WEEK 11 + TESTING AND INSPECTION C&G 2391

8/10/2019 WEEK 11 + TESTING AND INSPECTION C&G 2391

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Inspection and Testing 2 003 page 1

INSPECTION AND TESTING OF ELECTRICAL INSTALLATIONS

This book is intended to act as a working manual for those engaged inthe initial inspection or re-inspection of an electrical installation. It willalso assist trainees studying for City and uilds courses 23!0-1"

23!0-2" 23!0-C" 2#00" 23$1" %&' and (T)C. The ad*ice gi*en" and themethods suggested" are based on many years of practical e+perience andtherefore will hopefully not be considered too theoretical for those engagedin the )lectrical Contracting Industry.

If this most important aspect of an electrician,s work is to be successfullycompleted" testing and inspection acti*ities must be carefully prepared"e+ecuted and documented. The principles enclosed in this book will enablethat goal to be achie*ed with the minimum of errors.

eferences are made throughout to ( /!/1" better known as the 1!th.

)dition of the I)) egulations" and their associated guidance notes.ossession of these documents is essential for any test engineer hoping to

perform a high uality ser*ice.

It goes without saying that inspection and testing re uires a suitable range of instruments that will be regularly checked for accuracy and" when necessary"regularly re-calibrated. ee appendi+ 3 for details of instrumentre uirements

If the reader is unfamiliar with current terminology" he should turn to section24 where definitions of commonly used terms are gi*en.

5lan Croucher 2 00#

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It is not only a basic safety need" but also a re uirement of (. . /!/1 andthe )lectricity at ;ork egulations 1$


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Inspection and Testing 2 003 page #

applicable correction factors that will modify the tabulated current rating of installed cables. 6r" more simply" the temperature of the cable can bemeasured on full load. If the insulation is &C" the ma+imum permittedtemperature is /0 degrees Celsius. 5ny temperature below this indicates thecables are running at less than capacity. Aowe*er the *oltage drop could be

e+cessi*e if the circuit lengths are long" perhaps necessitating theinstallation of a larger cable.

ection 13 of this book will e+amine methods of assessing the pro*ision of o*ercurrent protection.

5n inspection of an installation is far from a simple matter" particularly if theinstallation has been in ser*ice for some years. ;ell-designed installationstend 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.

hysical defects cannot be detected by the application of instruments.Construction faults such as insecure fi+ings and inappropriate means of isolation can only be detected by *isual inspection" which will ha*e to beconducted in a systematic manner. (efore commencing the inspection and test" you are strongly ad*ised toprepare a schedule of work" which will not only organise the se uence of operations" but their detail. If the inspecting team arri*e on site without anypre-planning" lots of time will be wasted and ner*ous energy e+pended. Thismeasure is particularly important if the installation is already in ser*ice and

installation schedules and diagrams of practical *alue are non-e+istent.

It is also essential that an inspection ascertain that the installation is of sufficient capacity to supply the demand. 8oads could ha*e grown beyondthe intention of the designer. 9a+imum current should be measured" and for both distribution and final circuits. Comparisons are then made with thecurrent rating of the controlling o*ercurrent de*ice and the connected cable.

9a+imum demand may be measured by the use of a recording clamp meter left in position for 2# hours.

;e li*e in a litigious society - don,t take chances and o*erlook ano*erloaded installation - it could be expensive B

If the installation is new" the inspecting engineer will be re uired to inspectthe construction of the installation" ideally both during erection and oncompletion" to ensure that no hidden defects are left undetected.

)lectrical installation inspection and testing is a potentially ha ardouse+ercise. 5ny person responsible for this work must be =competent= in thesense that this term is used in the )lectricity at ;ork egulations 1$


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Inspection and Testing 2 003 page !

egulation /12-3 lists salient points re uiring particular attention" whichinclude the following"

Connection of conductorsE F Identification of conductorsE

F Current carrying capacity of conductorsE F 7etermination of *oltage dropE F resence of fire barriersE F resence of de*ices for isolation and switchingE F pace factor of conduits and trunking.

ee also appendi+ 5 of the ( /!/1 uidance %ote 3 for further ad*iceregarding *isual inspections and subse uent assessment.

There is a re uirement for a periodic inspection of installations" the details of which are gi*en in ection /## o f ( /!/1 and table 2.1.4 of % 3.

ome installations" which ha*e a mandatory re uirement for periodicinspections and tests" are listed below

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CinemasTheatresPetrol filling stationsCaravans and caravan sitesLeisure complexesPlaces of public entertainment

estaurants and hotelsPublic housesLaunderettes

5ll other installations ha*e only recommended periods between inspectionsand tests. Hor all buildings where people work" the)lectricity at ;ork egulations 1$


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)CTI6% 2

The re-test of an installation introduces ha ards not present in a deadinstallation and it is an important mandatory re uirement that the inspector shall be deemed to be =competent= )5; reg. 1! . The term indicates afamiliarity with the type of work" technical literacy and maturity. 7iagrams" charts etc. and any other rele*ant information will be re uired. Intheir absence - as pre*iously mentioned - a degree of cautious e+ploratorywork will be necessary.

