You have been called to a fixed-wired concrete-making vibrating plant where astaff member received an electric shock off the frame of the plant. The vibratingplant is driven by a 400V, three-phase, 1.5kW induction motor controlled by aDOL (direct-on-line) starter. The motor and starter are controlled by anadjacent isolator.You have isolated the vibrating plant, by turning off the isolator and proving theisolation on the load side of the switch with the prove-test-prove method.The only test instruments you have are a multimeter and clamp type ammeter.(a) (i) What is the first electrical test you would carry out?(1 mark)________________________________________________________(ii) What instrument would you use to carry out the test you have statedin (a)(i).(½ mark)________________________________________________________(iii) Describe how you would carry out the test you have stated in (a)(i)(1 mark)________________________________________________________________________________________________________________(iv) What is the minimum or maximum test result value would youconsider acceptable for the test you have stated in (a)(i)?(1 mark)________________________________________________________(b) (i) What is the second electrical test you would carry out?(1 mark)________________________________________________________(ii) What instrument would you use to carry out the test you have statedin (b)(i).(½mark)________________________________________________________
(iii) Describe how you would carry out the test you have stated in (b)(i)(1 mark)________________________________________________________________________________________________________________(iv) Given that the motor circuit is faulty, what test result value would youexpect for the test you have stated in (b)(i)?(1 mark)________________________________________________________________________________________________________________________________________________________________________(c) Explain why you would carry out the test stated in (a)(i) before the test youhave stated in (b)(i).(1 mark)________________________________________________________(d) In addition to a staff member receiving an electric shock off the frame ofthe vibrating plant, you were also told that in fact the motor had beentroublesome for some time. Sometimes the motor would run and at othertimes it would hum then trip the starter overload unit.State TWO possible reasons why the motor would run and, at other times,it would hum then trip the starter overload unit.(2 marks)(1) ________________________________________________________________________________________________________________(2) ________________________________________________________________________________________________________________
(a) (i) Protective earthing conductor test. (1 mark)(ii) The multimeter set on the lowestohms scale(½mark)To gain thismark the use ofthe multimeterhas to bequalified.(iii) Test between the earth connectionof the isolating switch to thevibrator frame.(1 mark)(iv) 1< or less (1 mark)(b) (i) Earth leakage test (1 mark) With this test,it is presumedthat theisolation isremoved andthe plantrelivened(ii) The clamp-type ammeter set on thelowest scale(½mark)To gain thismark the use ofthe ammeterhas to bequalified.(iii) . Start the motor running and (½mark). Measure the current in theprotective earthing conductor(½mark)(iv) Considerably more than 5 mA (1 mark)Alternative solution(i) Continuity test from each phase toframe of plant.(ii) Multimeter on low ohms.(iii) Test resistance from each phase tothe proven earth conductor (orplant) on both incoming andoutgoing phases of motor starter. (iv) A low enough resistance to indicatea possible connection of a liveconductor to the frame.(c) . To ensure the PEC is continuous (½ mark). And of low resistance (½ mark)(d) Any TWO of: (2 marks). Intermittent open circuit in thewindings on one phase.. Intermittent open circuit in onephase of the starter or isolatingswitch contacts.. Faulty bearings Intermittent mechanical fault indriven load
