Module Title: Electrical Installation II Laboratory Sheet:

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1 Vocational Training Council Hong Kong Institute of Vocational Education Department of Engineering Module Title: Electrical Installation II Laboratory Sheet: Student name: Course / Year: Subject: Date: Inspection & Earth Loop Impedance Test for Electrical Installation Objectives: To understand the principles and to have hands-on experience on:- Insulation Tests using a 500V Meggar Tester, Earth Loop Impedance measurement with a Phase-Earth Loop Impedance Tester, Apparatus: 500V Meggar (AVO model BM201) (x1) Phase-Earth Loop Impedance Test (Kyoritsu model 4141) (x1) 13A socket outlets (4 nos.) of correct & incorrect polarities (1 set) Components: a short run of cable with good insulation (x1) a short run of cable with defective insulation (x1) Ver Author(s) Date Remark 1.0 hcho 5-03 For Elect. Installation II Page 1 of 10

2 Part A: Background 1. Introduction Electrical installations may be categorised as being portable or fixed. Fixed electrical installation may be defined as being a low or high voltage electrical installation that is fixed to premises but does not include any electrical equipment that is supplied with electricity after passing through a socket of the installation at which the supply can be disconnected without the use of a tool. Otherwise, the electrical installation may be classified as portable. To ensure safety, all fixed electrical installation, no matter new or alteration, shall satisfy the inspection and testing procedures as set down by the Code of Practice for the Electricity (Wiring) Regulations. In particular, the testing procedures for Low Voltage (<1000 V) electrical installations are particular relevant to general fixed electrical installations. Examples of low voltage fixed electrical installations include distribution boards, lighting installations, socket outlet installations, etc. It is a statutory requirement that all fixed electrical installations, >200A industrial and >100A domestic, are to be tested regularly. Precautionary measures should be taken during testing and the method of tests should be such that no danger to persons or property can occur even if the circuit being tested is defective. The following tests, where relevant, are to be tested preferably in the sequence indicated below:- (i) continuity of ring final circuit conductors, (ii) continuity of protective conductors, including main and supplementary equipotential bonding, (iii) earth electrode resistance, (iv) insulation resistance, (v) polarity, (vi) earth fault loop impedance, (vii) functions of all protective devices, (viii) functions of all items of equipment. In the event of any test indicating failure to comply, that test and those preceding, the results of which may have been influenced by the fault indicated, should be repeated after the fault has been rectified. Among these tests, items (iv) and (vi) are studied in this experiment to highlight the key principles of the inspection and testing of electrical installations. Page 2 of 10

3 2. 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. There are two tests under this heading:- (i) (ii) insulation resistance (IR) between conductors and IR between conductors and earth. Before these tests are carried out, it is essential to disconnect any neons and capacitors from the circuit because they will upset the readings obtained. In addition, any control devices which contain semiconductor components must also be disconnected as they can be damaged by the test. IR between conductors For this test, all lamps have to be removed and all switches closed. The test instrument is then connected between phase and neutral (if the supply is single-phase) and the test voltage applied. The minimum acceptable value of IR is 0.5 MΩ for low voltage installations. In new and otherwise electrically healthy circuits, the reading will be higher than this value. If a zero reading is obtained, this indicates a shortcircuit. If a significant resistance reading is obtained, this might indicate, say, a lamp is left in the circuit (e.g. the filament resistance of a 100W lamp would be in the region of 600Ω). IR between conductors and earth For this test all fuses (or MCBs) should be in place, all switches closed (including the main switch if practicable) together. Again the minimum acceptable value is 1 MΩ. Figure A.1 Test for insulation-resistance to earth Page 3 of 10

4 Figure A.2 Test for insulation-resistance between conductors Table A.1 Minimum values of insulation resistance 3. Earth-fault loop impedance Earth fault loop impedance is the impedance of the earth fault-current loop starting and ending at the point of earth fault (Figure A.3). It comprises the following starting at the point of fault: (i) (ii) (iii) (iv) (v) the circuit protective conductor; the consumer s earthing terminal and earthing conductor; the earth return path (for TT system); the path through the earthed neutral point of the transformer and the transformer winding; the phase conductor from the transformer to the point of fault. Page 4 of 10

