earthing - questions answered

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WIRING MATTERS Autumn 2005 Issue 16 Electrical installations outdoors Ongoing accuracy of test instruments PAT testing of equipment EARTHING: Your questions answered

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Page 1: Earthing - Questions answered

WIRINGMATTERS

Autumn 2005 Issue 16

Electrical installations outdoorsOngoing accuracy of test instruments

PAT testing of equipment

EARTHING:Your questions answered

Removed Pages
To save downloading time the following pages have been removed: 1) Cover page adverts, front & back 2) Advert pages 3, 7 & 11-14
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IN THIS EXAMPLE, the user has requested a smallsupply be provided to a detached garage to feed lightingand socket-outlets. The supply to the dwelling is PME.

Initially we will assume that the garage containsno extraneous-conductive-parts, such as a metallicwater supply or other earthed metalwork.

Two methods of meeting the user’s requirementswill be discussed:

1. The preferred method, where the supply is takenfrom a spare way in the existing consumer unit

2. Where the supply to the garage will be spurredfrom an existing ring final circuit.

At the end of the article we will discuss:

what to do if the garage has an extraneous-conductive-part such as a metal water pipe

verifying the existing installation, including theassessment of the earthing and bondingarrangements

protection of the cables, and inspection, testing and certification.

The electrical contractor making the addition to theinstallation must ensure all the applicable require-ments placed by BS 7671 are met including theprovision of protection against both direct andindirect contact, the correct selection of protectivedevices, the correct sizing and routing of cables andthe earthing and bonding arrangements.

The requirements of all parts of the BuildingRegulations must be met and this work is notifiable.

1. Garage is to be supplied from a spare way in theconsumer unit and a small two-way distributionboard is to be provided in the garage (fig 1).A suitably sized circuit-breaker should be fitted in thespare way in the dwelling’s consumer unit and a cablerun out to the small two-way consumer unit in thegarage. The two-way consumer unit in the garageshould be fitted with circuit-breakers for the two finalcircuits such as 6A for the lighting circuit and 16A forthe socket-outlet circuit. The socket-outlet is very likelyto be used to supply portable equipment outdoors andRCD protection must be provided by an RCD with arated residual operating current not exceeding 30mA.RCD protection should be provided by means such as:

An RCBO in the dwelling’s consumer unit for thegarage supply, or

An RCCB in the dwelling’s consumer unit, or Selecting a device that includes RCD protection as

ELECTRICALINSTALLATIONSOUTDOORS: A SUPPLY TOA DETACHEDOUTBUILDINGBy John Ware

IEE Wiring Matters | Autumn 2005 | www.iee.org

In this article, we will consider therequirements when supplying a detachedgarage, or shed, from a dwelling

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the main switch for the small two-wayconsumer unit in the garage, or

Protecting the socket-outlet circuit inthe garage with an RCBO, or

Providing an SRCD (a socket-outletincorporating RCD protection) for thesocket-outlet in the garage.

2. Garage is to be supplied from a spurfrom the ground floor ring final circuit(fig 2).If the ring final circuit is suitable for theadditional load, the garage could be suppliedfrom a fused spur inserted in the ring and acable run out to the socket-outlet in thegarage. In the garage a fused connection unitcould be employed for the lighting circuit.Once again RCD protection should be provi-ded for the socket-outlet, which is very likelyto supply portable equipment outdoors.

Published by IEE 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, P Still | Chief Sub Editor Jim Hannah | Design SaBle Media SolutionsIEE Wiring Matters is a quarterly publication from the Institution of Electrical Engineers (IEE). The IEE is not as a body responsible for theopinions expressed.

©2005: The Institution of Electrical Engineers. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system,or transmitted in any form or by any means without the permission in writing of the publisher. Copying of articles is not permitted except forpersonal and internal use. Multiple copying of the content of this publication without permission is always illegal. Web-offset printing byWyndeham Heron, The Bentall Complex, Colchester Road, Heybridge, Maldon, Essex, UK

Co-operating Organisations The Institution of Electrical Engineers 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 | Office of the Deputy Prime Minister – K Bromley, 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 – N Gove | National Inspection Council for Electrical InstallationContracting | | ERA Technology Limited - M Coates | British Cables Association – C Reed

Fig 1: Garage supply taken from a spare way in the consumer unit

Fig 2: Garage supply taken from a spur off the downstairs ring final circuit

[Illu

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A Type B 6kA, 30mA, 1module, 32A, RCBO

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Where the garage contains an extraneous-conductive-part such as a metal water pipeAmong the options open to the installation designerare to make the installation in the garage part of a TTsystem or to provide main equipotential bonding tothe extraneous-conductive-part in the garage.