5 *isual inspection shall be carried out with the installation de-energised" as

far as practicalities will allow - embracing as much of the installed e uipmentas possible and attention being paid to the following factors?

afety 7oes the installation present a shock risk To ensure safety" measures employed to ensureprotection against o*ercurrent and earth leakage must be suitable for any gi*en situation.

;ear 7oes the installation show any sign of wear or abrasion Check portable tools and other e uipment.

)n*ironment The en*ironment to which an electrical installation is sub>ect may cause rapid deterioration instandards of safety. )n*ironmental factors will include high and low temperatures" e+posure tothe elements etc.

7amage 7amaged e uipment is dangerous e uipment. 5ll facets of the installation must be regularlychecked for signs of damage.

Corrosion Aas the en*ironment a corrosi*e atmosphere 7oes the electrical installation come into contactwith the elements If so" is the choice of e uipment suitable

5ge 5s with most things" an electrical installation will deteriorate with age - particularly conductor insulation based on organic compoundsJ This deterioration cannot necessarily be checked byan insulation resistance test - a *isual inspection is re uired.

uitability 5ll electrical e uipment shall be suitable for the use to which it is put. :nsuitable selection cancause a dangerous situation to arise.

It should be emphasised that the )lectricity at ;ork egulation 1# does not permit any live or"ing unless it is absolutely essential for the installation toremain energised. The a*oidance of incon*enience is not considered areason for in*estigating an installation li*e. This regulation - for reasons of con*enience - is commonly and lightly breached" but any accidents resultingthat attract the attention of the Aealth and afety Inspectorate could result ina prosecution.

ome installations that are getting on in years defy logic and no assumptionsshould be made regarding any installed e uipment. It is not unusual todisco*er that two fuses may control a particular point or une+pectedly ha*e#00& at its terminals.

%e*er assume that a circuit dead - particularly if the circuit is three phase.ItDs your life at stake" so use your *oltage tester intelligently. emember the*oltage between two points of the same phase of a li*e circuit is eroB (utthe *oltage with respect to earth will be 230&. The resistance of the human body is *oltage dependant @ the higher the

*oltage the lower the resistance. 5 #00& shock is of a much higher magnitude than the increase in *oltage would suggest.

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Inspection and Testing 2 003 page oined and the resistance of theloop measured at the distribution board. ee Hig. 2

Circuit length P 2$.# + + metres

;here P loop resistance P cable cross sectional area in mm .2

)+ample? the loop resistance of a lighting circuit" shorted out at the furthestpoint is found to be 0./ . If the c.s.a. of the cable is 1.0 mm .2" what is thecircuit length

olution 8 P 2$.# + 0./ + 1 P 20.! metres.

The *oltage drop may then be determined by reference to appendi+ # of (/!/1.

)+ample? if the abo*e circuit is carrying a current when fully loaded of 45"the *oltage drop will be?

&d P Ib + 8 + m&d P 4 + 20.! + ## P #.43 &olts 1 000 1 000

5ssumed conductor temperature of /0 o.

&oltage drop is within limits. The abo*e calculation assumed a single-phasecircuit" wired in single core cable enclosed in conduit. If the installation werea three-phase one" the procedure would be identical" using two of thephases instead of phase and neutral to determine the circuit length. It shouldbe remembered that calculated three-phase *oltage drops are line volt drops " meaning the *oltage difference between phases or lines" at the mainsand load ends of the circuit. 5 *oltage drop along the length of a cable iscalled a phase volt drop.

The difference between permitted single and three-phase *oltage drops is bya factor of 3 1./32 . The allowance for *oltage drop on a three-phase

I

7etermination of cable length and *oltage drop


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Hig.# 9easurement of circuit length

8ow

8ink


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hort circuit currents will produce a rise in temperature" causing an increasein resistance" which must be determined before R s is calculated 1. ee thee+planatory notes attached to the worksheet and ection 11.

If the installation has a distribution board to which are connected finalcircuits supplying both socket outlets and fi+ed e uipment - such as lighting -( /!/1 makes the following re uirements?

egulation #13-02-13 % 3the resistance of the circuit protective conductor associated ith the distribution board must not exceed the resistanceindicated in table 0*C4 from the earth bar to the point here the maine#uipotential bonding is connected.5

5lternati*ely" the local earth bar must connect to the same e+traneousconducti*e parts as the main e uipotential bonds"

Hor e+ample" a distribution board supplies a number of sockets" lighting"circuits and power circuits. The largest installed fuse is rated at 1005 fuse to

(


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Inspection and Testing 2 003 page 1oined to the neutral conductor of the otherE a continuity test is conducted between the un->oined ends andthe resistance noted.

