***Useful Information For The Working Sparky***

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17th EDITION REQUIREMENTS FOR THE TESTING OF RCDs :

The 17th Edition of the Wiring Regulations (BS 7671: 2008) will introduce a number of new requirements for the installation of
RCDs, therefore it is timely to look at the requirements within the17th Edition for verification of RCDs. The continuing effectiveness of these RCDs needs to be confirmed periodically. This article discusses the verification required where RCDs are
used to provide automatic disconnection of supply in the event of a fault and additional protection. It should be stated at this point that the 17th Edition does not introduce any significant changes in the requirements for the testing of RCDs even where they are installed to provide automatic disconnection in the event of a fault ,

Use of RCDs to achieve automatic disconnection in case of a fault :
411.3.2.1 requires (in most cases) that a protective device shall interrupt the supply to a line conductor of a circuit or equipment in the event of a fault of negligible impedance between said line conductor and an exposed conductive- part or a protective
conductor for the circuit or equipment within the appropriate required disconnection time. A disconnection time of 5 seconds
for distribution equipment and final circuits of rating exceeding 32A is permitted by 411.3.2.3. Similarly, a disconnection time of 1 second for distribution equipment and final circuits of rating exceeding 32 A is permitted by 411.3.2.4. : 411.3.2.2 states that the maximum disconnection times of Table 41.1 shall be applied to final circuits not exceeding 32 A. Table 41.1 gives the maximum
disconnection times for final circuits not exceeding 32 A of varying nominal voltages forming part of an
installation having either TN or TT system earthing. These disconnection times may be met by the use of fuses, circuit breakers (formerly known as MCBs) or RCDs. is used to meet the requirements of 411.3.2.2, that is, to provide the required disconnection time, the maximum values of earth fault loop impedance in Table 41.5 may be applied. The maximum permissible earth
fault loop impedances (Zs) to ensure RCD operation for non-time delayed RCDs protecting final circuits not
exceeding 32 A are given in Table 41.5, a new table introduced in the 17th Edition, which is reproduced below.
Where an RCD is employed to achieve the disconnection time required by Table 41.1, it is necessary to satisfy
ourselves that the maximum earth fault loop impedance (Zs) stated for a particular sensitivity of RCD in Table 41.5 is not exceeded in the circuit to which they provide protection. This is in effect the same procedure that we applied in earlier editions where fuses
or circuit breakers were used to achieve the necessary disconnection time and indeed continue to apply for fuses and circuit breakers in the 17th Edition. Regardless of which type of protective device is used to achieve the disconnection times required by Table
41.1, whether fuse, circuit breaker or RCD, there is no requirement to confirm that the required disconnection time can be achieved by testing the protective device. Rather, we confirm that the earth fault loop impedance of the protected circuit does not exceed the relevant tabulated maximum earth fault loop impedance for the type / sensitivity of the protective device intended to provide
the required disconnection time.

Maximum earth fault loop impedance (Zs) to ensure RCD operation in accordance with Regulation 411.5.3 for non-delayed RCDs to BS EN 61008-1 and BS EN 61009-1 for final circuits not exceeding 32 A

( 411.3.2.2 ) 230v
TN- Systems : The maximum disconnection time stated in table 41.1 shall be applied to final circuits Not-Exceeding 32Amp
( 411.3.2.3 )
TN- Systems : in a TN-system, a disconnection time Not exceeding 5sec is permitted for a distribution circuit and for a circuit Not covered by Regulation 411.3.2.2 ,
( the table you require TT-systems 41.5 p-50

Resistivity :

Double cable length – Double Conductor Resistance to 1.6 Ω, but halve insulation resistance to 50 MΩ
Halve . CSA – double conductor resistance to 1.6 MΩ, but insulation resistance is unaffected and remains 100 MΩ

Resistivity :
A twin cable has a Phase to Neutral résistance value of 100MΩ and an individual conductor résistance values of ( 0.8Ω )
Determine the values if the cable ….
(a) was double in length ,
(b) length as the same but the conductor cross-section areas was halved ,
Conductor Résistance
This is a function of the resistivity of the conductor material :
In other words , résistance is directly proportional to length and inversely proportional to area ,
So doubling length or halving the area will both double résistance , try it with some values ,

Let’s say the original length is 40m and the area is 2mm2 . The equation then , is :
0.8 = p x 40 ÷ 2 ( 0.8 x 2 ÷ 40 = 0.04 ( ignoring the units )
Double length : R = 0.04 x 80 ÷ 2 = 1.6Ω : / Halve area : R = 0.04 x 40 ÷ 1 = 1.6Ω
Beware the question that asks what happens if the diameter is varied , because is proportional to the diameter squared ,
Doubling diameter will increase the area by four times; halving the diameter will quarter the area ,
Insulation Résistance :
The Insulation between two conductors is considered to act as a ( Series of many high résistance in parallel )
Résistance ( because of the greater number of apparent parallel paths ) Taking the insulation résistance of the original length as R1 ,
Adding an identical extra length is like adding a second R1 in parallel , So :

…….... 1 ……..... 1
….. ─── = ─── so R1 = Rtotal = 100MΩ ( in the first instance )
……....Rtotal … R1

………………………. ……......... 1 ……..... 1……...... 1……...... 2
With double the length ….. ─── = ─── + ─── = ─── so new Rtotal = 50MΩ
…………………………......... Rtotal …... 100 ….... 100…...... 100

Changing the conductor CSA , should have no effect on the insulation résistance for the same value of voltage applied ,

Insulation Resistance Values
0.00 MΩ = dead short :
0.08 MΩ = low insulation resistance fault :
>200 MΩ = healthy circuit :

Notes: ensure all neon's are removed or isolated before commencing testing, as these will make test results appear low during insulation resistance testing.

Notes: Neon's will cause false readings, as will emergency or discharge lighting, so ensure these are all disconnected prior to commencing tests.

Prospective short circuit current (PSC) testing

The prospective short circuit or fault current at any point in an electrical installation is the current that would flow in the circuit if no circuit protection operated and a complete (very low impedance) short circuit occurred. The value of this fault current is determined by the supply voltage and the impedance of the path taken by the fault current. Measurement of PSC can be used to check that protective devices within the system will operate within safety limits and as per the safe design of the installation. PSC is normally measured between the phase and neutral at the DB or at a socket outlet.

 

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The 17th Edition of the IEE Wiring Regulations (BS 7671:2008) places much greater emphasis on the use of RCDs.
It is well-known that most circuits feeding 13A socket-outlets now require RCD protection but equally important are requirements for additional protection of wiring concealed in walls or partitions, which includes lighting circuits and their concealed switch wiring. This means that the vast majority of domestic circuits, power and lighting now require 30mA RCD protection :
There are also important requirements to minimise nuisance tripping and collateral risks due to a tripped RCD affecting other circuits, such as lighting.

Socket-outlets :
Rule 411.3.3 calls for additional protection by means of a 30mA RCD for all socket-outlets with a rated current not exceeding 20A for use by “ordinary persons”. The only exceptions allowed are for socket-outlets for use under the supervision of skilled or instructed persons (e.g. some commercial/industrial locations) or a specifically labelled socket provided for connection of a particular item of equipment (such as a freezer). :
This rule clearly covers the vast majority of domestic circuits feeding 13A socket-outlets, and any other socket-outlets (5A, 15A etc.) up to 20A. There is also a requirement for RCD protection of circuits feeding mobile equipment with a current rating up to 32A for use outdoors. :

Cables in walls or partitions
Rule 522.6.7 calls for RCD protection of wiring concealed in walls or partitions. All concealed wiring at a depth of less than 50mm now requires protection by a 30mA RCD unless it is provided with earthed mechanical protection, for example by metallic conduit or trunking. This applies to many lighting circuits and their switch wiring, including those installed in previously defined a “Safe Zones”. :

There is a further requirement for protection by means of a 30mA RCD where cables are concealed in walls constructed with metal stud partitions, irrespective of the depth from the surface, unless provided with protection in the form of earthed metallic covering, trunking, conduit or other mechanical protection so as to avoid damage to the cable during installation or construction of the wall. :
Special locations :

Rule 710.411.3.3 calls for RCD protection of all circuits in specific locations such as those containing a fixed bath or shower. This means that, in bathrooms or bedrooms with en-suite facilities, circuits feeding lighting, heating and showers must have RCD protection. :
Other “special installations and locations” as defined in Part 7 of the Regulations are also required to have RCD protection. These include swimming pools and saunas, agricultural premises, caravans and caravan sites, floor and ceiling heating systems. :