TT systemOne possibility the electrical contractor may decide uponis to supply the garage from a spare way in the consumerunit and make the small installation in the garage part ofa TT system (fig 3). A local earth electrode must beprovided at the garage. The metal water pipe must not beused for this purpose (Regulation 542-02-04). To achieveprotection against indirect contact for both circuits inthe garage an RCD must be employed. Main equipoten-tial bonding will need to be provided at the garage con-necting the metal water pipe and any other extraneous-conductive-parts to the earthing terminal in the smalldistribution board (Regulation 413-02-02). An exposed-conductive-part connected to one means of earthingmust not be simultaneously accessible with an exposed-conductive-part connected to another means of earthing(Regulation 413-02-03 refers). Where the installation inthe garage is supplied by an armoured cable, the armouror any protective conductor in the cable must not beconnected to and must not be simultaneously-accessiblewith any exposed-conductive-parts in the outbuilding.

Main equipotential bondingAnother possibility is to include the garage within theinstallation in the main dwelling and provide mainequipotential bonding, in accordance with Table 54H(PME conditions apply), in practice this would mean a

10mm2 main bonding conductor would berequired to connect the water pipe in thegarage with the Main Earthing Terminal inthe dwelling.

Verify the existing installation The electrical contractor must verify thatthe rating and condition of existingequipment, including that of the distributor,should be adequate for the additional loadand that the existing earthing and bondingarrangements are also adequate (Regulation130-07-01 refers). It is possible that theincreased load placed by the garage wouldoverload an existing ring final circuit if thesecond option (above) was adopted.

Protection of the cables The cable run from the house to the garagemust be suitably protected either by beingrun overhead (see Regulation 412-05-01) orburied in the ground (Regulation 522-06-03refers). Where cables are installed in areasinhabited by rodents, such as might befound in a garage, the wiring system mustbe capable of resisting damage caused bygnawing (Regulation 522-10-01).

Inspection, testing and certificationInspection and testing must be performed toconfirm the adequacy of the relevant partsof the existing installation, which willsupport the changed requirements, theupgrading of the existing installationnecessary to support the addition oralteration, and the addition or alterationitself. The requirements for initial verifi-cation are contained in Chapter 71 of BS 7671and further information on the requirementsfor inspection and testing is given in the IEEPublication Guidance Note 3 Inspection andTesting. Compliance with BS 7671 must beverified for every addition or alteration(Regulation 721-01-02 refers). Requirementsfor certification and reporting in respect ofelectrical installations are given in Chapter74. An Electrical Installation Certificatemust be provided to the owner of the instal-lation giving details of the extent of theinstallation covered by the certificate, with arecord of the inspection and the results of thetesting (Regulations 741-01-01 and 743-01-01).

Fig 3: Garage installation made part of a TT system

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The requirementsAnyone can make a mistake, even the mostexperienced of electricians. For this reason, allelectrical installations, alterations and additionsshould be tested, inspected and subsequently certifiedprior to handing over to the client for use. Thecertificate issued describes the current operatingparameters and will provide the basis for any futurealteration or addition to the installation. The resultsof testing, contained within the certificate, must be anaccurate representation of the installation. The testinstrument must, of course, be accurate and theobtained results must be consistent but what are therequirements?

The requirements of BS 7671BS 7671 requires that every installation shall beinspected and tested to verify that the Regulationshave been met before being put into service. Therequirements are stated in the following Regulations:

133-02-01 On completion of an installation or anaddition or alteration to an installation, appropriateinspection and testing shall be carried out to verify sofar as is reasonably practicable that the requirementsof this standard have been met.

711-01-01 Every installation shall, during erectionand on completion before being put into service beinspected and tested to verify, so far as is reasonablypracticable, that the requirements of the Regulationshave been met.

Precautions shall be taken to avoid danger topersons and to avoid damage to property and installedequipment during inspection and testing.

713-01-01 The tests of Regulations 713-02 to 713-13where relevant shall be carried out and the resultscompared with relevant criteria.

The tests of Regulations 713-02 to 713-09 whererelevant shall be carried out in that order before theinstallation is energised.

If any test indicates a failure to comply, that testand any preceding test, the results of which may havebeen influenced by the fault indicated, shall be

When undertaking electrical testing on aninstallation, how accurate are the test results?The readings obtained from a test instrumentwhen carrying out a measurement of earth-fault loop-impedance or the operating time of anRCD – is the reading shown on the instrumentcorrect? How do I know?

This, the second article in the series of testing &inspection, looks at what can go wrong and howto keep track of an instrument’s performance.

ONGOING ACCURACY OFTEST INSTRUMENTS By Mark Coles

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repeated after the fault has been rectified. Some methods of test are described in Guidance

Note 3, Inspection & Testing, published by theInstitution of Electrical Engineers. Other methods arenot precluded provided they give valid results.

Electrotechnical Assessment Scheme (EAS)Under the Electrotechnical Assessment Scheme(EAS), Annex 1, Test Instruments – Calibrationrequirements, it is a requirement that the Enterpriseshall have a suitable system in place to ensure that theaccuracy and consistency of all test instruments usedfor certification and reporting purposes is beingmaintained.

