Re-take - Useful Information for 2394 :

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Old-Amber is onRant-mode . When things can go Pear-shape . it’sall in the colour . On-Site .

Protective-earth - green / yellow .
Neutral- ( N ) black . Old .
Neutral- ( N ) blue . New .

Line. single-phase . Red . Old .
Line. single-phase . Brown . New .

Whenwe mix the colour together . Old / New.
3- Phase .
Red - phase . Yellow - phase . Blue - Phase .
Brown- phase . Black - phase . Gray - Phase .

Line. 3 - phase . ( L1 ) Red . Old .
Line. 3 - phase . ( L1 ) Brown . New .
Line. 3 - phase . ( L2 ) Yellow . Old .
Line. 3 - phase . ( L2 ) Black . New .
Line. 3 - phase . ( L3 ) Blue . Old .
Line. 3 - phase . ( L3 ) Gray . New .

Iwas taught when the . harmonization was bought in .

SWA. 3 - core & earth - usage of colour .

Brown- will always be . Permanent - Live .
Blackis always Phase or an Earth - sleeved green & yellow .
Gray is always Phase or a Neutral sleeved Blue.

Brown- will always be . Permanent -Live .
3- core & earth . Black - Switch - Live .
3- core & earth . Gray - Neutral sleeved Blue.
Plus. earth

Thatis the accepted method these days .

How you keep coming across-installations .using ( Black as Neutral ) it is because black used to be the . Neutral . to me that is the very-reason it should not beused as a Neutral - Gray is Neutral .

Onejob I was on . the foreman did say .
Brown- Permanent - Live
Blackis Earth - sleeved green & yellow
Gray is Neutral sleeved Blue.

At the time . Got three-new subbies on the job well the rest is history . (Black as Neutral & Gray as Earth ) Bang . Bang . Bang .
Everyone should use the recognised-colour-scheme . it will make sense for everyone where mains-voltage is concerned .

Whatwent wrong .

Single-phase - Red-phase & Black - natural ← ( Replaced ) Brown-phase & Blue - natural ←

Pointto Note : Cables are marked at the .Interface - ( L1 . L2 . L3 . ) for the Phases . etc .

Schematic of alternative-interface . Old toNew .

Old - Red- ( L1 ) ….. Harmonised - ( L1 ) Brown .
Old - Yellow - ( L2 ) ….. Harmonised - ( L2 ) Black .
Old - Blue - ( L3 ) ….. Harmonised - ( L3 ) Gray .
Old- Black ( N ) ….. Harmonised - ( N ) Blue .

Old - Red- ( L1 ) ….. ( L1 ) Brown . with indentation
Old - Yellow - ( L2 ) ….. (L2 ) Brown . with indentation
Old - Blue - ( L3 ) ….. (L3 ) Brown . with indentation
Old- Black ( N ) ….. Harmonised - ( N ) Blue .

PlainEnglish : Meaning -
Phase - 1 . ( L1)
Phase - 2 . ( L2)
Phase - 3 . ( L3)

Sowe turn back the clocks . IET .

Thechanges in the . United-Kingdom . to adopt “ Blue “ for neutral-conductor .& at least one black for a “ phase-conductor “ in a multi-phase-circuit . could if notaddressed . ( Introduce ) the possibility of ( Confusion ) with the black-conductor . & blue-phase-conductor inexisting-three-phase-distribution-circuit(s)

 

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Useful- junk . BS-1362 - Overload-current-protection

Inpractice-overload-protection is oftenprovided within an appliance . or the load is limited by the nature of the equipment .

BS-1362- cartridge-fuse ( Table - 4F3A ) Flexible-cables .

Wherea BS-1362 - cartridge-fuse is used for . Overload-current .co-ordination complying with . Regulation- 433.1.1. is required between the fuse . & the protected-circuit. compliance with condition (iii) ofthe regulation is afforded where therated-current ( In ) of the . BS-1362 - cartridge-fuse isselected such that it does not exceed ( 0.763 - times ) the current-carrying-capacity ( Iz ) of the lowest-rated-conductor . in the protected-circuit .

