2394: As the RCBO . is here to stray . Somefacts . ( iii )
Overcurrent - ( Instantaneous-tripping ) RCBOs- conforming to RCBO-61009-1. areclassified into three-distinct - Types . B . C . D . according to their ( Instantaneous-tripping-overcurrent ) Instantaneous-tripping-overcurrent is the ( Minimum-value of current-causing an RCBO ) tooperate . automatically without .intentional-time-delay . That is - within a time not exceeding - 0.1s or 100mS .
RCBO-61009-1. permits - Instantaneous-tripping to occur at a value of ( Overcurrent ) expressed as a . multiple of the rated - or nominal-current ( In ) within a range of values for each RCBO - type.
2394: As the RCBO . is here to stray . Somefacts . ( iv )
Although. manufactures may - design RCBOs tooperate ( Instantaneously ) within smaller-ranges of current . falling within the. Standard-ranges .
RCBO- B . Standard-range . B - above . ( 3 - In ) up to &including ( 5- In ) . RCBO- C . Standard-range . C - above . ( 5 - In )up to & including ( 10 - In ) . RCBO- D . Standard-range . D - above . ( 10 - In) up to & including ( 20 - In ) .
Regulation- 531.2.8. - Fault-Currents . BS-7671: requires that where an . RCD ( Without ) integral-overcurrent-protection. is used for . fault-protection bymeans of ( ADS )
Itis to be . verified that the RCD . canwithstand without damage the . ( Thermal & Mechanical stresses ) in the case of a fault-occurring on theload-side of the point at which it isinstalled .
Minor-Works: Additions& alterations to an installation . that do not extend to the provision of anew-circuit .
BS- Examples - include the addition of socket-outletsor lighting-point . to an ( Existing-circuit ) therelocation of a light-switch . Etc .
Accessories : The replacementof an accessory . such as asocket-outlet . ceiling-rose . lighting-switch . on a Like-for-like . may be regardedas . maintenance rather than . installation - work .
Theseundertaking such . maintenance-work . should carry out essential - Inspection & Testing .to ensure that the ( Accessory ) when replaced . is safe to use before beingput into service .
Testing- As a minimum . test to confirm that . shock-protection . has been providedare essential . These essential-tests . - Test- to establish the . Earth-fault-loop-impedance . the value of which needs tobe checked against the . characteristics of the - Protective-device . -Polarity . - Wherean . RCD . is used for fault-protection or additional-protection . Thecorrect-operation of the RCD .
Certification: Arecord of the replacement of the ( Accessory ) includingthe results of Inspection & Testing . should be issued for the benefit of both thePerson Ordering the Work . & the Installer .
TheMinor-work-certificate . is the most appropriate from for providing such a record.
-number of the . BS - that is BS-1363 . -rated-current . that is 13A . -rated-voltage . that is ( A.C. ) notgreater than - 250V . -nature of supply . ( A.C. - only ) -incoming ( in or supply ) terminals . -outgoing ( out or load ) terminals . -symbols to indentify . which terminals are . Line . Neutral . & Earth .
Installation:
Thedepth of ( Accessory- box ) shouldbe such that the cables . or conductors do not suffer damage or cause stress .to the terminals . Regulation- 522.8. and 522.8.3. - refer . BS-1363-4 . recommends that the depth of the box . should besuch that there is . adequate-wiring-space .
Fused-connection-units. are supplied with a cord-anchorage to enable the installer . to preventstress & strain on connections . asrequired - regulation - 522.8.5.
Switching:
- Fused-connection-units. are ( Not )designed for ( D.C.) supplies and only switch an ( A.C. - supply ) - Switched-fused-connection-unit. is a double-pole-device . & may therefore be used where it is necessary . to switch boththe . Line & Neutral-conductors . such as might be the case where . astep-up-transformer is used - Regulation - 555.1.3. -an ( Unswitched-fused-connection-unit is Not ) a double-pole-device . & may not be usedas such .
