Answers City and Guilds 2391 December 2009 ExamSection A- All questions carry equal marks. Answer all twenty questions 1. Electrical installations require inspection and testing throughout their working life. State three separate occasions when inspection and testing must be undertaken, excluding periodic inspection. Pg 15 - New installations, additional work, alterations, minor works 2. State three items of information that are required by a person undertaking the inspection and test of an installation, excluding information contained on diagrams, charts or tables. Pg18 - Maximum demand in amperes, number and type of live conductors, type of earthing arrangement, the nominal voltage and frequency, prospective fault current and Ze external earth fault loop impedance, the type and rating of the protective device. 3. State the legal status for each of the following documents. a)GS38 non-statutory b)GN3 non-statutory c)B S 7671 non statutory 4. State three different practical situations that may require the use of the sense of touch during the inspection process. Pg 27-32 Feel for excessive heat from circuit breakers or pvc/pvc cable. Feel for loose bolts connecting sections of cable trays, feel exposed bonding cable and clamps for tightness. 5. List three items to be checked, as identified on the Schedule of Inspections, where fault protection is provided by automatic disconnection of supply (ADS). Pg 22 & 92 Check for presence of Main bonding, earthing, circuit protective conductors, choice and setting of protective devices 6. State three requirements for test probes and/or leads as required by GS38 No more than 4mm exposed tip of probe 2mm preferred, finger guards, fused test probe, leads long enough for safe use, leads in good order no obvious damage 7. State the correct instrument for conducting each of the following tests. a)Continuity of ring final circuits. Low resistance ohmmeter pg 82-84 b)External earth fault loop impedance on a TN system. Earth fault loop impedance tester c)Earth electrode resistance for a generator installation. Earth electrode resistance tester 8. State why, for safety reasons, a) inspection must take place before testing Hazards such as exposed live parts, dangerous methods of erection and unexpected and dangerous environmental conditions b) BS 7671 identifies a sequence of tests to be followed before the installation is energised If protective conductor continuity is not confirmed then because this could be a break or a loose connection there would be no point in insulation testing as this would not go beyond the break c) the installation polarity is checked when the supply is connected for the first time. the supplier polarity could be reversed or the electrician may incorrectly connect the tails in reverse. 9. List three different protective conductors that may be present in an electrical installation. Earthing conductor, supplementary bonding conductor, main protective bonding conductor 10. State the effect each of the following would have on the measured resistance of a conductor. a)Increase in length. Increase resistance b)Decrease in CSA. Increase resistance c)Increase in temperature. Increase resistance 11 State d)a) two test methods to verify the continuity of protective conductors. Method 1 connect the line R1 to the cpc R2 at the distribution board and measure resistance at the end of the circuit between R1 and R2. Method 2 Wander lead method e)b) the test instrument to be used to conduct the tests in a) above. Low resistance ohmeter 12 List three items that would be recorded on a Schedule of Test Results, on completion of a fault-free ring final circuit continuity test. Resistance of R1&R2, End to End resistance of R1, 1

