Inspection And Testing Of Electrical Installations

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Inspection and Testing

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INSPECTION AND TESTING OF ELECTRICAL INSTALLATIONS

T

This book is intended to act as a working manual for those engaged in the initial inspection or re-inspection of an electrical installation. It will also assist trainees studying for City and Guilds courses 2360-1, 2360-2, 2360-C, 2400, 2391, NVQ and BTEC. The advice given, and the methods suggested, are based on many years of practical experience and therefore will hopefully not be considered too theoretical for those engaged in the Electrical Contracting Industry. If this most important aspect of an electrician’s work is to be successfully completed, testing and inspection activities must be carefully prepared, executed and documented. The principles enclosed in this book will enable that goal to be achieved with the minimum of errors. References are made throughout to BS 7671, better known as the 16th. Edition of the IEE Regulations, and their associated guidance notes. Possession of these documents is essential for any test engineer hoping to perform a high quality service. It goes without saying that inspection and testing requires a suitable range of instruments that will be regularly checked for accuracy and, when necessary, regularly re-calibrated. (See appendix 3 for details of instrument requirements) If the reader is unfamiliar with current terminology, he should turn to section 25 where definitions of commonly used terms are given. C 2004

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CONTENTS SECTION

1

Requirements for the initial testing of an electrical installation

SECTION

2

Inspection and testing of existing installations.

SECTION

3

Outline of instrument tests and the compilation of inspection check list.

SECTION

4

Determination of cable length and voltage drop.

SECTION

5

Tests of protective conductor continuity.

SECTION

6

Tests of main and supplementary equipotential bonds.

SECTION

7

Tests of Ring Circuit continuity.

SECTION

8

Tests of Insulation Resistance.

SECTION

9

Tests of Polarity.

SECTION

10

Tests of Earth electrode resistance and soil resisitivity.

SECTION

11

Tests of Line - Earth loop impedance.

SECTION

12

Tests of RCD effectiveness.

SECTION

13

Overcurrent provision survey.

SECTION

14

Organisation of a testing programme.

SECTION

15

Testing portable and transportable equipment.

SECTION

16

Measurement of earth leakage current.

SECTION

17

Operation of devices for isolation and switching.

SECTION

18

Testing of information technology equipment.

SECTION

19

Measurement of prospective short circuit current.

SECTION

20

Measurement of illumination.

SECTION

21

Testing of escape lighting.

SECTION

22

Discrimination between overcurrent protective devices.

SECTION

23

Urban distribution systems.

Appendix

1

Related principles.

Appendix

2

Definition of terms.

Appendix

3

Instrument requirements.

Appendix

4

Self-assessment exercise.

Appendix

5

Completion and inspection certificates

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SECTION 1 TESTING AND INSPECTION - BASIC REQUIREMENTS

I

It is not only a basic safety need, but also a requirement of B.S. 7671 and the Electricity at Work Regulations (1989), that any electrical installation shall be verified as safe to operate before being energised.

The term "safe to operate" means the user of the electrical installation will be free from the risk of fire, shock, burns and the injury from mechanical movement produced by electrical machines such as electric motors. Safe to operate also means that the user of the installation should not require technical knowledge in order to stay clear of the casualty department of the nearest hospital. This verification procedure requires: I ii iii

A validation of the installation design A visual inspection of the construction Instrument tests

A well-constructed installation will not necessarily function correctly or safely if cables, switchgear etc. have been incorrectly selected by the designer. Design assessment is not easily undertaken when the installation has been completed, the numerous facets of the installation requiring verification cannot be assessed by a simple visual inspection. The inspector will have to have an intimate knowledge of BS 7671, a good technical education and considerable experience. If, in the circumstances, an assessment of design viability is not a practical possibility, the designer of the installation - if known - will have to be requested to affirm that the installation design meets all regulation requirements. This affirmation will take the form of a signature in the appropriate section of the Electrical Installation certificate – which will be of a type published in Guidance note 3 - BS 7671 - certifying that all regulations have been conformed with. The foregoing assumes the installation is new. If not the inspection and testing process will be a periodic one not requiring knowledge of the original designer. Incorrect selection of fuses, circuit breakers, and cables are a prime example of a design defect that is not obvious to the casual observer. To assess overcurrent device efficiency for example, it will be necessary to have

