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

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O.S.G.. The use of other methods’ of determining Maximum Demand is Not Precludedwhere Specified by the Installation Designer

FirstlyI make no Apologies for the Way am Writing on any Matters . it can be a first day Apprentice or some one Needing aJog of Memory .
Sowe are all in the Same Boat . “ To Learn “

For the Apprentices . The Day we stop learning is the Day we hang Upour Tool-Bag

CookerDesign Current Calculations

Thefirst thing you have to do is get Your Head around the Calculations !!

(From a Design point of View ) 2392-10

DomesticInstallation Oven(s) & Hob(s) are to be Calculated upon their MAXIMUM LOADING
Startwith a simple Calculation ( An Oven has a rating of 2kW ) 2000

(I = P/V ) Formula … I = 2000 ÷ 230V = 8.70A …. Weare Using the Unit Amps


2392-10/ Domestic Installation Oven(s)

Ovenhas 4 Rings ( 2 x 1kW ) & ( 2 x 1.5kW ) & Grill ( 2kW ) & Oven (3kW )

-Controlled via a CookerSwitch with a Socket outlet .

Asa Designer . we’ll have to Apply Diversity ??

Important )- Diversity allowance to be Applied to the FULL LOAD CURRENT for CookingAppliances .

TheO.S.G. is telling us . Purpose of the Final Circuit fed from theConductors )
O.S.G.Table 1B p/97 – column (3) Cooking Appliances → At the Top of the Page Note : Type ofPremises ( 2392-10 → Household Installations ) Domestic Installation(s)

DomesticInstallation(s) Only O.S.G. - 10A + 30% f.l – Full Load ) of connected Cooking Appliances in the Excess of 10A+ 5A if a socket-outlet is incorporated in the Control Unit . ( C.C.U. ) – 45A + 13A Socket Switched with Neon .

Fromyour point of View ( The First 10A ofthe rated current plus 30% of the reminder ( Plus) 5A if the Control Unit incorporates s Socket.

Calculations)- You bank “ Hold OFF“ the first 10 Amps of the Maximum Load Current )
The10A will be used at the End of the Calculations’

-So your Work out the Total Power Rating & then calculate the Full Load Current

Calculations)- Power = ( 2 x 1 ) + ( 2 x 1.5 ) + ( 2+ 3 ) = 10kW

I= 10000 ÷ 230V = 43.48A … round it up to the first four numbers43.47826087 ( 48 ) 43.48A

UsingDiversity allowance stated ↑↑ ( 43.48A sub 10A = 33.48A )

I= 33.48 x 30 ÷ 100 = 10.04A

Youradding the ( 5A ) for Socket outlet . I = 10A + 10.04 + 5A = 25.04A )- Asa Designer this is your Expected Current Demand .

Remember )- Supply Cables Rated to suit DesignCurrent ( Iz )

 

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RCBOs:

EarthFault Current sensitivity - 30mA - I∆n
Tripping principles employed:- Electronic detection & Actuationfor Residual Current :43:

 

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RCBOs:
Withthe RCBO you have provided Additional Protection . :toetap05:

MCBs.

Requirementsfor Automatic disconnection – maximum disconnection times . Table 41A
411.3.2. – Automatic disconnection in “ Case “ of a “ Fault “

Normalcable ratings relate to continuous service under specified installationconditions .
Cableswill . of course . Carry Higher Currents – for Short Time without suffering permanent damage . Type –B . C . circuit breakers can generally be selected to achieve tripping timesthat will protect the circuit conductors against normal surge currents in accordancewith . BS-7671 .

