***Cont../ Useful Information for Electricians & Apprentices***

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Definitions– Part 2 . BS-7671:2008 :17:

Bonding Conductor . p/20 .
Aprotective conductor providing equipotentialbonding .

Class1 Equipment . p/21 .
Whereprotection against electric sock does not rely solely on basic insulation alone. Exposed – conductive parts being connected to a protective conductor whiththe fixed wiring of the installation . (See BS-EN 61140 )

Class11 Equipment . p/21 .
Whereprotection against electric shock relies on the application of additional orsupplementary insulation .

Thereis no provision for the connection of a protective conductorto exposed metalwork .

DoubleInsulation . p/23 .
DoubleInsulation ( Class 11 ) Insulation comprising both basic Insulation &supplementary insulation .

Earth. p/23
Theconductive mass of Earth . whose electricpotential at any point is conventionally taken as Zero. ◄◄

EarthElectrode . p/23
Conductivepart . which may be embedded in the soil or in a specific conductive medium .e.g. concrete or coke . in electrical contact with the Earth .

Aconductor or group of conductors in intimate contact with . & providing anelectrical connection to Earth .

EarthElectrode Résistance . p/23 .
Therésistance of an Earth electrode to Earth .

EarthFault Current .
Anovercurrent resulting from a Fault of negligible impedance between a Lineconductor & an Exposed – conductive – part or a protective conductor .

AFault current which flows to Earth .

EarthFault Loop Impedance . p/23
The impedance of the EarthFault current loop starting & ending at the point of Earth Fault . This impedance isdenoted by the symbol ( Zs )
Symbol( Z ) - Unit ( Ω )

TheEarth Fault Loop .

The impedance of the EarthFault current loop starting & ending at the point of Earth Fault . Consists of :-
• The circuit protective conductor ( C.P.C. )
• Consumers Earthing terminal & Earthingconductor
• for TN – Systems . Themetallic return path .
• for TT – Systems & IT Systems Earth return path .
• The path through the Earthed neutral point ofthe Transformer .
• The Transformer winding & phase conductor to point of Fault .

 

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543-Protective Conductors .

543– provides information on the Selection of “ BOTH “ type & Cross – sectional Area of Protective Conductors .
Protective Conductors . areconductors provided for the purposes of Safety . protectionagainst Electric Shock .

GenericTerm “ ProtectiveConductors “ :thinking2:

 

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2392-10. as designer(s) . Am making you Aware of the Facts .

Cross-sectional Areas of Protective Conductors .

Thereare two Methods that may be Employed when choosing a Protective Conductor asrequired by Regulation 543.1.1.

TheCross- sectional Areas ( C.S.A. ) of every ProtectiveConductor . other than Protective BondingConductors .

MustBe :-

► Selected . in accordance with Regulation 543.1.4. - :icon4:

Or

► Calculated . in accordance with Regulation 543.1.3. - :icon4:

 

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2392-10: Yes we still use it in Industry :icon4:

543.2.7. Earthing Tail Requirement .

WhereMetallic Conduit . Trunking . etc is used for a ProtectiveConductor . The Earthing Terminal ofeach Accessory is required to be connected by a Separate Protective Conductor to the Earthing Terminal incorporated in theassociated box ( Back – Boxes) or other Enclosure .

 

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Fault Protection . “ Simply “

Automatic Disconnection Supply . :icon4:

 

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Applicationsof Earthing : :vanish:

All Metallicenclosures & Extraneous conduciveparts . are at Equipotential .

Functionsof Earthing :

EquipmentEarth : Path for Fault Current . lowertouch voltage . protection against Electric Shock .
LightingEarth : Low résistance path to diversethe current . Under lighting attack .
TelecommEarth : Signal Earth . reduce noise & interference . stabilize D.C. supply voltage & prevent ElectricShock .
ComputersEarth : Reduce interference . maintain supply voltages .

 

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Typeof Earthing :- :vanish:

SupplySystem – Neutral / Earth .
SystemEarth
ElectricalSafety Earth .
LightingEarth .
GeneratorEarth .
ProtectiveEarth . - Surge arrestor .
Telecom/ Computer Earth .
ShieldingEarth .
IntegratedEarthing System -
ElectrostaticEarth – Clean / room – Hospital .

 

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Factors affect to the EarthImpedance . TT Systems

Soil.
Weather.
Electrodetype .
Electrodesize .
Near by Utilities .
Electrode in Parallel . -&-s
Distance between Electrode .

 

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:thinking2:TT Systems

EarthRésistance of an Electrode

Soil Exhibits a resistance to the flow an Electrical Current .
Not an “ Ideal “ conductor
Résistance – can never be Zero . betweenthe Earth Electrode & “ True Earth “
The résistance between the Earth Electrode & “ True Earth “

SoilResistivity in ( Ωm )
Buriedlength of the Electrode in ( m )
Diameter of the Electrode in ( m )

 

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Fault :icon4:

A circuitcondition in which current flows through an abnormal or unintended path .