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knowledge of the short circuit fault levels, maximum current demand and any applicable correction factors that will modify the tabulated current rating of installed cables. Or, more simply, the temperature of the cable can be measured on full load. If the insulation is PVC, the maximum permitted temperature is 70 degrees Celsius. Any temperature below this indicates the cables are running at less than capacity. However the voltage drop could be excessive if the circuit lengths are long, perhaps necessitating the installation of a larger cable. Section 13 of this book will examine methods of assessing the provision of overcurrent protection. An inspection of an installation is far from a simple matter, particularly if the installation has been in service for some years. Well-designed installations tend to deteriorate with the passage of time, due to both the natural effects of ageing but more usually because of the unwelcome attention of "electricians" of uncertain skills. Physical defects cannot be detected by the application of instruments. Construction faults such as insecure fixings and inappropriate means of isolation can only be detected by visual inspection, which will have to be conducted in a systematic manner. Before commencing the inspection and test, you are strongly advised to prepare a schedule of work, which will not only organise the sequence of operations, but their detail. If the inspecting team arrive on site without any pre-planning, lots of time will be wasted and nervous energy expended. This measure is particularly important if the installation is already in service and installation schedules and diagrams of practical value are non-existent. It is also essential that an inspection ascertain that the installation is of sufficient capacity to supply the demand. Loads could have grown beyond the intention of the designer. Maximum current should be measured, and for both distribution and final circuits. Comparisons are then made with the current rating of the controlling overcurrent device and the connected cable. Maximum demand may be measured by the use of a recording clamp meter left in position for 24 hours. We live in a litigious society - don’t take chances and overlook an overloaded installation - it could be expensive! If the installation is new, the inspecting engineer will be required to inspect the construction of the installation, ideally both during erection and on completion, to ensure that no hidden defects are left undetected. Electrical installation inspection and testing is a potentially hazardous exercise. Any person responsible for this work must be "competent" in the sense that this term is used in the Electricity at Work Regulations 1989.

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Inspection and testing is not a job for apprentices! It carries a heavy responsibility. Understand the job - act responsibly - act safely - get organised! Approved voltage testers should be available to ensure that any circuit assumed to be dead is in fact so. If the installation has to be attended to live, suitable precautions must be taken. These measures above all else require competence on the part of the test engineer. An electric shock at 400V is in quite a different league to one at 230V. Make sure that you don't have to learn the difference by experience. A competent person could be described as someone of relevant experience, technically qualified, mature, and trained to use all of the necessary instruments in a safe manner and, additionally, be able to accurately interpret the obtained test data. No inspection can be effectively carried out without the possession of current drawings of the installation and knowledge of the installation "characteristics". These characteristics will include: (i) (ii) (iii) (iv) (v) (iv)

The maximum current demand The prospective short circuit current at each distribution board The external P-E loop impedance at each distribution board The nature of the overcurrent device at each distribution board The means of earthing and equipotential bonding The presence of sensitive electronic devices.

All of the above will be examined in detail in later sections of this book. If drawings of an installation are not available, then possibly a substantial amount of exploratory work will be required. Tracing a myriad of outlets of differing kinds back to their parent circuit is a tedious and time-consuming business. However, an electronic cable tracer makes this work "comparatively" easy by the simple expedient of sending signals down the investigated cables. A receiver attached to the outlet under test will display the trace number on a LCD. Additionally, this instrument is extremely useful for conducting tests of polarity as explained later. No assumptions should be made that the installation has been logically designed and constructed. An essential for this exercise is a pad of adhesive labels for attaching to the various outlets, on which will be inscribed the circuit identification. In summary, the inspection and testing requirements can be quite simply stated thus: "The completed installation shall be inspected and tested to ensure that in all respects the requirements of BS 7671 have been conformed with". A tall order, but that's the objective.

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Regulation 712-3 lists salient points requiring particular attention, which include the following, * * * * * * *

Connection of conductors; Identification of conductors; Current carrying capacity of conductors; Determination of voltage drop; Presence of fire barriers; Presence of devices for isolation and switching; Space factor of conduits and trunking.

See also appendix A of the BS 7671 Guidance Note 3 for further advice regarding visual inspections and subsequent assessment. There is a requirement for a periodic inspection of installations, the details of which are given in Section 744 o f BS 7671 and table 2.1.5 of GN 3. Some installations, which have a mandatory requirement for periodic inspections and tests, are listed below ANNUALLY Cinemas Theatres Petrol filling stations Caravans and caravan sites Leisure complexes Places of public entertainment Restaurants and hotels Public houses Launderettes

All other installations have only recommended periods between inspections and tests. For all buildings where people work, the Electricity at Work Regulations 1989 regulation 4 (2) has a requirement for maintenance of the electrical installation, which will necessitate testing and inspection on a regular basis.