The Electrotechnical Assessment Scheme (EAS)specifies the minimum requirements against which anelectrical installation enterprise, or electricalcontractor, may be assessed in order to determine thetechnical competence of the enterprise. The ownershipand management of the EAS was taken over by the IEEand is administered by the EAS ManagementCommittee. An Electrotechnical Assessment Schemewas seen as necessary if the electrical industry andother interested parties were going to support theefforts of the then DETR in introducing electrical safetyin to the Building Regulations.

Subsequently, the Office of the Deputy PrimeMinister (successor body to the DETR) asked the EASManagement Committee to make recommendations forthe technical competence of contractors to carry outelectrical installation work in dwellings only withoutprior notification to building control.

The document ‘Minimum Technical Competence ofEnterprises that undertake Electrical Installation Workin Dwellings’ (MTC) was prepared for this purpose andaccepted by the Office of the Deputy Prime Minister.All authorised competent persons schemes are requiredto ensure that registered firms have the minimumtechnical competencies.

HSE guidance note GS 38Electrical testing leads and accessories must bedesigned to provide adequate protection againstdanger. The HSE guidance note GS 38, Electrical testequipment for use by electricians, gives guidance toelectrically competent people, along with advice ontest probes, leads, voltage indicating devices andmeasuring equipment.

What can go wrong with test instruments?Problems with test instruments are not alwaysimmediately obvious so it is important that anyirregularities are discovered as early as possible.

What can go wrong?

Damage to the instrumentConsider the scenario; a test instrument wascalibrated by a calibration house six months ago andis still within the stated 12-month calibration period.It appears to be working well but, unbeknownst to thecontractor, five months ago it had a collision with aconduit bender in the back of the firm’s van and isnow out of calibration. Many, many jobs have beentested with this instrument since the incidentoccurred and, hence, many certificates and formshave been issued. The certificates and forms, ofcourse, are worthless as the test results containedwithin would not be representative of the installation.Therefore, it is extremely important that theinstrument is regularly assessed.

Connecting leads and the importance of ‘nulling’Often, problems associated with test instruments canbe traced to faulty leads. The leads suffer a great dealof punishment, constant flexing and pulling,uncoiling and recoiling, squashed into boxes, etc. Astesting probes are repetitively changed and replacedwith gripping ‘crocodile’ clips, over time, contactresistance at the point of connection can increase,which could throw doubt on the results obtained. Thesprings that maintain the gripping pressure of thecrocodile clips can suffer fatigue with age or when notadequately maintained. Foreign bodies and particles,such as brick dust, can clog the connections andmoving parts, again, increasing contact resistance. Itis well documented that the leads should be ‘nulled’prior to use and there is a correct way of nulling.

Fig 1 shows the correct method of connection whennulling test leads. Note that the current flow is directlyfrom lead to lead.

Fig 2 shows the incorrect method of connectionwhen nulling test leads. Note that the current flow isacross two hinges. When the leads have been nulledwith this method of connection, a value of resistancewill be subtracted from the final test result.

Should the leads be re-connected in the correctmanner and a measurement of resistance taken, theinstrument may give an error message or show a valueof ‘negative’ resistance.

Fig 3 shows that the leads have been nulled with theincorrect method of connection. The instrument showsthe value of resistance as 0 Ω.

Fig 4 shows that when the leads are re-connected inthe correct manner, a value of ‘negative’ resistance maybe obtained.

The value of negative resistance is due to the

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resistance of the hinges subtracted from the value ofzero resistance. Hence, in this instance, the hingeswould subtract a value of 0.08 Ω from the final result.

Often, ‘hybrid’ connecting leads are assembled in anad hoc fashion from the remnants found in the bottomof a drawer. Obviously, these types of leads areextremely unreliable and the manufacturer’srecommended leads should always be used. Adequatenumbers of leads, along with a selection of the relevantspares should be retained.

Fused connecting leads and internal fusesThe probes and gripping clips of some connectingleads are fitted with fuses. The fuses protect the userand the equipment from high currents that may beunexpectedly encountered. Suitably rated high-breaking capacity (HBC) or high-rupturing capacity(HRC) fuses should be used, usually not exceeding 500mA but the manufacturer’s instructions shouldalways be followed.

Test instruments are protected by internal fuses,commonly rated at 440V 10kA. The impedance of thetest leads will limit the maximum current that couldflow through the internal fuse. In the case of a fault,this current will be limited to 10 kA.

If the manufacturer states that sufficient protectionis provided by the fuse within the instrument thenfused connecting leads could be omitted.

BatteriesOne final point on problems with items of testequipment is batteries. The test instrument may be ingood working order but will be unreliable unlessfitted with working batteries. Commonly, a lowbattery symbol will appear in the instrument’sdisplay panel, warning that the cells are nearlyexhausted. Sufficient quantities of new batteries

should be kept with the instrument as spares to beused for this purpose only.

