interfere with or endangerthe security of the bonding of theother part(s).

Regulation 547-02-01 and Table 54H ofBS 7671 gives sizing requirements formain equipotential bondingconductors. However, it isrecommended that the electricitydistributor or supplier should be askedto confirm their agreement to theproposed size(s) it is intended toinstall.

Regulation 514-13-01(ii) requires apermanent label to be fixed at or nearthe point of connection of every mainequipotential bonding conductor to anextraneous-conductive-part.

Supplementary equipotential bondingBS 7671 also has requirements forsupplementary equipotential bonding,which includes installations andlocations of increased shock risk suchas rooms containing a bath or shower,as shown in Figure 3.

Where supplementary equipotentialbonding is applied in a particularlocation within an installation, e.g. abathroom, it has the effect of re-establishing the equipotentialreference at that location for all theexposed-conductive-parts andextraneous-conductive-parts whichare bonded together locally. Thisfurther reduces any potentialdifferences that may arise betweenany of these parts during an earthfault.

Further information.For more information on earthingand bonding refer to IEE GuidanceNote 5. Also a new IEE Guidance Note8 specifically on earthing andbonding is due to be published shortlyby the IET.


earth

main equipotentialbonding conductor

main earthingterminal

extraneous-conductive-part

circuitprotectiveconductor (cpc)

U f

current-usingequipment

<2 m

R2

L

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originof

installation

exposed-conductive-part

If~– If

metal waste

Outside Zones

* Zone 1 if the space is accessible without the use of a tool. Spaces under the bath, accessible only with the use of a tool, are outside the zones.

Shaverunit

Cord

shower Switchfor fire

Radiant fire

metal pipes

Pull cord switch luminaire

m 4.2m 6.0

3.0 m 2.25 m

Zone 2

Zone 2 3 enoZ1 enoZ

Outside Zones

Ceiling

*

metalpipe

Zone 3

Zone 0

Figure 1: Illustration of main equipotential bonding

Figure 2: PME supply (TN-C-S system) Schematic of earthing and main equipotentialbonding arrangements. Based on 25 mm2 tails and selection from Table 54G.Note: An isolator is not always installed by the electricity distributor.

Figure 3: Supplementary bonding in a bathroom - metal pipe installation with soldered jointsproviding reliable electrical continuity


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Following these steps should ensure adownlighter will not pose a risk of firedue to overheating

1. Only use downlighters that conformto BS EN 60598, the British Standardfor Luminaires and ensure therequirements of BS 7671 are met

2. Follow the Manufacturer’sinstructions

3. Ensure the requirements of theBuilding Regulations (England andWales) are met

4. Provide space around thedownlighter

5. Fit the correct lamp

1. EN 60598. International StandardEN 60598 specifies generalrequirements for luminairesincorporating electric light sources foroperation from supply voltages up to 1 000 V. The requirements and relatedtests of this standard cover all aspectsof safety including electrical, thermaland mechanical in the areas ofclassification, marking, mechanical

construction and electricalconstruction.

BS 7671 Requirements for ElectricalInstallations requires, in Regulation 511,that electrical equipment, which includesluminaires, conforms to an applicableStandard. Section 422 ofBS 7671 gives requirements for protectionagainst fire and harmful thermal effectsand Regulation 422-01-02 applies to fixedelectrical equipment such as adownlighter that, in normal operation,has a surface temperature sufficient tocause a risk of fire or harmful effects toadjacent materials. The Regulation givesthree methods of preventing dangerwhich are (i) mounting within a suitableenclosure, (ii) screening or (iii) provisionof sufficient distance from adjacentmaterial. Refer to the Regulation for fulldetails.

2. Manufacturer’s instructions. Themanufacturer’s instructions suppliedwith the downlighter must be followed.The Manufacturer may require a

certain amount of space be left aroundthe back of the downlighter or that thedownlighter must not be covered withloft insulation, or only lamps of aparticular type and maximum wattagebe fitted or that a fire hood orintumescent hood be installed.