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SECTION 2 INSPECTION AND TESTING OF EXISTING INSTALLATIONS

The re-test of an installation introduces hazards not present in a dead installation and it is an important mandatory requirement that the inspector shall be deemed to be "competent" (EAWR reg. 16). The term indicates a familiarity with the type of work, technical literacy and maturity. Diagrams, charts etc. and any other relevant information will be required. In their absence - as previously mentioned - a degree of cautious exploratory work will be necessary. A visual inspection shall be carried out with the installation de-energised, as far as practicalities will allow - embracing as much of the installed equipment as possible and attention being paid to the following factors: Safety Wear Environment Damage Corrosion Age Suitability

Does the installation present a shock risk? To ensure safety, measures employed to ensure protection against overcurrent and earth leakage must be suitable for any given situation. Does the installation show any sign of wear or abrasion? Check portable tools and other equipment. The environment to which an electrical installation is subject may cause rapid deterioration in standards of safety. Environmental factors will include high and low temperatures, exposure to the elements etc. Damaged equipment is dangerous equipment. All facets of the installation must be regularly checked for signs of damage. Has the environment a corrosive atmosphere? Does the electrical installation come into contact with the elements? If so, is the choice of equipment suitable? As with most things, an electrical installation will deteriorate with age - particularly conductor insulation based on organic compounds> This deterioration cannot necessarily be checked by an insulation resistance test - a visual inspection is required. All electrical equipment shall be suitable for the use to which it is put. Unsuitable selection can cause a dangerous situation to arise.

It should be emphasised that the Electricity at Work Regulation 14 does not permit any live working unless it is absolutely essential for the installation to remain energised. The avoidance of inconvenience is not considered a reason for investigating an installation live. This regulation - for reasons of convenience - is commonly and lightly breached, but any accidents resulting that attract the attention of the Health and Safety Inspectorate could result in a prosecution. Some installations that are getting on in years defy logic and no assumptions should be made regarding any installed equipment. It is not unusual to discover that two fuses may control a particular point or unexpectedly have 400V at its terminals. Never assume that a circuit dead - particularly if the circuit is three phase. It's your life at stake, so use your voltage tester intelligently. Remember the voltage between two points of the same phase of a live circuit is zero! But the voltage with respect to earth will be 230V.

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The resistance of the human body is voltage dependant – the higher the voltage the lower the resistance. A 400V shock is of a much higher magnitude than the increase in voltage would suggest. The facets of the installation that require a visual inspection are the same as for an initial inspection with particular emphasis being given to switching devices and identification and notices and correct polarity. It is recommended by GN3 that a random sample of 10% of all switches and isolators shall be selected for an inspection, which will assess their electrical and mechanical condition. Where any defects are revealed, all switching devices shall be inspected and tested unless the cause of a defect can be clearly identified and be confirmed as locally confined. The test will be one of continuity, verifying complete isolation has taken place when the isolator is opened. It goes without saying that before any circuit is assumed to be dead it must be effectively tested with an approved voltage tester. Examples of voltage testers are illustrated below. The tester shown in Fig.1 is also a continuity indicator, giving a beep when connected in a circuit of resistance up to 500 000 . This tester will indicate the approximate impressed voltage by the illumination of the appropriate LED. A self-test and battery test facility is also incorporated. Fig.1 Approved voltage testers

The tester above will respond to the electric field that surrounds all energised conductors and requires no direct contact with live parts. However it will only indicate the presence of a voltage and not its value, nor will it precisely point to the energised conductor.