Types of instruments and the associated standardsWhen deciding which test instruments to purchase, it is important to ensure that the instruments are fit for purpose and fulfil the testing requirements ofBS 7671. The basic instrument standard is BS EN 61557: Electrical safety in low voltagedistribution systems up to 1000V a.c. and 1500V d.c.Equipment for testing, measuring or monitoring ofprotective measures. This standard includesperformance requirements and requires compliancewith BS EN 61010. BS EN 61010: Safety requirementsfor electrical equipment for measurement control andlaboratory use is the basic safety standard forelectrical test instruments.

The following list shows the test instrument along

Fig 1: Correct method of connection

Fig 2: Incorrect method of connection

Fig 3: Leads nulled in theincorrect manner

Fig 4: Leads reconnected in thecorrect manner

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with the associated harmonised standard:

Low-resistance ohmmeters BS EN 61557-4 Insulation resistance ohmmeters BS EN 61557-2 Earth fault loop impedance testers BS EN 61557-3 Earth electrode resistance testers BS EN 61557-5 RCD testers BS EN 61557-6 Voltage indicators Consult guidance note

GS 38 – Electrical test equipment for use by

electricians

Manufacturers will state which standard, or standards,the instrument conforms to. Other standards ordirectives that the instrument may conform to, such asemissions and immunity standards EN50081-1: 1992,EN50082-1: 1992 or EN61326-1: 1997 will be stated withinthe instruments documentation.

CalibrationHistorically, many electrical contractors have had testinstruments calibrated on an annual basis. Theinstrument would be sent away to a calibration houseand it would arrive back, some time later, with acertificate stating the date of calibration, the time periodfor which the certificate would be valid along with thefindings of the assessment in the form of a table ofresults. The certificate would state that the instrumentwas within calibration parameters at that time only; itcertainly could not guarantee that the instrument wouldstill be fit for purpose at any time after that.

Ongoing accuracy and maintaining recordsEstablishing an effective method of proving theaccuracy of test instruments is of paramountimportance. BS 7671 offers no guidance as to themethod that should be employed to ensureconsistency and accuracy other than requiring thatthe results of testing are compared with the relevantcriteria.

One method of assessing the on-going accuracy oftest instruments is to maintain records, over time, ofmeasurements taken from designated circuits ofreference. Before such a system is implemented, theaccuracy of each test instrument must be confirmed;this could only be carried out by a formal calibrationhouse. An important point to note is that test leadsshould be assessed at the time of calibration.

The following examples are methods in which theon-going accuracy of test instruments may beassessed.

In each instance, the designated circuit or socket-outlet must be used for every subsequent assessment.

To avoid ambiguity, the relevant testing points shouldbe labelled allowing other operatives, who may notusually be charged with the task of test instrumentassessment, to follow the system. Should the resultswaver by ±5%, the instrument should be recalibrated.If the results remain within the ±5% limits then theinstrument could feasibly go for a number of yearswithout the need of calibration assessment by acalibration house. For some businesses, this could bea major cost saving.

It is worth noting that changes to the electricalsupply network could affect the actual supplycharacteristics at the installation.

Many test instrument manufacturers produceproprietary ‘checkboxes’ (see fig 5) that incorporatemany testing functions, such as high and lowresistance, earth fault loop impedance and RCDtesting.

Such instruments could be used in conjunctionwith the following systems.

Other equally effective methods of assessmentcould be utilised.

Resistance OhmmetersOnce a month, take a measurement of each resistorand record the results in tabulated form. Over aperiod of time, the table will show how theinstrument is performing.

Fig 5: Proprietary checkbox [Image courtesy of Seaward Instruments. Model shown– Checkbox 16]

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i.) Low-resistance ohmmeters (fig 6)A set of suitable resistors could be used to assess theinstrument; suitable values could be 0.5Ω, 1.0Ω and 10Ω. i.) High-resistance (insulation resistance) ohmmeters (fig 7)A set of suitable resistors could be used to assess theinstrument; suitable values could be 0.5MΩ, 1.0MΩ and10MΩ.

The resistor values chosen merely reflect commonbands of resistance that are generally encounteredwhen testing electrical installations. Other values ofresistance, indeed, greater numbers of resistors, couldbe used to assess resistance ohmmeters across thespectrum of resistance.

Earth fault loop impedance test instruments Earth fault loop impedance test instruments could bechecked by carrying out tests on a designated socket-outlet, not RCD protected as unwanted tripping mayoccur. Once a month, take a measurement of earthfault loop impedance and record the result on a test-record sheet.

RCD test instruments RCD test instruments could be checked by carrying outa test on an RCD, ideally from a socket-outlet. Once amonth, perform an RCD test and record the trippingtime on a test-record sheet. The instrument could beset to test at the rated tripping current, In, ofthe RCD. An example of a testing record sheet is shown (right).

