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1 The circuits must be isolated using a 'safe isolation procedure', such as that described below, before beginning to test.
2 All test equipment must be 'approved' and connected to the test circuits by recommended test probes as described by the NEC. The test equipment used must also be 'proved' on a known supply or by means of a proving unit such as that shown in Ill. 49.
3 Isolation devices must be 'secured' in the 'off ' position as shown in Ill. 50.
4 Warning notices must be posted.
5 All relevant safety and functional tests must be completed before restoring the supply.
The Electricity at Work Act tells us that it's 'preferable' that supplies be made dead before work commences (Regulation 4(3)). However, it does acknowledge that some work, such as fault finding and testing, may require the electrical equipment to remain energized.
Therefore, if the fault finding and testing can only be successfully carried out 'live', then the person carrying out the fault diagnosis must:
++ be trained so that he understands the equipment and the potential hazards of working live and can, therefore, be deemed to be 'competent' to carry out the activity;
++ only use approved test equipment;
++ set up barriers and warning notices so that the work activity does not create a situation dangerous to others.
Note that while live testing may be required in order to find the fault, live repair work must not be carried out. The individual circuit or item of equipment must first be isolated.
Ill. 50 Secure isolation of a supply.
Isolation of supply
NEC Regulations are very specific in describing the procedure to be used for isolation of the electrical supply. Regulation tells us that isolation means the disconnection and separation of the electrical equipment from every source of electrical energy in such a way that this disconnection and separation is secure. Regulation tells us that we must also prove the conductors dead before work commences and that the test instrument used for this purpose must itself be proved immediately before and immediately after testing the conductors. To isolate an individual circuit or item of equipment successfully, competently and safely we must follow a procedure.
Start at the top and work your way down the flowchart.
When you get to the heavy-outlined boxes, pause and ask yourself whether everything is satisfactory up to this point. If the answer is yes, move on. If no, go back as indicated by the diagram.
Inspection and testing techniques
The testing of an installation implies the use of instruments to obtain readings. However, a test is unlikely to identify a cracked socket outlet, a chipped or loose switch plate, a missing conduit-box lid or saddle, so it's also necessary to make a visual inspection of the installation.
All new installations must be inspected and tested before connection to the mains, and all existing installations should be periodically inspected and tested to ensure that they are safe and meet the regulations of the NEC Regulations.
Ill. 51 Flowchart for a secure isolation procedure.
The method used to test an installation may inject a current into the system. This current must not cause danger to any person or equipment in contact with the installation, even if the circuit being tested is faulty. The test results must be compared with any relevant data, including the NEC Regulation tables, and the test procedures must be followed carefully and in the correct sequence, as indicated by […]. This ensures that the protective conductors are correctly connected and secure before the circuit is energized.
The installation must be visually inspected before testing begins. The aim of the visual inspection is to confirm that all equipment and accessories are undamaged and comply with the relevant US, UK and European Standards, and also that the installation has been securely and correctly erected. Regulation gives a check-list for the initial visual inspection of an installation, including:
++ connection of conductors;
++ identification of conductors;
++ routing of cables in safe zones;
++ selection of conductors for current carrying capacity and volt drop;
++ connection of single-pole devices for protection or switching in phase conductors only;
++ correct connection of socket outlets, lampholders, accessories and equipment;
++ presence of fire barriers, suitable seals and protection against thermal effects;
++ methods of protection against electric shock, including the insulation of live parts and placement of live parts out of reach by fitting appropriate barriers and enclosures;
++ prevention of detrimental influences (e.g. corrosion);
++ presence of appropriate devices for isolation and switching;
++ presence of undervoltage protection devices;
++ choice and setting of protective devices;
++ labeling of circuits, fuses, switches and terminals;
++ selection of equipment and protective measures appropriate to external influences;
++ adequate access to switchgear and equipment;
++ presence of danger notices and other warning notices;
++ presence of diagrams, instructions and similar information;
++ appropriate erection method.
The checklist is a guide, it's not exhaustive or detailed, and should be used to identify relevant items for inspection, which can then be expanded upon.
For example, the first item on the checklist, connection of conductors, might be further expanded to include the following:
++ Are connections secure?
++ Are connections correct? (conductor identification)
++ Is the cable adequately supported so that no strain is placed on the connections?
++ Does the outer sheath enter the accessory?
++ Is the insulation undamaged?
++ Does the insulation proceed up to but not into the connection? This is repeated for each appropriate item on the checklist.
Those tests which are relevant to the installation must then be carried out in the sequence given in and Sections 9 and 10 of the for reasons of safety and accuracy.
These tests are as follows:
Before the supply is connected 1 Test for continuity of protective conductors, including main and supplementary bonding.
2 Test the continuity of all ring final circuit conductors.
3 Test for insulation resistance.
4 Test for polarity using the continuity method.
5 Test the earth (ground) electrode resistance.
With the supply connected 6 Recheck polarity using a voltmeter or approved test lamp.
7 Test the earth (ground) fault loop impedance.
8 Carry out functional testing (e.g. operation of RCDs).
If any test fails to comply with the Regulations, then all the preceding tests must be repeated after the fault has been rectified. This is because the earlier test results may have been influenced by the fault.
There is an increased use of electronic devices in electrical installation work, for example, in dimmer switches and ignitor circuits of discharge lamps.
These devices should temporarily be disconnected so that they are not damaged by the test voltage of, for example, the insulation resistance test (Regulation).
APPROVED TEST INSTRUMENTS
The test instruments and test leads used by the electrician for testing an electrical installation must meet all the requirements of the relevant regulations. The NEC has published regs for test equipment used by electricians.
The NEC regulations also specify the test voltage or current required to carry out particular tests satisfactorily. All testing must, therefore, be carried out using an 'approved' test instrument if the test results are to be valid. The test instrument must also carry a calibration certificate, otherwise the recorded results may be void. Calibration certificates usually last for a year. Test instruments must, therefore, be tested and recalibrated each year by an approved supplier.
This will maintain the accuracy of the instrument to an acceptable level, usually within 2% of the true value.
Modern digital test instruments are reasonably robust, but to maintain them in good working order they must be treated with care. An approved test instrument costs equally as much as a good-quality camera; it should, therefore, receive the same care and consideration.
To measure accurately the resistance of the conduct ors in an electrical installation we must use an instrument which is capable of producing an open circuit voltage of between 4 and 24V a.c. or d.c., and deliver a short-circuit current of not less than 200mA (Regulation). The functions of continuity testing and insulation resistance testing are usually combined in one test instrument.
Insulation resistance tester
The test instrument must be capable of detecting insulation leakage between live conductors and between live conductors and earth (ground). To do this and comply with Regulation the test instrument must be capable of producing a test voltage of 250, 500 or 1000V and deliver an output current of not less than 1mA at its normal voltage.