If the cable used in the construction has an integral earth" it shouldsubstitute for the neutral and the test be repeated. It should be noted that thecross sectional area of a protecti*e conductor in p*cNp*c cables is less thanthat of li*e conductors and conse uently the resistance would be somewhathigher.

Copper protecti*e conductors enclosed in steel conduit or trunking must beformed into a ring. 9easurements of ring circuit continuity where cpcDs areenclose in metal conduit or trunking is rather tedious" re uiring thedisconnection of each socket from its bo+ and the disconnection of the earth

tails.

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The test procedure for test 1 is illustrated in fig.12(

5lternati*ely each of the circuit conductors may be tested separately.

Test two

The un->oined phase and neutral conductors are connected as shown infigure 1!. :sing the 135 adapter" a resistance measurement is madebetween " % and ) at each socket outlet. The resistance measurements for a gi*en pair of conductors should be the same in each case. If test oneincorporated phase and neutral connected together" the resistancemeasured will be a uarter of that measured in test 1. If the loops weretested separately the resistance e+pected in test 2 will be half of thatmeasured in test 1.

esistance *alues will *ary with the physical si e of the circuit. The following

table gi*es some guidance regarding e+pected measurements. 5ny recorded*alues widely at *ariance with those below will possibly indicate looseterminations. %ote" *alues shown indicate that test 1 tested the circuitconductors separately.

It should also be noted that test 2 will not gi*e consistent results whentesting between and ) if the cable used is p*cNp*c.

F 2.4 mm. 2 at 24 deg. Hor 1.4 mm. 2 multiply by 0.!. Hor #.0 mm. 2 multiply by1.!

T%"$+ ($.+e +en/" Te'" %ne Te'" " %

30 m. 0.22 ohms 0.11 ohms40 m. 0.3/ ohms 0.1< ohms

!0 m. 0.## ohms 0.22 ohms/0 m. 0.42 ohms 0.2! ohms


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ReroingU out lead resistance priorto taking measurements

Test one

Conductors disconnected in preparationfor the test

Test conducted 0.0#

Hig.13

emo*al of ring circuitconductors at the distributionboard


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Inspection and Testing 2 003 page 3#

%ote should be taken of the current rating of the o*ercurrent de*ice and the cross sectionalarea of the connected cables that form the ring circuit.

Test one may be conducted at the distribution board if more con*enient than testing at asocket.

Hig.1#


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Inspection and Testing. 5.;.Croucher 2 00# page 34

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The purpose of this test is to ensure that all li*e and protecti*econductors are effecti*ely separated from each other by means of highresistance insulation" hence ensuring safety by confining current to theintended path. Current flow directly to earth" to neutral" or to another

phase" will produce shock and fire risks. 5 leakage current of 200 m5 flowingto earth through a concentrated fault resistance of 1 000 will result in apower dissipation of #0 ;atts. )nough power to start a fire" but not enough toblow a fuse or trip a mcb. Think of the temperature attained by a filamentlamp of this power.

Insulation resistance can be regarded as consisting of a myriad of indi*idualparallel connected leakage paths. The larger the installation - in accordancewith the rules of parallel circuit theory - the lower will be the insulationresistance. This principle applies to indi*idual circuits and completeinstallations.

8ssume that the resistance values indicate the insulation resistance of individual circuits ithrespect to earth.

The overall insulation resistance of the distribution board ill be e#ual to

+.9/ ' 4 less than that of any individual circuit

( /!/1 re uirements are to conduct an insulation resistance test of thecomplete installation. ub di*ision will only be permitted between distributionboards" not between circuits.

rior to any test commencing" any electronic controls such as lamp dimmersshould be disconnected. The search for their location should be a thorough

T) T 6H I% :85TI6% ) I T5%C) egulations /13-# /13-4

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, , ,5 9 10 ,100

Hig.14


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one. The application of 400 & 7.C. usually means permanent damage andreplacement" which can remo*e much of your profit margin in less than asecond. 'a"e sure that those lamp dimmers and computers etc. aredisconnected.

If remo*al of electronic e uipment is not a practical possibility it should beshorted out for the duration of the test.(eware of thinking that a -) test will not cause damage. If a %-) short hasoccurred" a -) test will ha*e the same effect as a -% test if e uipment isleft connected" i.e. the test *oltage is applied across e uipment terminals.

9ethod

:mall single phase installations

5s pre*iously stated" the installation must be tested as a whole? sub-di*isioninto indi*idual circuits is not permitted unless fault finding.

% - ) test

This test should be conducted first in order to ensure that test *oltages arenot inad*ertently applied to sensiti*e e uipment.

9inimum I . acceptable - 0.4 megohms with the ualifications stated overleaf

- % test

5ll fuses must be inserted" all current-using e uipment disconnected" and alllighting switches on. Hor a 230& installation a test instrument capable of an

output of - ( % * m8 7.C. shall be applied and a minimum insulationresistance of .- megohms shall be obtained. It should" howe*er" be noted

8ow insulation resistancecan be caused by surfacetracking betweenterminals. The conducti*etrack will be caused byeither condensation orcarbon contamination.

