Trouble-shoot and repair faults in low voltage electrical apparatus and circuits

Transcription

1 UEENEEG108A Trouble-shoot and repair faults in low voltage electrical apparatus and circuits AS3000:2007 & AS3017:2007 Summary MANUFACTURING AND ENGINEERING DIVISI M & E UEENEEG108A

2 TESTING Before any installation is connected to the supply it is necessary for the electrician to determine that the installation is safe. To prove safety the following tests must be carried out. (a) Visual Inspection. - Use the checklist given in Clause to ensure all visual inspections are carried out. (b) Electrical Testing - There are six (6) Mandatory tests that must be carried out. These are 1. Earth Continuity Test Insulation Resistance Test Polarity Test This test completes the / Transposition Test not switched 4. Correct circuit connections Test This test completes the / Earth Transposition Test and belong to the same circuit / Earth Transposition Test. Earth Fault Loop Impedance Test Operation of RCD s Test UEENEEG108A Page 2

3 Test 1 Earth Continuity Test Pur p ose f or Test To keep exposed metal at earth potential To allow circuit protection to operate under fault conditions Test Inst r um ent :-Ohm m et er How t o Per f orm Test The resistance of a protective earthing conductor must be low enough to ensure the circuit breaker will trip within 0.4 seconds. The FAQ for AS 3000:2000 still applies. FAQ 009/2001: AS/NZS 3000:2000 CLAUSE & CLAUSE INSPECTI AND TESTING Test results Continuity of earthing system TESTING Mandatory tests Continuity of the earthing system Question 009/2001. Is there an appropriate single value of resistance that could be used to show compliance with the continuity of earthing system test? Answer. No. An appropriate value of resistance is related to the type and rating of the protective device used to protect the circuit under test. An appropriate table has been included in AS Disconnect the Earth from the neutral link Disconnect the from the neutral link If the circuit has an alternative earth path (eg the water pipes to a hot water system), then disconnect the sub circuit earth from equipment. Use ohms range of megger to measure earth resistance to every earth point. Results should comply with table shown. RECNECT THE EARTH TO THE HWS CE THE EARTH HAS BEEN TESTED Turn on all switches as you go Disconnect sub-main at sub-board Turn off sub-board isolator Table 8.2 AS id en t if yin g t h e m axim um p erm issib le resist an ces o f act ive (Rp h ) an d p rot ect ive eart h in g (Re) co n d uct o rs m easured at an am b ien t t em p. o f 20 C. LEAVE MAIN EARTH DISCNECTED LEAVE MAIN NEUTRAL DISCNECTED UEENEEG108A Page 3

4 Test 2 Insulation Resistance Test Pur p ose To avoid exposure to live components To prevent short circuits NOTE: DO NOT MEGGER TEST BETWEEN ACTIVE AND NEUTRAL OF FINAL SUBCIRCUITS, BUT YOU MUST TEST BETWEEN ALL CIRCUIT CDUCTORS OF MAINS AND SUBMAINS. Test Inst r um ent Insulation Resistance Tester (Megger) 00V see clause (noting exceptions 1 & 2) How t o Test range and hot water service. Earth AND are still disconnected from link. Switch ALL switches Turn ALL circuit breakers Bridge to Link Megger Test between the bridge (joined active / neutral) and Earth Reading should be greater than or equal to 1M If reading is less than 1M disconnect and conductors from Range and HWS. (See Note) Change any multi-position switches (eg two way) and repeat megger test. Megger heaters & busways individually Reconnect the active and neutral to the Note. Sheathed heating elements (as found in HWS and ranges) can have insulation resistances as low as 10 k Clause Number (i) &(ii) Insulation Resistance between conductors of mains and sub mains would be expected to be much higher - Say 0 M M RECNECT THE MAIN EARTH AND MAIN NEUTRAL TO THE NEUTRAL LINK UEENEEG108A Page 4

5 HAS THE MAIN EARTH & MAIN NEUTRAL BEEN RECNECTED? Test 3 Polarity Test for loaded circuits A) Circuits with load connected eg Water Heaters, Urns, Ranges Test Inst r um ent Ohm meter How to test: On active side you should get Resistance of the load R= U 2 P On side you should get zero ohms Ensure the Earth AND has been reconnected to the Link Switch off Circuit breaker off All load switches, including simmerstats and thermostats, on For wall ovens where no access to terminal block, make definite check prior to installation. UEENEEG108A Page

6 Test 3 Polarity Test - Also tests all other transpositions, circuit intermix and short circuits between active and neutral. B) Unloaded Circuits Including fluorescent luminaries Test Instrument: Ohmmeter How to test: 10 C/B Switch closed Infinite Switch open Close only one circuit breaker Disconnect only its corresponding Connect resistors as shown Test ALL points on this circuit Result other than those shown indicates a FAULT Faults indicated by readings other than above: Reverse Polarity and Transposition Earth reversed 10 C/B 3 3 being switched C/B being switched UEENEEG108A Page 6