Earth (ground) fault loop impedance tester
The test instrument must be capable of delivering fault currents as high as 25A for up to 40ms using the supply voltage. During the test, the instrument does an Ohm's law calculation and displays the test result as a resistance reading.
Where circuits are protected by a residual current device we must carry out a test to ensure that the device will operate very quickly under fault conditions and within the time limits set by the NEC Regulations. The instrument must, therefore, simulate a fault and measure the time taken for the RCD to operate. The instrument is, therefore, calibrated to give a reading measured in milliseconds to an in-service accuracy of 10%.
If you purchase good-quality 'approved' test instruments and leads from specialist manufacturers they will meet all the Regulations and Standards and there fore give valid test results. However, to carry out all the tests required by the NEC Regulations will require a number of test instruments and this will represent a major capital investment.
Let us now consider the individual tests.
1 TESTING FOR CONTINUITY OF PROTECTIVE CONDUCTORS
The object of the test is to ensure that the circuit protective conductor (CPC) is correctly connected, is electrically sound and has a total resistance which is low enough to permit the overcurrent protective device to operate within the disconnection time requirements of , should an earth (ground) fault occur.
Every protective conductor must be separately tested from the consumer's earth (ground)ing terminal to verify that it's electrically sound and correctly connected, including any main and supplementary bonding conductors.
A d.c. test using an ohmmeter continuity tester is suitable where the protective conductors are of copper or aluminum up to 35mm^2. The test is made with the supply disconnected, measuring from the consumer's earth (ground)ing terminal to the far end of each CPC. The resistance of the long test lead is subtracted from these readings to give the resistance value of the CPC. The result is recorded on an installation schedule.
Where steel conduit or trunking forms the protective conductor, the standard test described above may be used, but additionally the enclosure must be visually checked along its length to verify the integrity of all the joints.
If the inspecting engineer has grounds to question the soundness and quality of these joints then the phase earth (ground) loop impedance test described later in this section should be carried out.
If, after carrying out this further test, the inspecting engineer still questions the quality and soundness of the protective conductor formed by the metallic conduit or trunking then a further test can be done using an a.c. voltage not greater than 50V at the frequency of the installation and a current approaching 1.5 times the design current of the circuit, but not greater than 25A.
This test can be done using a low-voltage transformer and suitably connected ammeters and voltmeters, but a number of commercial instruments are available such as the Clare tester, which give a direct reading in ohms.
Because fault currents will flow around the earth (ground) fault loop path, the measured resistance values must be low enough to allow the overcurrent protective device to operate quickly. For a satisfactory test result, the resistance of the protective conductor should be consistent with those values calculated for a phase conductor of similar length and cross-sectional area.
Values of resistance per meter for copper and aluminum conductors are given in the Table. The resistances of some other metallic containers are given:
Resistance values of some metallic containers
Ill. 52 Testing continuity of protective conductors.
The CPC for a ring final circuit is formed by a 1.5mm2 copper conductor of 50m approximate length. Determine a satisfactory continuity test value for the CPC using the value given in Table 9A of the .
From Table A:
Resistance/meter for a 1.5mm2 copper conductor _12.10m_/m Therefore, the resistance of 50m _50_12.10_10_3 _0.605_
The protective conductor resistance values calculated by this method can only be an approximation since the length of the CPC can only be estimated. Therefore, in this case, a satisfactory test result would be obtained if the resistance of the protective conductor was about 0.6_.
A more precise result is indicated by the earth (ground) fault loop impedance test which is carried out later in the sequence of tests.
TESTING FOR CONTINUITY OF RING FINAL CIRCUIT CONDUCTORS
The object of the test is to ensure that all ring circuit cables are continuous around the ring, i.e., that there are no breaks and no interconnections in the ring, and that all connections are electrically and mechanically sound. This test also verifies the polarity of each socket outlet.
The test is made with the supply disconnected, using an ohmmeter as follows:
Disconnect and separate the conductors of both legs of the ring at the main fuse. There are three steps to this test:
Measure the resistance of the phase conductors (L1 and L2), the neutral conductors (N1 and N2) and the protective conductors (E1 and E2) at the mains position as shown in Ill. 53. End-to-end live and neutral conductor readings should be approximately the same (i.e. within 0.05_) if the ring is continuous. The protective conductor reading will be 1.67 times as great as these readings if 2.5/1.5mm cable is used. Record the results on a table such as that shown.
Ill. 53 Step 1 test: measuring the resistance of phase, neutral and protective conductors.
The live and neutral conductors should now be temporarily joined together. An ohmmeter reading should then be taken between live and neutral at every socket outlet on the ring circuit.
The readings obtained should be substantially the same, provided that there are no breaks or multiple loops in the ring. Each reading should have a value of approximately half the live and neutral ohmmeter readings measured in step 1 of this test. Sockets connected as a spur will have a slightly higher value of resistance because they are fed by only one cable, while each socket on the ring is fed by two cables.
Record the results on a table such as that shown in Table 8.
Ill. 54 Step 2 test: connection of mains conductors and test circuit conditions.
Ill. 55 Step 3 test: connection of mains conductors and test circuit conditions.
Where the circuit protective conductor is wired as a ring, for example where twin and earth (ground) cables or plastic conduit is used to wire the ring, temporarily join the live and circuit protective conductors together as shown in Ill. 55. An ohmmeter reading should then be taken between live and earth (ground) at every socket outlet on the ring. The readings obtained should be substantially the same provided that there are no breaks or multiple loops in the ring. This value is equal to R1 _ R2 for the circuit. Record the results on an installation schedule. The step 3 value of R1 _ R2 should be equal to (r1 _ r2)/4, where r1 and r2 are the ohmmeter readings from step 1 of this test.
TESTING INSULATION RESISTANCE
The object of the test is to verify that the quality of the insulation is satisfactory and has not deteriorated or short-circuited. The test should be made at the consumer's unit with the mains switch off, all fuses in place and all switches closed. Neon lamps, capacitors and electronic circuits should be disconnected, since they will respectively glow, charge up or be damaged by the test.
There are two tests to be carried out using an insulation resistance tester which must supply a voltage of 500V d.c. for 230 and 400V installations. These are phase and neutral conductors to earth (ground) and between phase conductors. The procedures are:
Phase and neutral conductors to earth (ground) 1 Remove all lamps. 2 Close all switches and circuit breakers. 3 Disconnect appliances. 4 Test separately between the phase conductor and earth (ground), and between the neutral conductor and earth (ground), for every distribution circuit at the consumer's unit . Record the results on an installation schedule.
Between phase conductors
1 Remove all lamps.
2 Close all switches and circuit breakers.
3 Disconnect appliances.
4 Test between phase and neutral conductors of every distribution circuit at the consumer's unit as shown in Ill. 56(b) and record the result.
