Re-take - Useful Information for 2394 :

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( 1 ) Inspection & Testing 2394 & 2395

Section B - All questions carry equal marks , Answer all three questions . show all calculations

Questions 4 to 6 all refer to the enclosed scenario , see source document . Ensure you read this scenario before attempting these questions

4 a) The loop length for the office ring final circuit is 60m and all the socket-outlets are connected directly into the ring , Determine , showing all calculations

i) the expected ( R[SUP]1[/SUP] + R[SUP]2[/SUP] ) value ... 7 marks

ii) the expected measured ( Zs ) value ... 4 marks

b ) When measuring ( Zs ) for the toilet lighting circuit the circuit breaker operates .
State why the circuit breaker tripped during the earth fault loop impedance test ... 4 marks

5) The circuit for the tree phase saw bench is installed to the local isolator for the saw.
a) A test of earth fault loop impedance is to be carried out on the saw circuit .

i) State the test instrument to be used . ... 2 marks
ii) State the document that specifies the requirements for the test leads ... 2 marks
iii) Describe , in detail , how the test would be carried out . .. 6 marks

b) i) Determine , using the information in Figures 1 and 2 , the expected ( Zs ) value for the circuit . Show all calculations .. 3 marks

ii) Explain , in detail , why the measured value for this circuit is lower than the value calculated in b) i) above .. 2 marks

6 ) Describe , with the aid of a fully labelled diagram , the earth fault loop path for the outside lighting circuit . .. 15 marks

 

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( 2 )

Section B -
Remember that the answers to the following questions must relate to the scenario contained in the Source Document .

4 ) a ) The loop length for the office ring final circuit is 60m and all the socket-outlets are connected directly into the ring , Determine ,
showing all calculations

i) the expected ( R[SUP]1[/SUP] + R[SUP]2[/SUP] ) test value .. 7 marks

Answer : r[SUP]1[/SUP] + r[SUP]2 [/SUP] = 60 x ( 7.41 + 12.10 ) 1000 = 1.17Ω .. ( sum in a sum ; 7.41 + 12.10 = 19.51 ; 60 x 19.51 ÷ 1000 = 1.17Ω )
R[SUP]1[/SUP] + R[SUP]2 [/SUP]= 1.17 ÷ 4 = 0.293Ω

There is more than one way to determine this value but any correct method would be given the marks . The use of a temperature correction factor in this calculation is not appropriate because the question asks for an expected " test " value and the Source Document states that testing is to be carried out at ( 20ºC ) which is the same temperature as that which applies to the mΩ/m values in Fig 2

The calculation could have also been laid out as show below ;

r[SUP]1 [/SUP]= 60 x 7.41 / 1000 = 0.445Ω
r[SUP]2 [/SUP] = 60 x 12.10 / 1000 = 0.726Ω
R[SUP]1[/SUP] + R[SUP]2 [/SUP]= 0.445 + 0.726 / 4 = 0.293Ω .... ( sum in a sum ; 0.445 + 0.726 = 1.171 ÷ 4 = 0.29275 . ( 0.293Ω )

4 a) ii) the expected measured ( Zs ) value .. 4 marks
Answer ; Zs = Ze + R[SUP]1[/SUP] + R[SUP]2 [/SUP]= 0.11 + 0.29 = 0.40Ω

C .
The answer to this question is dependent upon the answer given in, a) i) above . it is not normal practice to penalise a candidate twice for the same error. With this in mind , marks would be awarded for the correct formula , correct total , including units .

4 b) When measuring ( Zs ) for the toilet lighting circuit the circuit breaker operates , State why the circuit breaker tripped during the earth fault loop impedance test .. 4 marks

Answer :
The test current may be greater than the instantaneous tripping current of the circuit breaker causing it to operate .

C .
The examiner is looking for any statement that indicates an understanding of what has happened , Reference to an RCD tripping would not gain any marks as the circuit is not protected by an RCD , This information is shown in Figure 1 . in the Source Document .

5) The circuit for the three phase saw bench is installed to the local isolator for the saw .
a) A test of earth fault loop impedance is to be carried out on the saw circuit .
i) State the test instrument to be used ... 2 marks

5 a) ii) State the document that specifies the requirements for the test leads .. 2 marks

Answer : Leads to GS-38 .