5 Figure A.3 Path for earth fault current showing the complete earth fault loop impedance The significance of earth fault loop impedance is that a fast disconnection time of the protective device means that a high fault current is required to blow the fuse or to trip the circuit breaker. In order to allow sufficient fault current to flow in order to trip the protective device, the earth fault loop impedance must have a low value. The instrument commonly used is a phase-earth loop tester which has two indicating lamps, both of which must be ON before a test is carried out. One lamp indicates that the polarity of the circuit is correct, P-N, and the other ensures that a proven earth connection is available, P-E. There is usually a recommendation that there should be at least a 20-second interval between tests. This is to allow any heat generated in the current limiting resistor to dissipate. The instrument passes a current of around 20A into the earth loop path. The reading obtained must be compared with the maximum Z s values given in the table in the COP (Table A.2). These values vary according to the type of overcurrent protective device and its current rating and whether the circuit feeds socket-outlets or fixed equipment. If it exceeds the recommended figure, the circuit must be investigated to find out the underlying reason. The reading obtained should also be compared with the reading of the previous test (if this information is available) to see whether the Z s value is on the increase, which might indicate a potential dangerous condition appearing in the circuit. For example, if the maximum Z s was 2 Ω and the test reading was 1.2 Ω on a previous test but is now 1.8 Ω, this indicates that the next would produce an unacceptable reading with the possibility that an earth fault occurring in the circuit would not produce enough fault current to operate the overcurrent protective device in either 0.4 second and 5 seconds, thus increasing the risk of electric shock to persons using the installation. Page 5 of 10

6 Tables A.2 Maximum Earth Fault Loop Impedance Page 6 of 10

7 Tables A.2 (Cont d) Maximum Earth Fault Loop Impedance Page 7 of 10

8 Part B: Procedures 1. Insulation Test Figure B.1 Connection diagram for insulation test of a cable (i) (ii) (iii) (iv) Test the operation of the 500V meggar by shorting the two terminals. Use the meggar to test the cable with the undamaged insulation and take the reading directly in ohms. Replace the cable with defective insulation and take the reading of the meggar. Repeat the insulation resistance tests with the 30A TPN switch in the laboratory. Take the reading of R-E, R-Y, Y-B, B-R and RYB-N in turns and record all results in Table B.1. No Test Items Meggar Reading (Ω) 1 Cable with perfect insulation 2 Cable with defective insulation 3 30A TPN Switch (i) R-E (ii) Y-E (iii) B-E (iv) R-N (v) Y-N (vi) B-N (vii) R-Y (viii) Y-B (ix) B-R (x) RYB-N (xi) RYB-E Table B.1 Insulation test Remarks Page 8 of 10

9 2. Earth Loop Impedance Test Four socket outlets are provided. Only one of the four is correctly wired. Figure B.2 Connection diagram for earth loop impedance test (i) (ii) The 'Phase-Earth Loop Impedance' Tester is plugged into the 13A socket outlet at the bench. Press the button. The pointer will move the give a reading directly in ohms. This is the phase-earth loop impedance. Repeat procedure (i) for the 4 nos. 13A socket outlets. If the pointer moves to give a reading, record the results. If the pointer does not move or move to give an excessive reading, also note down the observations. Justify the results recorded in Table B.2. 1st 13A S/O 2nd 13A S/O 3rd 13A S/O 4th 13A S/O P-N LED (ON/OFF) Response of the Impedance Tester P-E LED (ON/OFF) Movement of Pointer Table B.2 Earth fault loop impedance Reading (Ω) By measurement, the earth fault loop resistance = Ω Page 9 of 10

10 Part C: Discussions 1. Comment on the results recorded in Table B.1. Suggest the minimum values of the test results to be acceptable. 2. What is the potential problem if the equipment/cable fails the insulation test? 3. Does all the earth fault loop impedance measured for the functional socket outlets in Part B.2 comply with the requirement as laid down in table A.2? - End - Page 10 of 10

2394 EXAM PAPER. 1. State THREE circumstances that would require a periodic inspection and test to be carried out on an installation

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