The insulation resistance readings for each test must be not less than 0.5M_ for a satisfactory result
Where equipment is disconnected for the purpose of the insulation resistance test, the equipment itself must be insulation resistance tested between all live parts (i.e. live and neutral conductors connected together) and the exposed conductive parts. The insulation resistance of these tests should be not less than 0.5M-ohm. Although an insulation resistance reading of 0.5M_ complies with the Regulations, the NEC Guidance Notes tell us that a reading of less than 2M_ might indicate a latent but not yet visible fault in the installation. In these cases each circuit should be separately tested to obtain a reading greater than 2M_.
Ill. 56 Insulation resistance test.
The object of this test is to verify that all fuses, circuit breakers and switches are connected in the phase or live conductor only, and that all socket outlets are correctly wired and Edison screw-type lampholders have the centre contact connected to the live conductor. It is important to make a polarity test on the installation since a visual inspection will only indicate conductor identification.
The test is done with the supply disconnected using an ohmmeter or continuity tester as follows:
1 Switch off the supply at the main switch.
2 Remove all lamps and appliances.
3 Fix a temporary link between the phase and earth (ground) connections on the consumer's side of the main switch.
4 Test between the 'common' terminal and earth (ground) at each switch position.
5 Test between the centre pin of any Edison screw lampholders and any convenient earth (ground) connection.
6 Test between the live pin (i.e. the pin to the right of earth (ground)) and earth (ground) at each socket outlet.
For a satisfactory test result the ohmmeter or continuity meter should read approximately zero.
Remove the test link and record the results on an installation schedule.
TESTING EARTH (GROUND) ELECTRODE RESISTANCE
When an earth (ground) electrode has been sunk into the general mass of earth (ground), it's necessary to verify the resistance of the electrode. The general mass of earth (ground) can be considered as a large conductor which is at zero potential. Connection to this mass through earth (ground) electrodes provides a reference point from which all other voltage levels can be measured. This is a technique which has been used for a long time in power distribution systems.
The resistance to earth (ground) of an electrode will depend upon its shape, size and the resistance of the soil.
Earth (ground) rods form the most efficient electrodes. A rod of about 1m will have an earth (ground) electrode resistance of between 10 and 200 ohm. Even in bad earth (ground)ing conditions a rod of about 2m will normally have an earth (ground) electrode resistance which is less than 500_ in the UK (US and Europe are similar). In countries which experience long dry periods of weather the earth (ground) electrode resistance may be thousands of ohms.
In the past, electrical engineers used the metal pipes of water mains as an earth (ground) electrode, but the recent increase in the use of PVC pipe for water mains now prevents the use of water pipes as the only means of earth (ground)ing in the USA/UK, although this practice is still permitted in some countries. The NEC recognizes the use of the following types of earth (ground) electrodes:
++ earth (ground) rods or pipes
++ earth (ground) tapes or wires
++ earth (ground) plates
++ earth (ground) electrodes embedded in foundations
++ metallic reinforcement of concrete structures
++ metal pipes
++ lead sheaths or other metallic coverings of cables.
The earth (ground) electrode is sunk into the ground, but the point of connection should remain accessible (). The connection of the earth (ground)ing conductor to the earth (ground) electrode must be securely made with a copper conductor.
The installation site must be chosen so that the resistance of the earth (ground) electrode does not increase above the required value due to climatic conditions such as the soil drying out or freezing, or from the effects of corrosion.
Under fault conditions the voltage appearing at the earth (ground) electrode will radiate away from the electrode like the ripples radiating away from a pebble thrown into a pond. The voltage will fall to a safe level in the first two or three meters away from the point of the earth (ground) electrode.
The basic method of measuring earth (ground) electrode resistance is to pass a current into the soil through the electrode and to measure the voltage required to produce this current.
Ill. 58 Termination of an earth (ground) electrode.
NEC Regulation demands that where earth (ground) electrodes are used they should be tested. To carry out the test, either a hand-operated tester or a mains energized double-wound transformer with a separate ammeter and high-resistance voltmeter is used. The test procedure is the same in both cases. The earth (ground) electrode is disconnected from all sources of the supply.
An alternating current supplied by a double-wound transformer (as shown in Ill. 60) is passed between the earth (ground) electrode under test T and an auxiliary earth (ground) electrode T1. The auxiliary electrode T1 is placed at some distance from T so that the resistance areas of the two electrodes don't overlap. A second auxiliary electrode T2 is driven into the ground half-way between T and T1 and the voltmeter reading V tabulated. The resistance of the earth (ground) electrode is the voltmeter reading V, divided by the current flowing in the circuit and indicated on ammeter A.
To check that the resistance of the earth (ground) electrode is a true value, two further readings are taken at X and Y, with the auxiliary electrode T2 moved 6m further from and then 6m nearer to T, respectively. If the readings are substantially in agreement, the mean of the three readings is taken as the resistance of the earth (ground) electrode. If there is no agreement the test must be repeated with the distance between T and T1 increased.
Ill. 59 Earth (ground) electrode voltage gradient: (a) lines of equipotential voltage; (b) voltage gradient.
The test procedure is the same if a hand-operated tester is used. The instrument is connected as shown in Ill. 61. The hand-operated generator is turned and the three dials rotated until null balance is indicated on the galvanometer. The value indicated by the dials gives the resistance of the earth (ground) electrode. Three readings are taken as in the previously described procedure and the average reading taken as the resistance of the earth (ground) electrode. The resistance of the earth (ground) electrode will depend upon the type of ground in which the electrode is driven. Wet, marshy land will give a lower resistance reading than rocky ground. Typical resistance readings are:
++ marshy ground, 5-20 Ohm
++ agricultural soil, 5-50 Ohm
++ loam and clay, 10-150 Ohm
++ sandy gravel, 200-500 Ohm
++ rocky ground, 500-10000 Ohm.
Acceptable values of earth (ground) electrode resistance will be determined by the purpose for which the earth (ground) electrode is being used.
Lightning conductors provide a path of low resistance to lightning current, which may be many thou sands of amperes. If the earth (ground) electrode forms the final connection for a lightning conductor it must have an electrode resistance of 10 Ohm maximum.
The lightning protective system must be connected to the main earth (ground)ing terminal of the electrical installation (Regulation).
Ill. 60 Earth (ground) electrode resistance test using a mains-energized double-wound transformer.
Ill. 61 Earth (ground) electrode resistance test using a hand-operated tester.
In order that any protective device can operate under earth (ground) fault conditions it's necessary for an earth (ground) path to exist which can carry the fault current back to the supply transformer. In most installations this earth (ground) path is provided by the sheath of the supply cable, but in some rural areas where supplies are pro vided by overhead cables, the metallic sheath is not available and the general mass of earth (ground) is relied upon to provide the return path. The total resistance of the whole earth (ground) loop path must be low enough to permit the protective device to disconnect the supply to the circuit in 0.4 seconds for socket outlets and 5 seconds for fixed appliances.