Table- 53-2 . of BS-7671: permits all . fused-connection-units . to be used as a ( Means of Isolation ) forTN - systems by ( Means ) of theremoval of the . ( Fuse-link) Removalof the . fuse-link . of an . ( Unswitched-fused-connection-unit ) however . is not suitable for . emergency-switching . or Function-switching. the relevant aspects - 53.2
Regulation- 610.4. requires that compliance withBS-7671 . is verified for every . addition or alteration . Chapter - 63contains requirements for .Certification & Reporting . inrespect of electrical-installation(s) .
Regulations- 631.1. and 631.3. which requires an . Electrical-Installation-Certificate - ( EIC ) . or Minor-Electrical-Installation-Works-Certificate . ( MEIWC ) Givingdetails of the extent of the installation covered by the Certificate . together with a record ( Schedule ) of theinspection & the results ( Schedule ) of testing . be issued following the .Verification of the installation . ( New or Changes to an Existing-installation ) required by . Chapter - 61 of BS-7671.
Domestic-Electrical-Installation-Certificate. may be used. under appropriate-conditions . ( As an alternative) to the ( EIC ) - Electrical-Installation-Certificate .
(MEIWC ) Where . Minor-Electrical-Installation-Work. does not include the provision of a new-circuit . Minor-Electrical-Installation-Works-Certificate. may be issued for each-circuit . to which an addition or alteration is made . instead of an . ( EIC ) Electrical-Installation-Certificate. Or (DEIC ) Domestic-Electrical-Installation-Certificate . regulation - 631.3.
An- ( EIC ) Electrical-Installation-Certificate . would be issued for . most additionsor alterations .
Where- an addition or alteration does-not include the . provision of a new-circuit . (MEIWC ) Minor-Electrical-Installation-Works-Certificate. may be issued instead of an - ( EIC) Electrical-Installation-Certificate .
(MEIWC ) are . - Addition of a socket-outlet . to a Ring or Radial-final-circuit . - Workcarried out on a . lighting-circuit .including an . alteration to the switching-arrangements . - Addition of a . lighting-point . - The replacement of an . Accessory or Luminaire.
Note: that the provision of a new-circuit . the ( EIC ) or where appropriate . & the ( DEIC ) must be used . Not the ( MEIWC )
Any. defects or omissions in the work covered by the . ( EIC) or the ( MEIWC ) must be made good before . the Certificate isissued - regulation - 632.4. & before the Installation is Energized . Thisone always comes up -&-s
(DEIC ) may be used under-appropriate-conditions. as an alternative to the ( EIC )
Allthe work of the . alterations or additions is to be covered by theCertificate-issued & all requirements of . regulation-group - 631 . & 632. for the issue of an ( EIC ) or a (MEIWC ) including those referred to . are applicable- Regulation - 633.1.
First-part :- A “Conductive-part “ Second-part:- “ Liable to introduce a potential . generallyEarth potential “
-Source of potential . -The question of whether the . potential is liable to be introduced . -Accessibility .
Extraneous-Conductive-Parts. are referred to extensively in BS-7671: Mostlyin relation to protective-equipotential-bonding . but also with regard toprotective-measures . such as placing out of reach &non-conducting-location .
Thedefinition - of an extraneous-conductive-part - given - part 2 . “ Extraneous-Conductive-Part “ A conductive-part . liable to introduce a potential . generallyEarth-potential . & not forming part of the electrical-installation “ 2394: decide whether or not a specific item . of metalwork or other-conducting-material . is an extraneous-conductive-part .
BS-7671:2011: Understanding the definition . Youare considered in the following - Three-parts. - First-part . “ Aconductive-part “ -Second-part . “ Liable to introduce apotential . generally Earth potential “ -Third-part . “ & not forming-part of the electrical-installation “
“ A conductive-part “ inmost-situations . only parts made of metal needs to be considered as “ Conductive-parts “
“Liable to introduce a potential . generally Earth potential “ in considering this part . of the definitionit is convenient to deal firstly with the . source of the potential & secondly with the question. of whether the potential is liable to be introduced .