0.5 M Ohm

1.0 M Ohm

10 M Ohm

0.5 Ohm

1.0 Ohm

10 Ohm

Fig 6: Set of low-value resisters Fig 7: Set of high-value resisters

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ELECTRICAL EQUIPMENTemploying small Variable Speed Drives (VSDs)is reducing in cost and size and is appearingmore frequently in equipment for non-industrial use. Examples include fitnessmachines for use in gymnasiums and potters’wheels used in schools and colleges. Theinstallation of such equipment often fallsunder the jurisdiction of Local Authorities,who, in many cases, have a standing policythat all items of electrical equipment aresubject to Portable Appliance Testing (PAT)before being put into service and annuallythereafter. VSDs are fitted with EMC filters,(see fig 1) and, often, there is a significantcurrent flow (around 6 mA) through the filtersto earth, under normal conditions. This can beindicated as a failure during an earth leakagecurrent measurement performed as part of aportable appliance test, although it is anormal characteristic of the equipment.Regulation 4(2) of the Electricity at WorkRegulations 1989 (EWR) requires that:

“As may be necessary to prevent danger, all systems shall be maintained so as toprevent, so far as is reasonably practicable,such danger.”

There is no specific requirement inlegislation detailing that in-servicemaintenance consisting of inspecting ortesting equipment must be performed onan annual basis. However the IEE givesguidance in its publication the ‘Code ofPractice for In-service Inspection andTesting of Electrical Equipment’ that asystem of maintenance includingperforming such testing on a routine basismay achieve compliance with Regulation4(2) of the EWR. The Code of Practicemakes it clear that inspection and testingalone will not result in compliance withthe law. The requirement is to maintainelectrical equipment as necessary toprevent danger. This means that therequirements of the Electricity at WorkRegulations are not met simply bycarrying out tests on equipment – the aimis to ensure that equipment is maintainedin a safe condition.

Paragraph 15.3 of the IEE’s guidance isentitled ‘In-service tests’ and proposesthat in-service testing should be precededby a preliminary inspection, as describedin paragraph 15.1 and summarised intable 1 (see p16).

Testing should then be performed andthe Code recommends that three tests aremade; Earth continuity, insulationresistance and functionality.

PAT TESTING OF EQUIPMENTHAVING HIGH PROTECTIVECONDUCTIVE (‘LEAKAGE’)CURRENT By Peter Still, Schneider Electric Ltd

Fig 1: Protectiveconductor currentdue to EMC filter

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Note that some electrical test devices apply teststhat are inappropriate and may even damageequipment containing electronic circuits, such asVSDs, possibly causing degradation of safety. Inparticular, while the Code of Practice includesinsulation resistance tests, equipment should not besubjected to dielectric strength testing (known as hi-pot testing or flash testing) because this may damageinsulation and may also indirectly damage lowvoltage electronic circuits unless appropriateprecautions are taken.

The limit stated in the Code of Practice forinsulation resistance for Class I equipment whentested at 500V d.c. is 1 megohm. An item of equipmentwithout input filters generally has an insulationresistance greater than 50 megohm and the use of theinsulation test in preference to the leakage currentmeasurement will therefore allow the equipment to be

put into service safely. Looking in detail at the insulation resistance test,

for some types of equipment it may not be appropriateto perform an insulation test at 500 V d.c. because ofthe risk of damage to the electronic components inthe equipment. The testing engineer may then decideto perform the protective conductor/touch currentmeasurement, which is an alternative to the in-service insulation test for use if the insulationresistance test either cannot be carried out or givessuspect test results. The protective conductor currenttest consists of measuring the current flowing fromlive parts to earth for Class I equipment, or from liveparts to accessible surfaces of Class II equipment.

For practical purposes the test voltage is thesupply voltage and the current should be measuredwithin five seconds after the application of the testvoltage, the supply voltage, and should not exceed the

Table 1: Preliminary inspection1 Disconnection Determine whether the equipment can be disconnected from the supply and

disconnect if, and only if, permission is received. If permission is not received todisconnect the supply do not proceed with any tests and record that the equipment has not been inspected and label accordingly

2 Visual inspection Appliance Thoroughly inspect the equipment for signs of damage. Paragraph 14.5 of the Code

of Practice gives full details

Cable Inspect the connecting cable for damage throughout its length. Look for cuts, nicks,

and kinks. If damage has been repaired with insulating tape, the cable should be

replaced. Ensure the cable is routed where it will not suffer damage in use

Plug Inspect the plug and check that the plug is suitable for the application. For 13 A

applications, a resilient plug marked BS 1363A may be necessary if the plug is

subjected to harsh treatment

Socket-outlet or flex outlet Inspect the socket-outlet or flex outlet of the fixed wiring system for any signs of

damage such as cracks and overheating such as discoloration

3 Assessment Assess if the equipment is suitable for the environment

Table 2: Testing1 Earth continuity test Earth continuity testing can only be applied to Class I equipment or cables.