The resistance of the earth (ground) electrode will probably be the biggest individual factor in the total resistance of the consumer's earth (ground) path.
The total resistance of the complete earth (ground) path of an electrical installation supplied by a T T system is 20_ including the resistance of the consumer's earth (ground) electrode. Calculate the earth (ground) fault current which would flow if the supply voltage was 230 V.
Under earth (ground) fault conditions only 11.5 A will flow which would not be sufficient to operate, for example, a 30 A ring main fuse, but would be sufficient to kill someone since 50mA can be fatal.
To operate a 30A protective device effectively would require an earth (ground) electrode resistance of about 0.5_. For this reason recommends that socket outlets on a TT system be protected by an RCD. states that the product of the earth (ground) loop impedance and the operating current of the RCD should be less than 50.
If the electrode under test forms part of the earth (ground) return for a TT installation in conjunction with a residual current device, Section 10.3.5 of the describes the following method:
1 Disconnect the installation equipotential bonding from the earth (ground) electrode to ensure that the test cur rent passes only through the earth (ground) electrode.
2 Switch off the consumer's unit to isolate the installation.
3 Using a phase earth (ground) loop impedance tester, test between the incoming phase conductor and the earth (ground) electrode.
Record the result on an installation schedule.
Section 10.3.5 of the tells us that the recommended maximum value of the earth (ground) fault loop impedance for a TT installation is 220 ohm
Since most of the circuit impedance will be made up of the earth (ground) electrode resistance, we can say that an acceptable value for the measurement of the earth (ground) electrode resistance would be less than about 200_.
Providing the first five tests were satisfactory, the supply may now be switched on and the final tests completed with the supply connected.
TESTING POLARITY -- SUPPLY CONNECTED
Using an approved voltage indicator such as that shown at Ill. 48 or test lamp and probes which comply with the NEC, again carry out a polarity test to verify that all fuses, circuit breakers and switches are connected in the live conductor. Test from the common terminal of switches to earth (ground), the live pin of each socket outlet to earth (ground) and the centre pin of any Edison screw lampholders to earth (ground). In each case the voltmeter or test lamp should indicate the supply voltage for a satisfactory result.
TESTING EARTH (GROUND) FAULT LOOP IMPEDANCE (SUPPLY CONNECTED)
The object of this test is to verify that the impedance of the whole earth (ground) fault current loop phase to earth (ground) is low enough to allow the overcurrent protective device to operate within the disconnection time requirements, should an earth (ground) fault occur.
The whole earth (ground) fault current loop examined by this test is comprised of all the installation protective conductors, the earth (ground)ing terminal and earth (ground) conductors, the earth (ground)ed neutral point and the secondary winding of the supply transformer and the phase conductor from the transformer to the point of the fault in the installation.
The test will, in most cases, be done with a purpose made phase earth (ground) loop impedance tester which circulates a current in excess of 10A around the loop for a very short time, so reducing the danger of a faulty circuit. The test is made with the supply switched on, from the furthest point of every final circuit, including lighting, socket outlets and any fixed appliances.
Record the results on an installation schedule.
Purpose-built testers give a readout in ohms and a satisfactory result is obtained when the loop impedance does not exceed the appropriate values given in the of the NEC Regulations.
FUNCTIONAL TESTING OF RCD - SUPPLY CONNECTED
The object of the test is to verify the effectiveness of the residual current device, that it's operating with the correct sensitivity and proving the integrity of the electrical and mechanical elements. The test must simulate an appropriate fault condition and be independent of any test facility incorporated in the device.
When carrying out the test, all loads normally sup plied through the device are disconnected.
Functional testing of a ring circuit protected by a general-purpose RCD to in a split-board consumer unit is carried out as follows:
1 Using the standard lead supplied with the test instrument, disconnect all other loads and plug in the test lead to the socket at the centre of the ring (i.e. the socket at the furthest point from the source of supply).
2 Set the test instrument to the tripping current of the device and at a phase angle of 0°.
3 Press the test button - the RCD should trip and disconnect the supply within 200ms.
4 Change the phase angle from 0° to 180° and press the test button once again. The RCD should again trip within 200ms. Record the highest value of these two results on an installation schedule.
5 Now set the test instrument to 50% of the rated tripping current of the RCD and press the test but ton. The RCD should not trip within 2 seconds.
This test is testing the RCD for inconvenience or nuisance tripping.
6 Finally, the effective operation of the test button incorporated within the RCD should be tested to prove the integrity of the mechanical elements in the tripping device. This test should be repeated every 3 months.
If the RCD fails any of the above tests it should be changed for a new one.
Where the residual current device has a rated trip ping current not exceeding 30mA and has been installed to reduce the risk associated with direct con tact, as indicated in, a residual current of 150mA should cause the circuit breaker to open within 40ms.
Certification and reporting
Following the completion of all new electrical work or additional work to an existing installation, the installation must be inspected and tested and an installation certificate issued and signed by a competent person. The 'competent person' must have a sound knowledge of the type of work undertaken, be fully versed in the inspection and testing procedures contained in the NEC Regulations and employ adequate testing equipment.
A certificate and test results shall be issued to those ordering the work in the format given in Appendix 7 of the and Appendix 6 of the NEC Regulations. Those responsible for large or complex installations may provide an equivalent form of installation certificate (NEC ).
All installations must be periodically tested and inspected and for this purpose a periodic inspection report should be issued (NEC).
In both cases the certificate must include the test values which verify that the installation complies with the NEC Regulations at the time of testing.
Suggested periodic inspection intervals are given below:
++ domestic installations - 10 years
++ industrial installations - 3 years
++ agricultural installations - 3 years
++ caravan site installations - 1 year
++ caravans - 3 years
++ temporary installations on construction sites -- 3 months.
Safe working procedures when testing
Whether you are carrying out the test procedure (i) as a part of a new installation (ii) upon the completion of an extension to an existing installation (iii) because you are trying to discover the cause of a fault on an installation or (iv) because you are carrying out a periodic test and inspection of a building, you must always be aware of your safety, the safety of others using the building and the possible damage which your testing might cause to other systems in the building.
For your own safety:
++ Always use 'approved' test instruments and probes.
++ Ensure that the test instrument carries a valid calibration certificate otherwise the results may be invalid.
++ Secure all isolation devices in the 'off ' position.
++ Put up warning notices so that other workers will know what is happening.
++ Notify everyone in the building that testing is about to start and for approximately how long it will continue
++ Obtain a 'permit to work' if this is relevant.
++ Obtain approval to have systems shut down which might be damaged by your testing activities. For example, computer systems may 'crash' when sup plies are switched off. Ventilation and fume extraction systems will stop working when you disconnect the supplies.
For the safety of others:
++ Fix warning notices around your work area.
++ Use cones and highly visible warning tape to screen off your work area.