Sourceof Potential . as indicated by this part of the definition . Thepotential generally of interest is “Earth-potential “ theelectrical-potential of the .conductive-mass of Erath . which isconventionally ( Taken as Zero ) This normally the potential of an item such as a “ Metal-pipe-buried in the ground “ or astructural-steel-column fixed in a foundation in the ground .
Other-potentials- however . may also be of interest .Example . Metalwork of anon-electrical-service or other-facility entering the premises from another-building . in which it may be “Bonded-to-the-main-earthing-terminal “ could under . earth-fault-conditions in theinstallation in that building . be ( At ) apotential-related to the fault
“ Thequestion of whether the . potential is liable to be introduced “ Electric-shock- to a person or livestock is caused bya potential-difference - ( a Voltage ) applied between two or more parts of the body . therefore . in order to be involved incausing electric-shock . a Conductive-part . must not only be at a ( Potential ) it mustbe liable to introduce that potential .to a person . or livestock . who may .simultaneously be in contact with . Another-potential .
Suchother potentials . may include the ( Potential ) of . - Exposed-conductive-part . - Extraneous-conductive-part - where liable to be at a potential . otherthan that of the . conductive-part under consideration . - Bare-live-part .
“ Accessibility“ Generally Inorder for a ( Conductive-part to beliable to introduce a potential ) to a person or livestock . in contact with thepotential . of any of the parts listed - above . theConductive-part must be ( Accessible ) tobe ( Touched ) by a person - or livestock .
Thepossibility of a conductive-part . introducing a potential to a person .through metalwork with which it is in Electrical-contact . should not be overlooked . this could occur where such metalwork is touchedby a person . who is also in contactwith an . Exposed-conductive-part . or an . Extraneous-conductive-part . or Metalwork incontact with one of these .
Third-part - & not forming-part of the electrical-installation “ this-part Definition- limits . Extraneous-conductive-parts . to being items which . do not form-parts ofthe electrical-installation .
Therefore. items which may be . Extraneous-conductive-parts . includethe following . referred to inregulation - 411.3.1.2. with ( Regard ) to . main-protective-equipotential-bonding.
Inorder to decide whether or not a specific item is an . Extraneous-conductive-part. the following should be considered . ( is the item-conductive ) is the item-conductive ) inorder to be - conductive . an item mustbe formed of ( Metal or some Other-conducting-material )
“Liable to introduce a potential . generally Earth potential “ That is to say . Isthe item liable to introduce a potential . to a person . or livestock . who may be in contact with another-potential.
Tointroduce a potential . to such a person . the item must not only be at a ( potential ) .but must also be able to ( Introduce ) thepotential . Introductionof the ( Potential) requires two-things . First- is that the item - or a conducting item in electrical-contact with it . mustbe ( Accessible) to be touched by a part of such apersons-body . Second- is that any electrical-résistance .through which the item is connected tothe source of it’s potential - such-résistance-connecting an item with Earth . must not be so great asto prevent the potential beingintroduced to a person - due to voltage-drop in the résistance .
Isthe Item-part of the electrical-installation: Onlyan item which does not form part . of the electrical-installation concerned maybe an . extraneous-conductive-part . however . items . serving a purely .non-electrical-purpose . within the installation should not be . ruled-out. Moreover. items forming part of another-installation are not precluded .
An- Unfused-spur . - Cable-branch of a 30A or 32A . ring-final-circuit . supplying 13A - (Accessories to BS-1363 ) may be wired with . thermoplastic - ( PVC ) or thermosetting-insulated-copper . Line andNeutral-conductor(s) as small as 2.5mm[SUP]2 [/SUP]. as is the casefor the ring-itself - Regulation - 433.1. refer .