Sub-division of circuits :
Rules 314.1 and 2 require that every installation should be divided into circuits as necessary to avoid danger and minimise inconvenience in the event of a fault. Designers are required to reduce the possibility of unwanted RCD tripping due to excessive protective conductor currents but not due to an earth fault. :
Separate circuits may be required for parts of the installation which need to be separately controlled in such a way that they are not affected by the failure of other circuits. The appropriate subdivision should take account of any danger arising from the failure of a single circuit, for example an RCD trip causing the disconnection of an important lighting circuit. :

This affects the configuration of the protective devices in a consumer unit. For example if a number of circuits are protected by a common 30mA RCD, lighting circuits need to be spread over more than one RCD. :
Rules 314.1 & 2 also call upon the designer to take steps to reduce the likelihood of unwanted RCD tripping due to excessive protective conductor currents, other than earth faults. :
Typical situations would include IT equipment with certain types of radio frequency interference suppression, and certain types of heating equipment. The cumulative effect of such loads can produce a standing earth leakage current that is beyond the threshold point of a normal 30mA RCD. However this situation is becoming more common in residential environments. :

Earth loop impedance ( 2392-10 )
It should also be noted that Chapter 41 of the Regulations includes revised earth loop impedance tables based on a nominal voltage of 230V (Previously 240V). This results in slightly lower values of earth loop impedance and could, in some situations, mean that RCDs will be required to achieve the required disconnection time where previously overcurrent protection devices would be considered adequate. :

17th Edition IEE Wiring Regulations :

Key Changes : RCD Protection to Socket Outlets ,

• Socket outlets rated not exceeding 20 A and intended for general use by ordinary persons must be protected with 30 mA RCDs(Residual Current Devices). This means that general purpose sockets in domestic and similar properties must have RCD protection. An exception can be made for socket outlets for specific purposes e.g. domestic freezer circuit, this socket should be suitably labelled or otherwise identified.
• External sockets rated not exceeding 32 A must also have 30 mA RCD protection.
Installation of cables – RCD Protection Requirements :
• Cables that are buried less than 50mm into a wall or partition and are not enclosed in earthed metallic covering or have mechanical protection capable of resisting nails or screws should be protected by a 30 mA RCD as well as being installed in the ‘safe zones’ as previously permitted.
• Similarly, irrespective of depth of cable, cables that are installed in metal framed walls require 30 mA RCD protection if not otherwise protected by earthed metallic covering. :

The above requirements do not apply to installations where the installation is intended to be under the supervision of a skilled or instructed person, such as in office buildings, large retail outlets and industrial premises. :

Bath/ Shower Rooms (containing a fixed bath or shower) :
• All circuits within a bathroom must have RCD (30mA) protection. Where this is provided and main equipotential bonding is used in the installation then supplementary equipotential bonding is not required.
• Bathrooms being modified or refurbished can either have their supplementary equipotential bonding extended or can be rewired with the installation of RCDs.
• Zone 3 has been removed
• 13A sockets in bathrooms are allowed providing they are installed at least 3m from the edge of the bath and protected by a 30mA RCD.
Under Floor Heating Systems :
Underfloor and ceiling heating systems are now covered as a special location/installation.
• A plan of the heating system shall be provided for each system (showing location, area, rating details, descriptions for use etc). A copy of the instructions for use should be fixed adjacent to the distribution board supplying the heating.
• Heating systems which do not have an exposed conductive covering or mesh must have installed on site a suitable exposed conductive part installed above the heating element . This could be in the form of an earthed metallic grid with spacing of not more than 30mm.
• A 30mA RCD must be used as the disconnection device.
• Heating systems of Class II (Double insulated) construction shall be provided with 30mA RCD protection.
•Floor heating systems in bathrooms the metal sheath, metal enclosure or fine mesh metallic grid should be connected to the protective conductor of the supply circuit.
Maximum Zs values
•There are new values based upon a nominal voltage of 230v and not 240v hence the values have been slightly reduced (i.e. 32A Type B - MCB was 1.50 now 1.44).
A new table has been provided giving the maximum values of earth fault loop impedance for RCDs.
Voltage Drop

•The new regulations now provide different voltage drop values for installation supplied from a public supply and private supply (i.e. Own generation). For a public supply the maximum values are 3% for lighting and 5% for other uses, for a private supply the maximum values are 6% for lighting & 8% for other uses.
Disconnection Times
•Final circuits not exceeding 32A shall have a maximum disconnection of 0.4 seconds for a TN-S or TN-C-S (PME) earthing arrangements.
•For TT systems the maximum disconnection time is 0.2 seconds, however a statement is included in the table (41.1) which states "Where disconnection is achieved by an overcurrent protective device, and the protective equipotential bonding, or main equipotential bonding, is correctly installed, the maximum disconnection times applicable to a TN system may be used."
New Sections on:

• 559 - Luminaries and Lighting
• 709 - Marinas and similar locations
• 711 - Exhibitions, Shows and Stands
• 712 - Solar Photovoltaic (PV) Power Supply Systems
• 717 - Mobile or Transportable Units
• 721 - Caravans and Motor Caravans
• 740 - Temporary Electrical Installations for Structures, Amusement Devices and Booths at Fairgrounds, Amusement Parks and Circuses

 

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17th Edition – ( this will help you with your 17th Chaps ,

Part 1: Scope, Object & Fundamental Principles ,

* (134.2.2 ) Designer of Installation responsible for specifying the interval to the first Periodic Inspection ,
* ( 135.1 ) Now makes a Positive Recommendation that every electrical Installation is subject to Periodic and Testing , in Accordance with Chapter 62 : p-162

Part 2 : Definitions ,
“ Competent Person “ ↔ p-22

Part 2 : Definitions , ↔ p-24
* Exposed Conductive part , A Conductive part of Equipment which can be touched and which is not live but which can
Become live when basic insulation fails , Example ( Metal Switch Plate , )
Part 2 : Definitions , ↔ p-24
• Extraneous-conductive parts , and its associated definition , remain unchanged :

************
* Line Conductor ( Replaces Phase Conductor Line is the Internationally used tem ) Do Not Confuse Line Conductor which can be a Neutral Conductor :

Chapter 41 : RCDs ,

* RCDs are now recognised as giving additional protection ( this term is now used instead of Supplementary Protection ) ←←←
* RCD now required for all general use socket-outlets rated up to 20A , allows for 2 Exceptions ←←←
(1) Socket-outlet used under supervision of skilled or instructed persons :
(2) Socket-outlet suitable identified for connection of particular item of equipment :
* To be recognised as giving additional protection The RCD must be rated at 30mA or Less and operate within 40mS
When tested at ( 5 x rated operating current )
Chapter 41 :
Revised Disconnection Times :
* TN-systems – 0.4 seconds ( Final Circuits up to 32Amp : ←←←
* TT-systems – 0.2 seconds ( Final Circuits up to 32Amp - Allows for 0.4s where all Protective bonding in place and
Disconnection is achieved by Overcurrent Device )
* Distribution circuits and circuits not covered by table 41.1 ( TN- = 5 seconds & TT = 1 second ,
* Supplementary bonding can be used were Disconnection Times can Not be Met . ←←←

Chapter 52 :
Selection & Erection of Wiring Systems :

* Chapter 52 now includes reference to Busbar Trunking systems and Powertrack systems :
* Max Value of Voltage Drop in Consumer’s Installations has Changed – Appendix 12 . p-358
Volt drop between origin and load terminals in LV system to be Less than ;
2392-10 ( Public Supply : Lighting 3% …….. Other Uses 5% ,
Private Supply : Lighting 6% …….. Other Uses 8% ,
( These Replace the Current 4% Requirement ) ←←←←←←←←

Chapter 55 :
Luminaires & Lighting ( 559 ) Regs
* Maximum circuit rating 16A for B15 , B22 , E14 , or E40 Lamp Holders ,
* Through wiring only permitted where light is designed for this ,
Chapter 55 :
Luminaires & Lighting ( 559 ) Regs
* 559 applies to selection & erection of luminaires & lighting installations fixed Installations and highway power supplies & street furniture ,
* Outdoor lighting includes : - Roads , Parks , Car-Parks , Gardens , Sporting Areas , Monuments , Floodlighting , Telephone Kiosks ,
Bus Shelters , Advertising Panels , Road Signs & Road Traffic Signals ,
• Excludes , Distributors Equipment & Temporary Festoon lighting ,

Part 6 :
* New part 6 Inspection & Testing Table 71A ↔ Now known as ( Table 61 p-158 )
-&- will ask you this one , ( 0.5MΩ SELV or PELV at 250 volts ) ←←←←

Chapter 62 :
Periodic Inspection & Testing
621.5 : Periodic Inspection & Testing shall be undertaken by a Skilled Person , Competent in such work ,
* Proof of Competence may be required ,

Section 701 : Bathrooms ,
* Section 7 Special Installations or Locations :
* Section 701 Locations, containing a bath or shower ,
* Zone ( 3 ) has been Removed : ←←←←←
* Suitable Equipment can be within 600mm of a bath ,
* Excluding 13amp Sockets to BS-1363 ( p-229 ) which must be 3 Meters from edge of bath or shower , -&- ,
* All circuits to be RCD Protected ,
* Supplementary bonding is NOT required – Provided any Required Protective Equipotential bonding has been installed ,

Part 7 :
Regulation ( 415.2 ) Supplementary Equipotential bonding : in relation to Section 701 , i.e. Locations Containing bath or Shower ←←:

Section 704 : Construction & Demolition Sites :
Section 705 : Agriculture & Horticulture :
• in both sections the reduced disconnection times of ↔ ( 0.2s ) ↔ and ↔ ( 25v ) ↔ Equation have been Removed : ←←←←

Section 708 : Caravan & Camping Parks :
* Sockets-Outlets must be provided individually with overcurrent and RCD protection for each pitch outlet : -&-
( Previously 1 RCD was allowed to protect not more than 3 pitch outlets ,

Tables 41.2, 41.3, 41.4

* If the measured Zs value exceeds 80% of the given values, a more precise measurement may have to be made to satisfy the requirements of BS-7671 . p361 :
* BS-7671 does not give maximum Zs values for BS 3871 mcb's. :
* If the maximum Zs value for a circuit in a TN- system cannot be met, the circuit may be protected by a 30ma RCD . 531.3.1
* If the maximum Ze value for a TN- system cannot be met, the installation may be protected by a 100ma RCD and treated as a TT- system. 531.3.1, 411.5.1, 411.5.2, 411.5.3 :

RCD :
the maximum disconnection time allowed for a RCD protected socket for a caravan/tent pitch :
Please refer to BS 7671:2008 Reg 708.553.1.13, this refers to Reg: 415.1.1, ( 30mA RCD 40mS at 5 x 30mA. )

17th Edition requirements for the testing of RCDs :

The 17th Edition of the Wiring Regulations (BS 7671: 2008) will introduce a number of new requirements for the installation of RCDs, therefore it is timely to look at the requirements within the 17th Edition for verification of RCDs.
The continuing effectiveness of these RCDs needs to be confirmed periodically. This article discusses the verification required where RCDs are used to provide automatic disconnection of supply in the event of a fault and additional protection. It should be stated at this point that the 17th Edition does not introduce any significant changes in the requirements for the testing of RCDs even where they are installed to provide automatic disconnection in the event of a fault.
Use of RCDs to achieve automatic disconnection in case of a fault 411.3.2.1 requires (in most cases) that a protective device shall interrupt the supply to a line conductor of a circuit or equipment in the event of a fault of negligible impedance between said line conductor and an exposed conductive- part or a protective conductor for the circuit or equipment within the appropriate required disconnection time. A disconnection time of 5 seconds for distribution equipment and final circuits of rating exceeding 32A is permitted by 411.3.2.3. Similarly, a disconnection time of 1 second for distribution equipment and final circuits of rating exceeding 32 A is permitted by 411.3.2.4.

Does the 17th Edition require a new test for RCDs ?

The rumour seems to have originated from Note 2 of Table 41.1 of the 17th Edition, with gives maximum permitted disconnection times for final circuits rated at up to 32A. The note states that: ‘Where compliance with this regulation is provided by an RCD, the disconnection times in accordance with Table 41.1 relate to prospective residual fault currents significantly higher than the rated residual operating current of the RCD.
BS 7671: 2008 (IEE Wiring Regulations 17th Edition) was published in January and comes into effect on 1st July. A rumour has been circulating amongst electrical contractors that the 17th Edition requires RCDs to be subjected to a test at twice their rated residual operating current (2 x I∆n ). Is this the case ?


As explained in this article, the familiar currents of 0.5 x I∆n , 1 x I∆n, and 5 x I∆n, (as applicable) should be all that are needed when testing RCDs in the vast majority of installations, as is the case under the 16th Edition.

A 2 x I∆n, test would be needed only in exceptional circumstances. But, even where this is the case, it does not necessarily mean that an RCD test instrument having a 2 x I∆n, test setting is required.

The rumour seems to have originated from Note 2 of Table 41.1 of the 17th Edition, with gives maximum permitted disconnection times for final circuits rated at up to 32 A. The note states that: ‘Where compliance with this regulation is provided by an RCD, the disconnection times in accordance with Table 41.1 relate to prospective residual fault currents significantly higher than the rated residual operating current of the RCD (typically 2 x I∆n ).’ However, Note 2 does NOT mean that a 2 x I∆n test is required.

 

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-&- ( 2391-301 Inspection, Testing and Certification of Electrical : Installations report – ( -&- March 2009 :

It would appear that candidates are still unaware that a working knowledge of Guidance Note 3 and BS 7671:2008 is required to achieve success in this qualification. Many candidates demonstrated significant gaps in both technical and underpinning knowledge of the subject matter. In particular the interpretation of information and test results and the validation process for test results are areas of concern.

From the information given by candidates it is apparent that they may be aware of the need for inspection and testing, but have little understanding of the reasons why it is done or the interpretation of the results obtained.
Many candidates did not display the required knowledge when answering the questions and this would suggest they do not have the necessary knowledge, understanding and experience when entering this qualification.

Use of correct terminology :
Correct terminology must be used when answering questions. Candidates continue to use incorrect terminology. This does indicate that candidates are not aware of the underlying requirements and processes. Typically candidates still referred to a periodic inspection and testing report as the type of inspection and the Electricity at Work Regulations is still incorrectly referred to as the Electricity at Work Act. Candidates also continue to use incorrect titles for documents and publications and to use inappropriate titles for the forms of certification
Documentation :
Some candidates were unable to identify the correct title of documents for certification, and many still refer to a Periodic Inspection Certificate. Candidates were unable to correctly identify the
Schedules by title with a Schedule of Items Inspected and a Schedule of Tests being common incorrect alternatives.
Questions which required candidates to identify where information is to be recorded on the forms of certification produced results which appear to indicate the candidates are not familiar with both the compilation and the content of these forms.
Many candidates did not consult with the third party (licensing authority) when determining the extent and limitations.
Inspection :
Question 22 asked candidates to identify three particular areas for investigation during inspection due to the nature of the location in the kitchen, laundry and residents rooms. The responses to this question were exceptionally poor with many candidates simply listing items such as sockets and switches for all locations giving no indication of the particular areas of investigation. Others included tests and many demonstrated a lack of understanding of the requirements for the locations. Many believed that some type of non-specific bonding was required.
Considering the information provided in the scenario there were very few candidates who identified inspection related to corrosion, appropriate IP ratings damage and the like. In the residents rooms many candidates considered the equipment being PAT tested etc as appropriate items for their inspection.
The response to this question generally indicated a lack of understanding of the inspection process and the information provided in GN3 related to the inspection of installations.
Question 4 required candidates to identify reasons why a survey would be required before a periodic inspection could be undertaken. Very few candidates were able to identify the relation to information not being available. Most identified events that could give rise to a periodic inspection being required.
Testing :
The common misunderstandings identified above show that ring final circuit testing is still a problem for many candidates. There is also some confusion as to the required tests fro particular circuits with many quoting the standard list, and the range of values expected for the tests undertaken. The test process and the expected results are fundamental to the requirements of those undertaking inspection and testing and for the candidates to complete the practical assessment.
Calculations :
Many candidates had problems with calculations related to the inspection and testing process. Cumulative insulation resistance values, determining values for stage 2 and stage 3 of the ring final circuit continuity test from given information caused some difficulty. These calculations are typical of the type of evaluations which may be required during the verification of test results. As these are fundamental to the activities of initial verification and periodic reporting such calculations should be within the abilities of the candidates. It would appear that a number of candidates do not understand the effects of resistances in series and parallel.

Describing test procedures :
When describing how to carry out a test, candidates were often confused as to what was required, unable to describe a logical approach and rarely used large clear diagrams to assist with their description. 2391-
Drawings and labels :
Question 25 required candidates to provide a fully labelled diagram of the earth fault loop path for a radial circuit. The scenario identified the system as TN-S. Whilst some candidates used the incorrect system there was a general lack of clear drawing and labelling.

Section A :

In Section A there were many fundamental errors identified in candidate responses. Typically many candidates

• identified a shock risk when testing main protective bonding conductors and not the tripping hazard. If the procedure is correct (isolation of the installation) there is no shock risk.

• could not correctly identify the reason for carrying out a continuity of ring final circuit test.

• could not identify special locations which were classified in zones; this including listing locations such as caravan sites and construction sites.

• were unaware of the IP requirement for Basic Protection with most examples related to ingress of liquids.

• were unable to determine the recorded RA value from given data, many adding the three values in parallel.

• did not know why R1 + R2 cannot be determined using Zs – Ze. Despite this information being given in Question 16 of Section A, many then went on in Section B to determine R1 + R2 using this incorrect method.

• were unable to identify the connection points for the instrument leads when measuring Ze when using a three lead test instrument.
• could not determine the maximum Zs for a 300 mA RCD to meet the requirements of BS 7671
• could not correctly determine the maximum permitted voltage drop for given circuits. Many used 16th edition values and/or determined voltage at equipment terminals.

Section B :
In Section B there were some areas where candidates were unable to demonstrate their understanding of the subject.

Many candidates were unable to

• identify operational aspects which would affect the inspection and testing activity with many simply identifying locations which were given in the scenario without identifying the aspect.

• identify the correct test sequence and relate instruments and ranges for the tests. For a radial final circuit the candidates included main protective bonding conductors, ring final circuit continuity and polarity with an approved voltage indicator in the dead tests, and PFC and RC in the live tests.

• describe the ring final circuit continuity test with may carrying out Stage 2 & 3 linking one pair (L1 & N2) and testing across the other pair for each test. Some included this as an extra stage wasting time and effort. A number were unable to determine the expected values.
• determine the calculated values of R1 and R2 correctly with a surprising number incorrectly using Zs – Ze despite the information in Part A that this method could not be used. Most candidates failed to appreciate that two of the circuits were ring final circuits and therefore failed to divide their result by 4.

Exam technique – time management :
Time management for candidates is important to ensure they have an opportunity to achieve the best possible result. Considering the number of marks available for each question and using this determine the extent and depth of the answer required would be useful. 2391-
As a guide, candidates should spend approximately one minute on an answer for each possible mark to be awarded. A question worth three marks for example should take approximately three minutes to complete. However many questions in Section A will take considerably less than this.

Exam technique – read the question
Careful reading of the question is important. Many answers did not include the information requested and candidates often provided answers which did not correspond to the question posed.
Candidates often did not answer the questions in both Sections A and B of the paper in relation to the given information and this often resulted in the loss of marks. Candidates must read the information given in the question. 2391-

 

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C&G 2382. 17th Edition ( BS7671:2008 ) Examination :

1. The Regulations (BS7671:2008) do NOT apply to
a. Residential Premises
b. Industrial Premises
c. Lightning Protection
d. Street Furniture
2. The Regulations do apply to
a. Offshore Installations
b. Mines & Quarries
c. Lift Installations
d. Low Voltage Generators
3. Which of the Following documents are deemed Non- Statutory
a. BS7671:2008
b. EAWR 1989
c. HASAW 1974
d. ESQCR 2002
4. Parts 3 – 7 of BS7671:2008 are explained in rudimentary terms within
a. Chapter 13
b. Chapter 12
c. Part 3
d. Appendix 5
5. Basic protection is defined as
a. Protection against shock under fault conditions
b. Protection against shock under fault free conditions
c. Protection against contact with live parts under fault free conditions
d. Protection against faults under sound electrical conditions
6. Equipment in which protection against electric shock does not rely on basic
insulation only is described as
a. Double Insulated Equipment
b. Class I Equipment
c. Class II Equipment
d. Class III Equipment
The7. The Earthing System illustrated in Figure 1 below would be identified as a
a. TN-S
b. TT
c. TN-C-S
d. IT
8. A Voltage of 250Volts AC (rms) would be defined as
a. Band I
b. Extra Low Voltage
c. High Voltage
d. Low Voltage
9. In determining Maximum Demand, ‘Diversity’ may be applies, which is
a. Taking the sum of all the protective devices from any CCU
b. Taking into account that not all loads will be switched on at the same time
c. Taking into account that all loads doubtless will be engaged at the same time
d. Ensuring that an economical and reliable design preference is utilised.
10. Every Installation is divided into circuits in order to
a. Ensue simplicity of isolation
b. Comply with European Standards
c. Avoid hazards and prevent inconvenience in the event of a fault
d. Allow individual energising of circuits which are not isolated
11. A building made entirely out of wood would be categorised for External Influences as
a. CA2
b. CA1
c. CB3
d. CB4
12. The Maximum Disconnection time for an a.c. TN circuit rated at 230V is
a. 0.04 seconds
b. 0.1 seconds
c. 0.4 seconds
d. 0.2 seconds
13. The Maximum Zs for a BSEN60898 Type C circuit breaker rated at 16Amps with
a 0.4second disconnection time is
a. 2.87Ω
b. 1.44Ω
c. 0.72Ω
d. 1.15Ω

14. For a TT System the Maximum earth fault loop impedance for a 100mA
BSEN61008-1 RCD in a 230Volt circuit is
a. 500Ω
b. 460Ω
c. 167Ω
d. 100Ω
15. Where, on electrical equipment, must the symbol in figure 2 be present
Figure 2
a. Where basic and supplementary earthing is present on an appliance
b. Where supplementary earth-bonding to an appliance is not present
c. Where electrical equipment has basic insulation only
d. Where Class I equipment is served from a sub-main CCU
16. Where Basic Protection is employed in the form of a barrier or enclosure, any
horizontal top surface must meet a protection level of at least
a. IPDXX
b. IP2X
c. IPXX3
d. IP4X
17. Except if made from adequate material, a luminaire rated at 200Watts should be
located away from combustible material by
a. 0.3m
b. 0.5m
c. 0.8m
d. 1.0m
18. To avoid burning, a non-metallic part intended to be touched but not hand held
cannot exceed
a. 80°C
b. 85°C
c. 90°C
d. 95°C
19. In relation to Voltage Disturbances, the resistance of the earthing arrangement
at the Transformer is referred to, within the area of symbols, as
a. RA
b. RB
c. RD
d. RE
20. Every core of a cable shall be identifiable at its terminations and preferably
throughout its length by
a. colour code only
b. letter code only
c. number code only
d. one or more of the above
21. An appropriate colour for a PEN conductor should be:
a. blue through its length with green markings at the terminations
b. green & yellow through its length with blue markings at the terminals
c. green & yellow through its length with brown markings at its terminals.
d. Green through its length with yellow markings at the terminals
22. A permanent label with the words ‘Safety Electrical Connection – Do Not
Remove’, complies with:
a. BS728
b. BS1363
c. BS951
d. BS423
23. A cable buried underground but not in conduit or ducting for mechanical
protection must incorporate
a. An earthed armour or metal sheath or both
b. A surface covering of 50mm thickness paving stones
c. A clear surface warning notice informing of its location
d. A PVC outer sheath
24. The de-rating factor for a cable surrounded by 50mm of thermal insulation is
a. 0.88
b. 0.78
c. 0.63
d. 0.51
25. In an L.V installation supplied directly from a public L.V distribution system the
maximum volt drop on a lighting circuit between the origin and any load point
should be no greater than
a. 6% Uo
b. 5% Uo
c. 4% Uo
d. 3% Uo

26. Every electrical inspection shall be accessible for inspection, testing and
maintenance purposes except for which of the following
a. A connection made in a junction box beneath floorboards
b. A connection made within a motor control unit
c. A connection designed to withstand fault current
d. A compound filled or encapsulated joint
27. The rated RCD operating current of such a device installed as a protection
against risk of fire in a TT system shall have a value of
a. 30mA
b. 100mA
c. 300mA
d. 500mA
28. The maximum prospective short circuit or earth fault current in a circuit should
not exceed
a. The operating current of circuit switching devices
b. The rated breaking capacity of any associated protective device
c. The design current of the circuit
d. The rated operating current of any RCD in circuit
29. Which of the following switching devices may be satisfactorily utilised for the
purposes of isolation?
a. BSEN60669-2-4
b. BSEN60669-2-3
c. BSEN60669-2-1
d. BSEN60669-1
30. When using bare conductors in extra low voltage lighting installations supplied
from a safety isolating transformer the minimum permissible cross sectional
area of conductors must be
a. 1.5mm2
b. 2.5mm2
c. 4mm2
d. 6mm2
31. Suspension devices for ELV luminaries must in any case be capable of
supporting at least
a. 5 Kg
b. 7.5 Kg
c. 10 Kg
d. 20 Kg
32. An automatic electrical safety service supply classed as medium break must, in
the event of losing the main supply, instate the safety service supply in a time
period of
a. between 0.15 & 0.5 seconds
b. between 0.5 & 5 seconds
c. between 5 & 15 seconds
d. greater than 15 seconds
33. The minimum value of Insulation Resistance for a 230Volt system must be
a. >0.25 MΩ
b. >0.5 MΩ
c. >1.0 MΩ
d. >2.0 MΩ
34. Correct Polarity must ensure that every ES lamp-holder have their outer or
screwed contacts connected to the neutral conductor, except for
a. E14 & E27 Lampholders
b. E14 & BSEN60895 Lampholders
c. E27 & BSEN61009 Lampholders
d. E11 & E24 Lampholders
35. To comply with PART 6 of BS7671, Periodic Inspection & Testing shall be
specifically undertaken by
a. A formally qualified Test Engineer
b. A person deemed as the ‘Duty Holder’ of the company carrying out the work
c. A expressly skilled person
d. A competent person
36. Zone 2 of a bathroom is restricted to the highest water outlet or the horizontal
plane lying above finished floor level by
a. 3.00m
b. 2.50m
c. 2.25m
d. 2.00m

37. In Zone 3 of a Sauna equipment must be able to withstand a minimum
temperature of
a. 100°C
b. 120°C
c. 125°C
d. 170°C
38. In marinas, equipment installed above a jetty or wharf, which is likely to
encounter water jets, shall be selected to comply with external influence levels
of
a. (AD4): IPX4
b. (AD5): IPX5
c. (AD6): IPX6
d. (AE6): IPX5
39. For a BS88-2.2 Fuse rated at 25A to obtain a 0.4sec disconnection time, it would
require a minimum prospective fault current of
a. 160A
b. 130A
c. 100A
d. 85A
40. A 30Amp Semi Enclosed BS3036 Fuse receiving a prospective fault current of
130A would disconnect in
a. 5.0sec
b. 1.0sec
c. 0.4sec
d. 0.2sec

Answers:
1. C Part 1 -110.2 Page 13
2. D Part 1 -110.1 Page 12
3. A Part 1 -114.1 Page 13
4. A Part 1 -120.3 Page 14
5. B Part 2 - DEFENITIONS
6. B Part 2 - DEFENITIONS
7. C Part 2 - DEFENITIONS
8. D Part 2 - DEFENITIONS
9. B Part 3 - 311.1 Page 38
10. C Part 3 - 314.1 Page 39
11. A Appendix 5 Page 319
12. C Part 4 - Table 41.1 Page 46
13. B Part 4 - Max Zs Tables - Part 4
14. B Part 4 - Table 41.5 Page 50
15. C Part 4 - 412.2.1 Page 55
16. D Part 4 - 416.2.2 Page 60
17. C Part 4 - 422.3.1 Page 67
18. A Part 4 - Table 42.1 Page 69
19. D Part 4 - 442.1.2 Page 80
20. D Part 5
21. B Part 5
22. C Part 5
23. A Part 5
24. A Part 5 – Table 52.2 Page 104
25. D Part 5
26. D Part 5
27. C Part 5
28. B Part 5
29. A Part 5
30. C Part 5
31. A Part 5
32. C Part 5
33. C Part 6 - Table 61 Page 158
34. A Part 6 - 612.6 Page 159
35. D Part 6 - 621.5 Page 162
36. C Part 6 - Page 169
37. C Part 7 - 703.512.2. Page 180
38. B Part 7 - 709.512.2.1.1 Page 193
39. A Appendix - Time/Current Graph - Page 248
40. C Appendix - Time/Current Graph -Page 245

Symbols :
In Rated current of the contacts - Expressed in amperes e.g. 100A.
I?n Sensitivity or residual operating current - Usually expressed in amperes e.g. 0.03A for 30mA

 

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Regulation 411.5.3 requires that for TT systems the
formula in 411.5.3 (ii) RA I∆n & 50 V.

RCD maximum Zs or RA values
……… maximum touch voltage
I∆n (A)………. 50V
0.01 …………. 5000Ω
0.03 …………. 1666Ω
0.1 ……………. 500Ω
0.3 ……………. 166.6Ω
1 ………………. 50Ω
3 ………………. 16.66Ω

Earth electrode resistances over 200Ω are likely to be unstable
( Measured Résistance should not exceed 100Ω ,

( RA I∆n ≤ 50 V : R ≤ 50v / 30mA , ( 50 ÷ 30 = 1667Ω
R ≤ 50v ÷ 160 = 0.31Ω ( 30mA RCD protects a.c Circuit rated residual current 30mA ,

411.5.4 : ( Zs x Ia ≤ Uo ) 32A = ÷ 230 = 0.13Ω
TT , 50v ÷ .3mA = 166.7 Ω , ( 30mA )
100mA = ) 50v ÷ 0.1mA = max Zs off 500Ω

612.9 : Earth fault loop impedance, Zs

This may be determined either by Direct Measurement at the further point of a live circuit OR by adding (R1 + R2) Ze

i.e. Zs = Ze + ( R1 + R2 ). In general, the Earth Fault Loop Impedance shall Not Exceed 100 Ω.

17th Edition Forms : 2392-10

1 Initial inspection and testing 2392-10

Forms 1 to 4 are designed for use when inspecting and testing a new installation, or an alteration or addition to an existing installation. The forms comprise the following:
1 Short form of Electrical Installation Certificate (To be used when one person is responsible for the design, construction, inspection and testing of an installation.)
2 Electrical Installation Certificate (Standard form from Appendix 6 of BS 7671)
3 Schedule of Inspections
4 Schedule of Test Results.
Notes on completion and guidance for recipients are provided with the form.

2 Minor works 2392-10
The complete set of forms for initial inspection and testing may not be appropriate for minor works. When an addition to an electrical installation does not extend to the installation of a new circuit, the minor works form may be used. This form is intended for such work as the addition of a socket-outlet or lighting point to an existing circuit, or for repair or modification.
Form 5 is the Minor Electrical Installation Works Certificate from Appendix 6 of BS 7671.
Notes on completion and guidance for recipients are provided with the form.

3 Periodic inspection 2391-10
Form 6, the Periodic Inspection Report from Appendix 6 of BS 7671, is for use when carrying out routine periodic inspection and testing of an existing installation. It is not for use when alterations or additions are made. A Schedule of Inspections (3) and Schedule of Test Results (4) should accompany the Periodic Inspection Report (6).
Notes on completion and guidance for recipients are provided with the form.

2392-10

Prospective short circuit current is the greater of the short-circuit current and earth fault current -&- :
Ze, the external impedance measured at the origin of the installation with the main bonding disconnected. -&- :

Continuity of protective conductors - Every protective conductor including bonding conductors shall be tested to verify it is sound and correctly connected :

Continuity of final circuit conductors - The sum of the resistance of the of the phase conductor (R1) and the protective conductor (R2 ) i.e. R1 + R2 , is to be inserted : This may be use, after temperature correction, by adding to Ze , to determine Zs.

Insulation resistance :

Equipment such as electronic devices shall, where necessary, be disconnected from the installation to avoid damage during testing. Where required, such equipment shall be tested separately.

17th Edition wiring regulations, explains : 2392-10

There is also a specific requirement for appropriate documentation for all installations.
Of particular interest to the health and safety manager, Regulation 134.2.1 requires that inspection and testing must be carried out by a 'competent person' to verify that standards have been met.
Importantly, a 'competent person' is defined as someone 'who possesses sufficient technical knowledge and experience for the nature of the electrical work undertaken and is able at all times to prevent danger, and where appropriate, injury to themselves and others'.
In practice, this means that inspection and testing should only be taken by experienced engineers that are qualified to the City and Guilds 2392 - 10 course 'Fundamental Testing, Inspection and Initial Verification'.
This course is now recognized as the qualification for competent persons carrying out initial inspection and testing of electrical installations.
For periodic inspection and testing, competent persons should successfully complete the C&G 2392 - 20 'Inspection, Testing and Certification of Electrical Installations' course in addition to the 2392 - 10 course.
Once the initial verification of the installation has been completed, which includes both inspection and testing, the regulations call for the issuing of an Electrical Installation Certificate, together with a schedule of test results and a schedule of inspections.
The certificate includes space for three signatures - the person responsible for the design, the person responsible for the construction and the person carrying out the inspection and test of the installation.
It should be emphasized that the signature for the inspection and test section is the person who actually carries out the inspection and test and not someone else who may be in authority.
In some cases, all three sections may require signature by the same person and this is perfectly acceptable.
However, the Electrical Installation Certificate should not be signed until any defects identified by the person responsible for inspection and test have been corrected.
An Electrical Installation Certificate (or a Minor Electrical Installation Works Certificate), stating the extent of the works covered, shall be issued once the inspector is satisfied that the works comply with the regulations.
Any defects found in related parts of the installation, not affecting the safety of the alteration or addition should be reported in writing to the person ordering the work.
If existing defects affect the new work then these defects need to be corrected before an Electrical Installation Certificate can be issued and before the new work can be put into service.
An example of this is where bonding or equipotential bonding is inadequate or omitted, as this would seriously affect the safety of the whole installation, including the new work.
The Electrical Installation Certificate should not be used for periodic inspections.
The 17th Edition regulations stipulate that the designer of the installation is responsible for specifying the interval to the first periodic inspection and test.
There is also the positive recommendation (Regulation 135.1) that every electrical installation is subject to periodic inspection and testing by a competent person (in accordance with Chapter 62).
For example, the IEE Guidance Note for periodic fixed installation test frequencies advise a maximum period of five years between inspections and testing for commercial offices, shops and hospitals - reducing to three years for industrial facilities, leisure complexes and theatres.
For some special installations, such as swimming pools, petrol stations and caravan parks, the maximum period between inspections and testing is one year.
This represents a substantial difference from the previous edition, which presumed that a programme of risk assessments, records and preventative maintenance could be adopted in place of periodic testing.
The Periodic Inspection Report form is only to be used for the inspection of an existing installation and should include both inspection and test results.
Again the extent and limitations of the report needs to be stated and recommendations of defects and their remedies should be made.
The report includes a numbering system for this purpose, as follows: 1 - Requires Urgent Attention; 2 - Requires Improvements; 3 - Requires Further Investigation; and 4 - Does Not Comply With BS 7671:2008 (although this does not necessarily imply that the electrical installation is unsafe).
A minor works is defined as 'work which does not include the provision of a new circuit'.
Testing is still essential and a number of tests are specifically identified as essential to confirm safety.
Also included on the form is space to allow the inspector to comment on the existing installation.

.

 

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17th Edition wiring regulations, explains :
“Earthing Arrangements “
1. Earthing Arrangements
2. In this section we are going to look at : -
The basics of shock .
Shock protection .
Class 1 & Class 2 equipment .
The three common earthing arrangements .
3. Earthing arrangements -
4. Definitions from Part 2 BS 7671
5. Bonding conductor A protective conductor providing equipotential bonding
6. Where protection against electric shock does not rely solely on basic insulation alone. Exposed-conductive parts being connected to a protective conductor within the fixed wiring of the installation. Class I equipment Class I insulation Single-layer insulation Live part Exposed conductive part
7. Class II equipment Where protection against electric shock relies on the application of additional or supplementary insulation. There is no provision for the connection of a protective conductor to exposed metalwork. Class II insulation Live part Two layers of insulation Exposed metalwork
8. Double insulation Double insulation (Class II) - Insulation comprising both basic insulation and supplementary insulation Symbol found on equipment
9. Earth The conductive mass of Earth, whose electric potential at any point is conventionally taken as zero
10. Earth Electrode A conductor or group of conductors in intimate contact with, and providing an electrical connection to earth
11. Earth electrode resistance The resistance of an earth electrode to earth
12. Earth fault current A fault current which flows to earth
13. Earth fault loop impedance The impedance of the earth fault current loop  starting and ending at the point of earth fault. Symbol Z Unit
14. The earth fault loop The earth fault loop starting at the point of fault consists of:
o The circuit protective conductor (c.p.c.)
o Consumers earthing terminal and earthing conductor
o For TN systems, the metallic return path
o For TT and IT systems the earth return path
o The path through the earthed neutral point of the
o transformer
o The transformer winding and phase conductor to point
o of fault
15. Earth leakage current A current which flows to earth, or to extraneous conductive parts, in a circuit which is electrically sound. This current may have a capacitive quality including that from the deliberate use of capacitors for noise filtration.
16. Earthed equipotential zone A zone within which exposed conductive parts and extraneous conductive parts are maintained at substantially the same potential by bonding, such that under fault conditions, the differences in potential simultaneously accessible exposed and extraneous- conductive parts will not cause electric shock.
17. Earthing Connection of the exposed conductive parts of an installation to the main earthing terminal of that installation
18. Basic contact (shock) Results from Making contact with parts of a circuit or system which are live under normal conditions
19. Earthing Connection of the exposed conductive parts of an installation to the main earthing terminal of that installation
20. Extraneous conductive part A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation.
21. Fault A circuit condition in which current flows through an abnormal or unintended path. This may result from an insulation failure or a bridging of insulation. Conventionally the impedance between live conductors or between live conductors and exposed or extraneous conductive parts at the fault position is considered negligible.
22. Functional earthing Connection to Earth necessary for proper functioning of electrical equipment Table 51A Functional earthing conductors to be coloured cream
23. Contact of persons or livestock with exposed-conductive parts which have become live under fault conditions. Fault contact
24. Protective conductors A conductor used for some measure of protection against electric shock and intended for connecting together any of the following parts
o Exposed conductive parts
o Extraneous-conductive parts

17th Edition : Earthing & Equipotential Bonding :

Earthing ,

Earthing ensures that in the event of a fault, adequate fault current will flow causing rapid operation of a Circuit Protective Device (fuse, circuit breaker, or RCD) promptly disconnecting the supply. This limits the duration of any shock that one might receive, dramatically reducing the risk of serious injury or death :

For example, suppose a poorly positioned live wire in a washing machine becomes abraded by a sharp metal edge when the machine is running and this has the effect of making the casework of the machine "live". Since the case is connected (via its 13A plug) to mains earth, a high current will flow which will either blow the fuse in the plug and/or trip the RCD protecting the circuit :

During fault conditions, earthing may also reduce the voltage rise of anything earthed, which in addition to the limiting of the shock duration described above can also reduce the shock risk :

On general purpose socket circuits, the size of earthing conductors, and the circuit protective devices used are chosen to ensure that a fault is cleared within 0.4 seconds (or 0.2 seconds if the installation uses TT Earthing). For submains or higher power circuits feeding fixed equipment the time limit is 5 seconds (or 1 with TT) :

Main Equipotential Bonding ( 17th Edition , Main Protective Bonding Conductor )

Main bonding is the electrical interconnection of incoming (metallic) services (e.g. water, gas, and oil pipes) plus any extraneous conductive parts of a building (like the metal framework used in some buildings, or the central heating pipework), to the main electrical earth. This ensures that under fault conditions extraneous conductive parts, such as pipework, are not able to take on a dramatically different electrical potential to that of the installation's earth connection :

Supplementary bonding ( 17th Edition , Supplementary Protective bonding conductors’ / where required ,

Supplementary, or cross bonding is usually found in special locations containing a bath or shower. Unlike earthing it is not designed to clear a fault. What it does is electrically tie together all accessible conductive parts (pipes, taps, electrical appliances etc) that could under fault conditions introduce a dangerous potential (voltage) into the room :

For example suppose an electrically heated towel rail develops a fault which makes it electrically live. (Of course this also supposes that it is not earthed properly: which should never happen but the regulations adopt a belt and braces approach). Without bonding, such a fault would result in the towel rail being at mains voltage, while adjacent basin taps might offer a path to earth via the water pipework. This would be a very dangerous situation since touching both towel rail and a tap would expose one to a 230V potential difference across the arms and chest (including heart) probably causing severe injury or death :

However if the pipework feeding both hot and cold taps is bonded together with that of the earth of any electrical circuits supplying the room, then the towel rail fault will try to bring both taps up to mains voltage (230V). However touching both rail and tap at the same time exposes one to a potential difference of zero volts :

(Actually the bonding may fail to tie all elements together at exactly the same potential, but it is designed to limit any potential difference to 50V or less) :

The 17th Edition and Earthing & Equipotential Bonding

* Before an addition or alteration can be made to an existing installation it must be ascertained that the earthing and bonding arrangements comply with the current version of BS7671 and any existing equipment including the incoming supply is adequate for the proposed addition or alteration. 131.8
* Every installation must be provided with a main earthing terminal. 542.4.1
* The main earthing terminal, all bonding conductor connections and connections to an earth electrode must be permanently labelled 'Safety Electrical Connection - Do Not Remove'. 514.13.1
* Every joint and connection must be accessible. 543.3.3, 526.3
* All circuits must have a cpc that is terminated at each wiring point and at each accessory. 411.3.1.1

 

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* Rigid metal conduit and the metal sheath or armour of a cable can be used as a protective conductor. 543.2.2, 543.2.5
* Where rigid metal conduit or the metal sheath or armour of a cable is used as a protective conductor, a separate protective conductor must connect the earthing terminal of any accessories to the appropriate metal backbox. 543.2.7, 543.7.2.1

* All joints in metal conduit must be continuous. 543.3.6
* The cpc of flat cables must be sleeved when the cable sheath has been removed. 543.3.2
* Protective conductors must be identified by the colours green & yellow. 514.4.2 The single colour green is not permitted. 514.4.5
* In most domestic installations switches are not to be used with a protective conductor. 543.3.4
* All exposed-conductive-parts of a TN installation must be connected to the main earthing terminal. 411.4.2
* All extraneous conductive parts in an installation must be connected to the main earthing terminal by main protective bonding conductors. This applies to the metallic sheath of a telecommunication cable where permission from the owner of the cable must be obtained. 411.3.1.2
Main Earthing Conductor :
* The minimum csa of the main earthing conductor must be determined by the adibiatic equation or selected from Table 54.7 543.1.1. If the adibiatic equation is used, the minimum csa of the main earthing conductor must be 6mm 544.1.1. Table 54.7 suggests a 16mm main earthing conductor for phase conductors with a csa of up to 35mm.
* The csa of the main earthing conductor where PME conditions apply should be not less than that for a main protective bonding conductor (10mm) for the same installation 544.1.1. Invariably the electricity supplier will provide a 16mm main earthing conductor for a PME supply in a domestic property.
Main Protective Bonding Conductors :
* All extraneous conductive parts in an installation must be connected to the main earthing terminal by main protective bonding conductors. This applies to the metallic sheath of a telecommunications cable where permission from the owner of the cable must be obtained. 411.3.1.2
* For TN-S or TT systems the csa of main protective bonding conductors must be a minimum of 6mm and not be less than half the csa of the main earthing conductor. 544.1.1
* For a PME system the csa of the main bonding conductors must not be less than that given in Table 54.8 i.e. a 10mm protective bonding conductor for a neutral conductor of 35mm or less. 544.1.1
* For a service pipe, the main bonding conductor should be connected as near as possible to the point where the service enters the building. The connection must be before any branched pipework and on the consumers side of any meter. If possible the connection should be made within 600mm of the meter outlet. Where the meter is outside, the bonding connection should be made at the point of entry of the service into the building. 544.1.2

* Main bonding conductors should not be supported by the service pipes they are connected to. 543.3.1
* Where a main bonding conductor loops in and out to connect to an extraneous-conductive-part, the conductor should be unbroken at the connection. 528.3.3
* It is not necessary to run a main protective bonding conductor to an incoming service where the incoming service pipe and the consumers pipework are both made of plastic. If the incoming service pipe is made of plastic and the consumers pipework is made of metal it is recommended to main bond any metal pipework. OSG p29
Supplementary Bonding Conductors :
* Supplementary bonding is not required in a bath or shower room if all the extraneous conductive parts of the installation are connected to the main equipotential bonding. p6, 701.415.2
It is not generally required to supplementary bond the following :
* kitchen pipes, sinks, draining boards, metallic kitchen furniture, boiler pipes, metallic parts supplied by plastic pipes or metal pipes to hand basins or wc's ( excluding metal waste pipes in contact with earth ). OSG p31
Earth Electrodes : ( 542.2 – 542.2.1 )
* All of the following can be used as earth electrodes :
* Earth rods or pipes * Earth tapes or wires * Earth plates * Underground structural metalwork embedded in foundations * Welded metal reinforcement of concrete embedded in the Earth (excluding pre stressed concrete) * Lead sheaths & metal cable coverings provided the following conditions are met :
* a - the cable covering must be in effective contact with Earth
* b - the permission of the cable owner must be obtained
* c - the owner of the cable must be able to inform the owner of the installation of any changes to the cable which may affect it suitability as an earth electrode
* metal gas or water pipe must not be used as an earth electrode. 542.2.4

Why is inspection and testing necessary ?

Periodic inspection and testing is necessary because all electrical installations deteriorate due to a number of factors such as damage, wear, tear, corrosion, excessive electrical loading, ageing and environmental influences. Consequently legislation requires that electrical installations are maintained in a safe condition and therefore must be periodically inspected and tested.
Licensing authorities, public bodies, insurance companies, mortgage lenders and others may require periodic inspection and testing of electrical installations.
The law and inspection and testing

17% of all house fires are caused by electrical faults, due to lack of maintenance, poor and / or DIY work.

Existing domestic electrical installations are recommended to be inspected and tested at least once every 10 years or 5 years for rental properties. The purpose of this is to ensure the ongoing safety and efficiency of the installation, and to rectify any faults or degradation identified during the work.

It is also recommended that all in-service equipment is also regularly tested and inspected, labelled and records kept

Many house buyers may require a report regarding the age, condition and suitability of the electrical installation for mortgage and / or insurance purposes. This type of report can also be used as a bargaining point where the installation is found to be substandard.

 

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Introduction to the Changes : Apprentices

BS7671:2008 is the current national standard for all electrical work undertaken in
the United Kingdom. The first edition of BS7671 was published in 1992 and has
so far been amended six times. The 17th Edition is the result of a complete
review of the 16th Edition and aims to standardise the UK standards with the
Harmonised Documents (HD’s) produced by the European Committee for
Electrotechnical Standardisation (CENELEC).
A new informative note in the preface advises that publication of the
2008 edition of BS7671 does not automatically mean that installations complying
with previous editions are unsafe for continued use or need to be upgraded.
Numbering. The Regulation numbers are changed so as to be in line with
International Electrotechnical Commission (IEC) Numbers. This enables users to
readily relate UK regulations with European HDs and IEC regulations.
Fundamental Principles. Requirements are added for protection for persons and
livestock against injury, and property against damage caused by voltage
disturbances and electromagnetic influences.
Protection against electric shock – chapter 41.
• The chapter has been rewritten. Many regulations are worded differently but
the requirements are not significantly changed.
• The terms ‘Direct Contact’ and ‘Indirect Contact’ are no longer used. They are
replaced with Basic Protection and Fault Protection. ‘Basic protection’ is
protection against touching live parts. ‘Fault protection’ is protection against
receiving a shock from conductive parts that have become live due to a
breakdown of insulation or damage to equipment.
• Socket outlets rated not exceeding 20 A and intended for general use by
ordinary persons must be protected with 30 mA RCDs. This means that
general purpose sockets in domestic and similar properties must have
RCD protection.
• External sockets rated not exceeding 32 A must also have 30 mA
RCD protection.
• Maximum permitted earth loop impedance – Zs values have been adjusted,
based on 230V nominal and this has slightly reduced these, for example for
32amp type ‘B’ MCB protection the maximum permitted is 1.44ohms and was
previously 1.5ohms.
Selection and erection of wiring systems – chapter 52.
• An important new regulation that particularly affects domestic and similar
installations, requires cables that are buried less than 50mm into a wall or
partition and are not enclosed in earthed metallic covering or have mechanical
protection capable of resisting nails or screws to be protected by a 30 mA
RCD as well as being installed in the ‘safe zones’ created by the position of
accessories etc. as previously permitted.
• Similarly, irrespective of depth of cable, cables that are installed in metal framed
walls require 30 mA RCD protection if not otherwise protected by earthed
metallic covering.
• The above requirements do not apply to installations that are under the control
of skilled or instructed persons, such as office buildings, large retail outlets and
industrial premises.
• Maximum permitted volt drop where supplied directly from a public
distribution system is now 3% for lighting and 5% for all other applications.

Part P of the Building Regulations

Part P came into effect in England and Wales on the 1st January 2005. It is now a legal requirement for all work on fixed electrical installations in dwellings and associated buildings to comply with relevant standards. The relevant UK standard is BS 7671: 2008, 'Requirements for electrical installations' (The IEE Wiring Regulations 17th Edition). BS 7671 covers requirements for design, installation, inspection, testing, verification and certification.

To what types of electrical work does Part P apply?
• In a dwelling
•
• In the common parts of buildings serving one or more dwellings, but excluding power supplies to lifts
•
• In a building that receives its electricity from a source located within or shared with a dwelling, and
•
• In a garden or in or on land associate with a building where the electricity supply is from a source located within or shared with a dwelling
•

The term dwelling includes houses, maisonettes and flats. It also apply to electrical installations in business premises that share an electricity supply with dwellings, such as shops and public houses with a flat above.

The common parts of buildings includes access areas in blocks of flats such as hallways and shared amenities in blocks of flats such as laundries and gymnasiums.

Part P applies to electrical installations located in outbuildings such as detached garages, sheds and greenhouses.

Part P applies to parts of electrical installations located on land around dwellings such as garden lighting.

Part P applies to electrical installations that operate at voltages not exceeding 1000 V a.c.

Notifiable work includes new installations, house re-wires, and the installation

of new circuits. Notifiable work also includes additions to existing circuits in kitchens, bathrooms, outdoors and in other special locations. (See below ).
Will all electrical work need Building Regulations approval?
No. In general, notification will need to be given to, or full plans deposited with, a building control body only if the work is major involving one or more complete new circuits, and is not being carried out by an electrical contractor registered with an authorised competent person self-certification scheme.
What types of electrical work are 'non-notifiable'?

The following types of work are non-notifiable:
• Replacing accessories such as socket-outlets, control switches and ceiling roses
•
• Replacing the cable for a single circuit only, where damaged, for example, by fire, rodent or impact (note a)
•
• Re-fixing or replacing the enclosures of existing installation components (note b)
•
• Providing mechanical protection to existing fixed installations (note c)
•
• Work that is not in a kitchen or special location and does not involve a special installation (note d) and consists of:
•
• Adding lighting points (light fittings and switches) to an existing circuit (note e)
•
• Adding socket-outlets and fused spurs to an existing ring or radial circuit (note e)
•
• Installing or upgrading main or supplementary equipotential bonding (note f)
•

Notes:

(a) On condition that the replacement cable has the same current-carrying capacity, follows the same route and does not serve more than one sub-circuit through a distribution board

(b) If the circuit's protective measures are unaffected

(c) If the circuit's protective measures and current-carrying capacity of conductors are unaffected by increased thermal insulation

(d) Special locations and installations are listed below

(e) Only if the existing circuit protective device is suitable and provides protection for the modified circuit, and other relevant safety provisions are satisfactory

(f) Such work shall comply with other applicable legislation, such as the Gas Safety (Installation and Use) Regulations
Special locations and installations
• Locations containing a bath tub or shower basin
•
• Swimming pools or paddling pools
•
• Hot air saunas
•
• Electric floor or ceiling heating systems
•
• Garden lighting or power installations
•
• Solar photovoltaic (PV) power supply systems
•
• Small scale generators such as microCHP
•
• Extra-low voltage lighting installations, other than pre-assembled, CE-marked lighting sets
•

What are competent person self-certification schemes?

Electrical contractors who register with a competent person self-certification scheme will be able to self-certify compliance with the Building Regulations whenever they carry out 'notifiable' work. Persons who are not registered with a self-certification scheme - including DIYers - will need to notify or submit plans to a building control body, unless the work is non-notifiable as described above.

How many electrical self-certification schemes have been approved?

On the recommendation of BRAC (the Building Regulations Advisory Committee), the Government has approved schemes to be operated by:
• BRE Certification Limited
•
• BSI - British Standards Institution
•
• ELECSA Limited
•
• NAPIT Certification Ltd
•
• NICEIC Certification Services Limited
•

 

[amberleaf]

With a TT- Function ,
a Test Current of 15mA or less is applied between Line – Earth ,
It enable Loop Measurement without Tripping most RCDs 30mA : * ( your also checking the sensitivity of the RCD ) -&- 3291-10

RCD Test can be Selected :
Selector , from Either the Positive ( Oº ) or from ,
The Negative ( 180º ) half-Cycle of Voltage ,
At Both Polarity Test Minimum ( best ) and Maximum ( Worst ) Trip Times :

For TN- systems the Earth Fault Loop Impedance is the sum of the following Impedances :
Impedance of the power transformer secondary winding :
Impedance of the phase conductor from the power transformer to the location of the fault.:
Impedance of the protective conductor from the fault location to the power transformer :


If an electrical installation is protected by over-current protective devices including circuit breakers or fuses,
the earth Loop Impedance should be Measured In the event of a fault the earth fault Loop Impedance should be
low enough (and the prospective fault current high enough) to allow automatic disconnection of the electrical supply by the
circuit protection device within a prescribed time interval Every circuit must be tested to ensure that the earth fault Loop Impedance value does Not exceed that specified or appropriate for the over-current protective device installed in the circuit :

Tester takes a current from the supply and measures the difference between the unloaded and loaded supply voltages. From this
difference it is possible to calculate the Loop résistance
For a TT system the earth fault Loop Impedance is the sum of the following impedances
▲ Impedance of the power transformer secondary winding
▲ Impedance of the phase conductor resistance from the power transformer to the location of the fault
▲ The Impedance of the protective conductor from the fault location to the earth system
▲ Resistance of the local earth system (R).
▲ Resistance of the power transformer earth system (Re)

The figure below shows in marked line the Fault Loop Impedance for TT system.
When the protective device is a residual device ( RCD ), Ia is the rated residual operating current I∆n . For example in a TT system protected by an RCD the maximum RA values are as follows:

Rated residual
Operating ………. 10 ………. 30 ………. 100 ………. 300 ………. 500 ………. 1000
current IΔn mA
Ra (at 50V) Ω …. 50000Ω …. 1667Ω …... 500Ω …… 167Ω ………100Ω ………. 50Ω

For this example the maximum value is 1667Ω , the Loop tester
reads 12.74Ω and consequently the condition RA is 50 / Ia is met. It also important to test the operation of the RCD using a
dedicated RCD tester in accordance with the international standard IEC60364 for a TN system :

The following condition shall be fulfilled for each circuit Zs – Uo / Ia where Zs is the earth fault Loop Impedance voltage is the
nominal voltage between phase and earth and ( Ia ) is the current that causes the automatic disconnection of the protective device
within the time stated in the following table :

Note:
▲When the protective device is a residual current device( RCD ),
Ia is the rated residual operating current I∆n , For instance in a TN system with a nominal mains voltage of
Uo = 230V protected by type gG fuses the ( Ia ) and maximum Zs values could be :

Principles of the measurement of line :
Impedance and prospective short circuit current Line Impedance on a single phase system is the Impedance measured between phase and neutral terminals. Measurement principles for line impedance are exactly the same as for earth fault Loop impedance measurement with the exception that the measurement is carried out between phase and neutral :

The protective short circuit or fault current at any point within an electrical installation is the current that would flow in the
circuit if no circuit protection operated and a complete (very low impedance ) short circuit occurred
The value of this fault current is determined by the supply voltage and the impedance of the path taken by the fault current
Measurement of prospective short circuit current can be used to check that the protective devices within the system will operate
within safety limits and in accordance with the safe design of the installation. The breaking current capacity of any installed
protective device should be always higher than the prospective short circuit current :

If the prospective fault current is measured , its value must be higher than the ( Ia ) value of the protective device concerned

* The maximum value of Zs for this example is 2.70Ω (16 amp gG fuse, 0.4 seconds). The Loop tester reads 1.14Ω and consequently
the condition Zs Uo / Ia is met :

Accordance with the International Standard IEC 60364 , for a TT system the following condition shall be fulfilled for each circuit
RA must be 50 / Ia
Where ;
RA is the sum of the resistances of the local earth system :
R and the protective conductor connecting it to the exposed :
Conductor part. 50V is the maximum voltage limit ( it May be 25V in certain circumstances ).
( Ia ) is the value of current that causes automatic disconnection : of the protective device within 0.1 seconds

Principles of RCD Measurement :
The RCD tester is connected between phase and protective on the load side of the RCD after disconnecting the load.
A precisely measured current for a carefully timed period is drawn from the phase and returns via the earth, thus tripping the
device. The instrument measures and displays the exact time taken for the circuit to be opened ,
An RCD is a switching device designed for breaking currents when the residual current attains a specific value It works on the
basis of current difference between phase currents flowing to different loads and returning current flowing through the neutral
conductor (for a single-phase installation). In the case where the current difference is higher than the RCD tripping current, the
device will trip and disconnect the supply from the current There are two parameters for RCDs; the first due to the shape
of the residual current wave form (types AC and A) and the second due to the tripping time (types G and S). A typical RCD is AC-G.
▲ RCD type AC will trip when presented with residual sinusoidal alternating currents whether applied suddenly or slowly
rising. This type is the most frequently used on electrical installations :
▲ RCD type A will trip when presented with residual sinusoidal alternating currents (similar to type AC) and residual pulsating direct currents (DC) whether suddenly applied or slowly rising. This type of RCD is not commonly used at present, however, it is
increasing in popularity and is required by the local regulations in some countries
▲ RCD type G. In this case G stands for general type (without trip-out time delay) and is for general use and applications
▲ RCD type S where S stands for selective type (with trip- out time delay).This type of RCD is specifically designed for


Auto Ramp : Check your RCD settings , 1 x / 5 x ,
The RCD should trip. Check Trip Out Current.
(1) Press the 0°/180°switch to change the phase and repeat step

Heres the Good New,s Chaps , am doing my 2392-10 on Friday 23/ 10/ 09
Am OFF the Air for One Week , I’ve left you some things to be going on with , PS kicking A--- Amberleaf
am in Manchester ,
PS, this could be a 6-pack ? on Friday