7 Circuit Intermix The neutral disconnected does NOT belong to the circuit being tested. 10 being switched 0 Transposition Earth C/B Wrong! When testing using this method with intermixed circuits it is possible to get some unexpected results as follows. If the circuits are supplied by combination RCD/MCB s, there will be no return path for an intermixed neutral. The active - earth test will show up as the expected 10 however the intermixed neutral will show up as an open circuit. This occurs because to isolate the actives, only the RCD/MCB for the circuit you are testing is turned on. There are several ways to get around this: 1. Bypass all RCD/MCB neutrals before you start with the polarity tests 2. If you find what appears to be an open circuit neutral, turn each combination RCD/MCB on, then off individually. If the multi-meter continues to show the neutral as an open circuit, then it probably is. If it is intermixed with another circuit, when you turn on the combination RCD/MCB with which it is intermixed the multi-meter reading will drop to a low value, and the intermix will be exposed. UEENEEG108A Page 7

8 Sub Board Sub Polarity Test: A) Where supplied via sub circuit earth Infinite SUB MAINS SUB BOARD These were opened from previous test. Zero Ohms Test: Ohmmeter E N = 0 E A = Infinite B) Where sub board is separate M.E.N. system Infinite SUB MAINS SUB BOARD These were opened from previous test. Reading * M 00 V Megger * Reading may be high, but a circuit will be indicated on the NEUTRAL. UEENEEG108A Page 8

9 Test 4 Short Circuit Test. Pur p ose To avoid damage to circuit components if excessive current flows due to short circuits Test Inst r um ent : Ohm m et er How t o Test Ohmmeter will indicate the resistance of these loads in parallel Remove test resistors Leave all switches Turn all circuit breakers Take reading between and Link Actual reading will depend on connected load Power circuit infinite 10.2 kw Range.3 ohms 3.6 kw HWS 16 ohms With all circuits connected loads will be in parallel, giving an even lower reading Individual circuits can be checked by turning all breakers OFF then turning each breaker and OFF in turn Fluorescent luminaires with p.f. correction capacitors may give low readings, increasing as capacitor charges. It also should also be noted that the polarity test conducted earlier with the resistors will also expose a short circuit between active and neutral as indicated above. The ohmmeter will read the same value on both terminals, which will be approximately 3.3 ohms, plus cable resistance. This occurs because the active to neutral fault places the and 10 ohm resistor in a parallel configuration to earth. UEENEEG108A Page 9

10 Test - Earth Loop Impedance Test This is required for circuits containing socket outlets that are not protected by a RCD. Purpose of Test To ensure that the resistance of the fault loop is low enough to allow the circuit protection to operate in the specified time (0.4 seconds) Test Instrument: Ohm meter How to Test No supply available. (If supply is available use test method (b)) This first method enables the fault loop impedance to be checked as part of the polarity test using and 10 ohm resistors. Close circuit breaker for circuit under test. Ensure resistors are connected from polarity test. Turn the switch controlling the socket outlet Measure the resistance between the and Earth at the socket outlet under test Note: this test needs only to be done on the most distant part of the circuit from the switch board The measured resistance must not exceed the values given in Table 8.2 of AS3000 (see below) Subtract 10 ohms from your measured result. To obtain the maximum allowable fault loop impedance see Table 8.2 Check your result to ensure the measured value is satisfactory Notes to Table 8.2 show how the values have been derived. Table 8.2 AS id en t if yin g t h e m axim um p erm issib le resist an ces o f act ive (Rp h ) an d p rot ect ive eart h in g (Re) co n d uct o rs m easured at an am b ien t t em p. o f 20 C. UEENEEG108A Page 10

11 The earth loop impedance test can also be carried out as follows without using the resistors by following this second method as outlined in AS3017 How to Test No supply available. (If supply is available use test method (b)) OFF Open circuit breaker for circuit under test. Connect the Load side of the Circuit Breaker to the Earth Link. Turn the switch controlling the socket outlet Measure the resistance between the and Earth at the socket outlet under test Note this test needs only to be done on the most distant part of the circuit from the switch board The measured resistance must not exceed the values given in Table 8.2 of the rules. Notes to Table 8.2 show how the values have been derived. Test 6 - RCD Test. (supply needs to be available) Test Equipment Test button on RCD or special test equipment. How to Test Supply must be available Operate the integral test device of the RCD Check that it trips the designated circuit. If using a special test device follow manufactures instructions DATE OF INITIAL ENERGISATI OF AN INSTALLATI The date of initial energisation of an installation should be available on-site, either by marking or in an accessible record. UEENEEG108A Page 11