Paragraph 15.4 of the Code of Practice gives further details of the testing

2 Insulation resistance test For many types of appliance, insulation resistance is generally checked by applying a test voltage and

measuring the resistance. This test may not always be suitable and the protective conductor/touch

current measurement of paragraph 15.6 of the Code of Practice may be a more appropriate alternative3 Functional check In its simplest form a functional check is a check to ensure that the appliance is working properly

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values indicated in table 3.However, equipment employing a VSD will, in most

cases, have a leakage current greater then 3.5mA dueto the filters at the input and hence will beunsuccessful when performing this test. Although thisdoes not show the equipment to be unsafe if this testis performed, the value of current should be recordedsince an increased current in any future tests can bedue to deterioration of components within theequipment.

The engineer performing the tests will then have todecide if the equipment is still safe for service andwill make his judgement based on whether theprotective conductor current increased from thepreviously recorded value or from the manufacturer’sadvised maximum current. The engineer must thenverify that such equipment with a protectiveconductor current designed to exceed 3.5mA complies

with the requirements shown in table 4. Further precautions need to be taken for equipment

with a protective conductor current exceeding 10mA,see Section 607 of BS 7671.

When appliances have high protective conductorcurrents, substantial electric shocks can be receivedfrom exposed-conductive-parts and/or the earthterminal if the appliance is not earthed. It is mostimportant that appliances with such high protectiveconductor currents are properly connected with earthbefore any supply is connected.

Connection to the mains supply via a portable cordset which does not meet EN 60309-2, such as a 13A plugand socket-outlet, is therefore not acceptable. Thesupply connection to the fixed installation will need tobe made using a fused spur connection unit to BS1363-4, or a plug and socket-outlet in accordance withBS EN 60309-2 and BS 7671.

Table 3: Maximum permissible protective conductor currentAppliance Class Maximum Current note (1)Portable or hand-held Class I Equipment 0.75 mAClass I heating appliances 0.75 mA or 0.75 mA per kW, whichever is the greater, with a maximum of 5 mAOther Class I equipment 3.5 mAClass II equipment 0.25 mAClass III equipment 0.5 mA

Table 4: Additional requirements for equipment having higher leakage current

Permanently wired or supplied by an industrialplug and socket-outlet

Protective conductors

Label The equipment should have a label bearingthe following warning or similar wordingfixed adjacent to the equipment primarypower connection (based on clause 5.1.7 ofBS EN 60950)

The equipment should have internal protective conductors of not less than 1.0 mm2 cross-sectional area (based on clause 5.2.5 of BS EN 60950), and

Notes to Table 3:

1. The values specified above are doubled:- if the applicance has no control device other than a thermal cut-out, a thermostat without an “off” position or an energy regulator without an ‘off’ postion.- if all control devices have an “off” position with a contact opening of at least 3 mm and disconnection in each pole.

2. Equipment with a protective conductor current designed to exceed 3.5 mA shall comply with the requirements of paragraph 15.12.

3. The nominal test voltage is:- 1.06 times rated voltage, or 1.06 times the upper limit of the rated voltage range, for applicanes for d.c. only, for single-phase applicances and for three-phaseappliances which are also suitable for single-phase supply, if the rated voltage or the upper limit of the rated voltage range does not exceed 250V;- 1.06 times rated line voltage divided by 1.73, or 1.06 times the upper limit of the rated voltage range divided by 1.73 for other three-phase applicances.

The equipment shall be permanently wired to the fixed installation, or be suppliedby an industrial plug and socket to BS EN 60309-2 (BS 4343)

WARNINGHIGH LEAKAGE CURRENT

Earth connection essential before connecting the supply

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This article, which is based on IEE Guidance notes, isintended to provide information that it is hoped willprove helpful.

BS 7671 lists five types of earthing system: TN-S, TN-C-S, TT, TN-C, and IT. T = Earth (from the French word Terre) N = Neutral S = Separate C = CombinedI = Isolated (The source of an IT system is either

connected to earth through a deliberately introducedearthing impedance or is isolated from Earth. Allexposed-conductive-parts of an installation areconnected to an earth electrode.)

When designing an electrical installation, one ofthe first things to determine is the type of earthingsystem. The distributor will be able to provide thisinformation.

The system will either be TN-S, TN-C-S (PME) or TTfor a low voltage supply given in accordance with theElectricity Safety, Quality and Continuity Regulations2002. This is because TN-C requires an exemptionfrom the Electricity Safety, Quality and ContinuityRegulations, and an IT system is not permitted for alow voltage public supply in the UK because thesource is not directly earthed. Therefore TN-C and ITsystems are both very uncommon in the UK.

EARTHING:YOUR QUESTIONS ANSWEREDBy Geoff Cronshaw

What are earthed and unearthed systems? What are the requirements of BS 7671? What are the advantages and disadvantages of the various types of earthing systems?