Rn and R2 and polarity would be confirmed. 13 The following results were recorded at Stage 1 of a fault-free ring final circuit continuity test. Line 0.36 Neutral 0.35 Cpc 0.59 State a)why the resistance of the cpc is greater than the resistance of the line and neutral conductors the greater resistance of the cpc is due to its smaller csa. b)the expected value of resistance at Stage 3, when the line and cpc are correctly interconnected at the distribution board r1+r2/4 = .36+.59/4 = .95/4 = 0.24 c)the pattern of results in b) above if the line and cpc were incorrectly interconnected at the distribution board. Progressive increase in resistance to midpoint then decreasing toward other end of ring. Look at the bottom of page 36 in gn3, not sure what you mean as a ring in the distribution has ends, anyway its gn3 quote, check it out 14 A lighting circuit has been securely isolated from the supply. State three actions which must be carried out to prepare the circuit before carrying out an insulation resistance test between live conductors.pg Disconnect any loads, all switches on, bypass or remove any voltage sensitive equipment and pilot lamps It says the protective device has been turned off for just that lighting circuit so not sure what the point is of saying all protective devices on as in this case its a dead test on that circuit turned off from a live supply 15 State the minimum acceptable value of insulation resistance for each of the following. a)400 V three-phase compressor. 1M b)650 V three-phase kiln. 1M c)12 V SELV lighting circuit. 0.5M 16 List three special installations/locations identified in Part 7 of BS 7671 :2008 excluding; locations containing a bath or shower, saunas, swimming pools and fountains. Restricted movement in Conducting locations, caravans, underfloor heating, marinas, circus 18 An earth electrode resistance test is to be carried out using an earth electrode resistance tester. State the recommended pg 46&47 a)soil conditions during testing At their driest to record highest resistance. b)distance between an earth electrode which is 3 m in length, and the furthest test spike 30m c)minimum number of test readings required. 3 readings and record the average 19 State a) the maximum test current to be applied to a 500 mA RCD 1x = 500mA b)why the test button on an RCD should be operated quarterly verifies operation of mechanical parts of the RCD are functional c)at what other time should the test button on an RCD be operated. At initial verification or Periodic inspection and Test 20 A prospective fault current test is to be carried out on a single phase electrical installation. State a)the two tests which are to be undertaken Test between the line and neutral for the maximum PSCC and between the line and earth for the maximum PEFC b)which value from a) above will be recorded as the PFC. The highest value above

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Answers City and Guilds 2391 December 2009 ExamSection B All questions carry equal marks. Answer all six questions. Show all calculations Question 21 to 26 all refer to the following scenario. A multi-level underground car park operating 24 hours a day has undergone a complete re-wire of the lighting installation within the car parking areas and stair wells. A new ring final circuit has also been installed in the office area. The car park is to remain operational whilst work takes place, and all relevant documentation for the new and existing installations is to be issued on completion of the work. The electrical installation forms part of a 400/230 V three-phase four wire TN-S system, with an external earth fault loop impedance value of 0.4 . A new TP&N distribution board fitted with BS EN 60898 type C circuit breakers has been installed for the new lighting circuits. The new fluorescent luminaires are surface mounted. The ground floor luminaires are controlled by a photocell, and the remaining luminaires are controlled solely by the circuit breakers. The installation is carried out in multicore steel wire armoured cables, having 70 C thermoplastic insulation cables with 2.5 mm2 copper conductors. These cables are clipped direct to cable tray throughout the installation. One of the cores of the cable is used as the cpc. The ring final circuit is wired in thermoplastic insulated single core copper cables which are installed in surface mounted pvc trunking and conduit. Conductor resistance (m /m) CSA(mm2) 1.5 2.5 12.1 7.41

21 State a)the two types of inspection and test required for this installation Initial Verification and Alterations and Additions Not sure if this is right, do you agree, I think you have to do additional because you should look at things like existing earthing, bonding etc. b)all the documents that need to be completed following the inspection and testing in a) above Electrical Inspection Certificate, Schedule of Inspection and Schedule of Test Results c)three characteristics of the supply that would need to be recorded on the documents in b) above Type of earthing system, number and type of live wires, Nominal voltage pg18 d)who is responsible for recommending the period to the subsequent inspection and tests. The designer pg 19 gn3 and BS7671 22 Describe, in detail, how an insulation resistance test would be carried out on one of the completed ground floor lighting circuits. Your answer should at least include initial safety considerations circuit preparation test procedure instrument to be used minimum acceptable value of insulation resistance test voltage to be applied final safety considerations. The person testing must ensure that the test instruments have been; Checked there is no damage Checked that the instruments battery is in a good healthy condition Checked that the test leads are not damaged, frayed, and that they comply with GS38 Standards, have finger guards, and that exposed tips do not exceed 2mm or 4mm if protected by retractable shroud 3

The person undertaking the testing must ensure the safety of him or herself and of others and livestock. The tester must ensure these isolation procedures are followed. Permission is sought to begin the testing from the client, owner, occupier Before starting testing, persons in the vicinity are informed of the testing about to commence. Warning notices, barriers and obstacles are in place The circuit is safely isolated/switched off at the distribution board The circuit is LOCKED OFF and the locking key is retained, and the warning notice tagged. If the isolation device involves the removal of a fuse, it must be kept with the person testing. Using an approved voltage tester, prove test instrument is working. Using the test instrument, test that the circuit is dead. Using an approved voltage tester, prove test instrument is working.