Thismay result from an Insulation failure or a bridging of Insulation .

Conventionallythe Impedance between Live conductors or between Live conductors & Exposed or Extraneous conductiveparts at the Fault position isconsidered Negligible.

 

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Regulation. Appendix 4 – ( 4D1A ) p/274 :17:

Caremust be taken tie sure that any Conductors’ which is operating above ( 70°C) is terminated into Equipment that canwithstand the higher temperature .
TheBritish Standard only requires that our Equipment is suitable for ( 70°C)

Ambient Temperature mustalways be taken into account .

TheCurrent Carrying Capacities given in the Tables in appendix 4 Calculated for a Cable operating in an Ambient Temperature of ( 30°C ) .

( 4D1A ) for4.0mm[SUP]2[/SUP] – Copper Conductor clipped direct can carry a currentof ( 37A ) at ( 30°C ) Pass a current of ( 37A ) through the conductor it would rise to ( 70°C )

Ambient Temperature were to increase to . ( 40°C ) then theTemperature of the conductor when we passed ( 37A ) through it would rise to ( 80°C )

Side– Effect . of this would be an Increase in Voltage Drop .

• The Maximumpermissible operating temperature of Cables are dependent mainly on the Type ofInsulation Material used in the Cable Construction

( 4D1A )
Ambient Temperature ( 30°C ) ◄◄
ConductorOperating Temperature ( 70°C )

Temperature is the Factor which dictates the size of theConductor in relation to the Number of Circuits Installed.

Thermoplastic : ( 70°C ) :44:

 

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Measurement of ExternalEarth Fault Loop Impedance ( Ze ) iscarried out between the Incoming Line & Protective Earth . :19:
Earthing Conductor Detachedfrom the ( MET ) & test probe clipped toit .

Measurementof the External Earth Fault Loop Impedance ( Ze ) at the Origin of theInstallation for compliance with regulation 612.9

EarthFault Loop Impedance :

Live Tests & great careshould be taken when carrying them out .

Thereare Two Measurements for Earth Loop Impedance .

i) One is for External Loop Impedance ( Ze )
ii) Other is for the Circuit Loop Impedance ( Zs )

Bothof these tests are carried out Using an Earth Fault Loop Impedance TestInstrument with Leads & Probes compliant to ( GS-38 )

(Ze ) Earth Fault Loop Impedance :

TheInstallation must be ( Isolated ) – “ Live Test “ ◄◄ -&-s & the Earthing conductor disconnected .
Connectone Lead to the disconnected Earthing conductor& then insert a Probe into the Terminal ofthe Incoming Line .

TheMeasured Value will be ( Ze )

I use the Megger 1552 : Two Leads .
►► Instrument has ThreeLeads then the Third Lead must be connected to the Incoming Neutral ofthe Supply .

Thisis what -&-s want to Hear . ▼

TheEarthing Conductor must be Reconnected beforere-energising the Supply to the Installation .

 

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(Zs ) Circuit Earth Fault Loop Impedance : :19:

Thisis a Live Test . ◄◄ for-&-s 2392-10 - ( At Socket Outlets)

“ Simple“ What is required . The Instrument is plugged into the Socketusing the Lead supplied & the resultRecorded .

TheHow’s & Why’s ??

Wherethe Circuit has NO socket outlets the instrumenthas to be ( Connected ) to the Exposed Terminalsof the Accessories on the Circuit being Tested .

ALL – POINTS must be tested & the Highest testresult Recorded as ( Zs ) for the Circuit

“ Certification& Reporting “
Onyour Test result sheet & add the Measured ( Ze ) to the Recorded ( R1 + R2 ) value . Then compare the totalvalue with the Measured ( Zs ) if it is Equal toor Lower then all is Fine .

Ifit is Higher then it may be that there is a Loose Connection & further Investigation is required .

Froma Testers point of View (- The Measured value will often be Less than theCalculated value due to the Presence of Parallel Path.

Measured( Zs ) must be compared to the Maximum value of ( Zs )

From-&-s point of View (- Ensurethat it complies with the Requirements for the circuit disconnection time .

 

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O.S.G.p/13 .
BasicInformation Required . 313.1. Beforestarting Work on an Installation . Etc

ForExisting Installations Electricians’ should Satisfy themselves as to the Suitability ofthe Supply including the Earthing Arrangement. :speechless:

 

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Regulations: Table 41.3 - Maximum Earth Fault LoopImpedance ( Zs )

Table 41.3 -For Circuit Breakers with ( Uo ) of 230V . ForInstantaneous operating giving Compliance with the ( 0.4s )disconnection time of Regulation 411.3.2.2.
BS-EN 60898 . / BS-EN 61009-1
MaximumMeasured Earth Fault Loop Impedance ( Ωs ) Overcurrent Protective Device is a Circuit Breaker .