2349:- Installerswill recognise that the . Current-Carrying-Capacity . of 2.5mm[SUP]2 [/SUP] thermoplastic-insulated-conductor(s)is . generally less than the . 30A or 32A rated-current of the . fuse orcircuit-breaker . of the ring-circuit . depending on the ( Cable ) installation--method & the Ambient-temperature . Etc The. same is true for 2.5mm[SUP]2 [/SUP]. thermosetting-insulated-conductor(s) such as in . 6242B - cable where operated at a conductor-temperature . not Exceeding 70°C as isusually necessary to suit the ( Temperature-rating-of-Accessories ) ◄◄
Consequently. the circuit-protective-device . cannot protect the . Unfused-spur-cable. against . ( Overload-current ) Thus. if the spur-cable becomes Overloaded .damage may be caused to the ( Insulation . Sheath . Surroundings of the Cable) or to connected-Accessories . due to the . temperature of the . conductors of the cable rising to above theirrated-value - such as 70°C . forthermoplastic-insulated-cables .
Ring-final-circuit.
2394: Why is a 2.5mm[SUP]2 [/SUP] conductor-size-allowed .
Thequestion therefore arises of why is it permittedto use 2.5mm[SUP]2 [/SUP] thermoplastic-insulated . Line &Neutral-conductors to wire an . Unfused-spur . Theanswer mainly lies in . condition (b) of the following . three-conditions that apply tothe wiring of a . ring-final-circuit .
(a). The Current-Carrying-Capacity - (Iz ) of the circuit-cable . includingthat in any . unfused-spur . must be not less than - 20A . taking into account the conditions of -Ambient-temperature . grouping & thermal-insulation . along the cable-run - Regulation - 433.1.5. refer .
(b) . The circuit must be designed such that the .load-current . in any part of the circuit . including - Unfused-spurs . is unlikelyto exceed the Current-Carrying-Capacity ofcable for long-periods - typically . ( 1 / hour ) under the intended-conditionsof use - Regulations - 433.1.5. refers .
(c). A conductor -size-larger than . 2.5mm[SUP]2 [/SUP] may berequired for reasons of . ( Vd ) -voltage-drop . Section - 525 refers .
Note : for an . Unfused-spur . compliance with . condition(b) leads to compliance with indent (ii) of . Regulation- 433.3.1. . This allows the . Omission of protection against . Overload for the .Spur-cable . on the basis that the .characteristic of the load on that cableare such that its conductor(s) are unlikely to carry-overload-current’s .
Sofar as an . Unfused-spurcable . is connected . condition(b) willgenerally be met by . designing the installation so that conditions (1) and (2) below are both met .
(1) . The spur-feeds-only. -One-single or one twin 13A - socket-outlet . - Oneitem of . fixed-equipment , supplied through a fused-connection-unit . - More than one item of fixed-equipment . eachitem being supplied through a separatefused-connection-unit . provided the total-load . on the spur-cable -taking-into account diversity . does notexceed the . Current-Carrying-Capacity . of the spur-cable .
(2) . Wherenecessary . to avoid to much-load being plugged-into a 13A - socket-outlet supplied through the spur-cable . a sufficient-numberof other - 13A socket-outlets . not suppliedthrough the same spur-cable . are installedin suitable-positions in the vicinity ofthe . socket-outlet .
Similarly. the question arises of will the . 30A or 32A . protective-device protect the 1.5mm[SUP]2 [/SUP] . the circuit-protective-conductor - ( CPC ) 2.5mm[SUP]2 [/SUP] of a BS-6004 flat-twin & Earth-cable . in an . Unfused-spur . against . Earth-fault-current . ( If )
Protectionis checked using the . Adiabatic-equation . in Regulation - 543.1.3.