Thisis more difficult to achieve with type D. devices . :43:
Whichmay “ Require a Lower earth Loop Impedance ( Zs ) to achieve the Operating Times required by Regulation. 411.3.2. – Automatic disconnection in “ Case “ of a “ Fault “

 

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FusedConnection Unit : ( Accessories ) :26:
A fusedconnection unit is defined in BS 7671 as a deviceassociated with the fixed wiring of an installation by which appliances may beconnected, and having provision for a replaceable cartridge fuse link. Fusedconnection units manufactured to BS 1363-4 haveprovision for a BS 1362 cartridge fuse-link andare available with or without an integral switch.
Fusedconnection units have a nominal voltage rating not exceeding 250 V a.c. at afrequency of 50 Hz, and a current rating not exceeding 13 A. Fusedconnection units are classified as: • switched or unswitched • flush, surface or panel-mounting • with or without provision for an outgoing flexible cableor cord • with or without indicator lamp. Fusedconnection units should, amongst other things, be marked with: • number of the British Standard, that is BS 1363• rated current; that is 13 A• rated voltage; that is a.c. not greater than 250 V• nature of supply; a.c. only• incoming (in or supply) terminals• outgoing (out or load) terminals • symbols to identify whichterminals are line, neutral and earth.Theincoming (supply) line and neutral terminals of fused connection units aredesigned for the connection, without special preparation, such as twisting, ofup to three 2.5 mm2 solid or stranded copperconductors, or up to two 4 mm[SUP]2[/SUP] strandedcopper conductors. Theincoming earthing terminal is designed to permit the connection, withoutspecial preparation, of up to three 1.5 mm2 or 2.5 mm2 solid or stranded copper conductors, or of up to two 4 mm2 stranded copper conductors. Theoutgoing (load) line, neutral and earth terminals are each designed for theconnection, without special preparation, of one 1.5 mm2or 2.5 mm2 solid or stranded copper conductor. Whereprovision is made by the connection unit for the fitting of a flexible cord,one copper conductor having a nominal cross-sectional area of 0.5 mm2 up to and including 1.5 mm2 may beused. InstallationThedepth of accessory box should be such that the cables or conductors do notsuffer damage or cause stress to the terminals, (Regulations 522.8.1and 522.8.3refer). BS 1363-4 recommends that the depth of thebox should be such that there is a adequate wiring space. Fusedconnection units are supplied with a cord anchorage to enable the installer toprevent stress and strain on connections, as required by Regulation 522.8.5. SwitchingFusedconnection units are not designed for d.c.supplies and may only switch an a.c. supply. Aswitch fused connection unit is a double-pole device and may therefore be usedwhere it is necessary to switch both the line and neutral conductors, such asmight be the case where a step-up transformer is used (Regulation 555.1.3refers). Anunswitched fused connection unit is not a double-pole deviceand may not be used as such.Table53.2 of BS 7671 permits all fused connection unitsto be used as a means of isolation for TN systems by means of the removal of thefuse-link. Removal of the fuse-link of an unswitched fused connection unit,however, is not suitable for emergency switching or functional switching. The relevant aspects ofTable 53.2

 

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RCBO designated type AC . is an RCBO for which tripping is ensuredfor residual sinusoidal alternating currents . :yesnod:
Whethersuddenly applied or slowly rising . an RCBO designated type ( A ) is an RCBOfor which tripping is ensured both for residual sinusoidal alternating currentsand residual pulsating direct currents . whether suddenlyapplied or slowly rising .

ConventionalTripping Current .
TheConventional Tripping Current of a circuit – breaker is a characteristicrelating to protection of the circuit againstoverload current . it is a specified value of circuit which causes the deviceto trip within a specified time [ known as the conventional time ] at the specified reference temperature . conventional tripping current isusually expressed as a multiple of the rated [ or nominal ] current ( In ) of the circuit breaker .

Residualcurrent breakers conforming to – BS-EN 61009 .all have a conventional tripping current of ( 1.45. In ) The related conventional time is ( 1 Hour ) forcircuit – breakers of rated currents ( In ) up to & including ( 63A ) . & ( 2 Hours ) for circuit – breakers of rated currentsgreater than ( 63A )

Thesymbol generally used for conventional tripping current is ( I[SUP]2[/SUP] ) rather than ( It )as is used in circuit – breakers standards . such as – BS-EN 61009 .
Thisis because “ conventional tripping current “ Means the same as the Term “ Current causing effective operation of theoverload protective device “ used in – BS-7671
Inwhich the symbol ( I[SUP]2[/SUP]) is Used .