5 a ) iii) Describe , in detail , how the test would be carried out .. 6 marks

Answer :
Supply on
Local isolator
• Access live terminals in the local isolator
• At the incoming terminals of the local isolator test
º L1 to isolator earthing terminal
º L2 to isolator earthing terminal
º L3 to isolator earthing terminal
• Close the local isolator cover
• Record highest result

 

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( 3 )

C .
The structure of the question is intended to help the candidate identify all relevant information .

A list has been used to answer a) iii) because it is easy to write down , easy to check and time efficient . Each statement must include sufficient information to make it clear how the test would be carried out . it is not necessary to include the instrument title and lead requirement as part of this answer as they have already been identified in i) and ii) above

The test is carried out between each line conductor and the isolator earthing terminal . if a candidate described the isolator earthing terminal as " the earthing terminal " earth " or " the cpc " this would also gain the marks.

if the wrong test was described in the answer , such as R[SUP]1[/SUP] + R[SUP]2 [/SUP]test and the results is then added to ( Zs ) then no marks would be awarded . The question specifically asks for an earth fault loop impedance test and not any test method that could be used to determine ( Zs )

Answers that include dangerous procedures would score zero marks .

5 b ) i) Determine , using the information in Figure 1 & 2 , the expected ( Zs ) value for the circuit . Show all calculations ... 3 marks

Answer : Zs = Ze + R[SUP]1[/SUP] + R[SUP]2 [/SUP] = 0.11 + 2 x 7.41 x 15 / 1000 = 0.11 + 0.22 = 0.33Ω

C .
Marks are awarded for each stage of the calculation . Setting out the calculation and showing each step makes it easy for the examiner to follow the process . it also helps the examiner award marks when the answer is not fully correct . Answers that are not clear , do not contain all steps and contain errors are likely to score fewer marks than an answer containing errors but is clearly laid out . This is because in the latter case the examiner can easily identify the correct parts of the calculation.

5 b ) ii) Explain , in detail , why the measured value for this circuit is lower than the value calculated in b) i) above .. 2 marks

Answer : Résistance ( R[SUP]2[/SUP] ) of is reduced due to steel conduit being in parallel with the cpc .

C .
Although the answer is only worth 2 marks , the question does state " in detail " The examiner is looking for a complete answer , Answers such as " parallel paths " will not score 2 marks as this is not in detail .

6 ) Describe , with the aid of a fully labelled diagram , the earth fault loop path for the outside lighting circuit 15 marks .

Answer : Drawing . TN-C-S System Earth Fault .

The fault path is :
from the point of fault
Along the cpc to the MET
from the MET via the earthing conductor to the supplier's PEN terminal
Along the PEN conductor to the supply transformer winding
Through the transformer winding , along the line conductor to the fault

C .
This answer contains both a fully labelled diagram and a description of the path , Providing the diagram is very clear and the path is shown on the diagram , as is the case above , then full marks would be awarded , The description of the path gives additional information for the examiner to consider when awarding marks . it is in the interest of the candidates to give as much information as possible . if an incorrect system is described / drawn then no marks would be awarded . This is also true if the diagram is incomplete and the type of system cannot be determined .

 

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Source Document . ◄ ▼▼▼
Scenario ( Section B - Questions 4 to 6 )

An existing building has been converted into a builders store on the ground floor with the offices above The electrical installation and supply to the building forms part of a 400 / 230V TN-C-S system . ( Ze is 0.11Ω ) and the prospective fault current ( Ipƒ ) is 4.1 kA

All circuits within the store area are wired in ( 70ºC ) thermoplastic single-core cables , with copper conductors , in surface mounted , galvanised steel trunking and conduit .

All circuits in the office area are wired in ( 70ºC ) flat thermoplastic multicore and cpc cable installed in pvc trunking and the ceiling voids .