Itcan also be shown that any of the . 30A or32A . Fuses or Circuit-breakers or RCBOs listed in Regulation - 433.1.5. will protect the - Bare - 1.5mm[SUP]2 [/SUP] CPC of a flat thermoplastic - orthermosetting-insulated & sheathed-twin-&-earth . unfused-spur-cable. provided the. Earth-fault-loop-impedance ( Zs ) at the point supplied by the spur . is withinthe . Maximum-value required - BS-7671: for a ( Disconnection-time - 0.4s )
Whathappens is a - Short-circuit-occurs . inthe Unfused-spur-cable . Inother-words . will the . 30A or 32A circuit-protective-device .protect against . thermal-damage . inthe event of a . Short-circuit-involving the . Line & Neutral-conductor(s) of the Spur .
Protectionis checked using the . Adiabatic-equation . in Regulation - 434.5.2.
Itcan be show that any of the . 30A or 32A - circuit-breakers . RCBOs . or fuses listed in . Regulation - 433.1.5. for use as the . protective-device . for aRing-Final-Circuit will protect . 2.5mm[SUP]2 [/SUP] thermoplastic - or thermosetting-insulated-conductor(s) of an - Unfused-spur. Cable against . Short-Circuit-Current .
Protective-earthing which is provided for . reasons of Safety . we lookat - Functional-earthing & at earthing for combined-protective & functional-purposes .
- Functional-earthing . by Definition - part 2 . p.28
Functional-earth. earthing of a point or points in a system or in an installation or inequipment . for purposes other than ( electrical-safety ) suchas for proper-functioning of electric-equipment .
Example- of Functional-earthing is alow-noise-earth . sometimes called a “ Clean-earth “ Thisis an . earth-connection in which the . level of conducted or included - interference from . External-sources does not produce an unacceptable-incidenceof malfunction in thedata-processing or similar-equipment to . which it is connected.
Earthingfor Combined-protective & functional-purposes . Asthe term suggests . Earthing for combined-protective & functional-purposes has two . main-purposes .
- Protective-purpose ( for reasons of electrical-safety ) - Functional-purposes ( to facilitate the proper-functioning of electrical-equipment )
Examples- of the use of conductor(s) for combined-protective & functional-purposes. - The protective-conductors of a circuit in an installation - supplyingequipment . having-high-protective-conductor-current . Regulation-group - 543.7. refer - Protective-conductor that is also used for . low-noise earthing in an installation .
Itis important to recognise that where . earthing is provided forcombined-protective & functional-purposes . the requirement s for . protective-measuresmust take-precedence . Regulation -543.5. where the requirements for protective & functional-purposes .cannot ( Both) be achieved by the same-earthing-system . separate-earthing-systems should be employed .
CoP:- let’s categorises . Electrical-equipment . Forsome . electricians that maybe thingabout . Pat-testing . it’s a start .
- Stationary-equipment: Either-fixed in position or heavierthan . 18kg & without a carrying-handle . such as - refrigerators & washing-machines . -Information-technology-equipment : Suchas computers . fax-machines . modems .telephones . printers . Etc . - Movable - Or transportable-equipment : Lighter than 18kg & not fixed . such as . electric-heaters . &equipment with wheels . castors . Etc to facilitate movement to perform. its intended-use . such as . compressors & air-conditioning-units . - Portable-equipment : equipment that is lighter than 18kg & moved in operation . such .as fans . table-lamps . kettles . toasters. vacuum-cleaners . -Hand-held-equipment : portable-equipment. intended to be held in the hand during normal-use . example . hairdryers . power-drills . soldering-irons .
Portable-appliances provided by Landlords.
Inaddition to the above - categories . the frequency & range ofInspection & testing will dependupon whether the equipment is - Class -1 . that is required to be earthed . Class- 11 . equipment having double or reinforced-insulation .