Example: consider a . 1.5mm[SUP]2 [/SUP] flexible-cord . with a current-carrying-capacity of 16A. the maximum - BS-1362 - cartridge-fuse rating -permitted is . 16 x 0.763 = 12.2A . Therefore a 10A - BS-1362 - cartridge-fuse would be suitable to give overload-current-protection . tothe 1.5mm[SUP]2 [/SUP] flexible-cord . a13A fuse would not be suitable .

Note : The application of the factor ( 0.763 ) is required because the minimum-fusing-current . ( I[SUP]2 [/SUP] ) for a BS-1362- cartridge-fuse is ( 1.9 / In ) Where the rated-current ( In ) of such a fuse is equal to ( 0.763 / In ) ( I[SUP]2 [/SUP] ) which is equal to (1.9 / In ) is also equal to . ( 1.9 x 0.763 / Iz ) i.e. - ( 1.45 In ) which is themaximum-permitted by condition (iii) of regulation - 433.1.1.

 

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Fault-protection. Protective-devices. For …… ( 2394- A better Understanding )

TT-system .

Foran installation forming-part . of a TT -system . fault-protection must be provided by one or more . RCDs and / or overcurrent -protective-devices . the former being preferred by BS-7671 -Regulation - 411.5.2. refer .

Thereason why RCDs are preferred by BS-7671 in a TT - system . is that the conditiongiven in Regulation - 411.5.4. cannot normally be fulfilled by usingan overcurrent-protective-device . .This condition is .

( Zs x Ia ≤ Uo )

Where:

Zs) . is the impedance . in ohms . of theearth-fault-loop .
Ia) . is the current in amperes ( A ) . causing automatic-operation ofthe disconnecting-device . within thetime specified in table - 41.1. ofRegulation - 411.3.2.2. or asappropriate Regulation - 411.3.2.4.
≤ ) . Means less than or equal to .
Uo) . is the nominal a.c. rms or d.c. line-voltage to Erath in volts ( V )

Thevalue of ( Ia ) for most overcurrent-protective-devices is some tens . hundreds oreven thousands of amperes . given also that the value of ( Zs ) in many - if not most . installations forming-parts of a TT - system is some tens of Ohms . it is easy to see why .overcurrent-protective-devices . cannotgenerally be used to fulfil the condition of Regulation - 411.5.4. - as the product of ( Zs & Ia ) is much higher then ( Uo )

ForRCDs on the other hand . ( Ia ) is equal to therated-residual-operating-current ( IΔn ) which isgenerally only afraction of an ( Ampere ) This makes it relatively easy to fulfil the condition required by Regulation - 411.5.3. which is. ( R[SUP] A[/SUP] x IΔn ≤ 50V )

Where: R[SUP] A[/SUP] ) is the sum of the resistances of theearth-electrode & theprotective-conductor . connecting itto the . exposed-conductive-parts - inOhms .

30mA ÷ 1000 = 0.03A . ( IΔn) which is generally only a fraction of an ( Ampere ) :13:

 

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Plain-English:

Overcurrent-protective-device. the value of ( Ia ) depends on the nominal-current ( In ) & time /current-characteristics of the device .
RCD. ( Ia ) is the rated residual-operating-current ( IΔn )


Overcurrent-protection-requirements.

Protection of the installation against the effects of overcurrent must not be with Chapter - 43 0f BS-7671 :unless another-device provides suchprotection . where an RCD neglected .where an overcurrent-protective-device is selected to provide-fault-protection . the device must also be . so selected as to protect against . overcurrent in accordance . with Chapter - 43 of BS-7671: Unless another device provides suchprotection . Where an RCD is used forfault-protection . then either thedevice must be of a type which . incorporatesOvercurrent-protection . such as an RCBO - BS-EN-61009-1 . or Overcurrent-protection must beprovided by a . separate overcurrent-device . or devices .