Inspection and testing is to be carried out in an ambient temperature of ( 20ºC )

detail of conductor résistance in mΩ/m at ( 20ºC ) are shown in ( Figure 2 )

( Figure 2 )
Conductor size mm[SUP]2 [/SUP]1.5 ............ Résistance in mΩ/m at ( 20ºC ) 12.10
Conductor size mm[SUP]2 [/SUP]2.5 ............ Résistance in mΩ/m at ( 20ºC ) 7.41
Conductor size mm[SUP]2 [/SUP]10 ............ Résistance in mΩ/m at ( 20ºC ) 1.83

One drawing to come . ► ( Figure 1 ) worst case scenario I'll type it out

 

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Question 2394 :

What is faster that Usain Bolt , My fingers typing . Gentlemen please hit the thank you button at your own leisure

 

[Richard Burns]

Source document for Section B scenario, the answers to the above questions must relate to this scenario.

 

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New :

(*1 ) 2394 : Q/As On Paper you're Inspecting & Testing . To prove yourself . As deemed by BS-7671: & GN-3

Answers , & Comments and advice where appropriate .

Section A
1 a) i) State one statutory document to which the inspector may refer whilst carrying out an inspection and test of an electrical installation .. (1/M)

Ans ; The Electricity at Work Regulation 1989 ◄ The Wording The correct meaning . end off

C;
because the question has asked of one answer , only the first answer given by the candidate would be considered , This is because candidates sometimes write down everything they can remember in the hope that one of the answers may be correct .
This is not showing ( KNOWLEDGE or UNDERSTANDING ) ........................ Make a statement

" ► " EaWR " or " EWR " are acceptable , ►►►►► Electricity at Work " ACT " ►►►►► is incorrect

( PS what does Point's make Prizes ) Your Ticket .

1 a) ii) State tow non-statutory publications to which the inspector may refer whilst carrying out an inspection and test of an electrical installation .. ( 2/M )

Ans ; BS-7671:2008(2011) & IET Guidance Note 3 .. ◄ Your Q . The Wording The correct meaning . end off

C;
Because the question has asked for two answers , only the first two answers given by the candidate would be considered , Other Guidance Notes would be acceptable but the MOST relevant is Guidance Note 3 .

The On-Site Guide is a perfectly acceptable alternative answer . ( Note not ◄► O.S.G. & GN-3 ) Full wording at all times .

 

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(*2 ) 2394 :

1 a) iii) State the title given in ( LAW ) to the inspector whilst carrying out the inspection of an electrical installation ... 1/M

Ans ; Dutyholder

C;
This question refers to the requirements of The Electricity at Work Regulation's and the term " Dutyholder " refers to anyone having a duty of care to other

1 a) iv) State the legal status of the inspector .. 1/M

Ans ; Competent

C;
Regulation 16 of The Electricity at Work Regulation's requires persons to be competent when carrying out any activity on an electrical system where the work may give rise to danger .

1 b) State the scope of the Minor Electrical Installation Works Certificate ... 2/M

Ans ; Alteration or addition to an existing circuit .

C;
A key part of this answer is the reference to " Existing Circuit " An alteration or addition to an existing installation is NOT within the SCOPE of a Minor Electrical Installation Works Certificate ............ 2394 : Take heed

Any description covering the essential points " alteration " or " addition and " existing circuit " would get the marks .

 

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(*3 ) 2394 : you will get this one . Yeah

2 a ) i) List five items to be inspected on PVC conduct system .. 5/M

Ans ;
• Conduit system overall
• Fixings
• Bends
• Draw-in points
• Joints

2 a) ii) Explain what is being inspected for each of the items identified in a ) i ) above .. 5/M

Ans ;
• Conduit system overall - conduit system is complete
• Fixings - Sufficient saddles fixed correctly
• Bends - Bending radius not too tight and bend not damaged
• Draw-in points - Sufficient number so that cables can be draw in without damage
• Joints - are secure and fit correctly between lengths and at boxes

C;
The question is in ( Two parts ) i) asks for what is to be inspected and ii) requires the candidate to identify precisely the nature of the ( INSPECTION ) Sometimes these ( Two parts ) are combined into one question

it is important for the candidate to state clearly what is being ( INSPECTED ) and what in particular they would be looking for

" Conduit system not damaged " " conduit complies with appropriate British Standard " , " expansion joints fitted where necessary " and " bushes are tight " are all acceptable answers

Any answer relating to cables , cable terminations and conduit capacity would not be acceptable , although the capacity of the conduit could have been addressed by reference to the conduit being the correct size specified by the designer

 

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(*4 ) 2394 :

3 a ) Explain , in detail , why an earth fault loop impedance test would need to be carried out on ( Existing circuits ) after the changing of a consumer unit within a domestic installation .. 5/M

You're Q here . ▼▼▼ Have you altered any characteristics ; On Paper you're Inspecting & Testing . To prove yourself . As deemed by BS-7671: & GN-3 . PS we are all sing out the same hymn sheet here .