Hig.1!


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Inspection and Testing. 5.;.Croucher 2 00# page 3/

that insulation resistance values of less than + megohms ould usually signal unacceptably lo insulation resistance for one or more of the tested circuits4unless the installation is well abo*e a*erage si e. If this is the case eachcircuit should be in*estigated to determine whether the low insulationresistance is concentrated in a particular circuit. It is strongly recommended

that the minimum insulation resistance measured on any individual circuit shall be not less than +. megohms. This re#uirement is automatically met if the overall resistance is + ' or more.

If e uipment disconnection is not a practical proposition" the controllingswitch may isolate e uipment. (ut be sure to make an on-site assessment of the chances of a switch line -neutral fault e+isting.

The insulation resistance tester shall be positioned - where*er practicable -at the mains end of the distribution circuit or" alternati*ely" at the distributionboard bus bars.

- ) tests

5ll fuses are inserted. Current-using apparatus may remain connected"assuming no %-) shorts e+ist. Controlling switches should be left on. Thepre*iously stated results and ualifications are applicable.

Hig.1/


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Inspection and Testing. 5.;.Croucher 2 00# page 3ected to the test *oltage and should therefore bedisconnected before any tests are conducted. uch de*ices will includedimmer switches" delay timers" power controllers and electronic starters for fluorescent fittings.

Certain classes of semi conductors are likely to be affected by the electricfield created by the test *oltage" e*en if these components are not part of thetest circuit. If such de*ices are installed" alternati*e methods of in*estigationshould be considered see ection 1! .

)lectronic components cannot be guaranteed safe from *oltages appliedduring an insulation resistance test" e*en when separated from the test*oltage by an isolating transformer. The initial pulse of *oltage will bereflected into the electronic e uipment" with uncertain results.

If any of the disconnected e uipment has a conducting enclosure" which isre uired to be connected to protecti*e conductors" an insulation resistancetest shall be conducted between li*e conductors and earth. In the absence of any applicable (ritish tandards" a minimum insulation resistance of 0.4megohms shall be obtained.

eriodic inspection

The test methods and procedures re uired are identical to that for a newinstallation" with the additional precaution of ensuring that the installation istruly dead.

)nsure that all means of isolation are identified and in good working order and no circuits are fed from two sources. ;on


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Inspection and Testing. 5.;.Croucher 2 00# page #0

Tests of Insulation resistance 1

Test of Insulation esistance between li*e conductors and earth

neutral

earth

9

5ll fuses in place or circuitbreakers switched on

Temporary links @ to be removedbefore s itching on

Connection between

phase and earth.%ote the test can beconducted at anypoint in theinstallation that iscon*enient

Insulation resistance tester

Hig.21


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Test of insulation resistance 3

Test of Insulation esistance between phases

neutral

earth

9

5ll fuses in place or circuitbreakers switched on

Temporary link @ to be removedbefore s itching on

Connection betweenphase and earth.

%ote the test can beconducted at any

point in theinstallation that iscon*enient

Insulation resistance tester

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0.029

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If bridging loads are connected is not possible to locate a fault by switching off circuit breakers.This principle is illustrated abo*e. 5 %-) fault e+ists on circuit V1. 5ll circuit breakers e+cept that ofcircuit V4 is switched offE the fault is ostensibly located on this circuit" indicated as a -) fault.To find the location of an insulation resistance fault all of the neutral conductors must be remo*edfrom the terminal block

Hig.2#


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Inspection and Testing. 5.;.Croucher 2 00# page ##

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The purpose of polarity testing is to ensure that all single pole de*ices suchas fuses" single pole switches" thermostats etc." are connected in the phaseconductor only and the three phases are correctly identified throughout the

eriodic testing

Tests shall be made using the described method to *erify that?

i 5ll single pole de*ices are connected in the phase conductor only.ii 5ll multi-pole de*ices are connected to the identified li*e conductors" e.g.

no phases are crossed.

If it can be established that no alterations or additions ha*e been made sincethe last test and inspection of the installation" the abo*e tests need onlyinclude 10L of the any installed points e+cluding socket outlet circuits.

5ll socket outlets must be tested for polarity.

If any re*erses of polarity are detected" all points on that particular circuitmust be tested for correctness of polarity and the sample testing on theremaining circuits connected to the distribution board increased in fre uencyto 24L. 5ny further cases of re*ersed polarity found in the 24L sample willre uire a 100L test of the completed installation.

5 simple" cheap" but effecti*e polarity-testing tool is illustrated in figure 30.

Consisting of few components" it can easily be made in less than an hour.To use the de*ice - having firstly ensured that the installation is dead - a $&battery is attached to the phase Q *e and neutral - *e bars of thedistribution board. 5ll switches are closed.

The p.d. will be distributed around the installation.