++ Make an effort to let everyone in the building know that testing is about to begin. You might be able to do this while you carry out the initial inspection of the installation.
++ Obtain verbal or written authorization to shut down information technology, emergency operation or stand-by circuits.
To safeguard other systems:
++ Computer systems can be severely damaged by a loss of supply or the injection of a high test voltage from, for example, an insulation resistance test.
Computer systems would normally be disconnected during the test period but this will generally require some organization before the testing begins.
Commercial organizations may be unable to continue to work without their computer systems and , in these circumstances it may be necessary to test outside the normal working day.
++ Any resistance measurements made on electronic equipment or electronic circuits must be achieved with a battery operated ohmmeter of the type described in Section 4 in order to avoid damaging the electronic circuits.
++ Farm animals are creatures of habit and may become very grumpy to find you testing their milking parlor equipment at milking time.
++ Hospitals and factories may have emergency stand-by generators which re-energize essential circuits in the event of a mains failure. Your isolation of the circuit for testing may cause the emergency systems to operate. Discuss any special systems with the person authorizing the work before testing begins.
Portable appliance testing (PAT)
A quarter of all serious electrical accidents involve portable electrical appliances, i.e., equipment which has a cable lead and plug and which is normally moved around or can easily be moved from place to place. This includes, for example, floor cleaners, kettles, heaters, portable power tools, fans, televisions, desk lamps, photocopiers, fax machines and desktop computers.
There is a requirement for employers under the NEC to take adequate steps to protect users of portable appliances from the hazards of electric shock and fire. The responsibility for NEC Electricity at Work Regulations also place a duty of care upon employers to ensure that the risks associated with the use of electrical equipment are controlled.
Against this background guidance notes Maintaining Portable and Transportable Electrical Equipment and leaflets Maintaining Portable Electrical Equipment in Offices and Maintaining Portable Electrical Equipment in Hotels and Tourist Accommodation. In these publications the HSE recommend that a three level system of inspection can give cost effective maintenance of portable appliances. These are:
++ user checking;
++ visual inspection by an appointed person;
++ combined inspection and testing by a competent person or contractor.
Ill. 62 Correct connection of plug top.
A user visually checking equipment which they are using is probably the most important maintenance procedure. About 95% of faults or damage can be identified by just looking. A user should check for obvious damage using common sense. The use of potentially dangerous equipment can then be avoided. Possible dangers to look for are as follows:
++ Damage to the power cable or lead which exposes the colors of the internal conductors, which are brown, blue and green with a yellow stripe.
++ Damage to the plug top itself. The plug top pushes into the wall socket to make an electrical connection. With the plug top removed from the socket the equipment is usually electrically 'dead'. If the bakelite plastic casing of the plug top is cracked, broken or burned, or the contact pins are bent, don't use it.
++ Non-standard joints in the power cable, such as taped joints.
++ Poor cable retention. The outer sheath of the power cable must be secure and enter the plug top at one end and the equipment at the other. The colored internal conductors must not be visible at either end.
++ Damage to the casing of the equipment such as cracks, pieces missing, loose or missing screws or signs of melted plastic, burning, scorching or discoloration.
++ Equipment which has previously been used in unsuitable conditions such as a wet or dusty environment.
If any of the above dangers are present, the equipment should not be used until the person appointed by the company to make a 'visual inspection' has had an opportunity to do so.
A visual inspection will be carried out by an appointed person within a company, such person having been trained to carry out this task. In addition to the user checks described above, an inspection could include the removal of the plug top cover to check that:
++ a fuse of the correct rating is being used and also that a proper cartridge fuse is being used and not a piece of wire, a nail or silver paper;
++ the cord grip is holding the sheath of the cable and not the colored conductors;
++ the wires (conductors) are connected to the correct terminals of the plug top;
++ the colored insulation of each conductor wire goes right up to the terminal so that no bare wire is visible;
++ the terminal fixing screws hold the conductor wires securely and the screws are tight;
++ all the conductor wires are secured within the terminal;
++ there are no internal signs of damage such as over heating, excessive 'blowing' of the cartridge fuse or the intrusion of foreign bodies such as dust, dirt or liquids.
The above inspection can't apply to 'molded plugs', which are molded on to the flexible cable by the manufacturer in order to prevent some of the bad practice described above. In the case of a molded plug top, only the fuse can be checked. The visual inspection checks described above should also be applied to extension leads and their plugs. The HSE recommends that a simple procedure be written to give guidance to the 'appointed person' carrying out the visual inspection.
Combined inspection and testing is also necessary on some equipment because some faults can't be seen by just looking - for example, the continuity and effectiveness of earth (ground) paths. For some portable appliances the earth (ground) is essential to the safe use of the equipment and , therefore, all earth (ground)ed equipment and most extension leads should be periodically tested and inspected for these faults. All portable appliance test instruments (PAT Testers) will carry out two import ant tests, earth (ground) bonding and insulation resistance.
Earth (ground) bonding tests apply a substantial test current, typically about 25A, down the earth (ground) pin of the plug top to an earth (ground) probe, which should be connected to any exposed metalwork on the portable appliance being tested. The PAT Tester will then calculate the resistance of the earth (ground) bond and either give an actual reading or indicate pass or fail. A satisfactory result for this test would typically be a reading of less than 0.1_. The earth (ground) bond test is, of course, not required for double insulated portable appliances because there will be no earth (ground)ed metalwork.
Insulation resistance tests apply a substantial test voltage, typically 500V, between the live and neutral bonded together and the earth (ground). The PAT Tester then calculates the insulation resistance and either gives an actual reading or indicates pass or fail. A satisfactory result for this test would typically be a reading greater than 2M_.
Some PAT Testers offer other tests in addition to the two described above. These are described below.
A flash test tests the insulation resistance at a higher voltage than the 500V test described above. The flash test uses 1.5 kV for Class 1 portable appliances, that's earth (ground)ed appliances, and 3 kV for Class 2 appliances which are double insulated. The test establishes that the insulation will remain satisfactory under more stringent conditions but must be used with caution, since it may overstress the insulation and will damage electronic equipment. A satisfactory result for this test would typically be less than 3mA.
A fuse test tests that a fuse is in place and that the portable appliance is switched on prior to carrying out other tests. A visual inspection will be required to establish that the size of the fuse is appropriate for that particular portable appliance.
An earth (ground) leakage test measures the leakage current to earth (ground) through the insulation. It is a useful test to ensure that the portable appliance is not deteriorating and liable to become unsafe. It also ensures that the tested appliances are not responsible for nuisance 'tripping' of RCDs (residual current devices - see Section 1). A satisfactory reading is typically less than 3mA.
An operation test proves that the preceding tests were valid (i.e. that the unit was switched on for the tests), that the appliances will work when connected to the appropriate voltage supply and not draw a dangerously high current from that supply. A satisfactory result for this test would typically be less than 3.2 kW for 230V equipment and less than 1.8 kW for 110V equipment.