3. Building Regulations (England andWales). The installer must be aware ofthe requirements of the BuildingRegulations in England and Walesbefore installing a downlighter. Forexample, before cutting a hole in thefabric of the building the installermust ensure that the structuralintegrity, fire resistance or otheraspects of the structure are notcompromised. Approved Document Bgives guidance on the precautions tobe taken to inhibit the spread of firewithin a building. Approved DocumentA deals with structure and the basicrequirement is that persons installingelectrical equipment must not cut,drill, chase, penetrate or in any wayinterfere with the structure so as to

Installing Downlighters Safely by John Ware

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cause significant reduction in its loadbearing capacity.

Regulation 4(2) states that, oncompletion of electrical installationwork, the building (and parts of theelectrical installations in the buildingthat were not the subject of work)should be no worse in terms of thelevel of compliance with the otherapplicable Parts of Schedule 1 to theBuilding Regulations than before thework was undertaken.

For example, one or moreperforations of a ceiling liningbeneath a floor – made toaccommodate recessed lighting orsimilar fittings – may have an adverse

effect on that floor’s performance interms of its resistance to fire andsound penetration. Due regard shouldtherefore be paid to the guidance inApproved Documents B and E onthe performance of compartmentfloors.

Regulation 4(2) also means that,when extending or altering aninstallation, only the new work mustmeet current requirements and thereis no obligation to upgrade theexisting installation unless the newwork would adversely affect the safetyof the existing installation, or the stateof the existing installation was suchthat the new work could not be

operated safely, or where there is arequirement to upgrade imposed bythe energy efficiency requirements ofthe Building Regulations.4. Provide space around thedownlighter. A downlighter candevelop significant heat and sufficientspace must be provided around it.

When installing the downlighter inthe void between the ground floorceiling and the upstairs floor, thereshould be sufficient space around thedownlighter as illustrated in Figure 1.The downlighter used must be markedwith symbol: . Building debris andother flammable material must beremoved from the void. Cables must be

F

Figure 1: Installing a downlighter in the void between a ceiling and an upstairs floorF

Figure 2: Installing a downlighter in a ceiling with a loft space aboveF


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secured such that they do not comeinto contact with the hot surfaces ofthe downlighter.

When installing a downlighter in aceiling with a loft space above,precautions must be taken to ensurethat loft insulation or other materialdoes not surround or come intocontact with the downlighter.Installing a board between two joistsas shown in Figure 2 will, and runningthe loft insulation over the top of theboard will, in most cases, ensure

sufficient air space around thedownlighter. Once again thedownlighter used must be markedwith the symbol: .

5. Fit the right lamp. Manydownlighters are designed either foruse with 230 V dichroic lamps fittedwith GZ10 caps or aluminised lampsfitted with GU10 caps. (See above).

As can be seen in Figure 3 a GZ10holder will accept lamps having a GZ10cap and lamps having GU10 cap. A

GU10 holder will only accept lampswith a GU10 cap due to the chamfer.

A luminaire employing a dichroiclamp will run hotter than anequivalent luminaire fitted with analuminized lamp.

However, lamps that can be purchasedare GZ10 - dichroic and aluminised andGU10 - dichroic and aluminised. Use ofdichroic lamps in a luminaire designedfor use with aluminised lamps couldcreate excessive heat within theluminaire leading to an unsafe situationand risk of fire.

The European standard EN 60598presently caters for this situation byapplication, on the luminaire, of asymbol warning against the use of coolbeam lamps (dichroic) (see Figure 4).

It is recognized that many peoplewill not know what the above symbolmeans nor will they know thedifference between dichroic lamps andaluminised lamps. To avoid theoccurrence of unsafe situations theLighting Association advises itsmembers to supply only luminairessuitable for both applications i.e. evenif fitted with a GU10 holder theluminaire design should accommodatethe additional heat produced by thepossible use of a dichroic lamp.

F

Dichroic: Light away from luminaire GZ10 holderHeat back to luminaire

Aluminised: Light and heat away GU10 holderfrom luminaire

aluminiumreflector

dichroicreflector

cool beam

Figure 3: GU10 and GZ10 bases

Figure 4: Cool beam or dichroic lamps forbidden

GZ10 holder: Accepts GZ10 & GU10 lamps

GU10 holder: Only accepts GU10 lamps

A serious fire occurred in a listed building when a new lamp was fitted in adownlighter that had not been working for years.