Disconnect any loads, all switches on, bypass or remove any voltage sensitive equipment such as the photocell and pilot lamps which may be damaged or effect the test results. Warn client of test to take place and post notices. Test line to neutral at the distribution board and operate all switches and record the highest reading. Test line to cpc at the distribution board and operate all switches and record the highest reading. The instrument is an insulation resistance tester, test voltage is 500vdc and insulation resistance not less than 1Mohm any reading less than 2Mohm should be investigated. After test Re-instate all connections and equipment, remove all bypasses and inform client about restoring power and re-instate power safely as in bullet points above. 23. State five items to be checked during the inspection, which relate specifically to the erection of the SWA cable in the car parking areas, excluding the construction of the cable tray system. Adequately supported and fixed, bending radii within limits, not to tight, termination of glands is correct, earthing via bolted earth tags with paint scraped away for metal to metal contact, shrouds on glands. 24. A continuity of ring final circuit conductors test is to be carried out on a circuit in the car park. The circuit is wired in 2.5 mm2 cable with a 2.5 mm2 cpc. The loop length of the cable is 18m. All socket outlets are connected directly on the ring. Using the information given in Figure 1, explain, in detail, the three steps required to confirm correct installation and connection of the circuit. Your explanation should include the test instrument and, at each step, the test connections and the expected readings. Show all calculations. Step 1-3 all require the use of a low resistance ohmmeter the can measure accurately to 0.2ohms with a resolution of 0.01ohms. The following diagram will show the connections necessary and the expected readings are below. These are dead tests so the circuit will have already been isolated. The meter needs to be checked for healthy batteries that it has no damage and complies to GS38 standard The meter leads should be nulled or zeroed to take account of the resistance of the leads Since the csa of all cables are the same the calculations regarding the expected reading in Steps 2 and 3 will be the same. When carrying out Steps 2 and 3 every socket must be measured and the socket that 4

Answers City and Guilds 2391 December 2009 Examhas the highest reading for R1 and R2 can be noted down for reference and comparison. For Step 3 find the highest reading after measuring every socket and record as R1 + R2 on the Schedule of Test Results. Calculations Step 1 all conductors same csa 2.5mm2. 18m x .00741= 0.13 for R1, Rn and R2 each measured end to end Step 2 .= R1+Rn/4= 0.13+0.13/4 = 0.065 should be the result at every socket Step 3 = R1+R2/4= 0.13+o.13/4 = 0.065 should be the result at every socket

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Determine the expected value for Z s for one of the lighting circuits where the ambient temperature is 20C and the circuit length is 35 m. 35m x .00741 per meter =0.26 pg51 Zs= Ze+(R1+R2) = 0.4+(0.26+0.26)= 0.4+0.52= 0.92 b)Explain why the measured value of Z s is likely to be less than the value determined in a) above. The calculated value does not take into account multiple parallel earth paths c)Determine the maximum acceptable measured value for Z s if the BS7671 tabulated value is 1.44 . Zs x 0.8 rule of thumb = 1.44 x 0.8 = 1.15 d)State the two conditions that are compensated for in c) above. 80% Rule of thumb compensates for the increase in temperature due to load current which increases the resistance of the cable and the 80% also accounts for parallel earth paths. 26. Draw a fully labelled diagram showing the complete earth fault loop path for one of the new lighting circuits installed in the car park. The diagram should identify the correct system, the complete fault path and all the component parts of the earth fault loop.

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