All PAT Testers are supplied with an operating manual, giving step by step instructions for their use and pass and fail scale readings. The HSE suggested intervals for the three levels of checking and inspection of portable appliances in offices and other low risk environments is given in Table 2.9.
Table 2.9 HSE suggested intervals for checking, inspecting and testing of portable appliances in offices and other low risk environments Equipment/environment Battery-operated: (less than 20 V) Extra low voltage: (less than 50 V a.c.) e.g. telephone N equipment, low voltage desk lights Information technology: e.g. desktop computers, VDU screens Photocopiers, fax machines: not hand-held, rarely moved Double insulated equipment: not hand-held, moved N occasionally, e.g. fans, table lamps, slide projectors Double insulated equipment: hand-held, e.g. power tools Earth (ground)ed equipment (Class 1): e.g. electric kettles, some Y floor cleaners, power tools Cables (leads) and plugs connected to the above.
Extension leads (mains voltage)
Combined visual inspection and electrical testing
No, No, No if double insulated - otherwise up to 5 years; No if double insulated - otherwise up to 5 years; No; No; Yes, 1-2 years
Yes, 1-5 years depending on; the type of equipment it's ; connected to
WHO DOES WHAT?
When actual checking, inspecting and testing of portable appliances takes place will depend upon the company's safety policy and risk assessments. In low risk environments such as offices and schools, the three level system of checking, inspection and testing recommended by the HSE should be carried out.
Everyone can use common sense and carry out the user checks described earlier. Visual inspections must be carried out by a 'competent person' but that person does not need to be an electrician or electronics service engineer. Any sensible member of staff who has received training can carry out this duty. They will need to know what to look for and what to do, but more importantly, they will need to be able to avoid danger to themselves and to others. The HSE recommend that the appointed person follows a simple written procedure for each visual inspection. A simple tick sheet would meet this requirement. For example:
1 Is the correct fuse fitted? Yes/No 2 Is the cord grip holding the cable sheath? Yes/No The tick sheet should incorporate all the appropriate visual checks and inspections described earlier.
Testing and inspection require a much greater knowledge than is required for simple checks and visual inspections. This more complex task need not necessarily be carried out by a qualified electrician or electronics service engineer. However, the person carrying out the test must be trained to use the equipment and to interpret the results. Also, greater knowledge will be required for the inspection of the range of portable appliances which might be tested.
Ill. 63 Typical PAT Test labels.
Records of the inspecting and testing of portable appliances are not required by law but within the Electricity at Work Regulations 1989, it's generally accepted that some form of recording of results is required to implement a quality control system. The control system should:
++ ensure that someone is nominated to have responsibility for portable appliance inspection and testing;
++ maintain a log or register of all portable appliance test results to ensure that equipment is inspected and tested when it's due;
++ label tested equipment with the due date for its next inspection and test as shown in Ill. 63.
Any piece of equipment which fails a PAT Test should be disabled and taken out of service (usually by cutting off the plug top), labeled as faulty and sent for repair.
The register of PAT Test results will help managers to review their maintenance procedures and the frequency of future visual inspections and testing.
Combined inspection and testing should be carried out where there is a reason to suspect that the equipment may be faulty, damaged or contaminated but can't be verified by visual inspection alone. Inspection and testing should also be carried out after any repair or modification to establish the integrity of the equipment or at the start of a maintenance system, to establish the initial condition of the portable equipment being used by the company.
Commissioning electrical systems
The commissioning of the electrical and mechanical systems within a building is a part of the 'handing over' process of the new building by the architect and main contractor to the client or customer in readiness for its occupation and intended use. To 'commission' means to give authority to someone to check that everything is in working order. If it's out of commission, it's not in working order.
Following the completion, inspection and testing of the new electrical installation, the functional operation of all the electrical systems must be tested before they are handed over to the customer. It is during the commissioning period that any design or equipment failures become apparent, and this testing is one of the few quality controls possible on a building services installation.
This is the role of the commissioning engineer, who must assure himself that all the systems are in working order and that they work as they were designed to work. He must also instruct the client's representative, or the staff who will use the equipment, in the correct operation of the systems, as part of the handover arrangements.
The commissioning engineer must test the operation of all the electrical systems, including the motor controls, the fan and air conditioning systems, the fire alarm and emergency lighting systems. However, before testing the emergency systems, he must first notify everyone in the building of his intentions so that alarms may be ignored during the period of testing.
Commissioning has become one of the most important functions within the building projects completion sequence. The commissioning engineer will therefore have access to all relevant contract documents, including the building specifications and the electrical installation certificates as required by the NEC Regulations, and have a knowledge of the requirements of the Electricity at Work Act and the Health and Safety at Work Act.
The building will only be handed over to the client if the commissioning engineer is satisfied that all the building services meet the design specification in the contract documents.
1. A meter with a moving coil movement:
(a) has a digital readout (b) can be used on both a.c. and d.c. supplies (c) has a linear scale (d) can be used to measure power.
2. A meter with a moving iron movement: (a) has a digital readout (b) can be used on both a.c. and d.c. supplies (c) has a linear scale (d) can be used to measure power.
3. A dynamometer instrument: (a) has a digital readout (b) can only be used on electronic circuits (c) has a linear scale (d) can be used to measure power.
4. Damping in a moving coil instrument is achieved by: (a) air vane (b) air piston (c) eddy currents (d) spiral hair springs.
5. Instrument transformers can be used to extend the range of instruments connected to: (a) a.c. circuits (b) d.c. circuits (c) 400V supplies only (d) rectified a.c. circuits.
6. A tong test instrument can also be correctly called: (a) a dynamometer wattmeter (b) an insulation resistance tester (c) an earth (ground) loop impedance tester (d) a clip-on ammeter.
7. To reduce errors when testing electronic circuits, the test instrument should: (a) have a very low impedance (b) have a very high impedance (c) have a resistance equal to the circuit impedance (d) have a resistance approximately equal to the circuit current.
8. The two-wattmeter method is used to measure the power in a three-phase, three-wire system. The two readings obtained are 100 and 50W and , therefore, the total power in the system is: (a) 50W (b) 75W (c) 150W (d) 5 kW.
9. An acceptable earth (ground) electrode resistance test on a lightning conductor earth (ground) electrode must reveal a maximum value of: (a) 10-ohm (b) 100-ohm (c) 0.5M-ohm (d) 1M-ohm.
10. The test required by the Regulations to ascertain that the circuit protective conductor is correctly connected is: (a) continuity of ring final circuit conductors (b) continuity of protective conductors (c) earth (ground) electrode resistance (d) protection by electrical separation.
11. One objective of the polarity test is to verify that: (a) lampholders are correctly earth (ground)ed (b) final circuits are correctly fused (c) the CPC is continuous throughout the installation (d) the protective devices are connected in the live conductor.