In the attic above, an old oily coat had been thrown down and was partiallycovering the non-working downlighter. The heat generated by the new lamp set fireto the coat and destroyed the upstairs and roof of the property.


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What is an uninterruptible powersupply (UPS)?Fundamentally, an uninterruptiblepower supply, or UPS, is a unit whichmaintains the electrical supply to apiece of equipment, or load, followingthe failure of the primary source ofsupply. The UPS is, therefore, installedbetween the source of the electricalsupply and the load.

BS EN 62040-1-1:2003 defines a UPSas a combination of converters,switches and energy storage devices(for example, batteries), constituting apower system for maintainingcontinuity of load power in case ofinput power failure.

Are there different types of UPS?Fundamentally, there are twocategories of UPS – rotary systemsand static systems.

Static Static UPS systems deliver theoutput voltage derived from a storedsource, e.g. a series of batteriesthrough an inverter.

With a static UPS there will be “novisible” loss of supply to the loadwhen the mains supply is lost; thefollowing, figure 1, shows an exampleof the layout of a static UPS system

Under normal circumstances, theelectrical supply can be routed directlythrough to the load whilst the rectifier“rectifies” the a.c. supply to d.c. tocharge the storage batteries.

In the event of loss of the electricalsupply, d.c. from the batteries isinverted back to a.c. and will supplythe load; the bypass switch opens andstops the inverted UPS supply frombeing routed back to the origin of theinstallation. This is known as a passivestandby system.

The bypass switch can be used foranother function. Should the electricalsupply to the installation, or load, benon-sinusoidal, e.g. the harmoniccontent is such that the supplywaveform is no longer considered to besinusoidal, the UPS unit may be used asa “smoothing” device and clean up thesupply for use on sensitive or vulnerableequipment and critical loads. In reality,the batteries will be charging whilstsupplying the load. This is known as anactive standby system. Operation in thismode will also compensate for dips orsurges in the supply.

Static UPS systems are available inmany different sizes, ranging from verysmall and simple to very large andcomplex. Small and autonomoussystems are available providing circa1kVA; large UPS units can be paralleledto provide, in excess of, 1MVA.

Rotary Rotary UPS systems consist ofone, or more, electrical rotatingmachines to provide the outputvoltage, e.g. a generator or multiple-synchronised generators.

UNINTERRUPTIBLEPOWER SUPPLIES by Mark Coles

The aim of this article is to give an overview of uninterruptiblepower supplies and how to meet the requirements of BS 7671.

Electricalmains supply

Rectifier Inverter

StorageBatteries

UPSunit

Bypassswitch AC output

(Load)

Figure 1: Example of a static UPS system


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The rotary UPS system generallysits dormant until it is required.Control equipment will sense the lossof mains supply and switch theinstallation over to be supplied by thegenerator. Usually, there will be aperiod of time when the load iswithout a supply; this could be aperiod of seconds, even minutes,whilst the prime-mover starts and thegenerator attains full speed. Thisknown as the automatic load transfertime. Figure 2, shows the layout of aninstallation with a back-up generatoror rotary UPS.

The requirements of BS 7671

Isolation and switchingA UPS is a source of energy and, tocomply with Regulation 460-01-01, anon-automatic means of isolation andswitching should be installed todisconnect the source from the load.BS 7671 lists four types of switching –Isolation, Switching off formechanical maintenance, Emergencyswitching and Functional switching.The concepts of isolation & switchingare examined here:

The definition of isolation is:

IsolationA function intended to cut off forreasons of safety the supply from all,or a discrete section, of theinstallation by separating theinstallation or section from everysource of electrical energy.

The definition of a switch is:

Switch A mechanical device capable ofmaking, carrying and breakingcurrent under normal circuitconditions, which may includespecified operating overloadconditions, and also of carrying for aspecified time currents under specifiedabnormal circuit conditions such asthose of short-circuit. It may also becapable of making, but not breaking,short-circuit currents.

Regulation 460-01-02 requires that wherean installation is supplied from morethan one source, a main switch shall beprovided for each source of supply anda durable warning notice shall bepermanently fixed in such a positionthat any person seeking to operate anyof these main switches will be warnedof the need to operate all such switchesto achieve isolation of the installation.Alternatively, a suitable interlocksystem shall be provided.