5ttending to the points to be tested" the red terminal is attached to the phasetermination and the black terminal to the neutral. If polarity is correct" thegreen 8)7 will be illuminatedE if not" the red 8)7 will be lit.

It is important" when testing for correctness of polarity" that the neutral andearth connections are physically disconnected if a discrimination is to bemade between these conductors. If the installation is single-phase" it ismerely necessary to open the main isolator. Aowe*er" if the installation isthree-phase" controlled by a 3-pole isolator" the neutral is unswitched and willha*e to be unbolted from its terminal block.

It is important to note that polarity cannot be *erified li*e using a *oltagedetector of the type illustrated in Hig.2!. If an %-) re*ersal has taken place itwill go undetected on a li*e test" resulting in load current flowing to earth. Theconse uences of this will be the opening of an C7 or the continual flow of earth current" with all the attendant ha ards this e*ent will possibly produce.

T) T 6H 685 ITG egulation /13-13


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olarity testers

olarity testers of a type that rely on the measurement of *oltage can falsely indicate correct polarity

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all lamps lit

5 proprietary polarity testerfor use on li*e circuits.Testers of this type do notmake a distinction betweenneutral and earth.

Hig.24


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Inspection and Testing. 5.;.Croucher 2 00# page #/

)CTI6% 10

f the supply system is TT" that is" a system without a mains distribution of protecti*e conductors - an earth electrode will ha*e to be installed locally.I

%o electrode can be in contact with the general mass of earth without aninter*ening resisti*e barrier. This resistance is called earth electroderesistance" which will be confined" to the immediate *icinity of the electrode.

The structure of electrode resistance is a complicated one" modelling series-connected concentric resisti*e hemispheres" each shell decreasing in

resistance with distance from the electrode.

ossibly the simplest concept of earth electrode resistance is that of twoconductors - one of which is earth - connected by a chain of resistors of steadily decreasing *alue. tarting at the low resistance end" in*estigationsoutwards would re*eal increasing resistance which is incrementally reduceduntil the point is reached when no further significant increase is detectable

see Hig.31 . 9easurable increase in resistance only occurs close to the electrode"occupying a finite area" known as the earth electrode resistance area.

In order to ensure that earth fault current is of sufficient magnitude to operateprotecti*e de*ices" earth electrode resistance must be measured andassessed.

5 simple earth electrode installation is unlikely to result in a resistance lowenough to meet the earth loop impedance re uirements of regulation#13-02-0


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Inspection and Testing. 5.;.Croucher 2 00# page #usted until the centre ero gal*anometer is balanced.The indicated resistance is that displayed by the decade resistors multipliedby the scale multiplier.

a

9easurement of soil resisti*ity

Hig.31


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The resisti*ity of the soil is calculated by the formula

P 2 a

;here is the resistance measured?

a P the distance in centimetres between electrodes.

P the soil resisti*ity in ohms-cm.

The depth at which the electrodes are to be buried should be 1N20 of that ofdistance a.

The resistivity is measured at depth a.

Hor e+ample" if the resistance measured was 100 and the distance betweenelectrodes #m. then using the abo*e formula" the soil resisti*ity will be"

241 32/ /cm.

Hig.32


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Inspection and Testing. 5.;.Croucher 2 00# page 4#

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T$:T: &= $8 T2 =8ALT L&&P I'P$;8NC$ egulation /13-10

If the dangers of earth fault currents are to be minimised" fault duration must

be carefully controlled by means of the circuit protecti*e de*ice" fuse" circuitbreaker" or C7. This control" howe*er" will only be effecti*e if the impedanceof the earth fault circuit is sufficiently low to generate the re uired operatingcurrent.

)arth fault currents are usually dri*en at 230&" which is" of course" a constant*alue. It follows therefore that *ariations in earth fault current are determinedonly by *ariations in -) loop impedance.

Ip P :oNRs ;here Ip P prospecti*e short circuit current:o P nominal *oltage to earth

Rs P phase-earth-loop impedance

The ma+imum fault duration for a 230 & socket outlet circuit likely to supplyhand held e uipment is 0.# seconds egulation #13 - 02 - 0< . Hor ma+imumdisconnection times for *oltages other than 230 &" see table #15.

If the installation is a temporary supply for construction site use" thedisconnection times are gi*en in table !0#5" and for agricultural andhorticultural installations" table !045 is applicable.

Tables #1(1" #1(2 and #17 indicate ma+imum operational *alues of - )

loop impedance permitted for a particular o*ercurrent de*ice" connected load"and current rating. If these *alues are not e+ceeded" the fault current will besufficient to produce the re uired disconnection time. It should be noted that*alues gi*en in these tables represent operational conditions not the limitsmeasured on a loop impedance tester. Hor tabulated limitations of measured*alues see % 3 tables 2( to 27.