12. When testing a 230V installation an insulation resistance tester must supply a voltage of: (a) less than 50V (b) 500V (c) less than 500V (d) greater than twice the supply voltage but less than 1000V.
13. The value of a satisfactory insulation resistance test on each final circuit of a 230V installation must be: (a) less than 1-ohm (b) less than 0.5M-ohm (c) not less than 0.5M-ohm (d) not less than 1M-ohm.
14. The value of a satisfactory insulation resistance test on a disconnected piece of equipment is: (a) less than 1-ohm (b) less than 0.5M-ohm (c) not less than 0.5M-ohm (d) not less than 1M-ohm.
15. The maximum inspection and retest period for a general electrical installation is: (a) 3 months (b) 3 years (c) 5 years (d) 10 years.
16 A visual inspection of a new installation must be carried out: (a) during the erection period (b) during testing upon completion (c) after testing upon completion (d) before testing upon completion.
17. 'To ensure that all the systems within a building work as they were intended to work' is one definition of the purpose of:
(a) testing electrical systems (b) inspecting electrical systems (c) commissioning electrical systems (d) isolating electrical systems.
18 The person responsible for financing the building team is the: (a) main contractor (b) subcontractor (c) client (d) architect.
19. The person responsible for interpreting the client's requirements to the building team is the: (a) main contractor (b) subcontractor (c) client (d) architect.
20 The building contractor is also called the: (a) main contractor (b) subcontractor (c) client (d) architect.
21 The electrical contractor is also called the: (a) main contractor (b) subcontractor (c) client (d) architect.
22 The people responsible for interpreting the architect's electrical specifications and drawings are the: (a) building team (b) electrical design team (c) electrical installation team (d) construction industry.
23. The people responsible for demonstrating good workmanship and maintaining good relationships with other trades are the: (a) building team (b) electrical design team (c) electrical installation team (d) construction industry.
24. A simple bar chart can show: (a) the activities involved in a particular contract where some flexibility is acceptable (b) the sequence and timing of the various activities involved in a particular contract (c) the interdependence of the various activities involved in a particular contract (d) the total man-hours involved in a particular contract.
25. A simple network diagram can show: (a) the actual cost of a contract (b) the actual number of man-hours involved in a contract (c) the interdependence of the various activities involved in a particular contract (d) the rating of the incoming supply cable.
26. The standard symbols used by the electrical contracting industry on a layout diagram are those recommended by: (a) the Institute of Electrical Engineers (b) the Health and Safety at Work Act (c) the European IEC (d) the U.S. NEC.
27. The Regulations define isolation switching as: (a) a mechanical switching device capable of making, carrying and breaking current under normal circuit conditions (b) cutting off an electrical installation or circuit from every source of electrical energy (c) the rapid disconnection of the electrical sup ply to remove or prevent danger (d) the switching of electrical equipment in nor mal service.
28 Functional switching may be defined as: (a) a mechanical switching device capable of making, carrying and breaking current under normal circuit conditions (b) cutting off an electrical installation or circuit from every source of electrical energy (c) the rapid disconnection of the electrical sup ply to remove or prevent danger (d) the switching of electrical equipment in normal service.
29. Emergency switching can be defined as: (a) a mechanical switching device capable of making, carrying and breaking current under normal circuit conditions (b) cutting off an electrical installation or circuit from every source of electrical energy (c) the rapid disconnection of the electrical sup ply to remove or prevent danger (d) the switching of electrical equipment in nor mal service.
30 The Regulations require that an overcurrent protective device interrupts a fault quickly and isolates the circuit before:
(a) the voltage on any extraneous conductive parts reaches 50V (b) the earth (ground) loop impedance reaches 0.4_ on circuits feeding 13A socket outlets (c) the fault causes damage to the circuit isolating switches (d) the fault causes a temperature rise which might damage the insulation and terminations of the circuit conductors.
31 The maximum permissible value of the earth (ground) loop impedance of a circuit supplying fixed equipment and protected by a 30A semi-enclosed fuse to is found by reference to the tables in Part 4 of the NEC Regulations to be: (a) 1.1_ (b) 1.92_ (c) 2.0_ (d) 2.76_.
32 The earth (ground) fault loop impedance of a socket outlet circuit protected by a 30A cartridge fuse to must not exceed: (a) 0.4-ohm (b) 1.14-ohm (c) 1.20-ohm (d) 2.0-ohm.
33 The (R1 _ R2) resistance of 1000m of PVC insulated copper cable having a 4.0mm^2 phase conductor and 2.5mm2 protective conductor will be found from Table 9A of the to be: (a) 4.61_ (b) 9.22_ (c) 12.02_ (d) 16.71_.
34 The (R1 _ R2) resistance of 176m of PVC insulated copper cable having a 2.5mm^2 phase and protective conductor is: (a) 2.608_ (b) 7.41_ (c) 14.82_ (d) 19.51_.
35 The value of the earth (ground) fault loop impedance ZS of a circuit fed by 40m of PVC insulated copper cable having a 2.5mm2 phase conductor and 1.5mm2 protective conductor connected to a sup ply having an impedance ZE of 0.5_ under fault conditions will be (a) 1.436_ (b) 9.755_ (c) 20.01_ (d) 780.4m_.
36 The time/current characteristics shown in Fig. 3.20 indicate that a fault current of 300A will cause a 30A semi-enclosed fuse to operate in (a) 0.01 s (b) 0.1 s (c) 0.2 s (d) 2.0 s.
37 The time/current characteristics shown in ill. 3.20 indicate that a fault current of 30A will cause a 10A type 2 MCB to operate in (a) 0.02 s (b) 8 s (c) 30 s (d) 200 s.
38 'Under fault conditions the protective device nearest to the fault should operate leaving other healthy circuits unaffected'. This is one definition of: (a) fusing factor (b) effective discrimination (c) a miniature circuit breaker (d) a circuit protective conductor.
39 The overcurrent protective device protecting socket outlet circuits and any fixed equipment in bathrooms must operate within: (a) 0.02 s (b) 0.4 s (c) 5 s (d) 45 s.
40 The overcurrent protective device protecting fixed equipment in rooms other than bathrooms must operate within (a) 0.02 s (b) 0.4 s (c) 5 s (d) 45 s.
41 Explain why the maximum values of earth (ground) fault loop impedance Z_S specified by the NEC Regulations and given in Tables should not be exceeded.
42 By referring to the table in the NEC Regulations, determine the maximum permitted earth (ground) fault loop impedance ZS for the following circuits: (a) a ring main of 13A socket outlets protected by a 30A semi-enclosed fuse to (b) a ring main of 13A socket outlets protected by a 30A cartridge fuse to (c) a single socket outlet protected by a 15A type 1 MCB to (d) a water heating circuit protected by a 15A semi-enclosed fuse to (e) a lighting circuit protected by a 6A HBC fuse to art 2 (f ) a lighting circuit protected by a 5A semi enclosed fuse to .