Characteristics of supplyAs with any installation, it is arequirement of BS 7671 that the natureof the supply parameters are assessed,e.g. Ze (Ω) and Ipf (A); UPS systems,which are a source of supply, are noexception.

Further, Regulation 551-02-02requires that the prospective short-circuit current and prospective earthfault current shall be assessed for eachsource of supply or combination ofsources which can operateindependently of other sources orcombinations.

Protection against electric shockAn important aspect of providingprotection against indirect contactwhich can be readily overlooked by thedesigner is the need to ensuresatisfactory operation of the relevantprotective device(s) when theinstallation, or part thereof, isenergised from a UPS. To be certainthat the requirements of BS 7671 forprotection against electric shock (andshort-circuit) will still be satisfied, thedesigner must obtain full informationfor the alternative supply and makethe necessary checks of the design,which will have been based upon thecharacteristics of the normal supplysource.

Regulation 551-04-04 requires thatwhere the conditions for automaticdisconnection of Regulation 413-02cannot be achieved for parts of theinstallation on the load side of thestatic inverter, supplementaryequipotential bonding shall beprovided on that side in accordancewith Regulations 413-02-27 and 413-02-28. The resistance (R) of thesupplementary equipotential bondingconductor between simultaneouslyaccessible exposed-conductive-partsand extraneous-conductive-parts shallfulfil the following condition:

R ≤ 50I

where: I is the maximum fault currentwhich can be supplied by the staticinverter alone for a period of up to 5 s.

Further, Regulation 551-04-05 statesthat precautions shall be taken or

Figure 2: Example of a rotary UPS system

Point of isolation for both supplies(see Regulation 460-01-02)

Dieselengine

Electricalmains supply

Bypass switch

AC output(Load)

Generator


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equipment shall be selected so that thecorrect operation of protective devicesis not impaired by direct currentgenerated by a static inverter or by thepresence of filters.

Protection against overcurrentRegulation 551-05-01 requires thatwhere means of detecting overcurrentof the generating set is provided, thisshall be located as near as practicableto the generator terminals. Agenerator control panel or UPSequipment may include self-protection, a feature of which is therapid collapse of output voltage to theload. This will inhibit the operation ofany fault protective device situatedbeyond the equipment terminals andthe feature cannot be assumed toprovide a fail-safe operationalarrangement for the user. Safety ofthe system as a whole must beensured by, if necessary, involving theequipment supplier.

Earth electrodeRegulation 551-04-03 requires thatprotection by automatic disconnectionof supply shall not rely upon theconnection to the earthed point of the

distributor’s network when thegenerator is operating as a switchedalternative to a TN system. A suitableearth electrode shall be provided.Clause 18.2.1 of BS 7430 statesgenerator earthing calls for theprovision of an independent earthelectrode. It is necessary that the earthloop impedance at any point of theinstallation is low enough to ensure operation of the earth fault protection,and this should be taken into accountwhen the earth electrode forms part ofthe earth fault loop. For independentearth electrodes associated with thelocal earthing of the star point ofgenerating plant, it is recommendedthat the earth resistance should notexceed 20 Ω.

Supplies for safety servicesSafety services, such as fire alarmsystems, sprinkler systems, etc., areoften supplied by UPS systems as lossof supply to such equipment couldresult in loss of life. BS 7671 defines asafety service as an electrical systemfor electrical equipment provided toprotect or warn persons in the event of a hazard, or essential to theirevacuation from a location.

BS 7671 recognises that UPSsystems may operate in a parallelconfiguration. Regulation 566-01-01requires that protection against short-circuit and against electric shockshall be provided whether theinstallation is supplied by either ofthe two sources or by both in parallel.Further, Regulation 566-01-02 requiresthat precautions are taken to limitcirculation currents, particularly thatof third harmonics or multiplesthereof, in the connection between the neutral points ofsources.