Complicated - but the flow chart included in this section will makeassessment relati*ely simple. The ma+imum disconnection time for circuits containing only fi+ed e uipment

is 4 seconds egulation #13-$ WiX . 5s pre*iously stated" if a distribution board has connected a mi+ture of fi+ede uipment and socket outlets" either?

>i? The impedance of the protective conductor associated ith thedistribution circuit shall not exceed that indicated in Table 0*C4 or4>ii? :upplementary bonding shall be installed at the distribution board that must connect to the same extraneous metal or" that is connected to themain e#uipotential bonds. >:ee regulation 0*9% +%*9?

Hor e+ample" if the largest fuse in such a distribution board were to be ratedat #05 (


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permitted protecti*e conductor resistance from the 7( earth terminal to themain earthing terminal is not to e+ceed 0.2$

To achie*e a disconnection time e+ceeding 0.# seconds" but not e+ceeding 4seconds" loop impedance *alues for a particular fuse or circuit breaker of a

stated current rating shall not e+ceed those indicated in tables #1(2 or #17as applicable.

9ethod

The measurement of -) loop impedance is essentially conducted on a li*ecircuit but egulation 13 of the )lectricity at ;ork egulations 1$


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(efore attaching the - ) loop impedance tester to the installation it shouldbe isolated. Hor safety reasons the instrument should not be directly appliedto li*e parts if at all a*oidable. 5fter attachment" the supply should beenergised and the test conducted.

9easurement of R e is a potentially dangerous business. The test is uiteoften conducted in a confined space" inade uately lit" with the additionalha ard of #00& e+posed to touch.

'easurement of B e must only be underta"en by confident and competent electricians.

The loop impedance tester should be of a modern design - older instrumentsare usually of low safety standards. Instrument leads should ha*e substantial

insulation and should be protected by in line A.(.C. fuses.To take into account an increase in temperature and hence resistance under full load conditions" as a rough rule of thumb" the recorded *alues should note+ceed 0./4 of the rele*ant *alue indicated in ( /!/1 tables, #1( and #17"which are based on a conductor temperature of /0 deg.

5 more precise assessment may be made of conductor resistance 1Q 2by consulting the table below" which sets out multiplying factors to correct for resistance increase due to the temperature rise occurring on full load current.

Test ambient temperature deg.C Correction factor 4 1.0!10 1.0#14 1.0220 1.009easured resistance is multiplied by the abo*e factor.The resulting *alue will then be multiplied by the factor gi*en below..Insulation type Correction factor

&C 1.20


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Inspection and Testing. 5.;.Croucher 2 00# page $#

)CTI6% 1$

It has long been the re uirement of wiring regulations that the prospecti*e short circuit currentshould not e+ceed the safe disconnecting capability of the controlling fuse or circuit breaker. Thedefinition of prospecti*e short circuit current is"

=That current which will flow between solidly bolted conductors of differing potential. In the caseof a three phase and neutral installation" the conductors in uestion will be all three phases.H

6b*iously" the prospecti*e short circuit current of an installation cannot be determined by the

abo*e methodE measurements will be taken using a dedicated prospecti*e short circuit currenttester. This instrument" when connected to the installation" will circulate a significant test currentthat will cause a small mains *oltage drop. This fall in *oltage and the current that produced itwill be integrated by the instrument" which will then calculate the mains impedance and thepotential resulting short circuit current.

rospecti*e short circuit current testers are usually designed to be attached to only two pointson the installation and therefore cannot be directly used to determine current resulting from asymmetrical three phase fault.

Theoretically it can be shown that a symmetrical three-phase fault will produce appro+imately

t ice the current produced by a single-phase faultE if a single-phase measurement of CC ismade" the recorded value should be doubled . 1hen using an instrument that connects to t olines >0 (? the correction factor is x *.*-

In reality" >oint and cable resistance and resistance changes produced by the ine*itabletemperature rise that results from current flow considerably attenuate short circuit current. Theactual three-phase fault le*el will be somewhat lower than that calculated" howe*er any error would be on the safe side. ;hen assessing the suitability of an o*ercurrent de*ice" ha*ing determined the apparent faultle*el" caution should be e+ercised. :pstream de*ices such as A(C fuses" miniature and

moulded case circuit breakers may be =current limiting=. This term means that the de*ice will =cutoff= before the prospecti*e current is reached. Hor e+ample" the prospecti*e short circuit currentat the bus bars of a distribution board may be assessed at 24 k5" but the upstream o*ercurrentde*ice may be designed to limit the current to $ k5 hence allowing the installation of cheaper downstream circuit breakers or fuses. It will ha*e to be ascertained of course by the designer that discrimination has been achie*ed for all fault le*els up to $k5" beyond which it will be lost.

hort circuit breaking capacity of differing o*ercurrent de*ices are indicated in the tableso*erleaf.