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1. Overview of earthing systems

1.1 TN-S system earthingA TN-S system, shown in fig 1, has the neutral of the source of energyconnected with earth at one point only, at or as near as is reasonablypracticable to the source, and the consumer’s earthing terminal is typicallyconnected to the metallic sheath or armour of the distributor’s service cableinto the premises.

Fig 2: Cable sheath earth(TN-S system). Schematic ofearthing and mainequipotential bondingarrangements. Based on 25 mm2 tails and selectionfrom Table 54G. Note: An isolator is notalways installed by theelectricity distributor.

Fig 1: TN-S system

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1.2 TN-C-S system earthingA TN-C-S system, shown in fig 3, has the supplyneutral conductor of a distribution main connectedwith earth at source and at intervals along its run.This is usually referred to as protective multipleearthing (PME). With this arrangement the

distributor’s neutral conductor is also used to returnearth fault currents arising in the consumer’sinstallation safely to the source. To achieve this, thedistributor will provide a consumer’s earthingterminal which is linked to the incoming neutralconductor.

Fig 3: TN-C-S system

Fig 4: PME supply (TN-C-Ssystem). Schematic of earthingand main equipotential bondingarrangements. Based on 25 mm2

tails and selection from Table 54G.Note: An isolator is not alwaysinstalled by the electricitydistributor.

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2. Requirements of BS 7671

Earth electrodesBS 7671 recognises a wide variety of types of earthelectrode. Regulation 542-02-01 lists the typesrecognised which include earth rods, earth plates and

Figure 5: TT system

Figure 6: No earth provided(TT system). Based on 25 mm2 tails and selectionfrom Table 54G. Note: An isolator is notalways installed by theelectricity distributor.Manufacturersrecommendations shouldbe sought with regards toconnections to earthelectrodes.

1.3 TT system earthingA TT system, shown above, has the neutral of the source ofenergy connected as for TN-S, but no facility is provided by thedistributor for the consumer’s earthing. With TT, theconsumer must provide their own connection to earth, i.e. byinstalling a suitable earth electrode local to the installation.

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underground structural metal work. The soil resistivity of the ground is probably the

single most important factor in the determination ofthe type of earth electrode. Rods can only be aseffective as the contact they make with thesurrounding material. Thus, they should be driveninto virgin ground, not disturbed (backfilled) ground.Where it is necessary to drive two or more rods andconnect them together to achieve a satisfactory result,the separation between rods should be at least equalto their combined driven depth to obtain maximumadvantage from each rod. In some locations low soilresistivity is found to be concentrated in the topsoillayer, beneath which there may be rock or otherimpervious strata which prevents the deep driving ofrods, or a deep layer of high resistivity. Only a test orknown information about the ground can reveal thiskind of information. In such circumstances, theinstallation of copper earth tapes, or pipes or plates,would be most likely to provide a satisfactory earthelectrode resistance value.

Whatever form an earth electrode takes, thepossibility of soil drying and freezing, and ofcorrosion, must be taken into account. Preferably,testing of an earth electrode should be carried outunder the least favourable conditions, i.e. afterprolonged dry weather. Further information onearthing principles and practice can be found in BS 7430 : 1998 ‘Code of Practice for Earthing’.

Earthing conductorsEarthing conductors which are defined in BS 7671 as aprotective conductor connecting the main earthingterminal of an installation to an earth electrode orother means of earthing must be adequately sizedparticularly where buried partly in the ground, andbe of suitable material and adequately protectedagainst corrosion and mechanical damage.

The size of an earthing conductor is arrived at inbasically the same way as for a circuit protectiveconductor, except that Table 54A of BS 7671 must beapplied to any buried earthing conductor. For a TN-C-S (PME) supply, it should be no smaller than themain bonding conductors.

Sizing of circuit protective conductorsThere are several factors which may influence ordetermine the size required for a circuit protectiveconductor. A minimum cross-sectional area of2.5 mm2 copper is required for any separate circuitprotective conductor, i.e. one which is not part of acable or formed by a wiring enclosure or contained insuch an enclosure.

An example would be a bare or insulated copperconductor clipped to a surface, run on a cable tray orfixed to the outside of a wiring enclosure. Such acircuit protective conductor must also be suitablyprotected if it is liable to suffer mechanical damage orchemical deterioration or be damaged by electro-dynamic effects produced by passing earth faultcurrent through it. If mechanical protection is notprovided the minimum size is 4 mm2 copper orequivalent.

BS 7671 provides two methods for sizing protectiveconductors including earthing conductors (see alsoTable 54A). The easier method is to determine theprotective conductor size from Table 54G but this mayproduce a larger size than is strictly necessary, sinceit employs a simple relationship to the cross-sectionalarea of the phase conductor(s).