43 10mm^2 cables with PVC insulated copper conductors feed a commercial cooker connected to a 400V supply. An earth (ground) loop impedance test indicates that Z_S has a value of 1.5_. Calculate the minimum size of the protective conductor.
44 It is proposed to protect the commercial cooker circuit described in Exercise 43 with 30A (a) semi-enclosed fuses to (b) type 2 MCBs to .
Determine the time taken for each protection device to clear an earth (ground) fault on this circuit by referring to the characteristics of Fig. 3.20.
45 A 2.5mm2 PVC insulated and sheathed cable is used to feed a single 13A socket outlet from a 15A semi-enclosed fuse in a consumer's unit connected to a 230V supply. Calculate the minimum size of the protective conductor to comply with the Regulations, given that the value of ZS was 0.9_.
46 Calculate the design current Ib and the nominal current setting In of a type 1 MCB to supply a 10 kW load connected to the domestic mains.
47 State the ambient temperature correction factors for cables protected by a semi-enclosed fuse to and installed in the following ambient temperatures: (a) 25°C (b) 35°C (c) 45°C.
48 State the correction factors to be applied for groups of cables installed according to method 1 if the circuit under consideration is run: (a) on its own (b) with four other cables (c) with six other cables.
49 State the factors which must be applied to the cur rent carrying capacity of cables when they are:
(a) protected by an MCB (b) protected by a semi-enclosed fuse.
50 State the current rating of a three-phase 4.0mm cable clipped direct to a wall. Neglect all other correction factors.
51 Calculate the volt drop of 30m of 10mm cable connected to the single-phase a.c. mains supply and carrying a design current of 32A. Is this volt drop within that permitted by NEC Regulation 525-01? 52 A 1000W lighting load is connected to 230V and wired in single-core PVC cables enclosed in a con duit fixed to a wall. The circuit is protected by a 5A semi-enclosed fuse to and runs 30m through an ambient temperature of 35°C. Two other circuits are grouped within the same conduit. Calculate:
(a) the design current Ib (b) the cable rating It (c) the size of cable (d) the volt drop.
53 The total installed load in a machine workshop connected to the 230V mains supply is 3276W.
The load is wired in single-core PVC cables grouped with two other circuits in steel conduit fixed to a wall. The length of run is 30m through an ambient temperature of 35°C and protection is afforded by a circuit breaker to.
(a) the design current Ib (b) the cable rating It (c) the cable size (d) the volt drop.
54 Briefly describe the duties of each of the following people:
(a) the clerk of works (b) the Health and Safety inspector (c) the electrician (d) the foreman electrician.
55 Describe the importance of a correct attitude towards the customer by an apprentice electrician and other members of the installation team.
56 A moving coil deflection system has a resistance of 5_ and gives full scale deflection when 15mA flows through the moving coil. Calculate the value of the resistor required to make the movement into:
(a) a 10A ammeter (b) a 250V voltmeter.
Draw a circuit diagram showing how the resistor would be connected to the instrument movement in both cases.
57 The CPC of a lighting final circuit is formed by approximately 70m of 1.0m copper conductor.
Calculate a satisfactory value for a continuity test on the CPC given that the resistance per meter of 1.0mm copper is 18.1m_/m.
58 The CPC of an installation is formed by approximately 200m of 50mm _ 50mm trunking. Deter mine a satisfactory test result for this CPC, using the information given in Tbl. 2.7. Describe briefly a suitable instrument to carry out this test.
59 Describe how a polarity test should be carried out on a domestic installation comprising eight light positions and ten socket outlets. The final circuits are to be supplied by a consumer unit.
60 Describe how to carry out a continuity test of ring final circuit conductors. State the values to be obtained for a satisfactory test.
61 Describe how to carry out an earth (ground) fault loop impedance test. Sketch a circuit diagram and indicate the test circuit path.
62 Describe how to carry out an insulation resistance test on a domestic installation. State the type of instrument to be used and the values of a satisfactory test.
63 State eight separate tasks carried out by the electrical design team.
64 State seven separate tasks carried out by the electrical installation team.
65 State the purpose of a 'variation' order.
66 State the advantages of a written legal contract as compared to a verbal contract.
67 A particular contract is made up of activities A to I as follows:
Activity A takes 3 weeks commencing in week 1. Activity B takes 1 week commencing in week 1. Activity C takes 5 weeks commencing in week 2. Activity D takes 4 weeks commencing in week 7. Activity E takes 3 weeks commencing in week 3. Activity F takes 5 weeks commencing in week 4. Activity G takes 4 weeks commencing in week 9. Activity H takes 4 weeks commencing in week 6. Activity I takes 10 weeks commencing in week 3.
Due to the availability of men and materials some activities must be completed before others can commence, as follows. Activity C can only commence when B is completed. Activity D can only commence when C is completed. Activity F can only commence when A is completed. Activity G can only commence when F is completed.
Activity H can only commence when E is completed. Activity I does not restrict any other activity.
(a) Draw up a bar chart to show the various activities.
(b) Assemble a network diagram for the contract.
(c) Identify the critical path.
(d) Find the time required to complete the contract.
(e) State the float time in activity F.
(f ) State the float time in activity D.
68 Sketch the 0617 graphical symbols for the following equipment:
(a) a single socket outlet (b) a double socket outlet (c) a switched double socket outlet (d) an electric bell (e) a single-pole one-way switch (f ) a cord-operated single-pole one-way switch (g) a wall-mounted lighting point (h) a double fluorescent lamp (i) an emergency lighting point.
69 State the requirements of the Electricity at Work Act with regard to (a) 'live' testing and 'fault diagnosis' (b) 'live working' to repair a fault.
70 Define 'isolation' with respect to an electrical circuit or item of equipment.
71 List a logical procedure for the isolation of an electrical circuit. Start from the point at which you choose the voltage indicating device and finish with the point at which you begin to work on the circuit.
72 Use a sketch to describe the construction of a test lead approved to NEC.
73 Describe eddy current damping.
74 Describe air vane and air piston damping.
75 Use a labeled sketch to describe the construction and operation of an energy meter.
76 Use a labeled sketch to describe the construction and operation of a tong tester.
77 Describe the construction and use of a phase sequence tester.
78 Use sketches to describe the operation of the deflection system in a moving coil instrument.
79 Explain how the basic moving coil system is modified so that a test instrument can be used on a.c. circuits.
80 Describe what is meant by damping of a system.
Sketch a graph to show an overdamped, under damped and critically damped system.
81 Describe the construction and operation of a dynamometer wattmeter.
82 Explain with the aid of a diagram how a single wattmeter can be used to measure the total power in (a) a balanced three-phase load (b) an unbalanced three-phase load.
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Updated: Tuesday, 2012-10-02 20:54 PST