 

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UsefulJunk .
( I[SUP]2[/SUP]t ) Characteristics . :carolers:

( I[SUP]2[/SUP]t ) ( Joule integral or “ Energy let-through )
The ( I[SUP]2[/SUP]t ) (Joule integral . also more commonly knownas the energy let-through ) of an RCBO is equal to the integral of the square of the FaultCurrent ( I ) over a given time interval ( t )

 

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VoltageRating :
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TheVoltage rating of the Overcurrent Protective Devicemust be at least Equal to or Greater than the circuit Voltage .

Voltagerating of an Overcurrent Protective Device is afunction of its capability to open a circuit under an Overcurrent condition .
Thevoltage rating determines the Ability of the OvercurrentProtective Device to “ Suppress “ & “ Extinguish “ the internal arcing that occurs during the openingof an Overcurrent condition .

Ifan Overcurrent ProtectiveDevice is used with a voltage rating lower than the circuit voltage . arcsuppression & the ability to Extinguish the arc will be Impaired & .under some Overcurrent conditions . The OvercurrentProtective Device may NOT clear the OvercurrentSafely .
Thevoltage rating is required to be marked on all OvercurrentProtective Device .

 

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RCBO is RCD+ MCB - ( RCBOs combines the functions of an MCB &RCD in One Unit )

RCD do Nothave Overload Protection . it needs a separate MCB t o backupprotect it . So an RCBO is formed :17:

Residual Current Breaker with Overload Protection

 

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ExposedConductive Parts : :juggle2:

Thesesare Metallic Parts of an Electrical System .which can be touched & which are not normally Live .
Thesecan become Live under Fault Conditions .

Metalcasings of “ Appliances“ ( Kettles . Ovens . )
Wiringcontainment systems such as Metal Conduit . Cable Tray & Trunking .

ExtraneousConductive Parts : :juggle2:

Theseare Metallic Parts within a Building . which do not frompart of the Electrical System . but canalso become Live under Fault Conditions .

Water & Gas Pipes .
AirConditioning .
Boilers .
AirDucting System .
StructuralSteel Work of the Building .

Purposeof Earthing .
Byconnecting to Earth all Metalwork not intended to carry current . a path is provided for leakage currentwhich can be detected & interrupted by RCDs . MCBs

CleanEarth return path from the Consumers Earth to the Supply Earth .

TheEarth Fault Loop.
Theconnection at the consumers Earth can be by means of either an Earth Electrodeat the building were the Earth isrequired or may be in the form of acable which runs back to the Generatoror Transformer & is then connected to an EarthPoint .

Generatoror Transformer at the point of supply always has an Earthpoint . a circuit is formed when Earth – Fault currents are flowing .
Ifthese Fault Currents are large enough they will operate the protective device . thereby isolating the circuit .

The “ Star point“ of the secondary winding in a Three phase - Four – wire distribution transformer is connectedto the Earth to maintain the ( Neutral at Earth Potential )

 

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OverloadCurrent :
Occursin a circuit which is Electrical Sound . p/27 -

FaultCurrent :
Isthe level of current flowing at a level up to & including the ( Prospective Short Circuit Current ) being the Maximum Current that the SupplyTransformer is capable of delivering .

Prospective Short Circuit Current ) must also be taken into account whenselecting the Type of Overcurrent Deviceto be installed .

FaultOccurs at the point between Line Conductors of negligible Impedance or betweena Live Conductor & an Exposed Conductive Part . [ it ismore serous than an Overload Current ]

Wherethe Earth Fault Loop Impedance for a circuit exceeds the maximum ( Zs ) for theOvercurrent device . is it permissibleto use an RCD .
Yes:- Regulation – 411.4.4. – 411.5.2.