 

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Cables :

Current-carrying-capacity. determination of .

- Introduction:
- Tabulated-current-carrying-capacity.
- Rated-factors.
- Worked-example.

1)Introduction :
The size of cable to be used for a particularcircuit can be determined without knowing its current-carrying capacity (I[SUB]z[/SUB]) However, there are situations in which it isnecessary to determine the current-carrying capacity of a cable in order toestablish that the requirements of BS 7671 havebeen met. For example:
- Regulation 433.1.5requires a check to be made that the current-carrying capacity of a cable of a30 A or 32 A ring final circuit supplying 13 A accessories is not less than 20A, and

- where a singleoverload device protects conductors in parallel sharing currents equally, thevalue of current-carrying capacity to be used in Regulation 433.1.1 is the sumof the current-carrying capacities of the parallel conductors (Regulation433.4.1 refers).

a cable(or busbar) does not have only one fixed value of current-carrying capacity;current-carrying capacity depends on a number of conditions, most of whichinfluence the rate at which heat produced by current in the conductors can bedissipated. This topic explains how those conditions are taken into account inorder to determine the current-carrying capacity of a cable.


- - - Updated - - -

2) Tabulated-current-carrying-capacity.
Tables of current-carrying capacity are setout in Appendix 4 of BS 7671, for cables installedin accordance with the installation methods shown in Table 4A2 of Appendix 4.The values of current-carrying capacity given are for continuous loading andare based on the following conditions:
i) . the values ofcurrent-carrying capacity apply to a single circuit .

ii) . the ambienttemperature is 30°C for non-sheathedand sheathed cables in air, irrespective of the installation method, or 20°C for buried cables, eitherdirectly in soil or in ducts in the ground .

iii) . the circuitcable is not surrounded with thermal insulation on all sides at any part of therun .

For a.c. operation, values of current-carryingcapacity apply where frequency is in the range 50 to 60 Hz and take no accountof harmonic content. For other frequencies guidance should be requested fromthe cable manufacturer. The current-carrying capacities forsingle-core armoured cables are for the condition where the armour is bonded toEarth at both ends. Alternative arrangements, if used, may affect thecurrent-carrying capacity of single-core armoured cables and further adviceshould be sought from the cable manufacturer. Rating-factors .

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3) Rated-factors.
Where the conditions in which a particularcable is installed are different to the installation conditions stated in (i),(ii) or (iii) of item 2, the tabulated value of current-carrying capacity hasto be multiplied by one or more rating factors, in order to give the effectivecurrent-carrying capacity of the cable in the conditions in which it isinstalled .
Appendix 4 of BS 7671provides the following rating factors :
(C[SUB]g[/SUB]):for groups of more than one circuit
(C[SUB]a[/SUB]):for ambient temperatures other than 30°C (or 20°C for cables burieddirect in the ground or in an underground conduit (or pipe duct) system).

In addition, Table 52.2 of BS 7671 gives values for a derating factor, Ci , forcables totally surrounded by thermal insulation for less than 500 mm.
The typeof protective device does not affect the current-carrying capacity of a cable.However, in the case of a semi-enclosed fuse to BS 3036,a factor of 0.725 has to be applied as a multiplier to the tabulated value ofcurrent-carrying capacity (I[SUB]z[/SUB]) for thepurpose of

co-ordinationbetween the circuit conductors and the overload protective device, so as toprovide effective protection against overload current (Regulation 433.1.101. refers).


- - - Updated - - -

4) . calculation of current-carrying-capacity .
The current-carrying capacity of a cable iscalculated using the formula in equation (1) :
( Iz = It x Ca x Cg x Ci )

Where .

Iz ) is the current-carrying capacity of a cable for continuousservice, under the particular installation conditions concerned . It ) is the value of current tabulated inAppendix 4 of BS 7671 for the type of cable andinstallation method concerned, for a single circuit in an ambient temperatureof 30°C or 20°C (for cables buried directin the ground or in an underground conduit (or pipe duct) system), as applicable. Ca ) is a rating factor to be applied wherethe ambient temperature differs from 30°C or 20°C (for cables buried direct in the ground or in anunderground conduit (or pipe duct) system), obtained from Table 4B1 or 4B2 inAppendix 4 of BS 7671, as applicable . Cg ) is a rating factor to be applied forgrouping .
Ci ) is a derating factor to be applied forthermal insulation .
Where a cable passes through variousinstallation methods and a variety of environmental conditions, and there isdoubt as to which set of conditions is the most onerous, the current-carryingcapacity for each condition is calculated separately. The current-carryingcapacity to be used for the cable is the lowest of the calculated values.

 

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5) . Worked-example.

A two-core 6 mm2 70°C armouredthermoplastic (pvc) insulated cable operating at 230 V 50 Hz is installed on aperforated cable tray in accordance with Installation Method 31 (ReferenceMethod E of Table 4A2 of Appendix 4 of BS 7671) ina single layer group of four circuits touching. All the cables in the group areof the same type and the same cross-sectional area and are expected to carrymore than 30% of their grouped current-carrying capacity. The ambienttemperature is 40°C.
At one point in the installation, the cablesand the cable tray pass through a layer of thermal insulation. The length ofthe run surrounded by the thermal insulation is 50mm. Find the current-carryingcapacity at that point. Column 4 of Table 4D4A indicates that thetabulated current-carrying capacity a two-core 6mm2cable installed according to Reference Method E is 53 A. Table 4B1 shows that the rating factor for acable with thermoplastic (pvc) insulation at an ambient temperature of 40°C for is 0.87.Table 4C1 indicates that the rating factor fora group of four circuits comprising a single layer of multicore cablestouching, on a perforated cable tray (Reference Method E), is 0.77. Table 52.2 indicates that the derating factorfor a 50 mm length of cable surrounded by thermal insulation is 0.88.Substituting the values in equation (1), weget: ( Iz = 53A x 0.87 x 0.77 x 0.88 = 31.24A )

 

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Thesize of cable to be used for a particular-circuit . can be determined withoutknowing its .current-carrying-capacity ( Iz ) there are situations in which it is necessary to determine the .current-carrying-capacity of a cable inorder to establish that the requirements of BS-7671: havebe Met .

Regulation- 433.1.5. / 433.1.103 . requires acheck
Tobe made that the . current-carrying-capacity of a cable of a . 30A / 32A ring-final-circuit supplying 13A- accessories is not less than . 20A.

Wherea . single-overload-device . protects-conductors in parallel-sharing-currents-equally . thevalue of current-carrying-capacity . to be used in . Regulation - 433.1.1. refer

433.1.:
Every-circuit shall be designed so that a small-overload . oflong duration is unlikely to occur .

433.1.1: The operating-characteristic of thea device-protecting a conductor against overload shall satisfythe following-conditions .
i). The rated-current of the protective-device ( In ) is not less than thedesign-current ( Ib ) of the circuit .
ii). The rated-current of the protective-device ( In ) does not exceed the lowest of the current-carrying-capacities ( Iz ) of any of the conductors of the circuit .
iii). The current ( I[SUP]2[/SUP]) causing effective-operation of theprotective-device does not exceed . 1.45 times the lowest of the . current-carrying-capacities ( Iz ) of any of the conductors of the circuit .

Ring-final-circuit(s).
Aring-final-circuit . supplying socket-outlets & / or fused-connection-unit to BS-1363. has to comply with . Regulation - 433.1.103 . in order to be deemed to meet the requirements of . BS-7671: for protection of the circuit-conductors against overload .

Theregulation requires the circuit to be wired with cable having .copper-conductors . the cross-sectional-area of which is not less than 2.5mm[SUP]2 [/SUP] .except for two-core mineral-insulated-cables . to BS-EN-60702-1 . for which the minimum is . 1.5mm[SUP]2 [/SUP]

The current-carrying-capacity ( Iz ) of the cable has to be not less than . 20A . Etc .

 

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Electricity is Invisible . this in itself makes it dangerous .

2394:

Additional-protection. RCD / RCBO . operation .

Where. RCD or RCBOs . are been used . theymust operate within . Set-parameters .

Basic-test. BS-EN-61557-6 .

Firstly. Rating of RCDs . 30mA . 100mA . 300mA . 500mA .

30mA- or test-instrument is set on .half-rated-trip ( 30 / 15mA ) RCD should not trip .

(30mA ) the test is made with a trip-current set to . 100% . fully-rated-tripping-current. ( x 1 ) thismeans in the eyes of GN-3 . thisfull-tripping-current must not flow for more than ( 2s ) you Megger - 1552 automatically controls this requirement . this requirement is for the ( 2s ) Your reason . for any timed-delayed RCDs .

- ForRCDs Only . ( 2s )
Discrimination- Ability of a . protective-device tooperate . in preference to another protective-device in Series .

Split-load-consumer-unit. ( CCU ) With a . Main-switch . and Two-independently fed - RCDs to allow for “ Discrimination “ against . Unwanted-tripping .
Main-switch- 100A . directly feeding a small number ofcircuit(s) .
The. Main-switch . also . independently feeds two- RCDs each with a number of circuit(s) 2 x 80A - 30mA RCDs .

Aswe all known - RCDs are designed tointerrupt ( If ) fault-currents . they will beat the circuit-breaker . so we have a case of . ( If ) to earth . RCD must trip within - 0.04s . Unless time-delayed-type .whereas circuit-breaker is allowed . 0.4s or 5s . depending on circumstances .

RCDs rated . 30mA or even 10mA . are designed to disconnect the supply within . 40mS at150mA . & within - 300mS . at rated-tripping-current to protect the User .
Forexams’ purposes .
BS- only . 200mS . ( G )
BS-EN- . 300mS . ( G )

5- times the rated-tripping-current of the RCD . 5 IΔn . x5 . must need exceed 40mS .

 

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5- times the rated-tripping-current of the RCD . 5 IΔn . x5 . must not exceed 40mS .

 

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Residual-current-devices( RCD ) in TN-systems .

Fora circuit using an RCD for fault-protection - Regulation - 411.4.5. requires that the product of theearth-fault-loop-impedance ( Zs ) & the rated-residual-operating-current ( IΔn ) of the RCD does not exceed ( Uo volts V )

Ittherefore follows that the maximum-permitted value of ( Zs ) is given by equation . ( Zs = Uo / IΔn Ω )

Where: IΔn ) isthe rated-operating- current of theRCD in amperes .

Equationis as follows : for a circuit ofnominal-voltage ( Uo - 230V ) supplied through an RCD . having residual-operating-current ( IΔn ) of 100mA - ( 0.1A ) the maximum-permitted-value of ( Zs ) produced by this equation is ( 2300Ω ) that is . 230V / 0..1A . 230V ÷ 0..1A = 2300Ω .

 

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Pleaseread - carefully .

Earth-fault-loop-impedance:
Theearth-fault-loop-impedance ( Zs ) at any point in a circuit . is made up of theearth-fault-loop-impedance ( Ze ) of that part of the system external to the installation plus the sum of the impedances . in the Line and protective-conductors in the installation ( Z[SUP]1[/SUP] & Z[SUP]2[/SUP] )

Note : that for circuits rated at . Less than 100A and operating at the supply-frequency not exceeding 50Hz . it is generally considered acceptable to ignore . Inductive-reactance & to replace ( Z[SUP]1[/SUP] & Z[SUP]2[/SUP] ) by ( R[SUP]1[/SUP] & R[SUP]2[/SUP] ) the résistances of the . Line and protective-conductors .

Zs = Ze + ( R[SUP]1[/SUP] & R[SUP]2[/SUP] )

 

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Basic: 2394 :

Protective-equipotential-bonding- ( Bonding for short ) is sometimes confused with .Earthing . even resulting occasionally in the use of the meaninglessterm - “ Earth-Bonding “

Bonding“ is quite distinct form . Earthing in its. Purpose . its general-arrangement . & inmany of the requirements of BS-7671: - That it has to Satisfy .

Thepurpose of ( Earthing ) whereused for the . protective-purposes . within aninstallation . ( is to limit the Duration of the voltages ) :icon_bs:

In ( Contrast ) the purpose of ( Bonding) is to limit the ( Magnitude of the voltages ) :icon_bs:

 

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Distribution-Network-Operator - ( D.N.O. ) :banghead:

2394. What we should NOT - DO .

(i). Responsibility for Earthing :

Where it is necessary for an installation to be . Earthed to meet the requirements for safety . it is the consumer’s -responsibility to ensure the installation is correctly-earthed . This is because . in order to receive a supply . the consumer is required to have an installation that . meets the safety-requirements of regulations . 26(1) & 26(2) of the . Electricity Safety. Quality & Continuity Regulations . 2002 - ( ESQCR )

Thesafety-requirements of . Regulation 26(1) & 26(2) are that the consumer’s installation is so constructed . installed . protected & used or arranged for use so as to prevent . so far is reasonably-practicable . danger or interference with the . D.N.O. or withsupplies to others . a consumer’s -installation that complies with . BS-7671 : is deemed to meet these requirements .

2394: this is the bit you are interested in .
Inpractice the . electrical-contractor . on behalf of the Consumer . should ensure that the installation iscorrectly-earthed . before issuing an .( EIC ) Electrical-Installation-Certificate. or where applicable . a Domestic-Electrical-Installation-Certificate. ( DEIC )

(ii). New-supply-connections - distributors. Obligation to provide an . Earthing-terminal .

D.N.O.- An electricity-distributor is obliged to provide anearthing-terminal for the consumer’s use. in the case of anew-supply-connection at ( LV) Low-voltage . except where this is inappropriate for . safety-reasons . - Regulation - 24(4) of the ( ESQCR refer ) a . distributor is NOT obliged to provide an .earthing-terminal in the case of an . existing-supply-connection . but may bewilling to do so.

Wherean . earthing-terminal is provided by the electricity-distributor . the distributor is responsible for ensuring that the terminal & its . Earthing-connection . tothe D.N.O. are installed & . so far as . reasonably-practicable . maintained so as to prevent danger . &are suitable for the purpose -Regulation - 24(1) of the ( ESQCR )

Nevertheless:- the Installer . or Contractor . on behalf of the consumer . must-ensure that theearthing-terminal is suitable for the requirements . of the installation concerned . & that it is properly-connected to . the Main-earthing-terminal - ( MET ) of the installation .

(iii). Basic-design .
Itis important to establish at an early-stage of the design . either of a New-installation . or an . Alteration or Addition to an Existing-installation . whetheran earthing-terminal for the consumer’suse . is already-available at the ( Service-position ) wherethere is ( NO) such terminal . it should be ascertained whether the distributor is prepared to make one-available .

2394: Do We Need a “ Reason “

Theeffectiveness of any existing-earthing-terminal . & its connections to the D.N.O. should be established . this should be done both by ( Inspection & by Measurement ) of the .External-earth-fault-loop-impedance - ( Ze )

2394: That’s why you get this on Exams . “ Recognition “

Forall installations . the Type of Earthing-systems . needs to be established . & the appropriate-arrangementsmade for connecting the ( MET ) main-earthing-terminal . of that installation with-Earth .

TN-C-S. system whereprotective-multiple-earthing - ( PME ) is provided . means have to be provided for the ( MET ) to be connected by the . D.N.O. to the Neutral of the source of energy . Regulation - 542.1.3. - Installation-earthing-arrangements .

( Whereno earthing-terminal is available at theservice-position ) the D.N.O. is NOT prepared to make-one-available . theinstallation will need to beconnected with Earth . by its own-electrically-independent-earth-electrode . Regulation - 542.1.2.3. in such . circumstances the requirements applicable to a TT- system have to be MET.

The “ Metal-work “ of a “ Gas. Water or other service-pipe must NOT be USED ) inany circumstances . be used as an .Earth-electrode for safety-purposes. this does not-preclude the bonding of such . metalwork whererequired by . Section - 411 . Regulation- 542.2.6. refers

(iv). Cable “ Sheath “ earthing . the contractor should NOT make the connection to the ( Metallic-sheath or Armour )

Where “ Cable-sheath “ earthing is provided ( TN-S system ) the earthing-connection to the distributors-metallic-cable-sheath or armour is made by the . distributor generally-prior to the electricity-metering-equipment beinginstalled . however . there areoccasions when such an earthing-connection has not been made & no earthing-terminal is available .

D.N.O.- Under-No-Circumstances . should the electrical-installation-contractor-clamp. sweat or in any other-way connect the consumer’s Earthing-conductor the distributor’s metal-cable-sheath or armour . the cable is the D.N.O. - property & the contractor is Not Authorized to Interfere with it .

Any-attempt by the contractor to make a ( Connection )to the metallic-cable-sheath or armour could result in an .Internal-fault . between the conductors. of the cable or between the metallic-sheath or armour . & one or more conductors . such-faults can lead to . injuries due to the . explosive-effects of arcing-current & / or by any associated fire or flames .

 

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Facts: BS-951 Earth-clamp . What to look out for .

Firstly: 27/9/12 . N***** - Inspector .because it is an Exiting-installation . he used an .Domestic-electrical-installation-condition-report .

Typeof Earthing-systems . My system is an . TN-S .

D.N.O. - Under-No-Circumstances . should the electrical-installation-contractor ( clamp ) .sweat or in any other-way connect the consumer’s Earthing-conductor the distributor’s metal-cable-sheath or armour . the cable is the D.N.O. - property & the contractor is Not Authorized to Interfere with it .

Someperson has done this . put on a BS-951 . Earth-clamp . the Inspector took a reading . ( Very-Low Ω ) he could move it by using two-finger . :icon13:

Inspector has putthis on . the Certificate . for the Attention of the “ Client “ . So its in Writing .

Metallic-cable-sheath or armour could result in an .Internal-fault . between the conductors .

 

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Electricity is Invisible . this initself makes it dangerous .

Stability of Résistance .

Clause 18.2.4.3. - of BS-7430 :1998 . States .

That an . Earth-electrode-résistance . which is in the excess of ( 100Ω ) may beunstable . although this statement is made in the contents ofan . Earth-electrode used for eathing a Generator. it is equally true for ( say ) the Earth-electrode of aninstallation forming part of a TT- system . However - Note-2 of Table 41.5 in BS-7671:2011: requires the résistance of the . Earth-electrode to be as . Low as practicable . & a value 200Ω . may not be stable .

The Use ofAdditional-Earth-Electrode(s)

Where the résistance of a single-electrode . is Unacceptably-high . a number of such electrodes’ may be connected in ( Parallel ) Clause- 10.2. of BS-7430 :1998 . States . that where a number of ( Vertical-rod or pipes-earth-electrode(s) are so connected . the combined résistance is then practically-proportional to the reciprocal of
thenumber-employed . provided thateach is situated outside the résistance-area . of any other . the same clause-states that . inpractice . it is often assumed that such-rods or pipes are outside of each-others-résistance-area . if the mutual-separation-distance is not ( Less than the driven depth ) the cause also mentions that little is to be gained by a separation beyond twice the driven-depth .


Useful-Junk: 2394: you will not get this on Exams .

Determining. Touch-Voltage - (Ze x Rated . mA / IΔn / RCD )

TT-system - Ze /200Ω < Main-RCD is rated ( 100mA )

200Ω x 100mA ÷ 1000 = 20V ……. ( 100mA x 20Ω ÷ 1000 = 2V /Touch-voltage .