Hig. !3

9)5 : )9)%T 6H 6 )CTI&) H5:8T CI C:IT C: )%T

eg. #3#-02-01


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Inspection and Testing. 5.;.Croucher 2 00# page $4

It should be noted that if a prospective fault level of + "8 ere to be measured at the origin of aninstallation4 a fault only one metre from that point4 connected by a copper cable of * mm.+4

ould attenuate the current to */.0 "8.

hort circuit breaking capacities for differing protecti*e de*ices.

5 symmetrical three-phase

fault


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Inspection and Testing. 5.;.Croucher 2 00# page $!

Types of de*ice (reaking capacity k5Huses to ( 13!1 type 1 1!.4Huses to ( 13!1 type 2 33.0Huses to (


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Inspection and Testing. 5.;.Croucher 2 00# page $/

)CTI6% 20

I88:9I%5TI6%? A6; 9:CA 8I AT

ecommended le*els of illuminance - measured in 8u+ - are specified by CI( ) in their 1$


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Inspection and Testing. 5.;.Croucher 2 00# page $or de*ice.This principle is illustrated in the diagram below.

4 5 designed loss of discrimination is allowed under certain circumstances. ee reg. #3#-03-01

5

9inor circuitbreaker

9a>or circuitbreaker

Hig.!4


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This ob>ecti*e is easily achie*ed when designing for o*erload. If de*ice 5 has a higher currentrating than that of ( then discrimination will be realised. 5 ma>or to minor ratio of current ratingsof not less than 2?1 should e+ist.

Aowe*er" when considering short circuit currents the picture may be different" particularly if

de*ices 5 and ( are of a different operating characteristics or principle. Hor e+ample" if 5 has arating of !3 5 and ( a rating of #05 and the prospecti*e short circuit current is 4 000 5" thiscurrent will simultaneously flow through both de*ices and will ob*iously be of sufficient le*el tooperate both 5 or (" but which will open first The answer to this uestion is of importance to theoperator of the installation. Clearly it will be most incon*enient if the ma>or de*ice were to openand isolate a large area of the building.

Correct design and assessment of o*ercurrent protection can only be undertaken if theprinciples are fully understoodE these principles will now be e+amined in detail.

Hirstly it must be realised that current alone will not cause a de*ice to operate. In order to

produce the necessary effects to open the circuit" current must flow for a period of time. Thistime delay will ideally be e+tended for o*erloads - which are often transient in nature - and beminimal for short circuits.

The rewirable fuse has no special pro*ision for discriminating between short circuits ando*erloads" and simply operates faster with increasing current" the precise characteristicsdepending upon the en*ironmental conditions of installation" such as ambient temperature and*entilation. A(C fuses in contrast are de*ices manufactured with great precision" with botho*erload and short circuit characteristics being predictable with a high degree of accuracy.

These fuses will be manufactured with a *ariety of attributes" tailored to suit that of the load. Hor

e+ample" they may be designed to tolerate o*erloads for a considerable time - relati*elyspeaking - as would be necessary for fuses protecting motors.

7uel element A(C fuse

The e+cess of current o*er the current rating re uired to blow a fuse is called the fusing factor.Hor e+ample 105 fuse ha*ing a fusing factor of 1.4 will re uire a minimum fusing current of 145to open.

uart

Tinned copper endcaps

Copper fuse elementil*er plug

Indicating bead esistance wire

Hig.!!


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It follows that the lower the fusing factor" the closer the degree of protection against smallsustained o*ercurrent. Hrom the characteristics of differing class of A(C fuse to (


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Hig. !/

Mnowledge of this circuit characteristic will enable the installation downstream of 91! mcbDse*en if a prospecti*e short circuit current test indicated a fault current of 20 k5.

If" on the other hand" the prospecti*e short circuit current at the distribution board was found tobe 4 k5 the fuse would not cut off and that current would be allowed into the system. It would stillmean of course that the 91! circuit breaker would be suitable.

;hen a short circuit current flows" that current will be of a common *alue throughout the circuit.The resistance of the *arious components of the circuit will *ary and hence the powerdissipated. 5nother common factor will be the fault duration. Therefore it can be stated that

I 2

t;ill be of the same *alue in all parts of the circuit. ;here I is the fault current and t the faultduration.

This unit is called the specific or admitted energy of the circuit that will be released under faultconditions. 9anufacturers of o*ercurrent protecti*e de*ices will design the short circuitcharacteristic in terms of specific energy and not current.

I2t characteristics of A(C fuses and circuit breakers are published by manufacturers and will be

used by designers to determine the short circuit relationship between fuses or circuit breakers

connected in series.