Harmonic distortionStatic UPS systems may createharmonics on the sinusoidalwaveform. Other than selecting theuse of low harmonic-producingequipment, there are two recognisedmethods of reducing harmoniccontent; install harmonic filterswhich are suited to the load of theUPS or increase the size of theneutral conductor. Regulation 524-02-02 requires that the neutral conductoris adequately sized to carry themaximum current likely to flow in itunder normal operating conditions.


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Small systemsSome small UPS systems, circa 1kVA,can be unearthed and effectivelyoperate as an electrically separatedsystem. Note that certain items ofequipment require a reliableconnection to the means of earthing tooperate, i.e. filters within the switch-mode power-supplies of personalcomputers. Prior to connectingequipment to a UPS, it must beensured that the equipment is suitablefor operation in such circumstances.Regulation 413-06-03 requires thatwhere only a single item of equipmentis supplied in this manner, thereshould be no connection between theseparated circuit and any othercircuit, or to Earth. The flexiblecable/cord supplying the load, whichis liable to mechanical damage, shouldbe visible throughout its length. It ispreferred that a separate wiringsystem should be used for theseparated circuit (although multicorecables without magnetic sheath orinsulated conductors in an insulatedenclosure are permitted if the ratedvoltage of the cables is not less thanthe highest voltage likely to occur andeach circuit is protected againstovercurrent). Every live part of eachseparate circuit shall be electricallyseparated from all other circuits to astandard not less than that providedbetween input and output windings ofan isolating transformer to BS 3535.

Regulation 413-06-04 requires that noexposed-conductive-part of theseparated circuit shall be connected toeither the protective conductor of thesource circuit, or to any exposed-conductive-parts of any other circuit.

Other considerations

Prolonged loss of supplyIn the UK, some areas are moresusceptible to power cuts than others,particularly rural areas. Shouldinclement weather bring downoverhead power lines, for example, themains supply could be interrupted for

quite some time, perhaps days. A staticUPS would not have the capacity tosupply the load for a period of daysbut it would, however, provide enoughtime to allow back-up of informationand data during the enforced poweroutage. This is known as the autonomytime. If the installation is located insuch an area, a static UPS systemcould be used for short term powerloss with a rotary UPS installed toprovide an alternative long-term back-up source.

Storage batteriesStatic UPS systems are usuallyequipped with storage batteries tomeet the power requirements of theconnected load; large loads requirelarge battery banks. Small UPSsystems may have maintenance freebatteries but large banks will consistof one of two types of rechargeablebattery, namely, lead-acid or alkaline.

Lead-acid batteries are the mostcommonly used rechargeable battery,they are found in such applications ascars, motorcycles and electric vehicles.Note that the correct battery must bechosen for the particular application.

Alkaline rechargeable batteries,such as nickel-cadmium, nickel-metal

hydride and lithium ion, are widelyused in small items such as laptopcomputers. Large capacity versions ofthese cells are now used in transportand UPS applications.

There are two different types oflead/acid and alkaline rechargeablebatteries: valve-regulated(‘maintenance-free’) and vented. Invalve-regulated batteries, anyhydrogen and oxygen produced duringcharging does not escape but isconverted back into water. Watercannot be added to these batteries asthey do not need topping up. Incontrast, vented batteries allow anyhydrogen and oxygen produced toescape into the surroundingatmosphere and they require regulartopping up with water. However,installation, commissioning andmaintenance should only be carriedout by a competent person trained inthis line of work and experienced withthe particular equipment.

Sources of further information1) BS 7671: 2001 (2004) Requirements

for electrical installations2) BS 7430: 1998 Code of practice for

earthing.3) The Selection and operation of

uninterruptible power supplies, HES107/1996. http://www.hse.gov.uk/research/crr_pdf/1996/CRR96107.pdf

4) Using electric storage batteries safely.http://www.hse.gov.uk/pubns/indg139.pdf

5) BS EN 88528-11:2004 Reciprocatinginternal combustion engine drivenalternating current generating sets -Part 11: Rotary uninterruptiblepower systems - Performancerequirements and test methods

6) BS EN 62040-3:2001 Uninterruptiblepower systems (UPS) - Part 3:Method of specifying theperformance and test requirements

Thanks to Uninterruptible PowerSupplies Ltd. for the images usedhttp://www.upspower.co.uk