Table 41.1.
MaximumDisconnection Times ( 0.4s ) - 411.3.2.2.
TheMaximum Disconnection Time stated in Table 41.1. shall be applied to FinalCircuit NOT Exceeding ( 32A ) - The regulations are using the Words . Final Circuit(s)

Apprentices. if we look at Table 41.1. - Maximum Disconnection Time stated in Table 41.1.is for a ( TN- system ) A.C. ≈ ( 0.4s )

MaximumEarth Fault Loop Impedance .
TheMaximum Earth Fault Loop Impedance is used to Calculate the available EarthFault Current within an Installation connected at ( Low – Voltage – below 1000V) :17:

 

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EarthFault Loop Impedance .

i) Electrical Earth Fault on a circuitcurrent flows from the Line conductor throughthe Fault . down the CPC.
Alongthe Earthing Conductor provided by the Electricity Company for . TN-S & TN.C.S. or through the EarthElectrode . for a TT System .

ii)The Circuit ( Loop ) then is the Transformers winding ( Impedance )
theSupply Company Line conductor in theInstallation & return Earth conductor ( Résistance )

iii)This Complete Circuit is called the ( Earth Fault Loop) & the ( Earth Fault Loop Impedance ) isthe value in Ohms . Measured

EarthFault Protection for an Electrical Installation .
- Thesupply must have ( One Pole connected to Earth at the Supply Transformer . (Single-Phase ) This creates what is known as a TN- System .
- ( Overcurrent ProtectiveDevice ) MCB . in the Line Conductorwill operate under Earth Fault Conditions provided that enough current flows .

Icant . Stress this Enough .
An“ RCD “ is for . 17[SUP]th[/SUP] Edition / Additional Protection “ Only “ :earmuffs:
433.1. Every Circuit shall be designed so that aSmall Overload of long duration is Unlikely to Occur .

Soif your Earth Fault Path is not good enough to allow sufficient current to flowto operate the MCB . Then you must Install a RCD . :17:

Apprentices:
Electricitytakes the path of Least Résistance . if the path of Least Résistance happens tobe Earth .

EarthElectrode Résistance TT .
The Electrode is the point where the Earth isconnected to Earth .
Measuringthis shows how well the System is connected to Earth . if the Résistance is to High then there is a chance that a Fault will find anEasier Path .

Why’s(- Measuring the Earth Electrode Résistanceshows how good the Earth is & whether it will Work .
EarthRésistance of the Fault Loop Impedance . ifthe Fault Loop Impedance in the Earth Conductor is to Highthen the Current may not rise to a Sufficient Level to cause the MCB to trip inthe Required Time .

 

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Prospective Short Circuit Current -( PSCC )
Thisis the Current you might get if you have a “ ShortCircuit “ in your Supply .

 

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O.S.G. . p/33 - Isolation :46:

Isolating & Switching :

Requirement( Means of Isolation should be Provided. 132.15.1. Every fixed Electric ( Motors ) Etc.

537.1.4.
Atthe Origin of the Installation .
Amain liked switch or Circuit Breaker should beprovided as a Means of Isolation & of Interruptingthe supply on Load.

ForSingle-phase household & similar supplies that may be Operated by UnskilledPersons . A Double - Poledevice must be Used for both TT& TN- Systems .

Fora Three-phase supply to an Installation forming part of a TT System . An Isolator must Interrupt the Line & NeutralConductors . ( DP ) in a TN-S or TN-C-SSystem only the Line Conductors need be Interrupted.

ForEvery Circuit : 537.2.1.1.
Otherthan at the Origin of the Installation . Every Circuit or Group of Circuits thatmay have to be Isolated without Interrupting the Supply to other circuitsshould be provided with its OWN Isolating device. The device must SWITCH all Live conductors . in a TT System & all Lineconductors in a TT System.

Apprentices.
TTSystem & all Line conductors in a TT System
►► Line conductors(s) ( Line & Neutral ) BothLive conductors . -&-s L/N

Asthe Regulation tells Us . p/118 : Isolation .

537.2.Note : Isolation is a “ Function “ intended tomake DEAD for reasons of Safety all or adiscrete section of the Electrical Installations by separating the ElectricalInstallation or Section from Every Source of Electric Energy .

 

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Loop Impedance Testing :

A “ Loop“ in NOT the same as aCircuit . :svengo:
A “ Circuit “ conforms to a Design :hurray:

A Loop may define itself byIncluding Unsuspected Elements where current has found Parallel Paths to Earth .

An Earth Loop determines the Effectiveness of the Protective Device

 

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Prospective Short Circuit Current ( PSCC ) this is theMaximum Short Circuit Current . That could flow in a Event of aFault . It is NecessaryInformation for the Correctsizing of Protective Devices .

The Maximum point of Test for ( PSCC ) is at the Service Entrance . 2392-10
Impedance is best Tested at the Farthest Point from theIncoming Supply .