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Despite its limitations this tester is a very good aid to safe working and does not require a potential difference to indicate a potential danger. It is important that the inspecting engineer realises that the condition of cable insulation cannot be determined solely by the application of an insulation resistance test. It is perfectly possible for the insulation to be totally absent; if the live parts are physically separated, an IR test will indicate a reading of infinite resistance. Insulation problems can result from ageing or overloading and it is absolutely essential that a thorough, systematic visual inspection be conducted. All parts of the installation shall be verified as being adequately provided by the necessary devices for isolation and, if required, emergency switching. It is not acceptable, for example, that a distribution board be isolated only by a fuse or circuit breaker. Isolating devices breaking either two or three poles depending upon the number of phases required, are needed. If the isolator is sited out of line of sight, it should be provided with a means of locking off. To maximise safety and convenience, notices or labels are required at the following points: * Where differing voltages exist, * Earthing and bonding conductors; * Where an RCD is fitted, * At socket outlets supplying equipment for use outdoors; * Caravan installations; * Where an installation is supplied from two differing sources, * Earth free locations. Additionally at each distribution board a schedule should be fixed to the inside of the lid indicating circuit destinations, number and type of points served and their location.

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Visual inspection. Shown below are photographs of defects discovered in the inspection process. Examine the photos carefully and determine the nature of the defects.

Fig.2

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OUTLINE OF TESTING REQUIREMENTS AND THE COMPILATION INSPECTION CHECK LISTS.

I

Instrument tests are required to reveal the "hidden" characteristics of an installation and must be conducted in a sequence indicated in section 713 of BS. 7671, reproduced below.

Continuity of protective conductors Continuity of final ring circuit conductors Insulation resistance Site applied insulation Protection by separation of circuits Insulation of non conducting walls and floors Protection against direct contact by enclosures during erection Polarity Earth - fault loop impedance Earth electrode resistance Operation of residual current devices Only those tests in italics will be examined in the book. The other tests are of only marginal interest to the working electrician. Additionally it may be necessary to measure: Prospective short circuit current levels Levels of illumination Portable appliance safety Emergency lighting effectiveness Maximum current demand For a re-test of an installation the previous sequence of tests has now been deleted. The testing sequence is largely determined by opportunity and appropriateness. When conducting a visual inspection, it is absolutely vital that observations are recorded at the time, not at a later date. The fallibility of memory makes recollections unreliable. Prepare and use check lists, not a loose-leaf pad.

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No inspection and test of an installation can be conducted without plans. This need is not only one of common sense and safety but also a requirement of BS 7671 reg. 711-01-02 and 514-09-01. If none exist they will have to be determined by an explanatory survey. You are expected to determine the location and current rating of all major fuses or circuit breakers controlling sub distribution boards and the associated isolators. All of this information can be summarised in a single line distribution diagram, shown below, to be provided on the reverse side. Fig.3 All sub distribution boards are TP&N lower ground floor

ground floor - north

A

B

ground floor - south

C

first floor

D

all distribution circuit cab are pvc/swa/pvc/4c

160 A

100 A

main distribution board

100 A

300 A x 3 switch fuse

CT KWH

TNC-S

E

63 A

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SECTION 4 Determination of cable length and voltage drop

I

If the client or a professional body requires a declaration of cable length and voltage drop, the following procedure can be adopted in the absence of any useful data.

The maximum permitted voltage drop allowable by BS 7671 is 4%, of nominal voltage, from the origin of the installation to the furthest point of utilisation. (Section 525) At 400V, permitted voltage drop is 16V, at 230V, 9.2V If the installation is a simple one consisting of a single distribution board, the procedure to measure voltage drop will also be simple. For each circuit when isolated - the P and N conductors are joined and the resistance of the loop measured at the distribution board. (See Fig. 2) Circuit length = 29.4 x R x S metres Where R = loop resistance S = cable cross sectional area in mm .2 Example: the loop resistance of a lighting circuit, shorted out at the furthest point is found to be 0.7 . If the c.s.a. of the cable is 1.0 mm. 2, what is the circuit length? Solution

L = 29.4 x 0.7 x 1 = 20.6 metres.

The voltage drop may then be determined by reference to appendix 4 of BS 7671. Example: if the above circuit is carrying a current when fully loaded of 5A, the voltage drop will be: Vd = Ib x L x mVd = 5 x 20.6 x 44 = 4.53 Volts 1 000 1 000 Assumed conductor temperature of 70 o. Voltage drop is within limits. The above calculation assumed a single-phase circuit, wired in single core cable enclosed in conduit. If the installation were a three-phase one, the procedure would be identical, using two of the phases instead of phase and neutral to determine the circuit length. It should be remembered that calculated three-phase voltage drops are line volt drops, meaning the voltage difference between phases or lines, at the mains and load ends of the circuit. A voltage drop along the length of a cable is called a phase volt drop.

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The difference between permitted single and three-phase voltage drops is by a factor of  3 (1.732). The allowance for voltage drop on a three-phase circuit is not more generous; it’s simply the same voltage measured from a different standpoint. An alternative means of determining voltage drop on a single-phase circuit is by the use of the formula: Vd = Ib x R x 1.29

Vd = 5 x 0.7 x 1.29 = 4.52 Volts

Where Ib is the load current and R the loop resistance. The correction factor of 1.29 is used to convert the resistance measured with the circuit dead at 20 deg. to that of the assumed cable operating temperature of 70 o. For a three-phase circuit the formula becomes, Vd = Ib x R x 1.12 (Where Vd = Line volts) For example, if the circuit current and loop resistance were the same as in the previous example, the voltage drop on a three-phase circuit would be, Vd = 5 x 0.7 x 1.12 = 3.92 Volts (line) or 3.92 /1.732 = 2.26 Volts (phase). It should be noted that for a given current and length of run the phase voltage drop on a three-phase circuit is only half that for a single-phase circuit. The easiest way of measuring voltage drop is to take a voltage measurement at the origin of the installation and others taken at points located at extreme ends of circuits. The voltage drop is then determined by simply subtracting locally measured voltage from mains values. The installation should, of course, be fully loaded when measurements are taken. Mains voltage must be measured. Don’t assume that it’s 230V or 400V: mains voltage continually fluctuates throughout the day. Mains and load voltages must be measured at the same time. If the installation is a large one, cable lengths will have to be determined in sections and the individual voltage drops added together to obtain the resultant voltage drop.

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Measurement of circuit length

Link

Low 

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SECTION FIVE (Reg. 713-02) TESTS OF PROTECTIVE CONDUCTOR CONTINUITY

The

purpose of these tests is to verify the continuity of all protective conductors and to obtain a measurement of the combined resistance of circuit phase and protective conductors at each point on every circuit. A shorting link at the distribution board between phase and earth will put these conductors in series, creating the circuit whose resistance is to be measured. This combination resistance is symbolised as R1 + R2. where R1 is the resistance of the phase conductor and R2 the resistance of the protective conductor. (See fig.6 P-E loop path.) The above data can usually be used to determine the phase - earth loop impedance for individual circuits, when the external P - E loop impedance (Ze) of the installation is known. Method As far as it is reasonably practicable to do so, remove all main and supplementary bonds before any continuity measurements are made. The reason for this measure is to minimise the lowering effect the equipotential bonds will have on the recorded resistance. They act as parallel paths to earth-fault current and hence reduce any measured resistance. It is required that earth-fault currents will be of sufficient magnitude due to the low impedance of the earthing construction, without reliance on the main and any installed supplementary bonding. Prior to the tests commencing, an insulation resistance test must be conducted to ensure that there are no short circuits between neutral and earth. A N-E short circuit will produce lower resistance readings than would otherwise apply, and of course a fault of this nature would not produce excess current when the circuit is energised, and hence go unnoticed. Having firstly ensured that the installation is dead, install temporary links between the phase bus bar(s) and the earth bar. (See Fig. 8) Isolate all circuits, except the circuit under test. Attending to all points of termination, measure the resistance between the phase conductor and earth, proving continuity. The resistance measured is that of R1 + R2. Record this value on an appropriate form - an example of which is included in this section. (See also section 24-25) A later test requires the live measurement of external P-E loop impedance (Ze). When this value is known for the distribution board under test, it will be possible to determine the values of phase - earth loop impedance for the individual final circuits.

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Short circuit currents will produce a rise in temperature, causing an increase in resistance, which must be determined before Zs is calculated1 . See the explanatory notes attached to the worksheet and Section 11. If the installation has a distribution board to which are connected final circuits supplying both socket outlets and fixed equipment - such as lighting - BS 7671 makes the following requirements: Regulation 413-02-13 - “the resistance of the circuit protective conductor associated with the distribution board must not exceed the resistance indicated in table 41C, from the earth bar to the point where the main equipotential bonding is connected.” Alternatively, the local earth bar must connect to the same extraneous conductive parts as the main equipotential bonds, For example, a distribution board supplies a number of sockets, lighting, circuits and power circuits. The largest installed fuse is rated at 100A fuse to BS 88 part 2. What is the maximum permitted resistance of the c.p.c.? The relevant table does not extend to a 50A fuse, so the formula given in reg. 41302-13 will be used. R2