Ans ;

Existing circuit protective devices have been removed and new protective devices have been fitted , The characteristics of the new devices are likely to be different for previous devices , Therefore it must be confirmed that the earth fault loop impedance is low enough so that the required disconnection times will be met .

C;
The question ask for an explanation " in detail " so the description needs to be comprehensive if full marks are to be awarded , Answers that referred to disconnection and re-connection of conductors and the reliability of those connections would score marks

3 b) i) Explain , why the earthing conductor in an installation must be disconnected from the MET when measuring ( Ze ) ... 2/M

Ans ; To remove parallel earth paths so that the intended fault path can be confirmed to be reliable

C;
if the parallel earth paths are not removed during the test then the reliability of the test result is in DOUBT .

 

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(*5 ) 2394 :

3 b ) ii) Explain why the earthing conductor is connected to the MET when measuring prospective earth fault current .. 2/M

Ans ; This is the condition that exists at the time of a fault and therefore is the only way to determine the maximum fault current to ( EARTH ) when carrying out the test .

C;
in this case the purpose of the test is to ( Determine ) the maximum prospective earth fault current , This will occur when the installation is energised and all earthing arrangements are in place , so the test must be carried out under these conditions .

3 c) i) List the measurements to be taken , at the main switch of a three-phase TN-S system , in order to determine the installation prospective fault current ( Ipƒ ) .. 3/M

Ans ;
• L1 to L2
• L1 to L3
• L2 to L3

C;
There is no need to carry out any other tests to determine ( PFC ) because on a three-phase system the largest fault current will be due to a symmetrical short circuit , that is a short circuit between all line conductors' at the same time . A test between line conductors is an acceptable approximation . if additional tests were included in the answer then this would not make the answer invalid .

The measurement of , L1 to N , L2 to N and L3 to N would be an acceptable Alternative method see answer to ( 3 c ii below )

3 c ) ii) Explain how the recorded value of prospective fault current ( Ipƒ ) is determined following the test in c) ii) above .. 3/M

Ans ; The highest value of the three measurements

C;
3 c ) i) and ii) are linked and the answer to ii) is dependent on the test method used in i) if the answer to i) was to measure between L & N then this answer would be " the largest value x 2 " if the answer to i) included tests to earth then this answer must state 2 the largest line to neutral value x 2 "

 

[Richard Burns]

From Post 753
6 ) Describe , with the aid of a fully labelled diagram , the earth fault loop path for the outside lighting circuit 15 marks .

Answer : Drawing . TN-C-S System Earth Fault .

The fault path is :
from the point of fault
Along the cpc to the MET
from the MET via the earthing conductor to the supplier's PEN terminal
Along the PEN conductor to the supply transformer winding
Through the transformer winding , along the line conductor to the fault

 

[Richard Burns]

Section B

 

[Richard Burns]

Distribution Layout: figure two

 

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Useful junk

Connection to the fixed wiring :

A luminaire is defined in Part 2 of BS-7671: P/30 As

‘ Luminaire. Equipment which distributes, filters or transforms the light transmitted from one or more lamps and which includes all the parts necessary for supporting, fixing and protecting the lamps, but not the lamps themselves, and, where necessary, circuit auxiliaries together with the means for connecting them to the supply .’

Means of connection at lighting points
At each fixed lighting point or position, Regulation 559.6.1.1 requires the fixed wiring to be connected to one of the following:
□ a ceiling rose to BS-67
□ a batten lampholder or a pendant set to BS EN 60598

Batten lampholders
Pendant sets .. Etc

Where a pendant set is installed, the accessory used as the means of connection with the fixed wiring installation is required to be suitable for supporting the mechanical load to be suspended Regulation 559.6.1.5

559.6.1.5 Adequate means to fix luminaires shall be provided .