5n e+ample for (


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Inspection and Testing. 5.;.Croucher 2 00# page 10#

F&'e #$"in/ in $,p'

Spe(i*i( ene#/-

10

100

1 000

10 000

100 000

1 000 000

App#%xi,$"e +e" " #%&/ ene#/- ( $#$("e#i'"i(' %* $ 'e+e("i%n %* S 88 @ C *&'e'7 Di'(#i,in$"i%n

i++ %(( &nde# ' %#" (i#(&i" (%ndi"i%n' i* " e "%"$+ di'(%nne("i%n ene#/- %* " e ,in%# *&'e d%e'

n%" ex(eed " e p#e!$#(in/ ene#/- %* " e ,$ %# *&'e

A#(in/ ene#/-

P#e $#(in/ ene#/-

F%# ex$,p+e i" ($n .e 'ee *#%, " e $.%)e ( $#$("e#i'"i(

$ 16A *&'e ($nn%" .e (%nne("ed in 'e# ie' i" $ *&'e $)in/

$ (#en" #$"in/ ',$++e# " $n 40A i* ' %#" (i#(&i" di'(#i,in$"i%n i' "% .e en'ed

16A 25A 32A 40A 63A 100A

;hen fuses are connected in series with circuit breakers" more complicated considerations arere uired in order to achie*e short circuit discrimination.

5 circuit breaker will usually ha*e magnetic detection of short circuit currents" the createdmagnetic field being used to acti*ate a mechanical circuit breaking mechanism. The speed ofdisconnection will depend upon the energy stored in the springs and the inertia of the parts. Inother words" circuit-breaking duration is not solely a function of electro-magnetic effects.

These characteristics will mean that at" and beyond" a certain fault le*el the speed ofdisconnection will be constant. 5t this point the disconnection is said to be =instantaneous=. Hor

most miniature circuit breakers" instantaneous means 0.01 seconds" or half a cycle. %o faultle*el regardless of magnitude will cause a reduction in this disconnection time.

It follows" therefore" that the short circuit characteristics of a particular circuit breaker - unlike anA(C fuse - cannot be simply e+pressed in terms of a gi*en *alue of specific energy. The currentproducing cut off will ha*e such a wide *ariation" from a minimum" for a particular circuit breaker"to its ma+imum breaking capacity. 9anufacturers will" howe*er" produce specific energycharacteristics based on a particular prospecti*e short circuit current and disconnection time. Inthis case half cycle or10 m .

5n e+ample of such data is shown o*erleaf

Hig.!


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5n e+amination of the graph will indicate that with a prospecti*e short circuit current of 10 k5" anmcb of types ( or C will let through a total energy of40 000 I 2t.

The smallest cable si e able to resist the thermal effects of the short circuit current is gi*en by"

JP I2 t Nk.

Hor e+ample if a circuit is protected against short circuit by a 1!5 mcb"2.4 mm. 2 cable will be needed if the prospecti*e short circuit current is 10k5 but if the

CC is only 1 k5" 2.4 mm. 2will suffice.

:p to a point a circuit breaker will ha*e an in*erse time characteristic similar to that of a fuse. Inthis area" thermal or magnetic-hydraulic de*ices will be utilised to detect o*erloads. (ut adistinct changeo*er point is reached for a particular circuit breaker in terms of fault current"where the magnitude is such that instantaneous disconnection takes place. This point isindicated by the cur*e going hori ontal.

;hen the ma>or de*ice is an A(C fuse and the minor de*ice a miniature circuit breaker" caremust be e+ercised in selection if short circuit discrimination is to be achie*ed. 5 study of thecur*es shown in fig.3# will indicate that for all fault le*els up to =+=" which is appro+imately !0times the current rating of the miniature circuit breaker" discrimination will be achie*ed. The mcbwill trip before the fuse blows. 5t point =+= the two cur*es cross and for *alues of prospecti*eshort circuit current beyond this point discrimination will be lost.


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x

fusem.c.b

in'"$n"$ne%&' di'(%nne("i%n

+i,i" %* in)e#'e "i,e ( $#$("e#i'"i(

(#en"

"i,e

An- ' %#" (i#(&i" (#en" in ex(e'' %* " $" #e$( ed $" p%in" B i++ ($&'e $ +%'' %*di'(#i,in$"i%n T e ,$ %# de)i(e ?*&'e i++ .+% .e*%#e " e ,in%# de)i(e ?, ( .%pe#$"e'

0 01 'e('

A++ ' %#" (i#(&i" (#en"' &p "% B i++ #e'&+" in (%##e(" de)i(e %pe#$"i%n

The temperature reached by a cable under short circuit conditions not e+ceeding fi*e seconds"is gi*en appro+imately by the formula"

T P ItN200

Hor e+ample" if the energy let through by a !35 mcb were to be /0"000 5 2 and the connectedcable 1! mm. 2E the resulting temperature rise would be 21 deg.

Hig.!$


107/137


Hig./0


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Inspection and Testing. 5.;.Croucher 2 00# page 10aws of the instrument should be as largeas possible and the resolution should be 0.015

Hig.


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%**

20,A

10A

10A CO< ,A

&n*&'ed 500 ,$x *&'ed

,e,

#$n/e

d( $(

DIGITAL A;TORANGE


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WEEK 11 + TESTING AND INSPECTION C&G 2391

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Transcript of WEEK 11 + TESTING AND INSPECTION C&G 2391