The second method involves a formula calculation.The formula is commonly referred to as the ‘adiabaticequation’ and is the same as that used for short-circuit current calculations (see Regulation 434-03-03).It assumes that no heat is dissipated from theprotective conductor during an earth fault andtherefore errs on the safe side. Even so, application ofthe formula will in many instances result in aprotective conductor having a smaller csa than that ofthe live conductors of the associated circuit. This isquite acceptable.

Regulation 543-01-03 states:The cross-sectional area, where calculated, shall benot less than the value determined by the followingformula or shall be obtained by reference to BS 7454.

where:

S is the nominal cross-sectional area of the conductorin mm2.

I is the value in amperes (rms. for a.c.) of faultcurrent for a fault of negligible impedance, which canflow through the associated protective device, dueaccount being taken of the current limiting effect ofthe circuit impedances and the limiting capability (I2 t) of that protective device.

Account shall be taken of the effect, on the resistanceof circuit conductors, of their temperature rise as aresult of overcurrent - see Regulation 413-02-05.t is the operating time of the disconnecting device in

S = I2tk

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seconds corresponding to the fault current I amperes.k is a factor taking account of the resistivity,temperature coefficient and heat capacity of theconductor material, and the appropriate initial andfinal temperatures.

3.0 Type of earthing systems, advantages and disadvantages

3.1 Protective multiple earthing (PME). Such a supply system is described in BS 7671 as TN-C-S.The advantage of this system is that it provides aneffective and reliable method of providing customerswith an earth connection. For example the maximumZe specified by a distributor is 0.35 Ω for TN-C-S

supplies compared to 0.8 Ω for TN-S supplies. However, under certain supply system fault

conditions (PEN conductor of the supply becomingopen circuit external to the installation) a potentialcan develop between the conductive parts connectedto the PME earth terminal and the general mass ofearth. However, since there are multiple earthingpoints on the supply network and bonding is providedwithin the building complying with BS 7671, the riskis considered to be small.

Special LocationsThe Electricity Association publications providesguidance on PME systems. Whilst PME systemsprovide an effective and reliable earth connection

Figure 7: Typical site distribution for aPME supply, separation from PMEearth at main distribution board

Figure 8: Typical site distribution for aPME supply, separation from PMEearth at pitch supply point

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precautions need to be taken when dealing withspecial locations. For example Regulation 9(4) of theElectricity Safety, Quality and Continuity Regulationsdoes not allow the combined neutral and protectiveconductor to be connected electrically to anymetalwork in a caravan or boat. This prevents PMEterminals being used for caravans or boat mooringsupplies, although they may be used for fixedpremises on the sites, such as the site owner’s livingpremises and any bars or shops, etc.

Petrol filling stations are another area whereprecautions need to be taken. The referencepublication is “Guidance for the design, construction,modification and maintenance of petrol fillingstations”, published by the Association for Petroleumand Explosives Administration (APEA) and theInstitute of Petroleum, which recommends a TTsupply for hazardous areas.

A separate earth electrode and RCD or otheralternative arrangement is required to ensure thesegregation of petrol filling area earthing and that ofthe PME earth of the distribution network. A PMEearth may be used for permanent buildings such asshops and restaurants.

Also, mines and quarries are another area. A supplytaken to an underground shaft, or for use in theproduction side of a quarry, must have an earthingsystem which is segregated from any system bondedto the PME terminal.

Finally, because of the practical difficulties inbonding all accessible extraneous-conductive-partselectricity distribution companies might not provide aPME earth to agricultural and horticulturalinstallations and construction sites.

3.2 TT systemWith TT, the consumer must provide their ownconnection to earth, i.e. by installing a suitable earthelectrode local to the installation. The circumstancesin which a distributor will not provide a means ofearthing for the consumer are usually where thedistributor cannot guarantee the earth connectionback to the source, e.g. a low voltage overhead supply,where there is the likelihood of the earth wire eitherbecoming somehow disconnected or even stolen.

A distributor also might not provide means ofearthing for certain outdoor installations, e.g. aconstruction site temporary installation, leaving it tothe consumer to make suitable and safe arrangementsfor which they are fully responsible. The electricitydistributor is required to make available his supplyneutral or protective conductor for connection to theconsumer’s earth terminal, unless inappropriate for

reasons of safety (Reg 24 of ESQCR). Constructionsite, farm or swimming pool installations might beinappropriate unless additional precautions aretaken, such as an additional earth electrode.

3.3 TN-S systemA TN-S system has the neutral of the source of energyconnected with earth at one point only, at or as near as isreasonably practicable to the source and the consumer’searthing terminal is typically connected to the metallicsheath or armour of the distributor’s service cable intothe premises or to a separate protective conductor of, forinstance, an overhead supply.

Large consumers may have one or more HV/LVtransformers dedicated to their installation andinstalled adjacent to or within their premises. In suchsituations the usual form of system earthing is TN-S.

More information on earthing and bonding isavailable in IEE Guidance Note 5. Also moreinformation on special locations is available in IEEGuidance Note 7.