2394: Devices for Protection against Overcurrent .533 . p.139
533.1.- A device for . protection againstOvercurrent . shall comply with . one ormore of the following :
-BS-88 . series . -BS- 646 . -BS-1362 . -BS-3036 . re-rewireable-fuse . - BS-EN-60898-1 & 2 . - BS-EN-60947-2& 3 . - BS-EN-60947-4-1. - 6-1 & -6-2 . - BS-EN-61009-1 . using to . circuit-breaker side of it .
Stillout there . BS-3036 .
533.1.1.3. : Afuse shall preferably be of the . cartage-type . Where a semi-enclosed-fuse .is selected it shall be fitted with an . element in accordance with the .manufactures-instructions . if any . in the absence of such instructions . itshall be fitted with a . single-element . of tinned-copper-wire . of theappropriate diameter specified in . table - 53.1.
Therestill out there . 5A . 15A . 20A . 30A . 45A . Re-wireable. semi-enclosed-fuses .
Older-installations.
Wherean installation having . rewireable-fuses . is periodically-inspected . altered or extended . the inspector - as applicable . will have to consider whether . BS-7671: still allows such . fuses to be used .or whether they should be replaced .
Inspectorsshould assess the degree of risk . to help him / her judge if there are any special-circumstances that would make rewireable-fuses . unacceptable in particular installation .
Fusing-factor. BS-3036 . allows a rewireable-fuse to have a . fusing-factor -the ratio of theminimum-fusing-current . to therated-current . of up to ( 2.0 ) this value is higher than the typical-value of . ( 1.25 ) to (1.6 ) for other types of device .commonly used for overload-protection . Suchas fuses - BS-88-3 . BS-88-2 . & Circuit-breakers to . BS-EN-60898 . depending on the type . Etc . 1.45 ÷ 2 = 0.725 - semi-enclosed-fuses
-&-sloves to use TT- systems . in Exams. ( Isolation)
TT- system . is always that - Both the Line & Neutral-conductor(s) should be Isolated
Why. in TT - system . the incoming-neutral-conductor. cannot reliably be regarded as being at Earth-potential . this means that for . TT- supplies . a Double-pole-device . Switching-device. which disconnects the . Line &Neutral-conductor(s) . Etc must be used as the - Means of Isolation .
TN - systems . 2394 : Question . Ask your self this .
Will make this an open ended Question .Why . did yourRCD trip when I was working on acircuit-isolated by a ( Single-pole-device )
Electricians . are sometimes puzzled by a problem that canarise when working on . a final-circuit that has been ( Isolated ) by means of a . single-pole .outgoing-protective-device . circuit-breaker-BS-EN-60898 . inthe ( CCU ) such that only the .Line-conductor is ( Isolated )
At One-stage . you’ll do one of thefollowing . Kid you not !!
• You’ll let or make - the Neutral & Protective-conductor of thecircuit ( Touch ) or suchas by twisting-them-together to push the cable into . the entry-hole of an . Accessory-box .
• Let the Neutral-conductor . touch-metalwork . such as a water-pipe . thatis connected to the . ( MET ) main-earthing-terminal . of the installation by a ( Bonding-conductor )
• Touch the Neutral-conductor with your hand .
You poor customer . TV has just gone off . “ Notme “ at this point . keep smiling . :vanish:
On investigating . you find that theRCD - protecting the circuit has tripped . what you were working on was ( Isolated !!! )
Question - 2394 : Did you do something Wrong . it must be “ Nuisance-tripping “ doyou still have faith in .Addition-protection - RCD
As you reducing you faith in RCDs . 2394 . What happened . was that . when the Neutral-touched the Protective-conductor . or other things connected to Accessories .
Youhave - Created a Parallel-path . for a proportion of any Neutral-currentsflowing in the . other-unisolated-circuits . this current . bypassed the current-balancecore of the RCD . causing the RCD to trip .
2394:Only-way to avoid the . tripping-problem is to work with the circuit - Isolatedby a Double-pole . in our case . the Neutral-conductor ofthe final-circuit - disconnected ( Single-pole ) at the (CCU )
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