Asthe Designer of an Installation . am I allowed to rely on the “ RCD element of an RCBO :49:“ to provide for Fault Protection in order toallow for Loop Impedance values greater than in table 41.3.

Yes:- so long as all the other ApplicableRequirements of the 17[SUP]th[/SUP] Edition ( asAmended ) are met . includingthose for Protection against Overload & Short Circuit .

Stillworking on an Electrical Installation that was designed before “ Amendment 1“ to BS-7671:2008 came into full Effect on ( 31 December 2011 ) to which requirements should theinstallation be ( Inspected . Tested . Verified & Certificated)

AnInstallation Designed before ( 31 December 2011 ) should be ( Inspected . Tested . Verified & Certificated) to BS-7671:2008 .That is to the Origin Version prior to the “ Amendment “

 

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MotorWindings : :49:

A Motorcan be manufactured with the Windings Internally Connected. if this is the case & there are Three Terminal Connections in the Terminal Block labelled .
( U . V .& W . ) you wouldexpect the Motor Windings to be connected in ( Delta Configuration)

MotorWindings with Delta Connection :
Theremay be four connections in the Terminal box labelled ( U. V . W . & N ) if this is the case . The Windings would bearranged to give a ( Star Configuration)

MotorWindings with Four Connections :

Alternatively. the Terminal block may contain Six Connections . ( U1. U2 . V1 . V2 . W1 & W2 . ) This is used where both Star & DeltaConfigurations are required .
TheTerminal connections can then be reconfigured for either Star or Delta .

MotorWindings with Six Connections :

• U1 . U2
• V1 . W2 .
• W1 . V2 .

 

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Cable Selection – Overview .

Requirements .

● The cable supplying the Load should be able to carry the current to the Load safety . After taking into account Diversity . without the cable overheating – ( 523 . App – 4 )
● Protective Devices should be able to provide protection for the cable from excess current ( 432.433. )
● The Length of the cable is Limited to ensure adequate voltage supplies the Load . ( 525 )
● Maximum Loop Impedance of a circuit is Low enough to ensure Disconnection within times specified by Table – 41.1.

● 0.4 – ( TN- ) 0.2 – ( TT ) for all circuits < 32A
5 sec . Disconnection time for Sub-mains distribution circuits

 

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612.7. Earthelectrode résistance .

Wherethe earthing system incorporates an earth electrode as part of the installation. the electrode résistance to Earth shall be measured .

NOTE : Where a measurement of( Ra ) is NOT practicable the measured value ofexternal earth fault loop impedance may be Met . :13:

 

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612.13. Functional testing :

Wherefault protection and/or additional protection is to be provided by RCD . theeffectiveness of any test facility incorporated in the device shall be verified.

612.13.2. Equipment . such as switchgear and controlgear assemblies . drivers . controls and interlocks. shall be subjected to a functional test to show that it is properly mounted .adjusted and installed in accordance with the relevant requirements of theseRegulations . :13:

 

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2392-10-

612.14. Verification of voltage drop .

Whererequired to verify compliance with Section 525 . the following options may beused .

(i)The voltage drop may be evaluated by measuring the circuit impedance .
(ii)The voltage drop may be evaluated by using calculations . for example . bydiagrams or graphs showing maximum cable length v load current for differentconductor cross-sectional areas with different percentage voltage drops forspecific nominal voltages . conductor temperatures and wiring systems .

NOTE : Verification of voltage drop is not normally requiredduring initial verification . – Still the Same :13:

 

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Certification& Reporting :

631.1.

Upon completion of the verification of a new installation orchanges to an existing installation . an Electrical Installation Certificate .based on the model given in Appendix 6 . shall be provided . Such documentationshall include details of the extent of the installation covered by theCertificate . together with a record of the inspection . the results of testingand a recommendation for the interval until the first periodic inspection . :rockon: